Doctoral Degrees (Crop Science)

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Cowpea seed quality in response to production site and water stress.
(2007) Odindo, Alfred Oduor.; Modi, Albert Thembinkosi.; Southway, Colin.
Cowpea (Vigna unguiculata. L) is an important African crop. However, it is also an underutilized grain legume. Consequently, there is not enough research data on cowpea seed physiology. Whereas there is evidence of cowpea being a drought tolerant crop, there is no evidence to associate plant drought tolerance with seed quality in response to water stress. This study sought to understand the effect of production site and water stress on cowpea seed quality development with respect to germination capacity and vigour. Patterns of raffinose family of oligosaccharides (RFO) during seed development to mature dry stage were used to physiologically relate seed performance to water stress. The effect of water stress and exogenous ABA on the accumulation of stress LEA proteins (dehydrins) in relation to seed quality development and germination was investigated. RFOs are known for their roles in desiccation sensitivity but no studies have shown their significance in cowpeas. Seeds of six cowpea cultivars were produced at two distinct growth sites characterised by irrigated and dry land conditions. The seeds were assessed during six developmental stages, for water content, dry matter accumulation, and performance. Harvested seeds were then planted in a pot experiment under controlled conditions to examine the effect of water stress on seed quality development and data collected during three developmental stages. Harvested seeds from the pot experiment were subsequently analyzed for changes in RFO accumulation during development using gas chromatography. The seeds were also used to investigate the effect of water stress and ABA on the accumulation of stress LEA proteins (dehydrins) in relation to seed quality development in cowpea. In addition, this study evaluated the use of image analysis as a method that can be used to objectively determine seed coat colour variation in cowpea. Statistical variation in individual seed’s solute leakage for cowpea cultivars differing in seed coat colour and produced under different environmental conditions was explored and correlations were done between seed conductivity test with other aspects of seed performance during germination. Furthermore the results of the conductivity test were compared with accelerated aging test, in relation to seed performance. The study provided evidence that cowpea seed lots produced under different environmental, and possibly management conditions may not differ with respect to seed quality as determined by germination capacity and vigour. However, significant differences between sites with respect to seed maturation patterns determined by water content and dry matter accumulation were observed. Adverse maternal environmental effects on the subsequent performance of seeds in a drought tolerant crop may not necessarily lead to poor performance. Cultivar differences in response to simulated drought conditions at the whole plant and tissue level can be considerable and highly variable; however, these differences may not have adverse effects on the germination and vigour of the seeds. Drought avoidance mechanisms at the whole plant level in cowpea are quite efficient in allowing the species to adapt to simulated drought conditions. These mechanisms may allow the cowpea cultivars to maintain metabolism and restore conditions for their continued growth under water stress; and produce few seeds of high germination capacity and vigour. Stachyose was found to be the predominant member of the raffinose family of oligosaccharides in cowpea. It is suggested that stachyose accumulation could be used as an indicator of stress tolerance in cowpea. However, the relationship between RFO concentration and the acquisition of desiccation remained as a matter of speculation in the present study and is still generally inconclusive. There was no evidence to suggest the acquisition of maximum desiccation tolerance is associated with maximum seed vigour. It is suggested in cowpea, which is drought tolerant, that maximum vigour does not necessarily imply the acquisition of maximum desiccation tolerance; rather there is a minimum level of desiccation tolerance that is required for the development of optimal seed vigour. The use of an in vivo approach in the study of LEA function in cowpea enabled the accurate comparison of two different groups of LEA proteins in developing cowpea seeds under conditions of water stress and in relation to germination and vigour. Both group 1 LEA and group 2 LEA (dehydrin) were shown to increase in concentration in response to water stress. In addition group 1 LEA protein was observed to be relatively abundant in cowpea seeds. A maternal influence on LEA protein gene expression under conditions of water stress, which may induce dehydrin accumulation vii during the earlier stages of seed development, was implied by the observation that dehydrin-like proteins were induced after two weeks of development in cowpea plants subjected to stress during the vegetative phase. In addition, the exogenous application of ABA delayed radicle protrusion; this was associated with a delay in the disappearance of LEA proteins and is suggestive of a relationship between LEA protein accumulation and the acquisition of desiccation tolerance. The study has demonstrated that image analysis can objectively discriminate seed coat colour variation in cowpea. Dark coloured seeds in general performed better than light coloured seeds; however seed coat colour was not always associated with better performance. A newly developed Aging Stress Differential Index (ASDI) has been used in this study to demonstrate a link between seed coat colour and sensitivity to water stress. The ASDI correlated well with the observations relating stress tolerance to stachyose accumulation. The skewed distribution patterns in individual electrical conductivity and the presence of extreme values may have implications with respect to the suitability of using standard statistical analyses which compare mean values to evaluate such data. In addition variation in individual electrical conductivity may also be influenced by cultivar differences and the chemical composition of the seed coat. Therefore associations between seed coat colour and electrical conductivity as a measure of performance should be treated with caution. The AA test does reflect changes in seed vigour, however ranked electrical conductivity values after AA did not consistently reflect differences in seed performance between cultivars and sites, and they did not correlate well with other aspects of performance.
Season effects on the potential biomass and sucrose accumulation of some commercial cultivars of sugarcane.
(2009) Donaldson, Robin Albert.; Greenfield, Peter L.
An experiment was conducted at Pongola (27°24´S, 31°25´E; 308m altitude) in South Africa to study the effects of season on growth and potential biomass and sucrose yields on nine commercial sugarcane cultivars. The treatments that were the focus in this study consisted of the cultivars NCo376, N25 and N26 ratooned in March, April, May, August and December. The crops were well fertilized and kept free of weeds and diseases. Irrigation applications were scheduled with a computer programme to keep the crops free of stress at all times. Shoot populations were counted regularly to study shoot density dynamics. Leaf appearance rates, sizes, numbers and senescence were measured to study the development of green leaf area. Green foliage, dead trash and stalk mass were measured at 4, 8, 10, 11 and 12 months in each of the starting times and also at 13 months in the March, April and May ratoon crops. The fibre, sucrose and non-sucrose content of stalks were determined on these harvesting occasions. Yields were calculated in terms of individual shoots and area (m‾²). The fraction of PAR intercepted by the developing canopies was measured until full canopy and daily intercepted solar radiation was interpolated for the entire crop. An automated meteorological station adjacent to the experiment site provided daily weather data. Shoot densities were described by thermal time, however, average peak shoot densities were lowest in the May ratoon (31.8 m‾²) and highest in the December ratoon (48.7 m‾²). Shoot senescence was most rapid in August and December ratoons. At the final harvest shoot densities were highest in the March, April and May ratoon (14.8 to 14.2 m‾²) crops. NCo376 (16.4 m‾²) and N25 (13.6 m‾²) had higher final shoot densities than N26 (10.5 m‾²). Leaf appearance rate was also well described by thermal time, however the first twelve leaves took longer to appear in crops started in December i.e. the first phyllochron was longer (109.5°C d) than in crops started at other times (80.4 to 94.5°C d). Leaves produced during the early stages of December and August ratoon crops were larger (e.g leaf number 13 of N26 was 443 to 378 cm²) than in other crops. April and May ratoon crops produced much smaller leaves (e.g leaf number 9 of N26 was 170 to 105 cm²). Leaf senescence was slower in April and May ratoon crops (0.36 to 0.46 leaves per 100°C d) than in March (0.51 to 0.59 leaves per 100°C d) or August and December ratoon crops (0.60 to 0.68 leaves per 100°C d). December ratoon crops produced very high green leaf area indexes (LAI) (>7.0) at the age of four months; all other crops had lower LAI (3.3 to 6.0) and most peaked later (8 to 11 months of age). The LAI of N25 peaked at the age of 8 months while NCo376 and N26 peaked when 10 to 11 months old. Seasonal fraction of solar radiation intercepted was high in the March ratoon crops (0.84) and declined to 0.63 in the May ratoon crops and was highest in the December ratoon crop (0.88). N26 intercepted lower fractions of PAR than NCo376 and N25, particularly in the May and August ratoon crops. Biomass accumulation, although initially slow, tended to be linear in the March, April and May ratoon crops in relation to intercepted radiation. In August and particularly in the December ratoons biomass accumulation was initially rapid, and RUEs were high (2.65 g MJ‾¹ at 114 days in the December ratoon crops). However, biomass accumulation slowed when these December ratoon crops experienced winter. Low growth rates after winter, as well as low shoot densities resulted in December ratoon crops having produced significantly lower above-ground biomass yields (4 886 g m‾² at the age of 12 months) than March, April and May ratoon crops (6 760 to 5 715 gm‾² at the age of 12 months). The December ratoon crops responded poorly to the better growing conditions in spring and second summer and accumulated little biomass after winter. N26 shoots grew rapidly during the first 6-8 months of the December ratoon crop and it yielded better than NCo376 and N25 at harvesting (biomass yields were 5.8 and 13.3% higher at the age of 12 months, respectively). April ratoons produced significantly higher biomass yields (6 760 g m‾²) than March, August and December ratoons. May ratoon crops produced the highest cane fresh mass yields (18 151 g m‾²) and April, May and August ratoons produced significantly higher sucrose yields than March and December ratoons. The highest sucrose yield was produced by the April ratoon crop of N26 (2 385 g m‾²). On average, across the five ratoon dates, NCo376, N25 and N26 produced similar sucrose yields (1 902 to 1 959 g m‾²). Foliage production was severely limited during winter while sucrose accumulation was less affected by the low temperatures, resulting in accumulation of sucrose in the top sections of the culm. Low temperatures slowed the development of canopies in March, April and May ratoon crops, but these crops were able to recover their growth rates and produced high biomass and sucrose yields at the age of 12 months. The December ratoons experienced low winter temperatures (<12°C) when they had already accumulated relatively high yields and became moribund during winter. They were unable to accumulate any significant amounts of biomass during final four months before the final harvest at the age of 12 months. NCo376, N25 and N26 all yielded poorly in the December ratoon crop. However, there are cultivars that appear to be less sensitive to the low winters and are able to yield relatively well when they are ratooned in December. Sucrose yields of March, April and May ratoons were increased substantially (10.6 to 22.7%) by harvesting at the age of 13 months rather than at the age of 12 months. The poor growth of December ratoon crops after winter is possibly due to the recently revealed feedback signaling by high sugar levels induced by low temperatures on photosynthesis. The incorporation of the effects of low temperature and the feedback signaling with the objective of better simulating yields of December ratoons is a proposed study at the South African Sugarcane Research Institute. Annual mean sucrose yields of NCo376, N25 and N26 crops were estimated to be 17% higher in March than in December ratoons. The suggested short term remedy therefore of the poor December yields is to shift milling seasons to include March and exclude December harvested crops in the northern irrigated regions. March crops grow vigorously during the months close to harvesting and therefore have lower levels of sucrose content which can be corrected with chemical ripeners.
