Horticultural Science

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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.
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.
Soilless cultivation of cucumbers and tomatoes under protection in Natal.
(1982) Smith, Irwin Elcoate.; Wolstenholme, B. Nigel.
No abstract available.
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.
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.
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.
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.
Some ecophysiological aspects of cashew (Anacardium occidentale L) with emphasis on possible flower manipulation in Maputaland.
(1994) Roe, Denis John.; Wolstenholme, B. Nigel.
There has been interest in developing a cashew industry in Maputaland, the far north-eastern corner of Natal/KwaZulu. Flowering and fruit development coincide with a rainy period, with accompanying serious flower diseases (Oidium anacardii and Colletotrichum gloeosporioides). Glasshouse studies were carried out at Pietermaritzburg, concurrently with field trials in Maputaland, in an attempt to manipulate flowering and growth of cashew trees. Two glasshouse trial were carried out. A factorial design with treatments 0, 3, 6 and 9 weeks of low temperatures (24°C day/9°C nights)(factor A) and 0, 3, 6 and 9 weeks of water deficit (Factor B) was used, with both factors in all combinations. During the second season the durations were increased to 0, 4, 8 and 12 weeks for both factors. No flowering occurred in this trial. Tree growth was not affected significantly by drought and/or cold duration. Temperature appeared to be the dominant factor at low temperatures, stomatal conductance and transpiration being suppressed by cold regardless of soil water potential. At more optimum temperatures for growth, stomatal conductance was dependent on soil water potential (r² = 0.756). Starch levels in the roots, dry matter production in the leaves, roots and stems, as well as leaf area were decreased significantly (P≤0.01) with increasing low temperature duration. Another glasshouse trial to test the effects of foliar urea at concentrations of 0, 1, 2, 4 and 8 g urea 100 l⁻¹, applied once, twice or thrice at fortnightly intervals was undertaken. The treatments were applied in late autumn/early winter of 1990 and 1991. Tree growth and flowering were monitored, and starch and leaf NH₃/NH₄⁺ analyses carried out. The highest urea concentration (8%) resulted in leaf scorch and abscission, extremely low stem diameter growth rates, and was too high for glasshouse trees. The starch contents of the 8% urea treatment were depleted significantly (P≤0.01) more than the other concentrations. The other urea treatments resulted in vigorous growth and high dry matter production. There were no significant effects of the number of sprays on cashew growth. Only seven trees flowered, and therefore no definite conclusions could be drawn regarding urea effects on flowering. Most hermaphrodite flowers (max. 76.8% hermaphrodite) opened soon after first anthesis of a panicle, and all terminal flowers of panicle branches were hermaphrodite. Flowers generally opened basipetally in a panicle, starting with hermaphrodite flowers and with progressively more male flowers. Urea sprays resulted in NH₃/NH₄⁺ build-up in the leaves, concentrations in flowering trees ranging from 100 to 700 μg g⁻¹ DM for approximately a month. A field trial at was carried out at Makatini Research Station to determine the effects of timing of a two month winter drought period on flowering and growth. An observational trial to determine the effects of girdling on growth and flowering was incorporated in the border rows of the irrigation trial. The trial tested five treatments (no irrigation during May and June, June and July, July and August, August and September, and a control treatment which received irrigation throughout Winter). Mean monthly temperatures were below 20°C, and mean minimum temperatures below 15°C for the 5 winter months during treatment application. There were no significant differences in tree growth, flowering, flushing, or yields between drought stressed treatments and control, indicating that, under the conditions at Makatini, autumn and winter temperature was the overriding factor controlling initial flower induction. Flowering occurred from early October (when mean temperatures exceeded 23 to 24°C) to late April (7 months - a prolonged flowering period), when mean monthly temperatures dropped below 23 to 24°C. Girdling of cashew trees in March and May, using girdle widths of 1, 5 and 10 mm was not successful in improving flowering and yields under the conditions of the trial. A field trial was carried out at Mosi Estate in Maputaland to test the following chemicals as tree and/or flower manipulators: foliar applied ethephon (50,100,200,500,2000 mg l⁻¹), KNO₃ (1 %, 2%, 4%), urea (1 %, 2%, 4%) and paciobutrazol (500, 1000, 2000 mg l⁻¹). A phenological model for cashew in Maputaland showed a dormant period during winter, followed by a generative flush, from which panicles and flowers were produced (peak November-January). The harvest period peaked in February and March. A strong post-harvest flush preceded the winter dormant period. Trunk starch levels were at their highest after the dormant winter period, and at their lowest following the harvest. Ethephon at high concentrations (500 and 2000 mg l⁻¹) resulted in excessive leaf drop, disturbed the root shoot balance and normal phenological patterns, and gave poor yields. The best ethephon concentrations were 100 to 200 mg l⁻¹. KNO₃had no significant effect on tree growth, flushing, flowering or yields when compared to control trees. Urea at 2% concentration gave a significant increase (P≤0.05) in flushing and simultaneous decrease in flowering. Paclobutrazol at 500 to 2000 mg l⁻¹ resulted in significantly lower growth rates, and early panicle production. The mean yields of all paclobutrazol treated treatments were higher than controls, despite a hail storm which damaged the flowers. From results of this trial, the use of these chemicals to improve yields and manipulate flowering may not be economically justified. The most promising chemical for further research was paclobutrazol.
