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Doctoral Degrees (Horticultural Science)

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    Ethephon-induced changes in macadamia nuts: implications for dietary indicators, nutritional quality, and postharvest shelf life.
    (2024) Aruwajoye, Noluthando Noxolo.; Tesfay, Samson Zeray.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.
    The thriving macadamia nut industry, prized for its nutritional value, grapples with the challenge of inconsistent abscission seasons, leading to variable harvest times and increased operational costs. To address this, the traditional application of ethephon induces uniform nut abscission and streamlines harvesting. However, it is crucial to investigate how ethephon impacts the nutritional quality and postharvest shelf life of macadamia nuts. This study focuses on the effect of ethephon on the postharvest quality of the ‘788’ and ‘Beaumont’ cultivars, comparing various categories: nuts that successfully abscised due to ethephon (ED), those manually picked from ethephon-treated trees (ET), nuts naturally dropped from control trees (CD), and those manually picked from control trees (CT). Guided by industryrecommended doses, Ethephon 480 SL® was administered using a Cima mist blower to facilitate nut abscission, specifically targeting physiologically mature nuts. The research explores the influence on fatty acids, dietary indicators, biochemical components, and polyphenol oxidase (PPO) activity. Various parameters, including total phenolics, flavonoids, antioxidant activities, sucrose, and total protein, were assessed across treatments using standardized procedures. samples were collected over a 72-day accelerated storage period. The study further assessed the effect of Ethephon on the physical attributes of macadamia during a 56-day accelerated storage, analyzing mass, diameter, colour (L*, a*, b*, and hue angle), and texture. Kernels were roasted at 125 °C for 15 minutes using a hot air oven dryer post drying and dehusking. Models were then developed to predict the firmness and colour parameters of macadamia nuts harvested with ethephon from the ‘788’ and ‘Beaumont’ cultivars based on drying days and storage conditions. The aim was to determine the minimal processing requirements for achieving satisfactory postharvest nut appearance. Input parameters considered included storage temperature, drying temperature, and storage days, vi while the resulting outputs were firmness and colour parameters (L*, a*, b*, and hue angle). The range of the input parameters spanned from -22℃ to 25℃ for storage temperature, 35 to 57℃ for drying temperature, and 0 to 70 days for storage duration. Significant changes were observed in the fatty acid profiles of macadamia nuts exposed to ethephon treatment. Notably, ethephon led to a substantial increase in stearic acid content, reaching 24,622 µg/g in (ED) after 72 days, compared to 16,764 µg/g in (CD), thereby impacting their nutritional quality. Conversely, ED nuts exhibited a reduction in unsaturated fatty acids (USFAs). Hierarchical clustering analysis identified strong correlations between ED and saturated fatty acids (SFAs) in both ‘Beaumont’ and ‘788’ cultivars, with coefficients of 0.78 and 0.80, respectively. This was accompanied by an increase in atherogenic indices, thrombogenic index, and saturation index, along with a decrease in the hypocholesterolemic/hypercholesterolemic ratio. Concurrently, a robust correlation (0.97) between sucrose and Polyphenol Oxidase (PPO) was observed in ‘Beaumont’, underscoring the influence of ethylene treatment. Notably, ED ‘Beaumont’ nuts exhibited the highest sucrose content at 18.63 mg/g, coupled with elevated PPO activity at 1.06 U g-L. For the physical attributes, ED exhibited a notable influence on the ‘Beaumont’ cultivar, particularly affecting parameters such as a*, b*, mass, and diameter. Conversely, in the ‘788’ cultivar, Principal Component Analysis (PCA) revealed that CD treatment held prominence, showing elevated values for firmness, a*, and b*. Furthermore, model development demonstrated significance, with high coefficients of determination ranging between 0.83 and 0.97. Notably, under reduced drying days and optimal storage temperature, the models predicted maximum L* values of 76.253 and 79.748 for the ‘788’ cultivar. This study provides a theoretical framework for identifying optimum set points crucial for the effective preservation of both the ‘788’ and ‘Beaumont’ varieties of macadamia nuts, particularly in preventing external discolouration. The proposed set points aim to enhance the quality and overall market value of these macadamia nut varieties. The overall findings offer a comprehensive perspective on the effects of ethephon application on macadamia nuts. Stakeholders, producers, and processors in the macadamia nut industry can leverage this research to make informed decisions that enhance overall efficiency and quality while ensuring optimal nutritional value and extended shelf life for macadamia nuts.
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    Efficacy of enhanced freshness formulation as a novel postharvest treatment for gold kiwifruit (actinidia chinensis)
    (2023) Mthembu, Sisanda Sibusiso Luyanda.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray .
    When exporting kiwifruit, the main limiting factors are excessive fruit softening and fungal decay. Furthermore, exposing kiwifruits to low temperatures induces the mechanisms involved in the softening process. In addition, kiwifruit become much more susceptible to fungal decay as they soften, which exacerbates deterioration of kiwifruit during transit. Fruit firmness is an important criterion for the market value of kiwifruit and storage life for the wholesale and retail trade. Therefore, loss in firmness is a serious problem resulting in postharvest and economic losses. Given that cold storage alone is not enough to optimise firmness retention of kiwifruit, the current study aimed to investigate the capacity of enhanced freshness formulation (EFF) to reduce kiwifruit softening. Kiwifruits were subjected to eight weeks of storage at 0 °C and 90 % relative humidity, followed by one week at ambient storage. The efficacy of EFF to regulate the mechanisms behind the softening process such as cell wall degradation, membrane deterioration and fungal infection was evaluated. Furthermore, the efficacy of EFF to preserve bioactive compounds was also evaluated to assess the effect of this treatment on these highly appreciated quality attributes. The first study evaluated the efficacy of EFF to delay softening of kiwifruit harvested at both the mid and late maturity stage. The influence of EFF on cell wall polysaccharides (CWPs) and cell wall degrading enzymes (CWDEs) was assessed. The findings showed that EFF optimised firmness retention of kiwifruit by preserving CWPs through the suppression of CWDEs which are responsible for initiating fruit softening. Furthermore, EFF maintained fruit quality and marketability, characterised by significantly lower mass loss, total soluble solids and higher titratable acidity. The second study examined the capacity of EFF to inhibit the excessive production of reactive oxygen species (ROS) which induce membrane damage, resulting in softening and senescence. EFF treatment effectively delayed the progression of kiwifruit senescence by reducing membrane deterioration and the accumulation of ROS. The mechanism by which EFF reduced membrane damage caused by ROS was attributed to its capacity to enhance the activities of antioxidant enzymes involved in neutralising ROS and suppressing Phospholipase D which destroys membrane integrity, resulting in improved storability of treated fruit. The third study assessed the efficacy of EFF to inhibit the fungal growth of Botrytis cinerea, which one of the major postharvest pathogens of kiwifruit that contribute to excessive softening. The findings show that EFF effectively suppressed the fungal growth of Botrytis cinerea in a dose -dependent manner. The results demonstrate that EFF can serve as a potential disease control strategy for kiwifruit at the postharvest stage. The mechanism by which EFF suppressed fungal decay can be attributed to the treatment’s capacity to induce disease resistance against Botrytis cinerea, by enhancing the synthesis of secondary metabolites and the action of defence-related enzymes, thus conferring greater protection against pathogen attack. The fourth study investigated the ability of EFF to retain the antioxidant quality of kiwifruit during prolonged storage. The bioactive compounds of EFF-treated kiwifruits were effectively maintained throughout the storage period. The findings of the present study further revealed that the mechanism by which EFF optimises antioxidant retention is due to its ability to regulate enzymes involved in the synthesis and oxidation of bioactive compounds. Despite the positive results, further research on transcriptome analysis is needed to elucidate how EFF regulates softening and antioxidant enzymes.
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    Assessing gaseous ozone and edible coatings as postharvest treatments for mango (mangifera indica L.) fruit.
    (2021) Bambalele, Nonjabulo Lynne.; Mditshwa, Asanda.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.