Taro (Colocasia esculenta L. Schott) yield and quality in response to planting date and organic fertilisation.
(2009) Mare, Rorisang 'Maphoka.; Modi, Albert Thembinkosi.
Despite the importance of taro (Colocasia esculenta L. Schott) as a food security crop, scientific research on it is scanty in South Africa. Production site, planting date and fertiliser regime affect crop performance and quality, particularly that of cultivars, because they tend to be adapted to specific localities. Storage temperature and packaging method on the other hand affect the shelf-life. To investigate performance and quality of three taro cultivars in response to planting date and fertilisation, a study was carried out at two sites in KwaZulu-Natal, South Africa (Ukulinga and Umbumbulu), during the 2007/2008 growing seasons. The effect of two storage temperatures (12oC and ambient temperature) and three packaging methods (polyethylene bags, mesh bags and open boxes) on cormel quality following storage was also investigated for three cultivars. Delayed planting negatively affected the number of cormels plant-1 and fresh cormel mass plant-1. Fertilisation and cultivar affected the number of cormels plant-1 and fresh cormel mass plant-1 only when planting was done in October and November at both sites. Fertilisation increased the number of cormels plant-1 for all cultivars except Dumbe-dumbe. Dumbe-dumbe had the lowest number of cormels plant-1 but the highest number of marketable cormels plant-1. Dumbe-dumbe showed the lowest fresh cormel mass plant-1 in October and the highest in November at Ukulinga. Fertisation increased fresh cormel mass plant-1 in October at Umbumbulu. Dry matter content was negatively affected by fertilisation at Ukulinga. The response of dry matter content, specific gravity, protein, minerals, reducing sugars and starch content was variable depending on cultivar. Delayed planting negatively affected starch content for Dumbe-dumbe and Pitshi at Ukulinga. Fertilisation decreased starch content of Pitshi, while delayed planting increased sugar content for Dumbe-dumbe and decreased it for Mgingqeni and Pitshi at Umbumbulu. Dumbe-dumbe had higher starch content and higher reducing sugars. Considering all growth and quality parameters, it is recommended that Dumbe-dumbe is the best taro cultivar for crisping and the best time to plant it is October with 160 kg N ha-1 of organic fertiliser and November with 320 kg N ha-1 at Ukulinga whereas at Umbumbulu the best time to plant Dumbe-dumbe is October with 320 kg N ha-1 of the fertiliser. Starch granules degradation, alpha-amylase activity and sprouting increased with storage time and storage temperature. Cormels of Mgingqeni stored in polyethylene bags showed highest alpha-amylase activity and sprouting. Reducing sugar content increased and starch content decreased with time in storage and decline in storage temperature. It is recommended that taro cormels be stored in mesh bags at 12oC. The chapters of this thesis represent different studies presented as different papers. Chapter 1 is a general introduction to explain the study background and hypothesis. Chapter 2 is a general review of literature. Chapter 3 is on growth, development and yield of taro in response to planting date and fertilisation. Chapter 4 is on the influence of planting date and organic fertiliser on crisping quality of taro cormels. Chapter 5 is on changes in the surface morphology of starch granules and alpha-amylase activity of taro during storage. Chapter 6 is on the effects of pre- and post-harvest practices on starch and reducing sugars of taro. The last chapter is a general discussion and conclusions.
Physiological basis of seed germination in Cleome gynandra (L.)
(2005) Ochuodho, Julius Onyango.; Modi, Albert Thembinkosi.
Dormancy characteristics and optimum conditions for germination of Cleome gynandra seeds have not been explained. Seed storage proteins were extracted, analysed with SDS-PAGE and sequenced. Seed proteins of Cleome were characterised by comparison with those of wild mustard (Brassica kaber). Wild mustard showed seed proteins composed of two α-chains of molecular weight (24-32 kDa) and another two β-chains of 18-22 kDa. The seed proteins of Cleome comprised two α-chain polypeptides of molecular weight (25-30 kDa), two β-chain polypeptides of molecular weight (18-20 kDa) and a smaller β-chain of 13-15 kDa. The storage proteins occurred in the seeds as dimeric complexes of molecular weight 40-65 kDa, which were broken into polypeptide chains of approximately 20 and 30 kDa by the reducing . action of DTT. Comparison with proteins in the proteome library and similarity index further confirmed that the seed proteins of Cleome had similarities with those of wild mustard. Two dimensional SDS-PAGE showed that the two species have nine similar polypeptides and four different ones. Events associated with dormancy release during seed germination still require explanation. Seeds of Cleome are characterised by low germination and there has been no explanation for this. Changes in protein expression during germination of Cleome in the presence or absence of light and at constant or alternating temperatures were examined. The germination of Cleome seeds at 20 degrees C was inhibited by light, but it was improved at 20 degrees C in darkness. There was no photoinhibition when seeds were germinated at constant 30 degrees C or alternating 20/30 degrees C (16 h night and 8 h day) for 10 days. Four proteins were observed to decrease in expression as germination progressed, but remained unchanged during photoinhibition. Photoinhibition was expressed more in seeds that were harvested late, after the pods had turned brown. These seeds showed a fifth, low molecular weight protein (13 kDa) that was absent from the immature seeds and embryos. Photinhibition is a pseudo-dormancy condition during which seed storage proteins are not utilised and the seed coat could partially play a role in it. The temperatures for the germination of Cleome in darkness have been determined. However, prior to this study the effects of temperature, light and pre-germination treatments (chilling, scarification, hydration and germination in the presence of KN0(3) or GA(3) on the germination of the seeds of this species have not been investigated. Seeds were germinated for 10 days and the final count of germination was used to determine seed performance. The highest germination percentage (60% and 80%, for a 2-year old and a l-year old seed lot, respectively) of untreated seeds was achieved when alternating temperatures of 20/30 degrees C (16 h/S h) in the dark or constant 30 degrees C in the dark were used. Among the pre-germination treatments, only scarification (puncturing of seeds at the radicle end) improved germination. Seeds were found to be negatively photoblastic, and the phenomenon was more pronounced when they were germinated at 20 degrees C and 12 h photoperiod or longer. Germination of photoinhibited seeds was, however, improved by treatment with GA(3) It is recommended that the germination of Cleome be undertaken under conditions of darkness and at either alternating 20/30 degrees C or continuous 30 degrees C. Seed lot vigour and seedling vigour are two important seed quality aspects that are used in defining the seed germination process. Seed germination is appropriately characterised by radicle protrusion and the attainment of normal seedling structures. However, the international rules for testing seeds combine radicle protrusion and normal seedling attainment in separating seed germination into the first and final counts. The challenge to a seed analyst testing the germination of a species whose first and final counts are unknown is that there is no statistical guideline to determine these important stages of seed germination. Cauliflower and broccoli, for which the first and final counts are published in the international rules for testing seeds and Cleome, for which there is no data on the first and final counts, were examined to determine the statistical significances of the first and final counts. Analysis of variance, logistic regression, 'broken-stick' regression models and survival analysis procedures were used. Analysis of variance showed that there were no differences between the germination percentages on the fourth, fifth and seventh days of germination. Low and stable standard deviations were recorded when evaluating germination after the fourth day. The germination curves of broccoli and cauliflower did not fit the Gompertz curve but fitted the exponential curve. The broken-stick model 'broke' the cumulative germination curve for the Cleome seed lots into two linear curves that were significantly different, but failed to break those for broccoli and cauliflower. However, this study confirmed the first and final counts for broccoli and cauliflower as determined by the international rules for testing seeds. Broken-stick modelling and life table analyses confirmed the fourth day as being appropriate to determine the first count for Cleome germination. There was no evidence of further seed germination after the seventh day as shown by probability density and hazard rate. It is suggested that for Cleome, the 'first count' and 'final count' be performed on the fourth and seventh day of the germination, respectively.