A prediction model to determine the cross-pollination ability of Citrus spp.
(1995) Barry, Graham H.; Wolstenholme, B. Nigel.; De Lange, Hannes.
Due to a greater emphasis having been placed on seedlessness by citrus consumers, a need developed in the southern African citrus industry to avoid unwanted cross-pollination and hence seediness. There was a paucity of knowledge upon which to base sound decisions in order to ensure optimal orchard layout to achieve the objective of the production of seedless or "commercially seedless" fruit. In vitro pollen viability of all locally available citrus cultivars/selections was determined and pollen of cultivars with strong and weak pollination potential were used for in vivo pollen tube growth and seed content studies of 'Nules Clementine' mandarin and 'Delta Valencia' orange. Data from the in vitro assays were rated and categorized into 'pollen germination potential categories', while the relationship between these categories and seed content of 'Nules Clementine' was determined by regression analysis. The development of the 'pollen germination potential categories' provides an additional facet to the evaluation and development of new citrus cultivars, providing a means to determine the pollination potential at an early stage of that cultivar's development. Citrus cultivars with a pollen germination percentage exceeding 2 % are likely to set too many seeds for the export of 'Nules Clementine' mandarins according to South African export requirements. In addition to the two previously known pollen sterile cultivars, viz. 'Navel' orange and 'Satsuma' mandarin, 'Star Ruby' grapefruit and 'Delta Valencia' orange have been identified in this study as pollen sterile cultivars that can be used as buffers to avoid cross-pollination between two pollen fertile cultivars. Furthermore, 'Delta Valencia' is also ovule sterile. An initial model is presented whereby the seed content of cross-pollinated 'Nules Clementine' fruit can be predicted (r2 = 0.9192) by determining the in vitro pollen viability of the cross-pollinating cultivar/selection and be calculated as follows: 2.619 x pollen germination category of compatible cultivars - 2.449.
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.
Some aspects of cold storage of 'Fuerte' avocados (Persea americana Mill.) grown in the Natal midlands.
(1995) Donkin, Derek John.; Wolstenholme, B. Nigel.; Cutting, Jonathan Garth Melville.