    This research examines the potential of gaseous ozone and edible coatings in preserving postharvest quality and extending the shelf-life of ‘Keitt’ mango fruit. A critical review of the literature focused on the recent postharvest technologies used to preserve the quality of mango fruit. The prospects of using non-chemical postharvest treatments such as gaseous ozone and edible coatings were also reviewed. A screening study was conducted to determine the optimum ozone (O3) application time for effectively maintaining fruit quality and extending shelf-life. Mango fruit were intermittently exposed to gaseous ozone for twelve, twenty-four, thirty-six, or forty-eight hours, and the control fruit were untreated. Fruit were stored at 10℃ for twenty-one days and seven days shelf-life at ambient temperature. The findings showed that the O3 treatment should be applied at the pre-climacteric stage to achieve optimum results. Ozone treatment for 24 or 36 hours effectively maintained firmness and carotenoids content, delayed color changes, decay incidence, and mass loss. Therefore, an ozone exposure time of 24 and 36 hours was adopted for the study. These exposure times were incorporated into edible coatings (moringa leaf extract and carboxymethyl cellulose) for further investigation. The study on the effect of gaseous O3 incorporated with edible coatings on sensory attributes and physicochemical parameters showed that EC and EC + O3 (36 h) were more effective in delaying the ripening process and maintaining the postharvest quality. Overall, consumers preferred the fruit coated with EC due to its attractive color, smell, and sweetness. The study on the postharvest effect of gaseous O3 and EC on antioxidants and the biochemical properties of mango fruit is discussed in Chapter Five. The findings of this study demonstrated that the treatment combination of EC and gaseous O3 (36 h) effectively maintained antioxidants, membrane integrity and enhanced the quality of mango fruit during storage. The effect of gaseous O3 and EC on postharvest diseases of mango fruit, specifically Colletotrichum gloeosporioides (anthracnose) and Lasiodiplodia theobromae (stem-end rot), was also investigated. This study revealed that EC + O3 (24 h) effectively controlled stem-end rot and anthracnose in mango fruit. The treatment combination of EC and O3 (36 h) reduced the mycelial growth and disease incidence of Lasiodiplodia theobromae and Colletotrichum gloeosporioides. The study of EC and O3 in the volatile compounds of mango fruit is discussed in Chapter Seven. The fruit treated with EC had a high content of volatile compounds compared to other treatments. The treatment combination of EC + O3 (24 h) was not effective in maintaining the volatile compounds of mango fruit during storage. The findings of the current study suggest that EC + O3 (36 h) can be used as postharvest treatment of mango fruit. Additional research is required to gain more insights in understanding the EC+ O3 mode of action in maintaining volatile compounds and controlling mango postharvest diseases.
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    Avocado seed physiology aspects.
    (2021) Abdalla, Mamoun Ahmed Arabi.; Bertling, Isa.
    The avocado seeds/seedling is needed as rootstock for other economic trees and loss of tress stand in orchids after establishment is of great commercial loss in avocado orchids around the worldwide and South Africa. The aim of this study was to evaluate and compare avocado seeds development of various seeds ages by investigating seeds germination percentage over three generations, as there is little information on avocado seeds growth and development, despite the importance of the seeds in avocado propagation. Seed harvesting was carried out over various developmental stages, from early fruit development to two-year-old seeds (Generation 1, 24 to 29 months after full bloom MAFB). Seed from current season (Generation 2, 12 to 17 MAFB) and newest seeds (Generation 3, 0 to 5 MAFB) of two cultivars (‘Hass’ and ‘Fuerte’) was analysed. Seed of three generations were analysed: ‘Hass’ Generation 1seed (seed from the oldest, commercially over-mature, fruit full bloom in July/ August 2017); Generation 2 (full bloom in July/ August 2018) and Generation 3 (full bloom in July/August 2019). Similarly, ‘Fuerte’ fruit of three generations were compared: from the avocado fruit, (Generation 1, full bloom in June/July 2017), to Generation 2 (full bloom in June/July 2018) to Generation 3 (full bloom in June/July 2019). Seed were extracted from fruit to determine seed parameters, such as germination percentage, seed viability, seed moisture content and seed respiration rate. Further, seed physiological parameters, such as cotyledonal sugars and starch concentrations, seed coat phenolic compound concentrations and polyphenol oxidase (PPO) concentrations were determined. Anatomical features of the seed coat, such as seed coat thickness and seed coat ultrastructure were also observed. In both cultivars, the germination percentage was higher in Generation 2, 12 to 18 MAFB), than in Generation 1, 24 to 29 MAFB) seed from June to September. From October to November Generation 3 (0 to 5 MAFB) had a higher germination percentage than Generation 2. Seed viability was higher in Generation 2 of both cultivars and lower for the Generation 1; similar results were found for the germination percentage, with seed from Generation 2 having a higher germination rate than seed from the Generation 1. Seed viability differed significantly between seed age, and the interaction between generations and months was statistically significant (P ˂ 0.001). The seed collected from fruit of the Generation 2 of both cultivars had a slightly higher moisture content and a higher germination percentage than the Generation 1. Seed moisture content ranged between 54.5 and 62.1 % in ‘Hass’ (Generation 2 seed age 12 to 15 MAFB), harvested in June to September, while the Generation 1 seed age 23 MAFB) seed had a lower moisture percentage (39.2%) in June. ‘Hass’ seed of (Generation 3 seed age 4 MAFB) harvested from October to November had a higher seed moisture than seed from (Generation 2,15 MAFB). ‘Fuerte’ seed showed a similar pattern with the highest moisture percentage (60.5%) in July and the lowest in June (33.2%). (Generation 2’ seed age 13MAFB seed had higher moisture percentages than Generation 1 from June to September, and (Generation 3, 3 MAFB) had higher moisture percentage than Generation 2. Seeds respiration rate, determined following fruit harvest, decreased over the time. Generally, Generation 2 respired more than the Generation 1, from June to September. From October to November 2019 the Generation 3 respired more than Generation 2 seed. The respiration rate of seed extracted from June to September 2018 Generation 2 declined rapidly. The Generation 3 (collected October to November 2019, seed age 4 to 5 MAFB) were characterized by a higher respiration rate than seed of Generation 2, seed age 16 to 17 MAFB; therefore, younger seeds generations respired more than older ones. It is concluded that the contribution of seeds respiration rate to avocado whole fruit respiration decreases with development over the time. The ability of the avocado seed to germinate quickly and produce seedlings is dependent on the carbohydrate reserves in the cotyledons, which make up the bulk of the avocado seed. In seed coats of both cultivars, phenolic concentrations inhibited seed germination of Generation 1, probably due to the higher level of phenolic concentrations in older seed coats. Seed coats generally contained high amounts of phenolics (2.3 mg GAE*g-1 DM for ‘Hass’ and 2.02 mg GAE* g-1 DM for ‘Fuerte’). Seed extracted from Generation 1 fruit in June to September 2018, had a higher amount of seed coat phenolics than those from Generation 2 fruit. In fruit from October to November 2019 the Generation 3 seed coat had lower phenolic concentrations than Generation 2 seed coats, confirming that older seed coats contain more phenolics than younger seed coats. Germination percentages of Generation 3 seed were higher than those of Generation 2 seed. The high phenolic concentration in the seed coats seems to be aligned with the seed turning dark brown upon maturation, probably due to sufficient oxygen present in the fruit to allow phenolic oxidation of the seed coat; the seed coat becoming entirely brown and very thin, could, therefore, be used as an indication that the fruit has reached physiological maturity. Seed at this stage of maturation are, however, characterized by a low germination percentage, possibly due to the seed coat phenolic compounds interfering with germination. This is supported by the positive correlation between lower seed coat phenolic compound concentration and higher seed germination rate for both cultivars (r = 0.11, P ˂ 0.61). Seed coat thickness of Generation 1 (24 to 28 MAFB) and Generation 2 (12 to 16 MAFB) ‘Hass’ seed coats differed, with the younger seed generation displaying thicker seed coats than the older ones (0.51 versus 0.11 mm, respectively). In ‘Fuerte’, in June and July older seed coats Generation 1, 24 to 25 MAFB, respectively) were thicker than Generation 2 (12 to 13 MAFB) (0.46 and 0.15 mm, respectively. There was, however, negative relationship between seed coat thickness and germination percentage (r = -0.11). Polyphenol oxidase (PPO) and phenolic concentrations of avocado seed coats were also investigated in the seed coat of Generation 1and Generation 2 ‘Hass’ and ‘Fuerte’ seed. Polyphenol oxidase (PPO) and phenolic concentrations of the avocado seed coats of the two avocado cultivars of Generation 1, 27 to 26 MAFB) and Generation 2, 15 to 14 MAFB) respectively, seed coats were investigated. During the colder (winter) season (June-August), Generation 1, fully mature ‘Hass’ seed coats showed higher polyphenol oxidase (PPO) concentrations than seed coats from the Generation 2. From October to November the Generation 3 seed coat also had a lower PPO concentration than those of Generation 2. Generation 2 ‘Hass’ seed coats had relatively low PPO concentrations in June /July, when fruit were 12 to 14 MAFB, but PPO concentrations increased thereafter and remained at a higher-level until October/ November. Generation 1 ‘Fuerte’ seed coat had a similar PPO concentration during all investigated months. Phenolic compounds were present in seed coats of both avocado cultivars, with seed coats of older seeds containing a much higher phenolic concentrations than the seed coats of the newer generation. The seed (cotyledons plus embryo) sugar profile was dominated by the C7 sugar perseitol, followed by the C6 sugar, sucrose, while mannoheptulose and glucose were present in very small amounts. Perseitol was present in in both cultivars with 14 months-old ‘Hass’ (September) cotyledons containing 9.8 mg*g-1 DM and 15-months-old ‘Fuerte (September) containing 10.3 mg*g-1 DM. Avocado cotyledons were found to also be a large starch source, probably providing carbohydrates for seed development and germination. The Generation 2, 14 to 15 MAFB) of ‘Fuerte’ and ‘Hass’ had a higher starch concentration than the Generation 1, 26 to 27 MAFB) and similarly, Generation 3, 4 to 5 MAFB, respectively, had higher starch concentration than Generation 2 for both cultivars, indicating the use of this carbohydrate reserve to sustain embryo development. The highest concentration of starch in ‘Hass’ seeds was detected in August as 88.8% of seed DM (Generation 2, seed age 13 MAFB), while for ‘Fuerte’ seed the highest starch concentration was in August at 90.5% of seed DM (Generation 2, 14 MAFB). Starch seems, therefore, more related to avocado seed development than to avocado fruit growth and development. Delaying fruit harvest to October (seed age 16 to 18 MAFB) allows seed to fully mature and to continue accumulating sugars and starch. To improve percentage and velocity of germination, seeds were soaked in various concentrations of aqueous moringa leaf extract (MLE, 0, 2.5, 5.0 and 7.5 % w/v) over different periods (0, 10, 30 or 120 minutes). Younger seed were stronger affected by the increasing MLE concentration. Soaking in 2.5% MLE tended to enhance the germination percentage more so than the other MLE concentrations. The lowest germination percentage was determined for seeds soaked in 7.5% MLE for 120 minutes, indicating that younger seed (from10 to 12 months after fruit set ‘Fuerte’ fruit harvested April to June) should be used as ‘nurse seed’. Overall, this study revealed that avocado seed germination and development do not coincide with the commercial fruit harvesting period, the avocado fruit needs 15 to 18 months to change from its flowering blooming period to a full harvest, and seed age12 MAFB can germinate for both cultivars. The study further confirmed perseitol as the dominant free storage sugar that assists in seed development, while starch is also an important energy provider for the developing embryo.
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    Rapid monitoring and quantification of unripe banana flour adulteration using visible-near infrared spectroscopy.
    (2021) Ndlovu, Phindile Faith.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.; Mphahlele, Rebogile Ramasele.
    A general lack of strict regulations in South Africa to monitor processed foodstuff increases chances of unfair producers and traders to intentionally mislabel and adulterate high valued food products with inferior lookalikes. Recently, unripe banana flour (UBF) has gained global attention and has been identified as a replacement for cereals flours due to its gluten free traits and resistant starch nutritional qualities, yet has no quality control standards. The objective of this research was to develop rapid prediction models based on a visible to near infrared (Vis- NIR) spectroscopy (Vis-NIRS) combined with multivariate analysis to classify, detect, and quantify different adulteration levels of staple flours (i.e. wheat and maize flours) in unripe banana flour. The other aim was to identify important biomarkers of unripe banana flour that could be used to discriminate unripe banana flour adulteration at different concentration levels. A critical evaluation of the portable Vis-NIR spectroscopy combined with chemometrics analysis indicated that it was possible to discriminate between unripe banana flour with wheat and maize flours and associated different adulteration levels. The partial least square (PLS) regression (PLSR) analysis quantified individually maize and wheat flours, based on different adulteration levels, showed that optimal PLSR detection models performances were obtained using the first derivative Savitsky-Golay (7-point smoothing, 2nd order polynomial) and the second derivative Savitsky-Golay (19-point smoothing, 2nd order polynomial). The study to optimise and test the handheld Vis-NIR instruments’ feasibilty to simulteniously develop a standard model for rapid solution to detect both maize and wheat flours adulteration indicated high classification and prediction accuracies could be achived through principal component analysis (PCA) and partial least squeres regression (PLSR). The study found that gluten could be utilised as a biomarker to test for unwanted adulteration of unripe banana flour with wheat flour, and showed good and reliable rapid spectroscopic PLSR model was achieved with high precision. Near infrared spectroscopy showed great potential to detect the nutritional changes of unripe banana flour during adulteration based on resistant starch content. The results of this investigation indicated that wheat adulteration is a threat to unripe banana flour importnt attribute as signification reduction of this parameter was observed with the increasing levels wheat adultearation. Vis-NIR spectroscopy with multivariate analysis detected the varying resistant starch concentration unripe banana flour samples successfully with high accuracy. The results and stability of the models developed in this study demonstrated clearly that the Vis- NIRS method has a potential of providing unripe banana flour processing industry with a rapid and non-destructive technique to manage unripe banana flour quality as well as adulteration by staple flours, therefore ensuring fair and safe trading of the product in retail markets of South Africa.
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    Investigating the effect of thermal processing on biochemical composition and kernel shelf-life of macadamia (macadamia integrifolia).
    (2018) Buthelezi, Nana Millicent Duduzile.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.
    Rancidity is a major limiting factor affecting the postharvest quality and consequently, the storability and market value of macadamia nuts. Initial high moisture content accelerates the primary stages of rancidity where hydroperoxides accumulate as main oxidation products, eventually breaking down to form low molecular weight oxygenated constituents such as alcohols, aldehydes, ketones and free fatty acids, eventually resulting in the development of off-odours and off-flavours. Hydroperoxides can also react with amino acid residues in the Maillard reaction, thereby initiating excessive browning. Kernel browning may be evident as surface discolouration or internal as ‘concealed damage’ of nuts. Internal browning may be accompanied by off-odours and off-flavours and is impossible to detect during processing, often with no visible signs. Such kernels are unacceptable to both the export and local market. The aim of this chapter was to review the potential of thermal processing on delaying the onset of rancidity and therefore, improving kernel shelf life and nutritional quality of macadamia nuts; and to review the reliability of visible to near infrared spectroscopy (Vis/NIRS) to non-invasively predict kernel rancidity. Keywords: Rancidity, browning, free fatty acids, near infrared spectroscopy
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    Pre- and post-harvest response of selected indigenous leafy vegetables to water stress.
    (2018) Maseko, Innocent.; Mabhaudhi, Tafadzwanashe.; Tesfay, Samson Zeray.; Ncube, Bhekumthetho.