Drought tolerance and water-use of selected South African landraces of Taro (Colocasia esculenta L. schott) and Bambara groundnut (Vigna subterranea L. Verdc)
(2012) Mabhaudhi, Tafadzwanashe.; Modi, Albert Thembinkosi.
Issues surrounding water scarcity will become topical in future as global fresh water resources become more limited thus threaten crop production. Predicted climate change and increasing population growth will place more pressure on agriculture to produce more food using less water. As such, efforts have now shifted to identifying previously neglected underutilised species (NUS) as possible crops that could be used to bridge the food gap in future. Taro (Colocasia esculenta L. Schott) and bambara groundnut (Vigna subterranea L. Verdc) currently occupy low levels of utilisation in South Africa. Both crops are cultivated using landraces with no improved varieties available. Information describing their agronomy and water–use is limited and remains a bottleneck to their promotion. The aim of this study was to determine the drought tolerance and water–use of selected landraces of taro and bambara groundnut from KwaZulu-Natal, South Africa. In order to meet the specific objectives for taro and bambara groundnut management, an approach involving conventional and modelling techniques was used. Three taro landraces [Dumbe Lomfula (DL), KwaNgwanase (KW) and Umbumbulu (UM)] were collected from the North Coast and midlands of KwaZulu-Natal, South Africa, in 2010. The UM landrace was classified as Eddoe type taro (C. esculenta var. antiquorum) characterised by a central corm and edible side cormels. The DL and KW landraces were classified as Dasheen (C. esculenta var. esculenta), characterised by a large edible main corm and smaller side cormels. A bambara groundnut landrace was collected from Jozini, KwaZulu- Natal, and characterised into three selections (‘Red’, ‘Light-brown’ and ‘Brown’) based on seed coat colour. Seed colour was hypothesised to have an effect on seed quality. Field and rainshelter experiments were conducted for both taro and bambara landraces at Roodeplaat in Pretoria and Ukulinga Research Farm in Pietermaritzburg, over two growing seasons (2010/11 and 2011/12). The objective of the field trials for taro and bambara groundnut was to determine mechanisms associated with drought tolerance in taro and bambara groundnut landraces. Experiments were laid out in a split-plot design where irrigation [fully irrigated (FI) and rainfed (RF)] was the main factor and landraces (3 landraces of either taro or bambara groundnut) were sub-factors. Treatments were arranged in a randomised complete block design (RCBD), replicated three times. Rainfed trials were established with irrigation to allow for maximum crop stand. Thereafter, irrigation was withdrawn. Whilst experimental designs and layouts for taro and bambara groundnut were similar, differences existed with regards to plot sizes and plant spacing. Trials were planted on a total land area of 500 m2 and 144 m2, for taro and bambara groundnut, respectively. Plant spacing was 1 m x 1 m for taro and 0.3 m x 0.3 m for bambara groundnut. Irrigation scheduling in the FI treatment was based on ETo and Kc and was applied using sprinkler irrigation system. Separate rainshelter experiments were conducted for taro and bambara groundnut landraces at Roodeplaat, to evaluate growth, yield and water-use of taro and bambara groundnut landraces under a range of water regimes. The experimental design was similar for both crops, a RCBD with two treatment factors: irrigation level [30, 60 and 100% crop water requirement (ETa)] and landrace (3 landraces), replicated three times. Irrigation water was applied using drip irrigation system based on ETo and Kc. Data collection in field and rainshelter trials included time to emergence, plant height, leaf number, leaf area index (LAI), stomatal conductance and chlorophyll content index (CCI). For taro field trials, vegetative growth index (VGI) was also determined. Yield and yield components (harvest index, biomass, corm number and mass) as well as water–use efficiency (WUE) were determined at harvest.Intercropping of taro and bambara groundnut was evaluated under dryland conditions using farmers’ fields at Umbumbulu, KwaZulu–Natal, South Africa. The experimental design was a RCBD replicated three times. Intercrop combinations included taro and bambara groundnut sole crops, a 1:1 (one row taro to one row bambara groundnut) and 1:2 intercrop combinations. The taro UM landrace and ‘Red’ bambara groundnut landrace selection were used in the intercropping study. Lastly, data collected from field and rainshelter experiments were used to develop crop parameters to calibrate and validate the FAO’s AquaCrop model for taro and bambara groundnut landraces. The UM landrace was used for taro while the ‘Red’ landrace selection was used for bambara groundnut. AquaCrop was calibrated using observed data from optimum (FI) experiments conducted during 2010/11. Model validation was done using observations from field and rainshelter experiments conducted during 2011/12 as well as independent data. Results showed that all taro landraces were slow to emerge (≈ 49 days after planting). Stomatal conductance declined under conditions of limited water availability (RF, 60% and 30% ETa). The UM landrace showed better stomatal regulation compared with KW and DL landraces under conditions of limited water availability. Plant growth (plant height, leaf number, LAI and CCI) of taro landraces was lower under conditions of limited water availability (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). The UM landrace showed moderate reductions in growth compared with the DL and KW landraces, suggesting greater adaptability to water limited conditions. The VGI showed a large reduction in growth under RF conditions and confirmed the UM landrace’s adaptability to limited water availability. Limited water availability (RF, 60% and 30% ETa) resulted in lower biomass, HI, and final yield in taro landraces relative to optimum conditions (FI and 100% ETa). For all trials, the DL landrace failed to produce any yield. WUE of taro landraces was consistent for the three irrigation levels (30, 60 and 100% ETa); however, on average, the UM landrace was shown to have a higher WUE than the KW landrace. Bambara groundnut landraces were slow to emerge (up to 35 days after planting). ‘Red’ and ‘Brown’ landrace selections emerged better than the ‘Light-brown’ landrace selection, confirming the effect of seed colour on early establishment performance. Plant growth (stomatal conductance, CCI, plant height, leaf number, LAI and biomass accumulation) was lower under conditions of limited water availability (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). The ‘Red’ landrace selection showed better adaptation to stress. Limited water availability resulted in early flowering and reduced flowering duration as well as early senescence and maturity of bambara groundnut landrace selections. The ‘Red’ landrace selection showed delayed leaf senescence under conditions of limited water availability. Yield reductions of up to 50% were observed under water limited conditions (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). Water use efficiency increased at 60% and 30% ETa, respectively, relative to 100% ETa, implying adaptabilityto limited water availability. The ‘Red’ landrace selection showed better yield stability and WUE compared with the ‘Brown’ and ‘Light-brown’ landrace selections suggesting that seed colour may be used as a selection criterion for drought tolerance in bambara groundnut landraces. The intercropping study showed that intercropping, as an alternative cropping system, had more potential than monocropping. Evaluation of growth parameters showed that taro plant height was generally unaffected by intercropping but lower leaf number was observed as compared with the sole crop. Bambara groundnut plants were taller and had more leaves under intercropping relative to the sole crop. Although not statistically significant, yield was generally lower in the intercrops compared with the sole crops. Evaluation of intercrop productivity using the land equivalent ratio (LER) showed that intercropping taro and bambara groundnut at a ratio of 1:1 was more productive (LER = 1.53) than intercropping at a ratio of 1:2 (LER = 1.23). The FAO’s AquaCrop model was then calibrated for the taro UM landrace and ‘Red’ bambara groundnut landrace selection. This was based on observations from previous experiments that suggested them to be drought tolerant and stable. Calibration results for taro and bambara groundnut landraces showed an excellent fit between predicted and observed parameters for canopy cover (CC), biomass and yield. Model validation for bambara groundnut showed good model performance under field (FI and RF) conditions. Model performance was satisfactory for rainshelters. Validation results for taro showed good model performance under all conditions (field and rainshelters), although the model over-estimated CC for the declining stage of canopy growth under RF conditions. Model verification using independent data for taro showed equally good model performance. In conclusion, the taro UM landrace and ‘Red’ bambara groundnut landrace selection were shown to be drought tolerant and adapted to low levels of water–use. The mechanisms responsible for drought tolerance in the taro UM landrace and ‘Red’ bambara groundnut landrace selection were described as drought avoidance and escape. The taro UM landrace and ‘Red’ bambara groundnut landraces avoided stress through stomatal regulation, energy dissipation (loss of chlorophyll) as well as reducing canopy size (plant height, leaf number and LAI), which translates to minimised transpirational water losses. This indicated landrace viii adaptability to low levels of water–use. The ‘Red’ bambara groundnut landrace selection showed phenological plasticity and escaped drought by flowering early, delaying leaf senescence, and maturing early under conditions of limited water availability. Performance of the ‘Red’ landrace selection lends credence to the use of seed coat colour as a possible selection criterion for drought tolerance in bambara groundnut, and possibly for other landraces with variegated seed. The taro UM landrace escaped drought by maturing early under conditions of limited water availability. The FAO’s AquaCrop model was successfully calibrated and validated for taro UM and ‘Red’ bambara groundnut landraces. The calibration and validation of AquaCrop for taro is the first such attempt and represents progress in the modelling of neglected underutilised crops. The calibration and validation of AquaCrop for taro requires further fine-tuning while that for bambara groundnut still needs to be tested for more diverse landraces.
Drought tolerance and water-use of selected South African landraces of Taro (Colocasia esculenta L. schott) and Bambara groundnut (Vigna subterranea L. Verdc)
(2012) Mabhaudhi, Tafadzwanashe.; Modi, Albert Thembinkosi.