The South African avocado industry is largely export orientated and export by sea to European markets necessitates cold storage for up to 4 weeks at temperatures around 5.5°C. Avocado fruit is subject to chilling injury which is manifested as mesocarp discolouration, and pitting and blackening of the rind. Of the South African cultivars exported, 'Fuerte' is the most susceptible to chilling injury, and accounts for > 50 % of avocado exports. A number of temperature regimes where temperature was reduced in a step-wise fashion from 8.5 or 7.5°C to 4.5 or 5.5°C during 3 to 5 weeks of storage were tested weekly throughout the 1993 and 1994 'Fuerte' harvesting seasons in the Natal Midlands (a cool mesic subtropical area), in attempt to find cold storage temperature regimes which would minimise chilling injury. No definite trends with regard to certain temperature regimes resulting in fruit with less chilling injury were evident. Overall, stepped down temperature regimes produced fruit of quality no better than storage for 5.5°C for 4 weeks. There was no significant difference in concentration of total phenolics in 'Fuerte' fruit mesocarp throughout the 1994 harvesting season (P < 0.05). Levels of ethylene evolution during 4 weeks of storage at 7.5 and 5.5°C ranged from 0 to 5 µl.kg(-l).h(-l), and peaked at 109 and 75 µl.kg(-l).h(-1) in fruit stored at 7.5 and 5.5°C respectively at room temperature on removal from cold storage. Rapid moisture removal from 'Fuerte' fruit after harvest and before cold storage by placing the fruit in glass jars to which a suction of -75 kPa was applied, resulted in increased susceptibility to external chilling injury, the severity of which was proportional to the amount of moisture removed from the fruit. Pre-storage heat treatments with a view to decreasing sensitivity of fruit to cold storage were carried out on 'Fuerte' fruit. Dry heat and warm water baths at temperatures of 36 to 40°C caused rind blackening of varying severity, depending on temperature and duration. Vapour heat treatments at temperatures of 36 to 48°C for 10 min to 48 h also caused rind blackening, with the exception of 10 min at 48°C and 1.5 and 3 h at 40°C which produced fruit of higher overall quality after 4 weeks of cold storage at 3.5°C than fruit not heat treated. These treatments however, could not be repeated in 1994 to confirm the results obtained as the harvesting season was over by the time the trial was completed.
Nutrition of containerised pine (Pinus patula Schlecht. et Cham.) seedlings grown in pine bark.
(1996) Jarvel, Leanne.; Smith, Irwin Elcoate.; Wolstenholme, B. Nigel.
Increasing demands for timber and timber products have resulted in the rapid expansion of containerised forestry seedling production. The lack of available information on pine seedling fertilization has forced nurserymen to use overseas recommendations which are based on peat and vermiculite as growing media, and not composted pine bark. Six fertilization trials were conducted in plastic enclosed "tunnels" to provide fertilization recommendations and optimum foliar nutrient concentrations for P. patula seedlings grown in composted pine bark. It was concluded that pre-enrichment of the pine bark should include micronutrients, preferably in the form of Micromax®, a soluble micronutrient fertilizer, at 1000 g.m(-3). Pre-enrichment with lime was unnecessary for pine bark media since pine bark contains sufficient Ca and Mg. At an unlimed pH of 4.5, the availability of micronutrients was adequate for pine seedling growth. The addition of an N-P-K fertilizer and scheduling of nutrients, to correlate with seedling growth stage, are essential for maximum pine seedling production. Slow release forms (SRF) of N-P-K fertilizers were not beneficial, although their use may be warranted when the cost of SRF is similar to that of conventional fertilizers. Nitrogen in the form of NH(4+)-N at 80 mg.l(-1) was shown to produce the best quality P. patula seedlings. The addition of Si, in the form of pre-enriched silicate slags and si nutrient solutions, reputed to improve growth by decreasing cuticular water loss and increasing plant rigidity, provided no added advantage to pine seedling growth under the trial conditions. It was concluded that individual nurseries should formulate a nutritional programme for their own situation, based on these guidelines. Care should be taken when extrapolating these results to other substrates and it is recommended that the medium be analysed before a nutritional programme is decided on.
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.
Aspects of avocado fruit growth and development : towards understanding the 'Hass' small fruit syndrome.
(1997) Moore-Gordon, Clive Scott.; Cowan, Ashton Keith.; Wolstenholme, B. Nigel.