    South Africa has wide diversity of African leafy vegetables (ALVs) rich in nutrients and adapted to marginal production. However, there is limited availability of ALVs in South Africa due to lack of cultivation owing to limited agronomic and postharvest management information. The increase in population growth, malnutrition and climate change necessitates production of more food using limited water resources. The aim of this study was to evaluate pre and postharvest response of Amaranthus cruentus (pigweed), Vigna unguiculata (cowpea), Corchorus olitorius (Jute mallow) and a reference crop B. vulgaris (Swiss chard) to varying irrigation regimes. The current study consisted of a literature review and five experiments (two agronomic studies and three post-harvest studies. In literature, the performance of ALVs is drawn in comparison to exotic counterparts grown under different conditions; yet agronomic and nutritional factors are only valid when crops are grown under the same condition. Hence in the four experiments of this study, Swiss chard was used as a reference crop grown under same locality. Swiss chard was chosen because it is an alien leafy vegetable that has been indigenised in sub-Saharan Africa and is highly nutritious (contains high levels of Fe, Zn and β-carotene). Before conducting experiments there was need to identify potential gaps and research priorities for this study and even for future research. This was done by conducting a literature review study (Chapter 2) on the status of production and utilisation of ALVs in South Africa for the period 1994–2017. Results of the review indicated that there is a decline in consumption of ALVs partly as a result of limited availability and negative perception. In order to promote ALVs, further research on agronomy, post-harvest handling, storage and processing is required in South Africa. Field and rain shelter experiments were conducted at Roodeplaat, Pretoria, over two summer seasons, 2015/2016 and 2016/2017 to evaluate growth, yield and water-use of selected leafy vegetables under varying water regimes. A randomised complete block design with three replicates was used. The treatments evaluated were: three irrigation regimes (30%, 60% and 100% of crop water requirement (ETc) on three ALVs – Amaranthus cruentus, Corchorus olitorius and Vigna unguiculata and a reference crop, Beta vulgaris. Seeds of A. cruentus and C. olitorius were obtained from the seed bank of the Agricultural Research Council (ARC) - Vegetable and Ornamental Plants (VOP), Roodeplaat, Pretoria seed bank. Vigna unguiculata (Bechuana white, a runner type) and Swiss chard (B. vulgaris L.) cultivar ‘Ford Hook Giant’ seeds were obtained from Hygrotech Seed Pty. Ltd., South Africa. Soil samples were taken from the field prior to land preparation and soil fertility analyses done at the Agricultural Research Council–Institute for Soil, Climate and Water (ARC–ISCW). Nitrogen, phosphate and potassium were applied according to the results and recommendations of the soil fertility analysis for both seasons. Seedlings of A. cruentus, B. vulgaris and C. olitorius were raised in commercial growing medium and covered with vermiculate to minimize water losses from above surface. Vigna unguiculata was sown directly. Seedlings were transplanted at four weeks after sowing. Irrigation scheduling was based on reference evapotranspiration (ET) and a crop factor for each crop. Data collection in field and rain shelter trials included plant height, leaf number, chlorophyll content index (CCI), chlorophyll fluorescence (CF) and yield. In A. cruentus, drought stress (30% ETc) reduced yield consistently in both field and rain shelter trials. Plant height and chlorophyll content index (CCI) were significantly reduced by water stress under field conditions. For C. olitorius, drought stress (30% ETc) reduced yield under rain shelter conditions while all measured parameters were not affected under field conditions. In V. unguiculata, stem fresh mass increased with increase in water application from 30%-60 ETc with no further significant increase under field conditions while all measured parameters showed a similar trend under rain shelter although the results were not significant. In B. vulgaris leaf number, plant height, CCI, yield, Mg, Ca, Na, Zn, and Mn were reduced by water stress for rainshelter. Using 60% ETc proved to be suitable for production of A. cruentus and B. vulgaris var. cicla whereas 30% ETc would be recommended for V. unguiculata. For V. unguiculata and C. olitorius application of 30% ETc is recommended while application of 60% ETc can be used under to slightly improve yield. Amaranthus cruentus and B. vulgaris were comparable in their response to water regimes while C. olitorius and V. unguiculata performed better than B. vulgaris under water stress, an indication of an opportunity to use these vegetables under drought conditions. The evaluation of nutritional quality of A. cruentus, C. olitorius, V. unguiculata and B. vulgaris was motivated by recommendations made in most agronomic studies based on biomass yield with no follow-up on nutritional value. Samples from each crop were collected from each of the three irrigation regimes (30%, 60% and 100%ETc) during each harvest (6, 8 and 10 weeks after transplanting for both seasons) and analysed for macro and micronutrients. Results from A. cruentus indicates that Ca and Mg were significantly higher under drought stress (30% ETc) while Na, K and Zn increased with water application up to 60% ETc with no further increase thereafter. Similarly, Ca and Mg were higher under drought stress and Zn under medium stress in C. olitorius. Calcium was high under drought stress condition in B. vulgaris while Na and Zn where high in medium stress; with a further increase in water application resulting in diminishing returns. Phosphate and potassium were high in medium stress condition in V. unguiculata while in water application up to 100% ETc the two elements showed diminishing returns. The high nutrients alternated between the most severe water stress (30% ETc) and medium stress (60 ETc) treatments across all crops in this study, an indication that although the crops can be grown under drought conditions, slight irrigation can lead to improved production. Leaf Fe, Zn, Mn, Mg and Ca increased with time of harvesting that increased from 6 to 8 weeks, with no further change in nutritional yield when crops were harvested at 10 weeks in A. cruentus, V. unguiculata and B. vulgaris. In C. olitorius, Fe, Zn, Mn, Mg and Na were high when harvested at 6 weeks compared to late harvesting (8 and 10 weeks). The first postharvest study investigated the effect of three irrigation regimes (30%, 60% and 100% ETc) and three drying (sun, oven, shade) methods on phenolic, flavonoid and gallatannin content of the four vegetables. Fully irrigated C. olitorius and subjected to sun drying (100% ETc x sun drying) had higher total phenolic content followed by medium stress subjected to shade drying (60% ETc x shade drying). Furthermore, water stressed plants that were then shade- or sun-dried retained better gallotannin content than other treatment combinations. Amaranthus cruentus grown under drought then shade- or sun-dried (30% ETc x shade and sun drying) retained better quality in all phenolic components measured. In V. unguiculata, phenolic content was high in water-stressed plants subjected to sun-drying (30% ETc x sun drying) while sun drying retained flavonoid and gallotannin than shade and oven drying. In B. vulgaris, well irrigated plants and shade- or oven-dried (100% ETc x shade/oven drying) had better phenolic content. Shade dried leaves had better flavonoid while drought-stressed plants had better gallotannins content compared to other treatments in B. vulgaris. All three ALVs can be grown under drought stress and subjected to sun or shade drying to retain nutrient compared to B. vulgaris. The second experiment on postharvest investigated the interaction of packaging (non-perforated and perforated), temperature [room storage, refrigerated storage (4℃), retail storage, 10℃] and storage duration (2, 4, 6, 8, 10 days) on C. olitorius. Plants rarely experience a single stress factor but are simultaneously exposed to multitude stress factors in their growing environment. The results showed that treatment combination of 4℃ with perforated packaging retains higher phenolic content followed by perforated packaging at 10℃ while lower phenolics were in treatment combinations that were stored at room temperature. Total phenolic content was higher at 2 days and 4 days storage in non-perforated packaging compared to all other treatments combinations. Furthermore, phenolic content decreased disproportionately with storage duration in non-perforated packaging treatment combinations, performing better than perforated in every storage duration. Flavonoid content and total phenolics decreased with increase in storage duration while better retaining these in any treatment combination of 4℃/10℃ compared to room temperature. Phenolic content was significantly higher from 2 to 4 days then declined from 6 through to 10 days at 4℃. At room temperature, phenolic contents decreased from 2 to 4 days storage durations but were higher at 6 and 8 days storage durations before dropping at 10 days. Antioxidant activity and overall acceptance was improved in any treatment combinations kept at 4 and 10℃ compared to room temperature for both types of packaging as storage duration increased. Antioxidant activity and overall acceptance degradation was reduced in treatment combination kept at 4 and 10℃ compared to room temperature for both types of packaging as storage duration increased. Corchorus stored at room temperature had a shelf life of 2 days, but 8 days at 4℃ and 10 days at 10℃ for both types of packaging.
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    Variations in growth, yield and metabolites of African ginger (Siphonochilus aethiopicus) in responses to irrigation regimes and nitrogen levels.
    (2017) Salmina, Mokgehle Ngoakoana.; Tesfay, Samson Zeray.; Araya, Hintsa Tesfamicael.