Issues surrounding water scarcity will become topical in future as global fresh water resources become more limited thus threaten crop production. Predicted climate change and increasing population growth will place more pressure on agriculture to produce more food using less water. As such, efforts have now shifted to identifying previously neglected underutilised species (NUS) as possible crops that could be used to bridge the food gap in future. Taro (Colocasia esculenta L. Schott) and bambara groundnut (Vigna subterranea L. Verdc) currently occupy low levels of utilisation in South Africa. Both crops are cultivated using landraces with no improved varieties available. Information describing their agronomy and water–use is limited and remains a bottleneck to their promotion. The aim of this study was to determine the drought tolerance and water–use of selected landraces of taro and bambara groundnut from KwaZulu-Natal, South Africa. In order to meet the specific objectives for taro and bambara groundnut management, an approach involving conventional and modelling techniques was used. Three taro landraces [Dumbe Lomfula (DL), KwaNgwanase (KW) and Umbumbulu (UM)] were collected from the North Coast and midlands of KwaZulu-Natal, South Africa, in 2010. The UM landrace was classified as Eddoe type taro (C. esculenta var. antiquorum) characterised by a central corm and edible side cormels. The DL and KW landraces were classified as Dasheen (C. esculenta var. esculenta), characterised by a large edible main corm and smaller side cormels. A bambara groundnut landrace was collected from Jozini, KwaZulu- Natal, and characterised into three selections (‘Red’, ‘Light-brown’ and ‘Brown’) based on seed coat colour. Seed colour was hypothesised to have an effect on seed quality. Field and rainshelter experiments were conducted for both taro and bambara landraces at Roodeplaat in Pretoria and Ukulinga Research Farm in Pietermaritzburg, over two growing seasons (2010/11 and 2011/12). The objective of the field trials for taro and bambara groundnut was to determine mechanisms associated with drought tolerance in taro and bambara groundnut landraces. Experiments were laid out in a split-plot design where irrigation [fully irrigated (FI) and rainfed (RF)] was the main factor and landraces (3 landraces of either taro or bambara groundnut) were sub-factors. Treatments were arranged in a randomised complete block design (RCBD), replicated three times. Rainfed trials were established with irrigation to allow for maximum crop stand. Thereafter, irrigation was withdrawn. Whilst experimental designs and layouts for taro and bambara groundnut were similar, differences existed with regards to plot sizes and plant spacing. Trials were planted on a total land area of 500 m2 and 144 m2, for taro and bambara groundnut, respectively. Plant spacing was 1 m x 1 m for taro and 0.3 m x 0.3 m for bambara groundnut. Irrigation scheduling in the FI treatment was based on ETo and Kc and was applied using sprinkler irrigation system. Separate rainshelter experiments were conducted for taro and bambara groundnut landraces at Roodeplaat, to evaluate growth, yield and water-use of taro and bambara groundnut landraces under a range of water regimes. The experimental design was similar for both crops, a RCBD with two treatment factors: irrigation level [30, 60 and 100% crop water requirement (ETa)] and landrace (3 landraces), replicated three times. Irrigation water was applied using drip irrigation system based on ETo and Kc. Data collection in field and rainshelter trials included time to emergence, plant height, leaf number, leaf area index (LAI), stomatal conductance and chlorophyll content index (CCI). For taro field trials, vegetative growth index (VGI) was also determined. Yield and yield components (harvest index, biomass, corm number and mass) as well as water–use efficiency (WUE) were determined at harvest. Intercropping of taro and bambara groundnut was evaluated under dryland conditions using farmers’ fields at Umbumbulu, KwaZulu–Natal, South Africa. The experimental design was a RCBD replicated three times. Intercrop combinations included taro and bambara groundnut sole crops, a 1:1 (one row taro to one row bambara groundnut) and 1:2 intercrop combinations. The taro UM landrace and ‘Red’ bambara groundnut landrace selection were used in the intercropping study. Lastly, data collected from field and rainshelter experiments were used to develop crop parameters to calibrate and validate the FAO’s AquaCrop model for taro and bambara groundnut landraces. The UM landrace was used for taro while the ‘Red’ landrace selection was used for bambara groundnut. AquaCrop was calibrated using observed data from optimum (FI) experiments conducted during 2010/11. Model validation was done using observations from field and rainshelter experiments conducted during 2011/12 as well as independent data. Results showed that all taro landraces were slow to emerge (≈ 49 days after planting). Stomatal conductance declined under conditions of limited water availability (RF, 60% and 30% ETa). The UM landrace showed better stomatal regulation compared with KW and DL landraces under conditions of limited water availability. Plant growth (plant height, leaf number, LAI and CCI) of taro landraces was lower under conditions of limited water availability (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). The UM landrace showed moderate reductions in growth compared with the DL and KW landraces, suggesting greater adaptability to water limited conditions. The VGI showed a large reduction in growth under RF conditions and confirmed the UM landrace’s adaptability to limited water availability. Limited water availability (RF, 60% and 30% ETa) resulted in lower biomass, HI, and final yield in taro landraces relative to optimum conditions (FI and 100% ETa). For all trials, the DL landrace failed to produce any yield. WUE of taro landraces was consistent for the three irrigation levels (30, 60 and 100% ETa); however, on average, the UM landrace was shown to have a higher WUE than the KW landrace. Bambara groundnut landraces were slow to emerge (up to 35 days after planting). ‘Red’ and ‘Brown’ landrace selections emerged better than the ‘Light-brown’ landrace selection, confirming the effect of seed colour on early establishment performance. Plant growth (stomatal conductance, CCI, plant height, leaf number, LAI and biomass accumulation) was lower under conditions of limited water availability (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). The ‘Red’ landrace selection showed better adaptation to stress. Limited water availability resulted in early flowering and reduced flowering duration as well as early senescence and maturity of bambara groundnut landrace selections. The ‘Red’ landrace selection showed delayed leaf senescence under conditions of limited water availability. Yield reductions of up to 50% were observed under water limited conditions (RF, 60% and 30% ETa) relative to optimum conditions (FI and 100% ETa). Water use efficiency increased at 60% and 30% ETa, respectively, relative to 100% ETa, implying adaptability to limited water availability. The ‘Red’ landrace selection showed better yield stability and WUE compared with the ‘Brown’ and ‘Light-brown’ landrace selections suggesting that seed colour may be used as a selection criterion for drought tolerance in bambara groundnut landraces. The intercropping study showed that intercropping, as an alternative cropping system, had more potential than monocropping. Evaluation of growth parameters showed that taro plant height was generally unaffected by intercropping but lower leaf number was observed as compared with the sole crop. Bambara groundnut plants were taller and had more leaves under intercropping relative to the sole crop. Although not statistically significant, yield was generally lower in the intercrops compared with the sole crops. Evaluation of intercrop productivity using the land equivalent ratio (LER) showed that intercropping taro and bambara groundnut at a ratio of 1:1 was more productive (LER = 1.53) than intercropping at a ratio of 1:2 (LER = 1.23). The FAO’s AquaCrop model was then calibrated for the taro UM landrace and ‘Red’ bambara groundnut landrace selection. This was based on observations from previous experiments that suggested them to be drought tolerant and stable. Calibration results for taro and bambara groundnut landraces showed an excellent fit between predicted and observed parameters for canopy cover (CC), biomass and yield. Model validation for bambara groundnut showed good model performance under field (FI and RF) conditions. Model performance was satisfactory for rainshelters. Validation results for taro showed good model performance under all conditions (field and rainshelters), although the model over-estimated CC for the declining stage of canopy growth under RF conditions. Model verification using independent data for taro showed equally good model performance. In conclusion, the taro UM landrace and ‘Red’ bambara groundnut landrace selection were shown to be drought tolerant and adapted to low levels of water–use. The mechanisms responsible for drought tolerance in the taro UM landrace and ‘Red’ bambara groundnut landrace selection were described as drought avoidance and escape. The taro UM landrace and ‘Red’ bambara groundnut landraces avoided stress through stomatal regulation, energy dissipation (loss of chlorophyll) as well as reducing canopy size (plant height, leaf number and LAI), which translates to minimised transpirational water losses. This indicated landrace adaptability to low levels of water–use. The ‘Red’ bambara groundnut landrace selection showed phenological plasticity and escaped drought by flowering early, delaying leaf senescence, and maturing early under conditions of limited water availability. Performance of the ‘Red’ landrace selection lends credence to the use of seed coat colour as a possible selection criterion for drought tolerance in bambara groundnut, and possibly for other landraces with variegated seed. The taro UM landrace escaped drought by maturing early under conditions of limited water availability. The FAO’s AquaCrop model was successfully calibrated and validated for taro UM and ‘Red’ bambara groundnut landraces. The calibration and validation of AquaCrop for taro is the first such attempt and represents progress in the modelling of neglected underutilised crops. The calibration and validation of AquaCrop for taro requires further fine-tuning while that for bambara groundnut still needs to be tested for more diverse landraces.
Quantitative and qualitative studies on grain sorghum for traditional beer (dolo) production in Burkina Faso.
(2012) Palé, Siébou.; Taonda, Sibiri Jean-Baptiste.; Mason, Stephen.; Bougouma, Boniface.; Kaboré, Daniel Pato.