Persea americana Mill. cv. Hass is predisposed towards producing a high proportion of undersized fruit. Reasons for phenotypically small 'Hass' fruit are obscure, but it does appear to be aggravated by adverse growing conditions. A detailed study of the metabolic control of avocado fruit growth was carried out to determine the underlying physiological reasons for the appearance of the 'Hass' small fruit phenotype. Furthermore, the application of a mulch was evaluated as a possible management strategy to increase 'Hass' fruit size. Anatomical and morphological comparisons were made between normal and small 'Hass' fruit in an attempt to characterise the 'Hass' small fruit phenotype. Small fruit always contained a degenerate seed coat and fruit size was closely correlated with seed size. Kinetic analysis of changes in cell number and size during fruit development revealed that growth was limited by cell number in phenotypically small fruit. Analysis of endogenous isopentenyladenine (iP) and abscisic acid (ABA) revealed that ABA concentration was negatively correlated with size of similarly aged fruit. Calculation of the iP:ABA ratio showed a linear relationship with increasing fruit size. Qualitative and quantitative differences in mesocarp sterol composition were observed between normal and phenotypically small fruit. Both the normal and small-fruit phenotypes were used to probe the interaction between end-products of isoprenoid biosynthesis and activity of mesocarp 3-hydroxy-3- methylglutaryl coenzyme A reductase (HMGR) in the metabolic control of avocado fruit growth. In phenotypically small fruit, a 70% reduction in microsomal HMGR activity was associated with a substantial rise in mesocarp ABA concentration at all stages of development. Application of mevastatin, a competitive inhibitor of HMGR, via the pedicel reduced growth of phenotypically normal fruit and increased mesocarp ABA concentration. These effects were reversed by co-treatment of fruit with either mevalonate, iP or the synthetic cytokinin (CK) analogue, N-(2-chloro-4-pyridyl)-N-phenylurea, but were unaffected by gibberellic acid. Likewise, in vivo application of ABA reduced fruit growth and HMGR activity, and accelerated abscission at all stages of development, effects that were reversed by co-treatment with iP. In contrast, the effect of sterols on mevastatin-induced inhibition of fruit growth was temporally different. Application of either stigmasterol or cholesterol during phase I caused a decline in growth, accelerated fruit abscission and exacerbated the effects of mevastatin whereas during phase II and III, stigmasterol reversed inhibition of fruit growth. Stigmasterol did not however, reverse the inhibitory effect of mevastatin on HMGR activity - presumably as a result of mevastatin-induced increased endogenous ABA. It was therefore concluded that ABA accumulation downregulates mesocarp HMGR activity and that in situ CK biosynthesis modulates the effect of ABA during phase I of fruit growth whereas, both CK and sterols perform this function during the later stages to sustain the developmental programme. The effect of an altered CK:ABA ratio on solute allocation, cell-to-cell communication and plasmodesmatal structure was investigated in 'Hass' avocado fruits to determine the relationship between a change in hormone balance and expression of phenotypically small fruit. Exogenous application of ABA induced early seed coat senescence and retarded fruit growth, and these effects were negated in fruit co-injected with ABA and iP. The underlying physiological mechanisms associated with ABA-induced retardation of 'Hass' avocado fruit growth included: diminution of mesocarp and seed coat plasmodesmatal branching; gating of mesocarp and seed coat plasmodesmata by deposition of apparently proteinaceous material in the neck region; abolishment of the electrochemical gradient between mesocarp and seed coat parenchyma; and arrest of cell-to-cell chemical communication. In addition, solute allocation in ABA-treated fruit resembled closely that of phenotypically small fruit confirming that elevated ABA concentration had contributed to the decline in postphloem symplastic continuity. In a field trial in the KwaZulu-Natal midlands, root growth was substantially increased throughout three seasons by the application of a coarse composted pinebark mulch. Mulching resulted in a significant 6.6% increase in mean fruit mass, in spite of 14.7% more fruits per tree. The combined effect was a 22.6% increase in overall yield. Differences in productivity between treatments closely correlated to levels of bark carbohydrate reserves. Data collated during this study to suggest that mulching at least partly ameliorated tree stress included: a reduction in the incidence of premature seed coat senescence and pedicel ring-neck, both of which are considered to be advanced symptoms of the stress syndrome; a lowering of mean foliage temperatures; and a reduction in the degree of photoinhibition during the heat of the day.
Banana (Musa AAA; Cavendish sub-group) cultivar/density trials in three bioclimatic groups on the north coast of KwaZulu-Natal.
(1997) Lagerwall, Gary Brian.; Wolstenholme, B. Nigel.