    Medicinal plants are valuable natural resources used as traditional medicine and have economic significance. African ginger (S. aethiopicus) (Schweinf.) B.L. Burtt is one of the most important rhizomatous plants, highly-valued for its medicinal properties and wide distribution in many regions of southern Africa. The plant is currently listed on the Red List of South African endangered species due to overharvesting. The increased demand for plant material has led to extinction in other areas of South Africa. The loss of wild populations harvested will destroy the natural habitats and genetic diversity in the long term. The demand for S. aethiopicus plant parts, particularly the rhizome is associated with the medicinal remedies possessed by the plant. The rhizomes have been traditionally used for the treatment of coughs, colds, asthma, headaches, pain, inflammation and malaria. Currently, there is limited scientific evidence on the cultivation and response of secondary metabolites of S. aethiopicus to agronomic practices. Cultivation of medicinal plants is a good approach to conserve species biodiversity and meet current demands for plant based products. This study investigated the variations in growth, yield and metabolites of S. aethiopicus in response to cultivation practices for commercial production and further development of medicinal products. In this study, total phenolic content, flavonoid content and antioxidant activity of S. aethiopicus leaf, rhizome and root from varying areas (Mpumalanga, KwaZulu-Natal, Limpopo and North West) were evaluated. Total phenolic and flavonoid contents were investigated by Folin-Ciocalteu and aluminium chloride (AlCl3) colorimetric methods, respectively. Antioxidant activity in different parts of S. aethiopicus was evaluated by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity and ferric reducing power (FRAP). Furthermore, the study determined the variations in soluble sugars in the leaf, rhizome and root as influenced by varying growing areas. The results showed high concentration of sucrose, glucose and fructose in the leaf and root as influenced by different growing areas. A higher content of both total phenolics and flavonoids were found in the root from Mpumalanga (54.5±2.0 mg GAE/g and 14.83±0.06 μg QE/g, respectively) compared to the leaf and rhizome from other growing areas. KwaZulu-Natal also exhibited high flavonoids in the leaf (12.72±1.18 μg QE/g), rhizome (14.21±1.98 μg QE/g) and root (12.88±0.57 μg QE/g) compared to other growing areas. In both methods, the leaf exhibited higher antioxidant activity than the root and rhizome. The high antioxidant activities exhibited in the leaf from Mpumalanga suggest its adaptive capabilities to different environments. S. aethiopicus parts could be used as a potential source for antioxidant properties and encourage cultivation under different growing areas to conserve its biodiversity and increase species populations. The effect of nitrogen levels and irrigation regimes on biomass yield, stomatal conductance, chlorophyll content and leaf area index was investigated under the rainshelter for two growing seasons. The results of this study conclusively reveal that the plant height and number of leaves per plant were significantly higher towards maturity. Plants grown with 50 and 100 kg N/ha had greater plant height, number of leaves per plant, LAI, SPAD values and biomass yield that eventually resulted in higher dry matter production. Stomatal conductance was higher throughout the growing period and decline in response water stressed treatment. The high amount of water utilized from well watered treatment (30% ADL) compared to moderate (50% ADL) and severe (70% ADL) treatments could be attributed to improved water availability and superior plant canopies. Further experiments should be conducted to evaluate different combinations of agronomic practices to fully exploit the growth of S. aethiopicus under different conditions. The high amount of water utilized from the well watered treatment (30% ADL) compared to moderate (50% ADL) and severe (70% ADL) treatments could be attributed to improved water availability and superior plant canopies. The well watered treatment (30% ADL) had a significantly higher total biomass, fresh and dry rhizome yield compared to other water stressed treatments. The response of water stress and nitrogen levels showed significant accumulation of plant flavonoids and phenolics in leaf, rhizome and root. In plant carbohydrates, root had high sucrose content with the application of low N under severely stressed (70% ADL) treatment. The investigation of volatile components of leaf, rhizome and root in response to irrigation regimes and nitrogen levels were analysed by GC-MS. The results showed that the highest volatile components in the root and rhizome were terpenes, as compared to the increased components of aliphatic acids, benzenoids and aliphatic aldehydes in the leaf. In all treatments and parts, the odorant sesquiterpene (1E)-5-Methyl-1-(2, 6, 6-trimethyl-2, 4-cyclohexadien-1-yl)-1, 4-hexadien-3-one was the most abundant volatile compound. The 4-Hydroxy-4-methyl-2-pentanone was detected under severely stressed (70% ADL) treatment with the application of 100 kg N/ha. Severely stressed (70% ADL) treatment with minimal application of N induced the terpenes components in all plant parts. The study showed that volatile components of S. aethiopicus vary with plant sources, water stress and mineral nutrient deficiency. Knowledge on the impact of S. aethiopicus parts will provide a useful guide for selection towards identifying profiles of volatile compounds and explore the additional bioactive compounds for therapeutic use. Taken together, this study represents the importance of cultivation methods as an alternative approach to wild harvesting, conserving S. aethiopicus for commercial production and exposure to water stress conditions for high secondary metabolites.
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    Effect of canopy position and non-detructive determination of rind biochemical properties of citrus fruit during postharvest non-chilling cold storage.
    (2017) Olarewaju, Olaoluwa Omoniyi.; Magwaza, Lembe Samukelo.; Tesfay, Samson Zeray.; Fawole, Olaniyi Amos.; Opara, Umezuruike L.
    No abstract provided.
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    Induced polyploidy as a tool for the development of novel South African indigenous crops.
    (2015) Hannweg, Karin Fiona.; Bertling, Isa.
    Polyploidy is a naturally-occurring phenomenon in plants and has been reported as an important pathway for evolution and speciation; it is estimated that a large percentage of flowering plants are polyploid in origin. Although the first plant polyploid was discovered over a century ago, the genetic and evolutionary implications of polyploidy have not been fully elucidated. On a more practical level, there are many opportunities for utilizing induced polyploidy as a valuable tool in traditional plant breeding programmes. South Africa has the highest recorded plant species density in the world, however, many of these species have only marginal potential due to size and other constraints. Induced polyploids may be expected to exhibit one, or more, of the following characteristics resulting in the improvement or the development of new economically important plants: larger tuber, rhizome or root size; increased flower or fruit size; enhanced flower colour intensity, improved drought tolerance, increased bio-mass; improved photosynthetic capacity; larger and/or thicker leaves; dwarfism; increased secondary metabolite production, e.g. medicinal compounds. Several plant species (Crocosmia aurea, Tetradenia riparia, Siphonochilus aethiopicus and Plectranthus esculentus) were selected for the induction of polyploidy and various horticultural characteristics evaluated. Methods for the successful induction of polyploidy were developed for all selected species. By evaluating various horticultural characteristics of the induced polyploids it was determined that flower size, plant vigour and nematode resistance, as well as essential oil content and bioactivity could be significantly improved in all tested species. Induced polyploidy could, therefore, have a significant impact on the development of economically-viable novel crops indigenous to southern Africa.
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    Optimization of flower and seed crop production in temperate eucalyptus orchards in South Africa through site selection and environmental manipulation.
    (2014) Gardner, Robin Arthur William.; Bertling, Isa.; Savage, Michael John.
    Temperate eucalypts are an important part of the commercial forestry landscape in South Africa, comprising approximately 50% of the total Eucalyptus planted area. The majority of the commercial temperate eucalypts grown in South Africa are reticent, shy flowerers, and subsequently erratic seed producers. Disadvantages associated with sub-optimum (inconsistent and sparse) flowering in Eucalyptus orchards include decreased levels of out-crossing and compromised quantity and (genetic) quality of the seed produced. Genotype, physiological age and a range of environmental factors are known to influence flower bud production in temperate eucalypts. To date, winter cold and paclobutrazol (PBZ), a plant growth regulator, remain the most effective treatments for encouraging early and prolific flowering in temperate eucalypts. Disadvantages associated with the use of PBZ in the outdoor environment include the toxicity and recalcitrant nature and persistence of the chemical in soils, the high cost of PBZ and its orchard application, and the need to re-apply the chemical approximately every five years. The main aim of this study was to provide a practical solution to the problem of shy flowering and seed crop production in important temperate Eucalyptus species in South Africa. The study focused on investigating key environmental factors associated with optimum flower bud production in temperate eucalypts. The resultant data were used to achieve optimization of flower bud production in temperate eucalypts in the summer rainfall forestry areas of South Africa, via informed site selection and/or manipulation of the environmental conditions. A subsidiary aim of the project was to lessen the dependency on PBZ for achieving satisfactory flowering levels in temperate eucalypts via improved site-orchard matching and environmental manipulation. The key objectives in the study included definition of summer rainfall area site conditions for maximal floral bud production in two important species, viz. E. nitens and E. smithii, investigation of the effects of optimum flowering environmental conditions on post-initiation floral development and seed maturation in temperate eucalypts, and development of a method for supplementing winter cold and increasing flower bud production in orchards located at marginal winter chilling sites.
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    Ovule and seed studies in relation to fruit growth in Carica papaya L. cl. honey gold.
    (1979) Pereira, Abel Luiz.; Alan, Peter.
    No abstract available.
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    Optimization, validation and application of radioimmunoassays for plant growth substances in avocado (Persea americana Mill.) fruits.