In the Central Plateau of Burkina Faso, grain sorghum [Sorghum bicolor (L.) Moench] is the major cereal crop used to produce the traditional beer commonly called dolo. Improvement of the dolo chain supply that requires quantity and quality grain for dolo production to improve the supply chain constitutes a big challenge for processors and police-makers. To that end, studies were conducted (1) to determine the best cropping practice to optimize grain yields and grain quality for IRAT9 and Framida (two red grain sorghum varieties) for dolo production, through experiments combining water management techniques and fertilizer treatments and conducted from 2003 to 2005, (2) to evaluate sorghum malt and dolo quality criteria and parameters affecting quality and (3) to assess costs and profits of malt and traditional beer (dolo) production through surveys conducted from December 2006 through January 2007. Results indicated that combination of water management techniques and fertilizer treatments largely influenced grain yield production and grain quality of the two red grain sorghum varieties. The best cropping system to optimize grain yield of Framida and IRAT9 was the use of tied-ridges and application of microdose with additional phosphorus and nitrogen. Tie-ridging technique produced the highest yield benefit of 241 kg ha-1 for Framida and 395 kg ha-1. Microdose with additional phosphorus and nitrogen produced the highest grain yield increases from 420 to 756 kg ha-1 for Framida and from 812 to 1346 kg ha-1 for IRAT9. Previous studies suggested a diastatic power of at least 70 mg of maltose equivalent per g of dry malt weight per minute for commercially acceptable sorghum malt in Burkina Faso, though the diastatic power of grain sorghum malt produced under traditional conditions was found to be 53.13 mg of maltose equivalent per g of dry malt weight per minute. Results from these studies indicate that for Framida the combination of water management techniques such as scarifying, tie-ridging, manual zai, mechanized zai or dry soil tillage with application of microdose + additional 20 kg P ha-1 and 30 kg N ha-1 produced a sorghum malt with higher diastatic power than the commercially acceptable one (Table 4.4) . For IRAT9, this targeted dp is only achieved in an agronomic practice combining scarifying and microdose + additional 20 kg P ha-1 and 30 kg N ha-1 (Table 4.5). Production of sorghum grain and malt with the needed characteristics for high dolo quality would be the use of water management techniques that sufficiently improve soil water conditions in combination with a microdose application with additional phosphorus and nitrogen to provide sufficient nutrients and particularly nitrogen to the crop. The malt and traditional beer (dolo) quality assessment study indicated that the major quality criteria for malt quality assessment were perceived to be taste (82% of respondents) and presence/absence of roots in the malt (76%). Taste (82%), alcohol content (73%) and wort sufficiently cooked (63%) were perceived as major criteria for the dolo quality assessment. The major parameters affecting malt quality were perceived to be malt production period (100% of respondents), proportions of grain and the amount of water entering malting (100%), presence of pesticide residues in the malting grains (62%) and age of grain (64%). Processing method (100%), yeast source (100%), proportions of the components (crushed grain, water, mucilage, yeast) entering dolo production (97%), malt quality (97%), wort temperature at time of inoculation (96%), amount of energy available for cooking wort and sediment boiling time (92%), quality of mucilage (78%), malt with non-sweet taste (75%), presence/absence of roots in the malt (73%) and ease of filtering crushed malt (64%) were perceived as major parameters affecting the dolo quality. The economic study showed differences in costs, sales and profits in the dolo chain from one group of members to another and from one category to another within each group. The study also indicated that, though equipment and raw materials were readily available throughout the year, their high cost limited accessibility and acquisition. Actions must be undertaken by policy-makers and developers to make credit available for farmers to produce quality sorghum grain and women processors to purchase equipment needed for malt and dolo processing, conservation and distribution, thus increasing profits. Other important aspects to consider when designing programs to improve the dolo supply chain were the organization of malt and dolo production and marketing systems and suitable training programs to the benefit of all members to improve production skills and increase profit per unit cost in all dolo activities. Results from this study will help in the improvement of the dolo supply chain in Burkina Faso by providing more reliable information for (1) development of best cropping practices to improve grain quality, and providing better selection criteria for sorghum breeding programs, (2) development of training programs for efficient dolo brewing processes and (3) development of training programs to improve marketing systems and skills for chain members. It is expected that results from this study would further help increase the economic potential of sorghum in Burkina Faso and neighboring countries since a commercialized traditional product has a greater chance of being popular and culturally acceptable than an exotic or novel product.
An evaluation of source-sink relationships in three dry bean (Phaseolus vulgaris L.) cultivars.
(1989) Liebenberg, Andries Johannes.; Lea, J. D.
The effect of intensities and times of source or sink related stresses on the growth and development of a determinant (Teebus) and two indeterminant (NEP 2, Bonus) dry bean (Phaseolus vulgaris L.) cultivars was measured in a series of field experiments at Potchefstroom Research Station. Variation in stress levels was attained by defoliation, thinning, light intensity manipulation (shades and reflectors) and removal of reproductive organs. No permanent detrimental effect on vegetative or reproductive organs was observed when source was reduced or increased during the vegetative period (V1-V6f) . Flower initiation (V6f-R1) was identified as the period most sensitive to defoliation as expressed in vegetative growth and economic yield. The negative effect of shading on vegetative development was reduced by an extended growing period . Thinning during flower initiation increased the vegetative and reproductive sink. A source stress (defoliation and shading) during the flowering period (R1-R5) restricted partitioning to the reproductive organs reducing seed yield and harvest index values. Reduced interplant competition during flowering favoured partitioning to the reproductive organs. Source size had a direct relationship with economic yield during flowering. This was confirmed by the absence of a yield response to partial depodding. A lack of response to defoliation (NEP 2) and shading (Bonus) may indicate a limited sink size in these two cultivars. During seed filling (R5-R9) Bonus was very sensitive to defoliation while NEP 2 was insensitive. Bonus was less sensitive to shading than Teebus. Thus in certain cultivars the level of current photosynthesis had a significant effect on seed yield throughout seed filling. The lack of a yield response to thinning in all cultivars during this period indicated that the potential sink size was set before R5. The results provided strong evidence supporting the concepts of yield component compensation in dry beans. Pod number was most seriously affected by defoliation during flower initiation and flowering. The potential sink size was determined mainly through the number of pods per plant which was in balance with the source unless some stress factor was present. The number of seeds per pod responded to current photosynthesis as well as the previously set pod number. Seed size was the least responsive yield component and it had a consistent negative relationship with the number of pods per plant.
Modelling deficit irrigation of wheat in Zimbabwe.
(1993) MacRobert, John Findlay.; Savage, Michael John.
Wheat is grown in Zimbabwe during the relatively dry, cool winter with irrigation. On most large-scale farms, land resources exceed irrigation water resources. Consequently, the efficient use of water is of prime concern. This has led to the development and adoption of deficit irrigation techniques, with the aim of maximizing net financial returns per unit of applied water rather than per unit land area. This often requires that less water be applied than that required for maximum yields, which implies that water deficits are allowed to develop in the crop. Although the basic principles of deficit irrigation are known, there exists no systematic procedure for advising farmers on whether or not to, or how to, employ such a management option in Zimbabwe. This research was therefore undertaken to develop an interactive computer programme that would assist farmers in determining optimum irrigation strategies for wheat. The CERES-Wheat version 2.10 crop simulation model (WHV21) was chosen as the basis for this programme. In order to validate and modify, where necessary, WHV21, a series of field experiments were conducted at a number of wide-ranging locations in Zimbabwe during the period 1986 to 1992. These included sowing date x cultivar, sowing date x seeding rate, growth analysis and irrigation experiments. In all, 122 data sets were collected, of which 47 were used for model validation and 75 used for calibration and modification of WHV21. The initial validation of WHV21 showed that the model gave biased and imprecise predictions of phenological development, particularly under deficit-irrigated conditions. The simulation of tillering was poor and the model tended to over-predict dry matter accumulation and under-predict leaf area indices. The yield component and grain yield predictions were also generally imprecise. On the other hand, for most data sets, the simulated soil water contents were similar to measured soil water contents. These inconsistencies prompted a revision of the phenological and growth subroutines of the model. In the phenological subroutine, new thermal time durations and base temperatures (Tb ) for all growth phases were determined from regressions of the rate of phasic development on mean air temperatures. For growth phases one, two and three, a Tb of 4°C was established, whereas for growth phases four and five, a Tb of 3°C was used. The revised model included the prediction of leaf emergence (as apposed to leaf appearance) and first node appearance (Zadoks growth stage 31). In order to hasten plant development under conditions of soil water deficit stress, daily thermal time was made to increase whenever the actual root water uptake declined below 1.5 times the potential plant evaporation. These changes improved the prediction of crop phasic development: for example, the Index of Agreement for the prediction of physiological maturity was improved from 0.643 with WHV21 to 0.909 with the revised version. Many changes were made to the growth subroutine, inter alia: 1. the extinction coefficient in the exponential photosynthetically active radiation (PAR) interception equation was reduced from 0.85 to 0.45; 2. an allowance was made for the interception of PAR by the wheat ears during growth phases four and five, with the statement IPAR=1-EXP(-O.004*TPSM), where IPAR is the proportion of PAR intercepted and TPSM is the number of tillers m-2; 3. the area to mass ratio of leaves was increased from 115 cm2 g -l to 125 cm2 g -l during growth phase two and this was allowed to decrease under conditions of water deficit stress; 4. tiller production during growth phase one was made a function of daily thermal time, total daily solar radiant density and plant density, moderated by high air temperatures and a new soil water deficit factor that takes the dryness of the surface soil layer into account; 5. a cold temperature routine was added to reduce kernel numbers whenever the exposed minimum air temperature decreased below 0°C during the period ear emergence to the start of the linear kernel growth phase (cold temperatures during anthesis occasionally cause reductions to kernel numbers in Zimbabwe); and 6. the kernel growth rate was gradually increased during growth phase four, and the rate of kernel growth was increased under conditions of water deficit stress during growth phase five. The modifications made to the growth subroutine of WHV21 improved predictions of tillering, ear density, yield components and yield on the independent validation data set. The modified model (WHVZIM22) was used to evaluate wheat sowing date and irrigation strategies on ten-year sets of weather data from representative locations in Zimbabwe. The results indicated that the highest yields were obtained with sowings during the latter half of April and the early part of May at all tested locations. Yields were greater for each sowing date and irrigation regime at the high altitude (1480 m) location than at warmer, lower altitude locations. The response of wheat yield to irrigation application was typically curvilinear, particularly on the soil with a high water holding capacity. Maximum yields were attained with the application of 400 to 500 mm (net) water. Soils with low water holding capacities produced lower mean yields than soils with a high water holding capacity. Maximum financial returns tended to occur with the application of less water than that required for maximum yields, particularly on the soil with a high water holding capacity. However, the variance of financial returns increased with reductions in the amount of water applied. These simulation results corroborated field observations and, taken together with the improved predictive ability of WHVZIM22 over WHV21, provided sufficient justification to use the revised model as a basis for the development of a pre-season irrigation optimization computer programme. This programme seeks the intraseasonal irrigation regime that maximizes the total gross margin for a particular soil, cultural and weather scenario, within the constraints of land and water availability. The programme is written in Microsoft QuickBASIC 4.00 and can generate an optimized irrigation regime within 4 to 5 minutes when executed on an IBM AT-compatible 80486 computer running at 25 MHz. It is envisaged that the programme would be used as a pre-season management tool, but the literal application of the results in the field is not recommended in view of the fact that the WHVZIM22 model has a number of inherent limitations and is therefore not a perfect predictor of crop growth and yield. The optimum irrigation solution generated by the programme simply provides a basis from which a farmer can plan irrigation management strategies. The actual intraseasonal irrigation schedule would necessarily depend on the real-time crop, soil and weather conditions.