The North Coast of KwaZulu-Natal is a relatively new banana production area, for which there is an absence of local norms, specifically for choice of cultivar and population density. Three cooperative split-plot banana cultivar/density trials were established in December 1991, January 1992 and February 1992 on farms at Eshowe, Nkwaleni and Mposa, respectively. These sites represent Phillips' (1973) Bioclimatic Groups 2,10 and 1 respectively, and have widely divergent climatic potential, ranging from cool, to warm, to hot subtropical. Each randomised trial block, 0.92 ha in extent, comprised five replications of five cultivar main plots, each of which was split into three density sub-plots. The five cultivars planted represent those registered at the time of planting, viz. 'Dwarf Cavendish', 'Williams', 'Grand Nain', 'Valery' and 'Chinese Cavendish'. Sub-plots were planted at 1 666, 2 105 and 2 500 plants ha(-1) and tissue culture plants were used to establish all three trials. The field trials were evaluated over three full cropping cycles and culminated in October 1996. Morphological differences such as pseudostem height and circumference, leaf length and width, number of functional leaves at flowering and harvest, as well as phenological differences such as monthly leaf emergence rates, emergence-to-harvest intervals and harvest cycles, were evaluated. The yield component data comprised measurements of bunch mass, number of hands per bunch and number, length and mass of fingers on the third hand. Productivity was expressed as tonnes per hectare per annum (t ha(-1) an(-1). Each cultivar and density treatment was evaluated independently. However, it was the evaluation of the cultivar/density interaction which formed the basis of the recommendations for the three different Bioclimatic Groups. At the relatively cool Eshowe site, 'Williams' proved to be the most productive cultivar (471 ha(-1) an(-1) over all densities. The highest production (49.8 t ha(-1) an(-1) was achieved from a density of 2 500 plants ha(-1), but due to lower costs per hectare, the intermediate density of 2 105 plants ha(-1) generated the highest gross margin. When the cultivar/density interaction was evaluated, the combination producing the highest gross margin was 'Williams' at 2 105 plants ha(-1). This substantiated the independent evaluations and is consequently the recommended combination for Bioclimatic Group 2. However, the faster cycling of 'Chinese Cavendish' could conceivably result in this cultivar outperforming 'Williams' in future ratoons. At the warmer Nkwaleni site, 'Grand Nain' (57.8 t ha(-1) an(-1) proved to be the most productive cultivar. The density of 2 500 plants ha(-1) was the most productive (64 t ha(-1) an(-1) and also generated the highest gross margin. However, the cultivar/density interaction indicated that 'Williams' at 2 500 plants ha(-1), was the highest producing combination yielding the highest gross margin, and is consequently the recommended combination for Bioclimatic Group 10. At the hot Mposa site, 'Chinese Cavendish' (54.21 ha(-1) an(-1) proved to be the most productive cultivar. The density of 2 500 plants ha(-1) outproduced (57.4 t ha(-1) an(-1) the lower densities and also generated the highest economic returns. However, when the cultivar/density interaction was evaluated, 'Chinese Cavendish' at the lower density of 2 105 plants ha(-1) realised the highest gross margin and is consequently the recommended combination for Bioclimatic Group 1.
Bowen ratio and surface temperature techniques for measuring evaporation from cabbages.
(1997) Lukangu, Gastao.; Savage, Michael John.; Johnston, M. A.
Good irrigation water management requires accurate, automated, non-destructive and simple techniques to measure crop water consumption. The actual evaporation from a cabbage crop was measured using the Bowen ratio energy balance technique (BREB), the surface temperature technique and the Penman-monteith method. All models used the shortened energy balance equation to estimate latent heat in which the advected energy is assumed to be negligible. Four irrigations were applied and 17 rainfall events were recorded during the experiment. The soil at the experimental field was a clay loam. An attempt to detect and reduce measurement error that could result from using inaccurate sensors was performed by calibrating the sensors. Data from inaccurate sensors were not used to compute the latent heat. Error and sensitivity analyse were performed, and the integrity of the weather data using the estimates of weather data from an appropriate model were checked. In addition, a comparative study showed that, for daily totals, there was a very small error in the latent heat calculations when fixed "constants" (density of air, specific heat capacity of air, psychrometric constant, slope of the saturation water vapour pressure vs temperature relationship and specific heat capacity of soil) were used instead of calculated ones. The Bowen ratio (β), a fundamental input of the BREB technique, was estimated accepting the Similarity Principle and excluding nighttime data. However, an error in β was also observed during the daytime measurement of the profiles entities because the sensors were wet and the stability condition was different from neutral conditions under which the Similarity Principle could not be observed. Negative values of β were observed when there were strong winds advecting sensible heat into the field under study. Data were rejected during mornings, and during strong