    (1984) Cutting, Jonathan Garth Melville.; Wolstenholme, B. Nigel.; Lishman, Arthur William.
    The objective was to develop, optimize and validate radioimmunoassays (RIA) for several plant growth substances (PGS) and then apply the RIA's to determine PGS trends in 'Fuerte' avocado fruits from fruit set to fruit ripening. Antibodies to the cytokinin isopentenyl adenosine (IPA) were obtained from rabbits inoculated with a periodate-derived IPA-BSA conjugate. The antiserum cross-reacted (25%) with only 2iP (isopentenyl adenine). The RIA measuring range was from 0,1 to 100 ng. Anti-IPA serum was used to develop a RIA for 2iP, with a measuring range from 0,5 to 100 ng. Using Dowex 50W-X8 and cellulose TLC purified avocado fruit extract, 20 samples per day could be processed. The RIA for abscisic acid (ABA) was developed from rabbit antibodies from an inoculated carbodiimide-derived, (+ -) ABA-BSA conjugate. The free active ABA component was isolated prior to quantitation by RIA by solvent partitioning and silica gel TLC. The indole-acetic-acid (IAA) RIA was established from sheep-produced antibodies to a formaldehye-derived IAA-BSA conjugate, after repeated inoculations. For both the ABA and IAA RIA's, contaminants in the avocado tissue were removed by solvent partitioning. Developing avocado fruits, and especially young fruits were rich sources of IAA and 2iP in particular with seed concentrations exceeding those of the fruit flesh. The concentration of ABA rose throughout fruit development, reaching 100 ng g(-1) in the flesh at fruit maturity. Just prior to seed and fruit maturity, relatively high levels of IAA, 2iP and IPA were associated with the thick, fleshy testa, rapidly declining to zero as the testa dried. The avocado fruit is physiologically dependent on the seed right up to this stage, and testa maturity correlates well with minimum "legal" maturity of 80% moisture content of flesh. In ripening avocado fruit, the concentration of free ABA rose as softening progressed. Total ABA concentrations fell initially, and then rose after a firmometer reading between 50 and 60 (100 coincides with eating ripeness). Later harvested fruit had double the ABA concentration of early harvested fruit. The ripening stimulus appeared to be related to moisture stress in the fruit. This study has confirmed the prime advantages of sensitivity, specificity and rapidity of RIA, as well as its usefulness in multi-PGS studies and batch-type analysis.
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    Tissue culture studies on the genus Rosa with special reference to the shoot tip.
    (1969) Jacobs, Gerard.; Allan, P.; Bornman, Chris H.
    A modified Berthelot (1934)-Knop (1865) solution was found to support the growth of rose pith segments while Murashige and Skoog's (1962) medium did not. Possibly the NH(4)+ content of the latter medium is toxic to rose tissue. The best callus growth was obtained with indolebutyric acid (IBA), which was superior to naphthaleneacetic acid (NAA), indoleacetic acid (IAA) and 2,4 dichlorophenoxybutyric acid (2,4-DB) at the concentrations tested, while gibberellic acid (GA) greatly enhanced callus formation in the presence of both IBA and kinetin. Occasional differentiation of shoots was observed both in cultured pith segments and in callus formed at the basal cut surface of the shoot tips. However, the precise culture conditions required for differentiating rose pith and callus tissue remain unknown. An interaction was found between NAA and kinetin with regard to root and leaf development in shoot tips. Root formation took place only in the absence of kinetin and in the NAA range of 0.5 to 2.0 mg/l, while normal leaf development occurred only in the absence of NAA and in the presence of 4.0 to 18.0 mg/l kinetin. Neither any combination of NAA and kinetin nor the sequential application of growth substances induced both root and shoot growth. Furthermore, shoot tips sampled in late summer formed roots much more readily than tips sampled in late winter. GA reduced the favourable effect of high kinetin treatments on leaf development. Different species of auxin affected growth of the shoot tip in different ways. IAA did not inhibit growth of the shoot tip in the same manner as was observed for NAA, IBA and 2,4 dichlorophenoxybutyric acid (2,4-D). It is concluded that further experimentation with different ratios of various species of auxin and cytokinin, as well as the sequential administration of growth substances, may lead to the successful culture of intact plantlets from rose shoot tips and shoot apical meristems, and ultimately to possibly virus-free rose plants.
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    Soilless cultivation of cucumbers and tomatoes under protection in Natal.
    (1982) Smith, Irwin Elcoate.; Wolstenholme, B. Nigel.
    No abstract available.
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    Some aspects of water relations on avocado Persea americana (Mill.) tree and fruit physiology.
    (1985) Bower, John Patrick.; Wolstenholme, B. Nigel.
    The effect of long-term irrigation on water relations of 'Fuerte' avocado trees, and the consequences for fruit ripening and physiology, particularly physiological disorders, were studied. Four irrigation regimes were used, namely dryland relying on rainfall, occasional (irrigation when soil moisture tension reached 80 kPa), regular (soil moisture tension 55 kPa) and frequent (soil moisture 35 kPa). Seasonal tree leaf water potential was studied. This became more negative during the dry season (winter and spring) and less negative during the period of summer rainfall. During the dry periods, the dryland and 80 kPa treatments had considerably more negative leaf water potential than the 55 kPa and 35 kPa regimes. These differences decreased during the summer rainfall period. Acclimation was studied by measuring leaf osmotic pressure and osmotic pressure at zero turgor. A pattern similar to seasonal leaf water potential emerged. Further, the dryland treatment showed higher osmotic pressure, particularly at zero turgor, during winter and spring. It was concluded that these trees may have acclimated. Diurnal cycles of stomatal resistance, transpiration and leaf water potential on a summer, two winter and a spring day were monitored. Dry land trees showed acclimation, with delayed reaction to environmental water demand and decreasing soil moisture. Trees of the 80 kPa treatment showed greatest stress. Fruit water potential became more negative between April and July, with fruit softening becoming more rapid. Treatment differences were inconclusive. Polyphenol oxidase activity (PPO), soluble and total, was measured. For fruit picked in April and July 1983, the 55 kPa treatment showed the lowest activity and the 80 kPa the highest. Storage at 5,5⁰C for 30 days increased the activity, while fruit softening decreased it. July activity was higher than the April-harvested fruit. The same pattern emerged for fruit harvested in April 1985, although treatment differences were not significant. Rainfall was considerably higher during the fruit development period of 1985 fruit as compared with that of 1983. A significant interaction between restricted container ventilation during ripening and irrigation history was obtained, the 80 kPa fruit showing higher PPO activity than 55 kPa fruit. Ethylene evolution during ripening showed a normal climacteric pattern for 55 kPa and dryland fruit, but a delayed peak for 80 kPa fruit. Fruit calcium concentrations showed rapid changes between 7 and 16 weeks after fruit set thereafter remaining constant to harvest. There were no clear treatment differences. Fruit abscisic acid levels at 50% soft (100% is eating soft) were lowest in 55 kPa fruit, and highest in 80 kPa. A significant correlation between these values and soft fruit PPO activity was found. A preliminary fruit quality prediction model is suggested.
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    Temperature as a factor in nectarine production in the Western Cape.
    (1994) Linsley-Noakes, Gavin C.; Alan, Peter.