Fluctuation of non-structural carbohydrates in the stem and ears of maize (Zea mays (L.)) during grain fill as influenced by water stress.
(1991) Shanahan, Paul Edward.; Greenfield, Peter L.
Stems of maize plants may serve as reservoirs for photosynthate produced in the leaves which may then be utilized for cell growth and maintenance requirements of the plants, and in particular for grain requirements during grain fill. Experiments were designed to ascertain the extent to which non-structural carbohydrates accumulate and are depleted in the stem and ears of locally cultivated maize hybrids during grain fill under conditions of water stress. Maize plants were grown: (i) under field conditions; (ii) under a rain-out shelter; and (iii) in pots placed inside a growth tunnel during grain fill. In the latter experiment whole maize plants were exposed to (14)C0(2) at selected intervals during grain fill. In the field trial large differences in the accumulation and depletion of total non-structural carbohydrates (TNC) were found between the six hybrids tested. The water stress conditions that prevailed from mid-grain fill (MGF) to physiological maturity (PM) resulted in TNC content levels being lower at PM than at anthesis in all hybrids except for SR 52. Total non-structural carbohydrate content in the whole stem of PNR 6427, CG 4602 and PNR 473 declined from anthesis to PM. In contrast TNC content in the whole stem of SA 60 and HL 1 declined from anthesis to MGF and then increased substantially in SA 60 and marginally in HL 1 from MGF to PM. In the rain-out shelter trial, water stress resulted in a 38 % reduction in final grain yield in SA 6 compared to 25 % in K78Y x I137TN. The greater tolerance to water stress of the more modern hybrid K78Y x I137TN compared to the obsolete hybrid SA 6 may be attributed to a number of factors, namely: (i) K78Y x I137TN recorded a higher leaf area index throughout grain fill under stress and non-stress conditions compared to SA 6; (ii) it did not partition as much non-structural carbohydrate to the stem during the first three weeks of grain fill as did SA 6 and did not markedly deplete stem non-structural carbohydrate pools to the same extent as did SA 6 under stress and non-stress conditions; and (iii) in the last week of grain fill as the leaf water potential of K78Y x I137TN increased sharply under stress conditions, it exhibited an ability to deplete stem non-structural carbohydrates to supplement the supply of current photosynthate to the grain. In the 14(C)-labelling pot trial, the maize single cross hybrid B254W X M162W generally depleted TNC in vegetative organs in the latter half of grain fill under stress conditions, while under non-stress conditions TNC continued to accumulate in vegetative organs until PM. Both stressed and non-stressed plants assimilated less 14(C) on consecutive labelling occasions during grain fill. The amount of 14(C) assimilated at six weeks after anthesis was only 12,1 and 16,3 % of that assimilated at anthesis in stressed and non-stressed plants, respectively. Stressed and non-stressed plants labelled at anthesis translocated a smaller proportion of assimilated 14(C) to the grain during grain fill than plants labelled later. Consequently, stressed and non-stressed plants labelled at anthesis recorded the highest proportion of whole plant 14(C) recovered in the whole shoot at PM compared to plants labelled on any of the other occasions. At anthesis the primary ear was not yet established as the major sink for photosynthate and much of the 14(C) assimilated at anthesis was utilized for final structural growth of the whole shoot including the cob and husks of the primary ear. Stressed and non-stressed plants assimilated similar amounts of 14(C) at anthesis and two weeks after anthesis, however, stressed plants assimilated less 14(C) than non-stressed plants at four and six weeks after anthesis. Forty-eight hours after each labelling occasion, the stressed plants had partitioned a higher proportion of assimilated 14(C) to the grain than the non-stressed plants. However, by PM the non-stressed plants had partitioned an equal or greater proportion of whole plant 14(C) recovered at PM to the grain compared to the stressed plants. Radioactivity associated with component non-structural carbohydrates, was determined using ion-exchange column chromatography and thin-layer chromatography. These procedures provided detailed data of the partitioning of 14(C) among glucose, fructose, sucrose and starch.
Soil amelioration and boron nutrition effects on the growth of sunflowers (Helianthus annuus L.) on an Avalon medium sandy loam.
(1975) Blamey, Frederick Paxton Cardell.; Nathanson, Karl.
Studies with sunflowers (Helianthus annuus) on an Avalon medium sandy loam, both in pots and in the field, indicated that the poor growth of this crop on this soil was largely due to soil acidity. Marked improvements in plant growth were brought about by liming as shown by measurements of emergence, seedling mass, population, plant heights, leaf areas and yield. Based largely on soil and plant analyses, it was concluded that improved growth with soil amelioration resulted from reduced aluminium toxicity and, since lime was more efficient than gypsum in neutralizing toxic Al, lime was markedly superior to the latter in improving plant growth. An annual application of 2 400 kg agricultural lime/ha increased seed yields >5 fold in the first and >10 fold in the second season in which the field experiment was carried out. In the pot experiment, no benefit of liming above the level required to neutralize toxic Al (-pH (N KCl) 4.5) was recorded and, on the contrary, yields tended to be depressed above this level. Highly significant linear relationships between yield and exch. Al (meq100g) were recorded in the pot and field experiements and, averaged over two seasons, seed yields in the field were increased 12% for each 0,1 meq/100g reduction in exch. Al. Another factor which decreased sunflower seed yields on this soil was boron deficiency and symptoms of B deficiency in the field were identified and described. At the levels of boron in the unfertilized soil, this deficiency affected the reproductive, rather than the vegetative stage of growth and correction of B deficiency by applying 10 kg borax/ha per annum increased seed yields by 38% and 18% in the two seasons, respectively. Soil amelioration had only a slight effect on the boron nutrition of sunflowers in this study. The chemical composition of the plant tissue was found to be a suitable means of quantifying B deficiency in sunflowers, the B concentration being a slightly superior method in most cases to the Ca:B ratio in plant tissue. Using field data, critical B concentrations in (i) month-old seedlings, (ii) the topmost, fully-mature leaf at flowering and (iii) in the seed were determined. The relationships were established between the amount of borax applied to the soil and the B concentration in plant tissues and these relationships could be used as a basis for recommending corrective B fertilization.
Potassium studies on an Avalon medium sandy loam.
(1970) Farina, Martin Patrick Warner.; Hulme, Sampie A.; Sumner, Malcolm E.
Abstract available in pdf file.
Maize crop growth and development related to solar radiation.
(1976) Kaiser, Heinz Werner.; De Jager, James M.