advection periods. Data were also rejected when the sensors were wet because of rain or irrigation. In this experiment, only 35 % of data were valid for determining latent and sensible heat estimated using the BREB technique. Comparative analysis showed that the BREB technique overestimated the latent heat by 17 % compared to the Penman-Monteith method. However, both the Penman-Monteith method and BREB technique could not be trusted because of the presence of advection, a component of the energy balance equation normally assumed to be negligible. Either the surface to air temperature differential or the aerodynamic resistance, or both, were the source of overestimation of latent heat using the surface temperature technique. The surface to air temperature differential was large in magnitude when there were high wind speeds and drier conditions in the upwind field . It was small with lighter wind speeds and wetter surface conditions. An error of less than 5 % was attributed to the use of fixed air density and specific heat capacity and acceptance of 2 % and 20 % error in measuring the net irradiance and soil heat flux density, respectively. A comparative study showed that the surface temperature latent heat was overestimated in relation to the Penman-Monteith and BREB latent heat. Generally, the technique has been reported to overestimate evaporation, although to a lesser extent than the 57 % error reported in this experiment when compared to the BREB technique. An analysis of the energy balance closure, taking the Penman-Monteith and BREB as standards, suggested that the surface temperature technique overestimated the consumption of sensible heat from the air. This observation was also confirmed when the eddy correlation technique was used to compare sensible heat estimated using the surface temperature technique. The effect of placement height of air temperature sensors suggested that the consumption of sensible heat would be overestimated if the sensor was placed far from the crop surface. This overestimation in consumption of sensible heat resulted in an overestimation of latent heat. Irrigation water management was analysed using the crop water stress index (CWSI). The CWSI was calculated using the actual to potential evaporation ratio estimated using the Penman-Monteith method and the surface temperature techniques. The estimated and measured actual surface to air temperature differential, and the estimated potential and non-transpiring surface to air temperature differential were also used to estimate the CWSI using the Penman-Monteith method, the surface temperature technique and empirical method. The estimates of the CWSI using these techniques were inaccurate because of the poor correlation between the surface to air temperature differential and the water vapour pressure deficit (or water vapour pressure deficit and net irradiance). However, use of the CWSI estimated using the actual to potential evaporation ratio (CWSI = 1 - λ(a)/ λE(p) compared well to the standard CWSI determined using the Penman-Monteith approach. The actual canopy resistance was estimated using an empirical equation based on the potential canopy resistance, solar irradiance, soil water content and the shelter factor. A value of 50 s m(-1) was estimated for potential (minimum) canopy resistance of the cabbage crop. The soil water content was poorly correlated to CWSI, while the canopy resistance was well correlated. Comparative analysis showed that the estimated soil water content using the soil water balance equation was underestimated in relation to the soil water content measured using the ThetaProbe (frequency domain reflectometry technique) when the evaporation component was overestimated, and vice versa. Soil water content was underestimated throughout the experiment when evaporation from the surface temperature technique was used. There was an underestimation of soil water content in the early stages and overestimation in later stages of the experiment when the BREB and Penman-Monteith evaporation were used. Use of the estimated soil water content using the soil water balance with the overestimated evaporation would result in an early date of irrigation application, an unnecessarily large irrigation amount and frequent irrigations. More research is needed to find the cause of overestimation of evaporation using the surface temperature technique. The robustness of the equipment allowed a long period of measurement without frequent maintenance, as was required when using the BREB technique. The technique can monitor evaporation and irrigation management aspects at a regional scale. A combination of the Penman-Monteith, surface temperature and empirical method can assist the estimation of the crop water requirement by determining the CWSI. Future research would focus on quantification of sensible and latent heat advection, and analysis of additional resistances to water vapour flow from the surface to the atmosphere. The equipment for the BREB should be refined so that it measures actual latent heat under adverse weather conditions for a protracted period. A precise use of the soil water balance equation for water management should take into consideration runoff, vertical flow of soil water through a profile, intercepted water on plant surfaces and an accurately determined evaporation.
Soil boron application for the alleviation of boron deficiency of avocado (Persea americana Mill.) in the KwaZulu-Natal Midlands.
(1997) Bard, Zac Jon.; Wolstenholme, B. Nigel.; Cowan, Ashton Keith.