    Phenological and climatic data were collected from nectarine orchards containing the cultivars Sunlite, Flavortop and Fantasia, growing in six climatically divergent areas in the Western Cape, South Africa. Shoots were forced at 25⁰C and bud volumes measured in order to develop an index of when the endodormancy requirement for each cultivar in each area had been satisfied. Flower bud break gave the only consistent indication of when the chilling requirement had been satisfied and 10% flower bud break after 14 days at 25⁰C was adopted as the best index of end of endodormancy. Unlike vegetative bud break, flower bud break was not influenced by forcing period or gibberellic acid (GA3) application. Comparison of winter chilling models using the temperature data for the endodormancy period, showed that the 'dynamic' model (DP) was more accurate than the currently used 'Utah' model (UCU), for estimating the rest requirement of the nectarine cultivars Sunlite, Flavortop and Fantasia. The DP model failed, however, to identify orchards displaying 'delayed foliation' symptoms as a result of insufficient winter chilling. The 'dynamic' model not only takes into account the positive effect of cool temperatures and negative effect of high temperatures on rest completion that the 'Utah' model does, but also the positive effect of moderate temperatures and the fact that chilling negation is subject to time constraints. The nectarine cultivars tested required about 33 ± 5; 46 ± 8 and 46 ± 8 'dynamic' portions of chilling for Sunlite, Flavortop and Fantasia, respectively. Modification of the UCU model, using principles from the DP model, resulted in a substantial increase in its accuracy. The modification assumes that the negating effect of high temperatures is confined to the diurnal cycle and that there is not a carry-over effect of chilling negation from one day to the next. Coefficients of variation for the estimated chilling requirements of 'Sunlite', 'Flavortop' and 'Fantasia' were reduced from 24.9%, 21.7% and 23.8% to 19.7%, 17.9% and 20.1 % respectively. This modification, now called the 'daily positive Utah chill unit model (PCU) , is currently used in the Western Cape Province to determine winter chilling efficiency in deciduous fruit orchards. The PCU requirements for 'Sunlite', Flavortop' and 'Fantasia' were 566± 111, 807± 145 and 817 ± 165 respectively. Correlations were then made between the heat component during endodormancy (degree hours > 10⁰C [D H]) and the apparent chilling requirement. The subsequent regressions gave almost parallel slopes regardless of cultivar or chilling requirement, indicating that in milder winter areas, heat accumulation could also play a vital role in endodormancy. As an approximation, an elevation in mean air temperature during the main endodormancy months of May and June, resulted in an apparent reduction in the chilling requirement of 20% for every 10⁰C increase within the range of 11-14°C. The post-endodormancy heat requirement for bud development in spring was highest in the milder winter areas and lower in areas receiving more winter chilling than their endodormancy requirement. Although there are already 65 automatic weather stations operating in South Africa's Western Province deciduous fruit industry, the variation in microclimate is much too large to make accurate extrapolations and alternative methods of estimation of chilling are required. Tables for the expeditious determination of daily chill unit accumulation from minimum and maximum temperatures were developed, using the assumption that air temperature follows either a straight line between extremes or that it follows a sine curve during the heating cycle followed by a logarithmic cooling phase. Both tables gave an average accuracy of 95 % for. determining monthly or total winter chilling in seven divergent locations over three seasons. The sine- logarithmic table appeared to be slightly more accurate under mild winter conditions. Daily positive Utah chill units are now used extensively by the South African deciduous fruit industry and colour maps depicting average PCU chilling values for the region were in the process of being plotted at the time of going to press.
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    Ecophysiological studies and tree manipulation for maximisation of yield potential in avocado (Persea americana Mill.)
    (1994) Whiley, Anthony W.; Wolstenholme, B. Nigel.; Schaffer, B.
    Tree fruit crops generally consist of scion and rootstock components, which through interactive synergism affect tree performance. Coupled with tree architecture, sink/source relationships (both spatial and temporal), genotypic responses to environments, and carry-over seasonal effects present a high level of complexity which often confounds research results. The development, description and use of pheno/physiological models as research and crop management tools is a new holistic approach to reduce complexity and improve understanding of the critical factors which influence crop productivity. A pheno/physiological model is described for cv. Hass avocado growing in a cool, mesic subtropical environment in S.E. Queensland, Australia. Seasonal shoot and root growth had bimodal periodicity with root growth offset and delayed with respect to shoot growth. The priority sink strength of developing shoots compared with roots was confirmed with 14(C) studies. Root growth in summer extended through until late winter when there was a substantial decline following anthesis - a critical time in fruit development with competition between reproductive and vegetative sinks for limited resources. Delayed harvesting of fruit over several seasons resulted in alternate bearing patterns, while removal of fruit at the minimum legal maturity of 21 to 24% dry matter sustained successive high yields. With cv. Hass, production was directly related to starch concentrations in trunks or shoots in July (midwinter) immediately prior to anthesis. However, seasonal starch concentration fluxes in trunks were much lower in coastal subtropical Australia compared with those previously reported from interior areas in more southerly latitudes (7.5% vs. 18% maximum). Current assimilate from over-wintered leaves was necessary to bridge the gap in early spring between the depletion of starch reserves by new reproductive and vegetative shoot growth, and the sink/source transition of the spring shoot growth. Net CO2 assimilation of summer grown leaves reached ca. 17 µmol CO2 m(-2) s(-1), approximately twice as high as previously reported rates on container-grown plants or trees in minimum temperatures were < 10⁰C for 50 days, this being the first report of this phenomenon in field-grown avocado trees. Partial recovery occurred prior to senescence of previous season's leaves in spring after minimum temperatures increased above 10⁰C. The plasticity of the light response was high with the compensation point for net CO2 assimilation at 30 µmol quanta m(-2) s(-1) and the light saturation point at 1270 µmol quanta m(-2) s(-1). Net CO2 fixation from fruit photosynthesis was always less than losses through respiration but was highest during the first few weeks of ontogeny, perhaps contributing to the fruit's own carbon economy at a time when competition for assimilates was greatest. In general, CO2 assimilation studies with current technology applied to orchard trees in non-restrictive soils have elucidated efficiencies more akin to deciduous than evergreen trees - thereby compensating for short-lived leaves and energy expensive fruits. Pheno/physiology models were used to substantiate the most effective timing for trunk injection of ambimobile phosphonate fungicides for the control of Phytophthora root rot, a serious disease of avocados, viz. at the completion of the leaf expansion phases when leaves were strong net exporters. Preliminary studies demonstrated potential yield increases when the assimilation efficiency of photoinhibited over-wintered leaves was improved through increased nitrogen concentration, and spring shoot growth was partially suppressed with foliar sprays of the growth retardant paclobutrazol.
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    Post-harvest manipulation of rind colour in 'Mauritius' litchi (Litchi chinensis Sonn.) fruit.
    (1996) Kaiser, Clive.; Wolstenholme, B. Nigel.
    Litchi fruit are non-climacteric, and are able to endure relatively low storage temperatures compared to other subtropical fruits. Unfortunately however, the litchi rind is relatively thin and lacks a thick, durable cuticle. Consequently, post-harvest desiccation is a major factor, and rind colour changes rapidly from red to brown, unless counter measures are taken immediately after harvest. Presently, the South African industry uses sulphur fumigation to prevent browning, but sulphur treatment is undesirable in many respects, only partially successful, and some overseas markets have lowered the permissible level of sulphur to 10 mg.kg-1 in the fruit flesh. Alternatives to sulphur fumigation were accordingly researched. The author tested the hypothesis that, in order to preserve the desirable red rind colour, it was necessary to break down rind cell membrane integrity, so that the vacuole-bound anthocyanin pigments can be exposed to zero pH solution, which effects rind colour preservation. Thereafter, rind desiccation must be reduced. A 2 s steam (95°C) treatment followed by 4 min immersion in zero pH solution resulted in fruit which retained excellent red rind colour, with normal pulp characteristics and tasted similar to control fruit after 28 days storage at 1°C. Ultrastructural studies showed that 2 s steam (95°C) treatment resulted in rind cell membrane breakdown, and this was enhanced when used in conjunction with 4 min in zero pH solution. In addition, electrolyte leakage studies showed that rinds of untreated control fruit had lowest electrolyte leakage, while those of fruit subjected to 2 s steam (95°C) had highest electrolyte leakage, making the previously compartmentalized and vacuole-bound pigments available for preservation in the desirable red colour. Polyphenol oxidase in litchi rinds was strongly inhibited by 2 s steam (95°C), but even more so when fruit were subjected to 2 s steam (95°C) followed by 4 min in zero pH solution. Energy dispersive x-ray microanalysis studies found that chlorine concentrations were relatively high on both the inner and outer surfaces of fruit subjected to 2 s steam (95°C) followed by 4 min in zero pH solution. Similarly, sulphur concentrations were high in rinds of sulphur-fumigated fruit, but this element was also present at low concentrations in nonsulphur- fumigated fruit. Rind colour of untreated control fruit lightened when stored at 30°C and hue changed from red to reddish orange. Rinds of fruit subjected to 2 s steam (95°C) only, lost colour rapidly and were a pale yellow hue 24 hr after treatment. The hue of fruit rinds subjected to 2 s steam (95°C) followed by 4 min in zero pH solution changed from reddish orange to red within 4 hr and then darkened up to 24 hr after treatment. Red colour was preserved in fruit held at 30°C for 72 hr, but lightened after 24 hr. HPLC of anthocyanin pigments found that the presumed cyanidin-3-rutinoside, pelargonidin-3-glucoside and pelargonidin-3,5-diglucoside all decreased in untreated fruit over 5 days storage at 30°C. Concentrations of presumed cyanidin- 3-rutinoside in fruit subjected to 2 s steam (95°C) followed by 4 min in zero pH solution increased immediately after treatment, peaked 24 hr later, but then decreased to about double the concentration of fruit treated on the day of harvest after 4 days at 30°C. Furthermore, no copigmentation or self-associations of anthocyanins took place in rinds of fruit subjected to 2 s steam (95°C) followed by 4 min immersion in zero pH solution. Semi-commercial trials showed that the steam: acid dip treatment is feasible, and has the potential to replace sulphuring as a fungicidal treatment. It also has the advantage of more permanently preserving the desirable rind colour, and in a more intense red colour.