The interception and reflection of solar radiant energy by maize crops was measured at Cedara using tube solarimeters. From the measured data the amount of PHAR that was intercepted was related to the L value of the crop. K(v), the extinction coefficient of PHAR thus obtained was found to vary with row spacing, plant population density and cultivar, whereas growth stage, time of day and weather conditions did not appear to effect the K(v) value markedly. On clear days the albedo (R(v)) of a ma1ze crop was of the order of 0,05 to 0,08 during the middle of the day. However, at low sun angles in the early and late hours of the day, R(v) increased to values of the order of 0,2. On overcast days, the diurnal variation was considerably lower. The rate of photosynthesis in a ma1ze crop was measured using the plastic enclosure technique. The rate of CO2 uptake was related to the amount of PHAR absorbed by the crop. A rectangular hyperbola (Eqn. 2. I) was found to describe this relationship well. The photosynthetic response of a crop to population density, plant spacing and temperature at different developmental stages is discussed. The seasonal distribution of dry matter in the maize crop was studied using normal growth analysis techniques. From these data functions describing the partitioning of DM to the various plant organs (e.g. roots, leaves, grain) at different growth stages were developed. The functions describing the absorption of light by the crop canopy, its photosynthetic rate and the partitioning of photosynthate were incorporated in a computer simulation model, which was used to estimate the growth of a maize crop from meteorological radiant flux density and temperature data. The functioning of the model and its possible use are discussed.
The use of climatic data for maize grain yield predictions.
(1972) Mallett, John Bertram.; De Jager, James M.
The development and testing of a mathematical model for maize grain yield predictions is described. The model is based upon daily considerations of soil moisture, atmospheric evaporative demand and stage of crop development. Final yield predictions depend upon a knowledge of yield decrement due to moisture stress and the number of occasions that stress is recorded. This information was determined in the following manner:- (i) Stress imposed in lysimeters before and after anthesis was found to reduce grain yields by 3,2% and 4,2% per stress day respectively. (ii) A stress day was identified with the aid of mass-measuring lysimeters and a U.S. Weather Bureau Class A evaporation pan for measuring atmospheric evaporative demand. A nomogram constructed in terms of evaporative demand and available soil moisture, which discriminates between stress and non-stress days, was obtained for the Doveton soil used in the lysimeters. The model was applied to Cedara rainfall and evaporation data and yield probability patterns for three planting dates were obtained. It was found that highest yields (8,5 Mg ha(-1)) and least seasonal yield variation, may be expected from the earliest planting data 15/10. The Cedara : Doveton yield prediction model was also applied to climatic records for two other Natal stations (Estcourt and Newcastle) and six stations outside Natal (Bethlehem, Potchefstroom, Hoopstad, Standerton, Ermelo and Krugersdorp). Interesting comparison of the suitability of their respective climates for maize production was obtained. A method which uses the predicted number of stress days and the resultant yield decrement to determine the most effective and economic irrigation scheduling is developed and described. The effect of moisture holding characteristics of various soils upon the shape of the discriminating curve is discussed, and a method of obtaining discriminating curves for other soils by modifying the Doveton curve is described.
Some effects of environment, age and growth regulating compounds on the growth, yield and quality of sugarcane in southern Africa.
(1974) Rostron, Harold.; Nathanson, Karl.; Thompson, G. D.
An assessment has been made of the potential for increasing yields of well grown irrigated sugarcane crops beyond their present maxima. The effects of age at harvest and artificial growth regulators on yield have also been investigated. Measurement of the apparent maximum yield of cane fresh weight (tc/ha) and total dry matter (t.dm/ha) of three varieties grown on a specially prepared site, indicated that actual cane yields were 63-70% of the estimated potential maximum. Differences in morphology between two extreme varieties, NCo 376 and CB 36/14, were insufficient to affect growth and yield under good growing conditions. The average productivity over a period of one year of plant and first ratoon crops of three varieties was 65 t.dm/ha and 150 tc/ha. This is a crop growth rate of 17,0 g/m(2)/dy, representing an average conversion into plant dry matter of 1,9% of total incident radiation (ri.) or 4,3% of visible ri. This level of productivity is equivalent to rates reported for sugarcane growing in more favourable parts of the world and for other highly productive crops. In an experiment in which a series of crops were ratooned at different times of the year and harvested at ages ranging from 32 to 72 weeks, sugar yield and all measures of cane quality were closely correlated with cane yield for crops of all ages. Sugar yield varied with age at harvest, according to the cycle of weather conditions experienced by the crop. Crops ratooned in January and February produced the highest yields of 23 tons estimated recoverable sugar per hectare ( ters/ha) at 64-72 weeks of age. The average rate of sugar production at this time ranged from 1,3-1,5 ters/ha/month. At 12 months of age crops ratooned in July and September had the highest yields of 17,2 ters/ha, but they then made little further growth. High rates of sugar production of 1,4-1,6 ters/ha/month were obtained from crops ratooned between June and September and harvested between May and. August at 40-56 weeks of age. Considering data for all crops, cane yield was correlated with weather conditions only up to 40 weeks of age. After this, increments of cane yield were correlated better with the amount of growth already made than with either crop age or the average weather conditions experienced. Artificial chemical ripening was successful on young immature crops harvested in May at the beginning of the milling season, but there was only a small response under less favourable growing conditions and when the crop was older and more mature. Ethrel and Polaris were the most successful of several ripeners tested, Ethrel being more active than Polaris. Cane quality was improved and sucrose storage was increased despite reductions in rates of photosynthesis and sheath and lamina size. The ripening response varied with variety, condition of the crop at the time of spraying, rate of chemical application and the time interval between spraying and harvesting. From this work it is concluded that it will probably be easier to raise the sugar yield of existing high yielding varieties by altering the proportioning of photosynthate in favour of sucrose storage, rather than by increasing cane yield.
Nutritional studies with a spanish-type groundnut on an avalon medium sandy loam soil.
(1972) Snyman, Jacobus Wilhelmus.; Nathanson, Karl.
Calcium requirements of a Spanish-type groundnut cultivar on an Avalon medium sandy loam soil. The effect of different levels of application of agricultural lime, dolomitic lime and gypsum on yield and quality of the Nelson Spanish groundnut cultivar was studied in a field experiment. The effect of the applied treatments was studied by means of detailed soil and plant analyses. The results reported were obtained in a season with below normal rainfall. The so-called "typical drought damage" symptoms were found to be largely due to nutritional deficiency conditions. A linear increase in kernel yield was obtained as a result of increased levels of exchangeable calcium in the soil. Calcium uptake by the fruit proceeded more efficiently where gypsum was used as calcium carrier. The calcium content of the fruit was increased as a result of increases in the level of exchangeable calcium in the soil. The gypsum treatments resulted in a marked increase in shelling percentage, percentage ovarian cavities filled and increased kernel yield. The mechanism of action of the gypsum treatments was not solely improved calcium nutritional conditions. A poor correlation between calcium content of the soil and the groundnut fruit, and shelling percentage and percentage ovarian cavities filled was obtained in the case of agricultural lime and dolomitic lime treatments. The level of exchangeable soil calcium and soil pH on the majority of groundnut fields investigated was found to be low enough to expect considerable increases in yield following on applications of agricultural lime. Such an increase in pH would result in an improved nitrogen status of the plants as a result of more efficient nitrogen fixation. Applications of agricultural lime should be supplemented with a supply of calcium in a more soluble form, such as gypsum. The more soluble calcium carrier would provide a supply of readily available calcium for uptake over periods of moisture stress. The relationship between chemical composition of the soil and plant, and yield and quality factors were studied. The occurrence, cause and control of hollow heart and black plumule damage in groundnut kernels Hollow heart and black plumule are symptoms of nutritional abnormalities in groundnut kernels. Although the occurrence of these symptoms are associated with the occurrence of drought conditions, the calcium and boron status of the Avalon medium sandy loam is low enough to result in the occurrence of these symptoms on a small scale under normal rainfall conditions. The supply of available boron in the soil is decreased as a result of fixation into an unavailable form during dry periods. Decreased boron concentrations in the plant under these conditions results in the occurrence of boron deficiency symptoms in the kernels (hollow heart damage). At the same time, it is suggested, that due to the nutritional association between calcium and boron in the plant, the decreased levels of boron in the plant results in a decreased metabolic activity of calcium indicated by the occurrence of black plumule damage (a calcium deficiency symptom). The critical level of boron in the kernel as far as both hollow heart and black plumule damage is concerned, appears to be between 10,2 and 13,9 ppm. Applications of gypsum resulted in a marked decrease in hollow heart and black plumule damage. This treatment resulted not only in increased levels of calcium in the kernel, but at the same time appears to prevent the fixation of boron thus allowing normal uptake of boron by the plant. Applications of agricultural and dolomitic lime had no effect on the occurrence of either form of damage. These treatments were associated with a marked increase in calcium content and a decrease in boron content of the kernels. The ratio Ca content of the kernel (ppm)/K content x Mg content of the kernel was found to be fairly closely correlated with the occurrence of black plumule damage. The calcium, potassium and magnesium status of the soil was of little value for the purpose of predicting hollow heart damage. The intensity of hollow heart and black plumule damage was increased by applications of urea. Cultivars differed in their susceptibility to hollow heart and black plumule damage. An application of 26 kg/ha borax virtually eliminated both forms of damage.
Relationships between climatic indices and soil properties that reflect leaching and weathering in the Natal Midlands, and their use in the assessment of afforestation potential.
(1991) Donkin, Michael John.; Fey, Martin Venn.