The avocado tree's requirement for additional boron in B deficient soils has traditionally been met solely by foliar sprays in South Africa. Since boron is regarded as poorly phloem translocated in most plants including avocado, foliar applications are unlikely to cater for the requirement of the entire tree. Foliar sprays are made prior to leaf analysis so that artificially high readings are likely. A survey of the boron status of four KwaZulu-Natal avocado orchards showed all soils to be in the deficient range, viz. <1 mg kg(-1). Leaf analysis records on these estates appeared inflated with more than occasional spurious results. Despite marginally adequate leaf boron concentrations, widespread deficiency symptoms were noted in all orchards. For foliar application, leaf analysis of spring flush leaves does not provide a true indication of orchard boron status. Soil applications of borax (11 % B) in the range 0 to 60 g m(-2) (soil canopy area) year(-1) split into three applications, succeeded in increasing orchard B levels to above the recommended optimum of 40 mg kg(-1) without any deleterious effects visible on the feeder roots or tree, except at the highest rates. Initial uptake of soil B was slow, particularly in older orchards and with standard rates as developed in Australia (typically between 5 and 20 g m(-2) year(-1), split into at least 3 applications). Higher application rates (40 and 60 g m(-2) year(-1) showed greater effectiveness at raising leaf boron concentrations, particularly in the second season. Toxicity occurred with 40 and 60 g m(-2) year(-1) rates, 18 months after initial applications were made. High application rates indicated the tolerance of established avocado orchards to very high soil B concentrations. Soil applications increased fruit yield through increased fruit size in younger 'Hass' trees. Older, more deficient orchards did not show increased fruit size within the experimental timespan. Glasshouse trials supported findings in that soil B applications significantly increased leaf B concentrations (P < 0.001) proportional to soil application rate. Recently grafted young potted trees were extremely sensitive to soil boron applications which were not split, with toxicities occurring at low application rates. 'Edranol' seedling, a rootstock of Guatemalan origin was shown to be ca. 40 % more efficient in boron uptake than clonal 'Duke 7', the widely used rootstock in South Africa. Results indicate that boron deficiency is primarily the result of soil deficiency rather than poor rootstock uptake and translocation. On the Inanda soil type used and under the conditions of the experiments, it is suggested that application rates do not exceed 20 g borax m(-2) year(-1) (split into 3 applications) in severely deficient trees (10-30 mg kg(-1) B leaf analysis), and rates of ca. 10 g borax m(-2) year(-1) would be adequate in marginally deficient trees.
Abscisic acid metabolism in a Citrus Sinensis flavedo enzyme system.
(1998) Kalala, Maloji.; Cowan, Ashton Keith.
This research project had its major objective the unequivocal demonstration that the plant growth regulator, abscisic acid could be produced in vitro from labelled precursors. In addition, this project was intended to demonstrate the metabolic relationship between β, β-xanthophyll turnover and ABA biosynthesis. Finally attempts were made to isolate the enzyme responsible for the cleavage of the immediate xanthophyll precursor to ABA, 9'-cis-neoxanthin. This was achieved using a cell-free system developed from Citrus flavedo which demonstrated formation of xanthoxal and abscisic acid from zeaxanthin, antheraxanthin, violaxanthin and neoxanthin. In addition product abscisic acid was shown to incorporate label from mevalonic acid lactone establishing the terpenoid origin of this plant growth regulator. 9'-cis-neoxanthin cleavage enzyme was present in the Citrus cell-free system, partially purified, and separated from lipoxygenase activity and shown to convert 9'-cis-neoxanthin into xanthoxal and related but unidentified neutral products.
Vapour heat treatment of 'Hass' and 'Fuerte' avocado (Persea americana Mill.) fruit for extending storage life.
(1998) Weller, Philip Lawrence.; Wolstenholme, B. Nigel.