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    A multi-factor study of cabbage production in the Umlaas River valley.
    (1995) Askew, Derek James.; Smith, Irwin Elcoate.; Le Roux, Fanie.; Wolstenholme, B. Nigel.
    A research and extension programme monitored 59 cabbage crops and many factors associated with cabbage production in the Umlaas River Valley of KwaZulu-Natal, from July 1991 - June 1993. A large data bank was developed, accumulating all information regarding soils, weather patterns, farming practices, management, crop water stress, yields, plant populations, weeds, pests, diseases, soil fertility, leaf nutrient concentrations and the presence of hormone herbicides in the rain. Analysis of this large data bank of information enabled the identification of limiting factors and optimized production practices. Excessively high or low levels of soil nutrients and soil acidity problems, reduced cabbage yields in the Umlaas River Valley. Identification of critical values and an ideal cation ratio in soils, and recommendations to rectify limiting factors, could drastically improve yields. Wide variation and excesses of soil nutrients were seen from soil analysis of 59 cabbage crops in the Umlaas River Valley. Soil P > 155, K > 486 and Mg > 568 mg 1(-1), exchangeable acidity> 0.11 cmol(c) 1(-1), acid saturation> 2 % and pH < 4.51 resulted in lower yields. Top yields were found between a certain range of soil nutrient levels and critical values lay within this range. Optimal yields resulted with an ideal ratio of Ca, Mg and K (65 :25: 10) in plots monitored. A soil acidity complex affected cabbage yields in the Umlaas River Valley. High exchangeable acidity, acid saturation and low pH increased Mn and Zn availability in the soil. Both Mn and Zn were absorbed in large quantities by the plant and maximum levels of 406 mg kg(-1) and 114 mg kg(-1) respectively were recorded in most recently mature (MRM) cabbage leaves at headform. Mn concentrations > 100 mg kg(-1) at headform and 50 mg kg(-1) at harvest reduced yields. Nutrient analysis results of a MRM leaf at headform, and a 1/8 head slice of a mature head at harvest, generated leaf nutrient norms and crop removal figures for cabbage. Adequate nutrient levels of MRM leaf at head form were: N = 3.3 - 4.8 %, P = 0.32 - 0.55 %, K = 2.1 - 4.2 %, Ca = 1.3 - 2.5 %, Mg = 0.25 - 0.65 %, Mn = 15 - 100 mg kg(-1), Zn = 18 - 60 mg kg(-1) and Cu = 4 - 100 mg kg(-1). Nutrient concentrations above or below adequate ranges resulted in lower yields. Nutrient removal figures were: 1.9 kg N, 0.3 kg P, 2.03 kg K, 0.43 kg Ca, 0.19 kg Mg, 2.8 g Mn, 2.6 g Zn and 0.4 g Cu t(-1) fresh material. These leaf nutrient norms were developed for reliable identification of soil fertility problems and the crop removals for improved fertilizer recommendations. A study of fertilization practices showed that most farmers supplied enough (+ 200kg) N to cabbage crops but used incorrect pre- and postplant proportions (1/4 and 3/4) for top yields. For maximum yields, 2/3 of the total N should be applied preplant and 1/3 at 4-6 weeks. Topdressings of N should be applied once only and not split as is common practise in the area. Application of P on cabbage crops was proportional to soil requirements but was generally excessive. Sufficient quantities of K were applied but scant attention was given to soil analysis results for K, Ca and Mg requirements. Input costs could be decreased and high yields maintained if soils were fertilized up to critical values and no further nutrients added when soil test values exceeded critical values. Fertilization was not generally based on results of soil analysis and few farmers limed their soils in spite of soil acidity problems. The more acidifying NH4 containing fertilisers were mostly used in preplant applications. The study recommended using more N0(3) based fertilisers to slow down acidification or, at least, corrective applications of lime. Good pest and disease control is essential for maximum yields in the Umlaas River Valley. This study identified optimum infection periods and maximum and minimum temperatures for infection by blackrot, club root , ringspot, damping-off, sclerotinia, downy mildew and alternaria leaf spot; and evaluated disease effects on yields. Clubroot, blackrot and damping-off significantly reduced cabbage yields. Clubroot incidence was generally associated with soil acidity problems, waterlogged lands or sandy soils subjected to slight over-irrigation. Blackrot was reduced by adequate N and K fertilization, increased with higher concentrations of Mn in most recently mature leaves (MRM) at head form (also associated with soil acidity) and always occurred with the warm, wet conditions of summer. Aphids and thrips occurred throughout the year; cutworm, bollworm, greater cabbage moth, leafminer and diamond-back moth, occurred mostly during the warmer season; and webworm over the cooler period in autumn. Aphids, thrips and webworm were the most important cabbage pests, causing serious losses especially during the first six weeks after transplanting. Factors such as amounts of fertilizer applied, soil acidity problems, slope of the land, infiltration rate, soil nutrient status, water stress, leaf concentrations of each element, weeds and disease were all associated with the incidence of various pests. This study highlighted the importance of good management practices, which affect all these aspects of cabbage production, and therefore pest incidence. An awareness of these factors and recommendations made in this study, could improve cabbage management practices in the Umlaas River Valley and therefore, reduce pest infestation. The study also evaluated relationships between weather patterns, hormone herbicide-like symptoms which occurred simultaneously on various garden trees and shrubs and vegetable seedlings, and pest and disease outbreaks or hormone herbicide symptoms on field crops of cabbage. Hot days with temperatures> 30°C, high VPD and high solar radiation followed by a rapid drop in temperature, high relative humidity and low solar radiation (stress weather cycles) were characteristic of weather conditions with an approaching frontal low in the Umlaas River Valley. All the dates of hormone herbicide-like symptoms on some garden shrubs and trees and vegetable seedlings in the nursery (indicator plants) were correlated with stress weather cycles, however not all stress weather cycles were correlated to these symptoms on indicator plants. It appeared that an unknown factor (Factor X) occurred with these weather cycles, and together with the stress induced by the harsh weather conditions, caused these symptoms on the indicator plants. A Canary creeper (Senecio tamoides DC.), fig tree (Ficus natalensis Hochst.) and an indigenous Cape chestnut (Calodendrum capense (L.F.) Thunb.) showed symptoms of leaf bubbling, twisting, burning, deformities and leaf drop. Lettuce and cabbage seedlings were twisted, etiolated and suffered from downy mildew outbreaks. All trees, shrubs and nursery crops experienced these symptoms 1 - 4 days after the stress weather cycles and Factor X. It is possible that low levels of hormone herbicide < 25 ng 1(-1)) deposited during dynamic fumigations (associated with approaching frontal lows), were Factor X. However, when examining the relationship between field cabbages and all related factors, no clear correlation could be established between weather patterns, Factor X, cabbage plant health, and pest and disease incidence. An overall study examined the effects of a range of factors on cabbage yield including: soil analysis data; management and crop water stress ratings; total nutrients supplied; weed, pest and disease indices; tillage operations; soil characteristics; plant populations; percentage marketable yield and headmass. Poor water management, high incidence of weeds, pest and disease, excessive nitrogen and phosphorus applied as fertilizer, and too many tillage operations, were associated with low yields. Conversely, good water management, good liming and soil fertility practices, higher levels of soil potassium and better weed, pest and disease control, resulted in higher yields. A greater percentage of marketable cabbage heads was more important than head mass for higher cabbage yields. The methodology used in this study provided an efficient, reliable method of identifying factors which limited and maximized vegetable crop production and provided data for the compilation of production guidelines for cabbage. The research and extension aspects of this study also provided an opportunity for the rapid transfer of technology to the farmer and excellent farmer participation.