In the summer rainfall regions of Southern Africa, knowledge of the mean annual effective rainfall (MAER) pertaining to a site is critical for the assessment of that site's afforestation potential. The ability to estimate MAER at a site from soil properties was sought for the forestry regions of the Natal midlands. The S-value (sum of basic cations) expressed per unit mass of clay (a diagnostic property in the South African soil classification system) is currently used for this purpose, but until now this usage has not been validated. Jenny's conceptual model of state factors of soil formation was employed to demonstrate, within a climosequence, the empirical relationships occurring between soil properties and climatic indices. Weather stations from around the Natal midlands were selected on the basis of the reliability of their rainfall and temperature records and 33 of these sites were subsequently incorporated into a conceptual climosequence. At each site, representative soil profiles were excavated, described and intensively sampled. Soil samples were analysed for a variety of properties indicative of the degree of leaching and/or weathering. At each site, indices of climate (mean annual precipitation, drainage from various depths and sub-horizons in the soil profile, and MAER) were calculated using an agrohydrological water-budgeting model (ACRU). Selected soil properties were systematically related to these climatic indices. The results showed that within the climosequence concerned, of all the soil properties considered, the effective cation exchange (ECEC, sum of basic and acidic cations) of the B1 horizon, expressed per unit mass of soil, was best related to the climatic indices.
The effect of maize-legume cropping system and nitrogen fertilization on yield, soil organic carbon and soil moisture.
(2015) Sebetha, Erick.; Modi, Albert Thembinkosi.; Owoeye, Lawrence.
Crop rotation and intercropping are regarded as better cropping in terms of yield improvement of both cereal and legume crops as compared to monocropping. A factorial experiment was carried out at three dryland localities of Northwest province (Potchefstroom, Rustenburg and Taung) from 2010/11 to 2012/13 planting seasons. The experiment consisted of three cropping systems, monocropping, intercropping and rotational cropping. Two rates of nitrogen fertilizer, zero and optimum levels based on soil analysis results prior to planting were applied on maize and cowpea plots. Soil moisture content was evaluated during three growth stages at different depths of the soil (0-15, 15-30, 30-60 and 60-90 cm) using gravimetric method. Parameters considered for the study included the followings: 100% tasseling/flowering, days to physiological maturity, plant height, number of leaves per plant, leaf area in maize, stem diameter in maize, ear length in maize, ear mass , kernel number per ear, hundred seed mass, grain yield in maize, LER, plant population at harvest and stover yield in maize, number of leaves and nodules per cowpea plant, pod length, seed per pod, pod mass at harvest, grain, field biomass yield at harvest, cowpea leaf, immature pod, seed protein content and maize seed protein, oil, starch and phosphorus content. The analysed soil chemical properties included soil organic carbon using Walkley Black method, soil Bray 1-P; N-NO3, N-NH4 and exchangeable K. Cropping system had significant effect (P < 0.05) on the growth and yield of maize. Cowpea-maize rotation and monocropping maize had tasseled earlier, reached days to physiological maturity earlier, had large leaf area, higher number of leaves per plant, ear mass, kernel number, seed mass, grain yield and stover yield. Maize-cowpea rotation and monocropping cowpea had significantly (P < 0.05) higher number of leaves per plant, seed per pod, pod mass, grain yield and field biomass yield than intercropped cowpea. Cropping system had significant effect (P < 0.05) on soil organic carbon; Bray 1-P and soil nitrate (N-NO3). The interaction effect of cropping system on cropping system x nitrogen x site on maize yield, cowpea growth, protein content and soil N-NO3 contributed towards significant of this study. The chapters of this thesis represent different studies presented as different articles. Chapter 1 is a general introduction to explain the study background and hypothesis. Chapter 2 is on the effect of maize-cowpea cropping system on soil moisture content. Chapter 3 is on crop rotation and intercropping cowpea with maize: maize growth and yield. Chapter 4 is on crop rotation and intercropping cowpea and maize: cowpea growth and yield. Chapter 5 is on the effect of crop rotation and intercropping on cowpea crude protein. Chapter 6 is on the maize seed quality in response to crop rotation, intercropping and nitrogen fertilization. Chapter 7 is on the effect of maize-cowpea cropping system on soil chemical composition. The last chapter 8 is a general discussion and conclusion.
Agronomic and physiological approaches to improving productivity of selected sweet potato (Ipomoea Batatas L.) cultivars in KwaZulu-Natal : a focus on drought tolerance.
(2014) Mgcibelo, Motsa Nozipho.; Modi, Albert Thembinkosi.
Sweet potato (Ipomoea batatas L) is a resilient food security crop with wide adaptation characteristics and hence can fit well under smallholder production. Its importance as a food security crop in relation to drought is still underestimated and fails to attract sufficient attention from agricultural researchers. The adaptive responses of different sweet potato cultivars to different agro-ecological areas may vary, and sweet potato is an important crop for small-holder farmers in KwaZulu-Natal (KZN) of South Africa, which has diverse agro-ecological areas. Adaptive responses of sweet potato cultivars in KZN’s ecological regions are not known. The possible varying adaptive responses may impact on the food and nutrition security role of sweet potato. This study evaluated the ecophysiology, growth, yield and nutritional composition of three locally bred sweet potato cultivars in response to a range of climates and soils from KZN, South Africa. With the help of smallholder farmers, field experiments were conducted at three sites located in three different agro-ecological areas (Deepdale, Umbumbulu and Richards Bay) of KZN. Agronomic, physiology and yield data were collected. Harvested roots were further analysed for selected nutrients (starch and β-carotene) and other metabolic responses to drought stress. A separate study on physiological and yield response of sweet potato to water stress was conducted under controlled environment. Metabolic analyses were conducted continuously during plant growth. Plant growth, physiological responses and yield were significantly (P ≤ 0.05) influenced by growth environment. Drought stress in Richards Bay resulted in poor plant growth, low yields and low nutritional content (starch and β-carotene content). High temperatures and evapotranspiration (ETo) were associated with drought stress. The other locations (Deepdale and Umbumbulu) where ETo was low showed increased plant growth, yields and nutritional content. The cultivars’ ecophysiology, growth and yield were not suitable for the Richards Bay agro-ecology/bioresource group. They were more suitable for Umbumbulu and Deepdale agro-ecology/bioresource groups. Under controlled conditions, the cultivars adapted to water stress through reduced canopy size. When fully-irrigated, they increased vegetative growth than storage root growth, thus resulted in low storage root yield. This suggested that the cultivars were drought tolerant and suitable for production in marginal areas. Leaf phytochemical content was high in sweet potato leaves compared to other common leafy vegetables. It was even higher in leaves of water stressed plants. This indicated that both leaves and storage roots can be utilised for improved food and nutrition security. Under marginal areas where storage root yield is compromised, the leaves can contribute to food and nutrition security. In areas where rainfall is not limited, communities can benefit from both storage roots and leaves.
Quantifying productivity and water use of sorghum intercrop systems.
(2015) Chimonyo, Vimbayi Grace Petrova.; Modi, Albert Thembinkosi.
Rural sub-Saharan Africa (SSA) faces the challenge of achieving food security under water scarcity amplified by climate change and variability. Under these conditions, it is necessary to adopt cropping systems that have a potential to improve productivity. The aim of the study was to assess the feasibility of a sorghum-cowpea-bottle gourd intercrop systems with a view to determine the resource use efficiencies. This was achieved through a series of studies which included conducting critical literature reviews, quantifying water use and water use efficiency of sorghum-cowpea-bottle gourd, and modelling such systems using Agricultural Production Systems Simulator (APSIM). Field trials were conducted at the University of KwaZulu–Natal’s, Ukulinga Research Farm over two seasons (2013/14 and 2014/15) under varying water regimes [full irrigation (FI), deficit irrigation (DI) and rainfed (RF)]. Intercrop combinations considered were sole sorghum, cowpea and bottle gourd as well as intercrops of sorghum–cowpea and sorghum–bottle gourd. Data collected included soil water content, plant height/vine length, leaf number, tillering/branching, leaf area index, relative leaf water content, stomatal conductance and chlorophyll content index as well as biomass accumulation and partitioning. Yield and yield components, water use (WU) and WUE were calculated at harvest. Extinction coefficient, intercepted photosynthetic active radiation (IPAR) and radiation use efficiency (RUE) for biomass and grain were also determined. Land equivalent ratio (LER) was used to evaluate intercrop productivity. Growth, yield and water use (ET) of the sorghum–cowpea intercrop system were simulated using APSIM. The validated model was then used to develop best management practices for intercropping. The review showed that aboveground interactions within intercrop systems have thoroughly been investigated while belowground interactions were mostly limited. The review showcased the potential of bottle gourd as a versatile food crop. The field trials established that sorghum yields were stable across different water regimes. This was mainly achieved through facilitative interaction within the intercrop systems which allowed for greater eco-physiological adaptation resulting in improved water capture and use. Improved water capture and use also increased WUE (50.68%) and RUE (8.96%). The APSIM model was simulated growth, yield and WU of an intercrop system under varying water regimes satisfactorily. The model over–estimated biomass (6.25%), yield (14.93%) and WU (7.29%) and under–estimated WUE (-14.86%). Scenario analyses using APSIM showed that the development of best management practices should be agro–ecology specific to ensure dynamic climate change adaptation strategies and increase resilience. It was concluded that intercropping results in improved productivity, especially under water–limited conditions. As such, it that can be used by farmers located in semi-arid and arid regions as an adaptation strategy for increased productivity. Dynamic agronomic management practices should be adapted to further increase the system’s resilience to predicted climatic uncertainties. Future studies on intercropping should consider root interactions and possibly different plant populations and planting geometry as factors that might influence resource capture and use. Decision support systems should be promoted within farming communities to better manage risks associated with on-farm decision making.

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