In an attempt to extend storage life of 'Hass' and 'Fuerte' avocado {Persea americana Mill.) fruit postharvest vapour heat treatments were used. Vapour heat treatment temperature regimes were 36, 38, 40 and 42°C for either 1, 2, 4 or 8 hours. After vapour heat treatment, fruit were cold stored at 3.5°C, for 5 or 6 weeks each. On removal from cold storage fruit were evaluated for firmness, and ripened at room temperature. Once ripe fruit were evaluated for heat/cold damage, days to ripening, weight loss and physiological disorders. These experiments were conducted during the 1996 and 1997 'Fuerte' and 'Hass' avocado seasons. Electron microscope analysis of vapour heat treated fruit revealed heat damage to the epidermis and fruit organelles of certain treatments. The effect of vapour heat treatment on protein synthesis on 'Fuerte' and 'Hass' avocado was investigated using gel electrophoresis, to determine if presumed heat shock proteins were synthesized under the treatment conditions. The activity of pectin methyl esterase was also investigated on the heat treated 'Fuerte' and 'Hass' avocados, to investigate if heat alters the activity of this enzyme. All experiments showed that vapour heat treatment extended 'Fuerte' and 'Hass' avocado storage and shelf life. Evaluation of the time/temperature combination for vapour heat treatment for each cultivar indicated that extension of shelf life was, in some cases, limited by rind heat damage. For the 1996 season, the best time/temperature combination for 'Fuerte' was at 38°C between 4 to 8 hours, 40°C between 4 to 8 hours and 42°C between 2 to 4 hours. For 'Hass', best results were achieved at 38°C between 4 to 8 hours. For the 1997 season, 'Fuerte's best time/temperature combination were at 36°C for 8 hours, 42°C between 1 to 2 hours and marginal results at 38°C between 4 to 8 hours. For 'Hass' the best time/temperature combination were achieved at 38°C and 40°C for 8 hours. Analysis of fruit flesh protein indicated changes associated with vapour heat treatment, and even induction of new proteins, perhaps heat shock proteins, giving 'Fuerte' and 'Hass' avocados beneficial characteristics such as an extension of shelf life. There was a decline in activity of pectin methyl esterase after vapour heat treatment, which was associated with a longer shelf life.
In vitro culture of avocado : a model system for studying the biochemistry of fruit growth.
(2000) Mitchell, Dudley Ian.; Cowan, Ashton Keith.
The avocado (Persea americana Mill.) is considered by many to be a horticultural problem. Cultivars in use today have several drawbacks associated with yield, disease susceptibility and fruit quality. Breeding programs to counter these disadvantages have been met with limited success due mainly to the crop's heterozygosity, outbreeding nature and long juvenile period. Furthermore, genetic information regarding current commercial scions and rootstocks is limited and crosses are made based on parental phenotypic characteristics which are not always additive. It is proposed that the development of a protoplast-to-plant system for avocado would not only provide a means for plant breeders to overcome these problems, but would also present researchers with a useful tool for studying biochemical and physiological mechanisms operating within the plant. An investigation into the development of an in vitro system for use in metabolic studies was carried out. This technology was then used as a model system for studies into the metabolic control of cell growth. An attempt was made at developing a protoplast system from the mesocarp tissue of 'Hass' avocado. It was found that the purity and activity of the cellulase preparation in the protoplast isolation medium was critical. Failure to generate a protoplast system from mesocarp tissue prompted an investigation into the development of cell cultures. Mesocarp, seed and embryo tissue was subjected to various treatments in an attempt to induce callus for use as a source material for cell cultures. Callus derived from nucellar tissue of 'Hass' avocado seed at high concentrations of a-naphthalene acetic acid (NAA)(5 mgL-1 ) and isopentenyladenine (iP)(5 mgL-1 ) in Murashige and Skoog media (MS) proved to be the most amenable to subculture into liquid medium. Cell suspensions initiated from this callus grew fastest in MS media supplemented with NAA (5 mgL-1 ) and iP (1 mgL-1) These cell suspensions were maintained through subculture and were selected for use in metabolic studies. Cytokinin-dependent cell cultures from avocado seed callus were used to study the involvement of isoprenoid products in cell division. Addition of mevastatin, a competitive inhibitor of the key enzyme in the isoprenoid pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), caused a reduction in cell growth at low concentrations (0.01 µM, 0.1 µM and 1 µM) and cessation of growth at higher concentrations (10 µM and 40 µM). Co-treatment with the isoprenoid compounds mevalonic acid lactone (MVL)(6 mM) and farnesyl diphosphate (FDP)(10 µM) completely reversed the effects of mevastatin at the 1 µM and 40 µM levels. The addition of stigmasterol (10 µM) to cell cultures treated with mevastatin (1 µM and 40 µM) resulted in a slight positive growth response indicating partial alleviation of inhibition. However, the response was not significantly different from the control suggesting that sterols played a minor role in cell division. It was concluded that isoprenoid-derived products played a critical role in the regulation of the cell cycle. Furthermore, it was suggested that mevastatin-induced HMGR inhibition gave rise to a response, most likely ABA-mediated, that acted antagonistically to regulatory mechanisms controlled, in part, by isoprenoid compounds.

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