Doctoral Degrees (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.
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.
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.
Post-phloem transport and metabolism of sucrose in avocado.
(2001) Cripps, Ryan F.; Smith, Michael Trevor.; Cowan, Ashton Keith.
In South Africa, and in several other sub/tropical countries, the avocado represents a commercially important crop. Very little is currently understood about the metabolism of sugars in this fruit. The variety 'Hass' is a popular cultivar that is grown extensively in South Africa. However this. cultivar has a tendency to produce two distinct fruit phenotypes: a normal sized variant and a small, undersized variant. Current literature suggests that the small fruit phenotype is characterised by an elevated abscisic acid (ABA) to cytokinin ratio and altered isoprenoid metabolism. The results presented in the current investigation represent the findings from a detailed study into the metabolism and transport of sugars in 'Hass' fruit in an attempt to characterise solute allocation in developing avocado fruit. Furthermore, the activities of sugar metabolising enzymes, routes of solute movement and polyphenolic contents of normal, small and ABA-treated fruit were compared and contrasted to evaluate the potential role of ABA in the induction and expression of the small fruit phenotype. The enzymes invertase, sucrose synthase (SSy) and sucrose phosphate synthase are involved in the metabolism of sucrose (Suc) and, hence, phloem unloading, post-sieve element transport and fruit growth. Although not the major sugar present, Suc was found in avocado phloem sap, and the enzymology for its metabolism was shown to exist in avocado fruit. It appears that sink strength is established during early fruit growth by high acid invertase activity, especially during the period of rapid cell division. As fruit growth progresses the activity of SSy and an enzyme responsible for the oxidation of perseitol (tentatively termed perseitol dehydrogenase) increases, suggesting that these enzymes play an important role in the supply of carbon during the linear phase of fruit growth. All Suc metabolising enzyme activity diminishes as the fruit approaches maturity. With the exception of SSy (in the cleavage direction), all enzymes assayed showed a general increase in relative rates of activity in small and ABA-treated fruit. Similarly, ABA-treatment of seed coat discs in vitro resulted in the elevation of insoluble and soluble acid invertase, SSy (in the synthesis direction), and sucrose phosphate synthase activity. Furthermore, both small and ABA-treated fruit were characterised by elevated total soluble sugars, glucose and fructose levels. These observations suggest that altered sugar metabolism, as a consequence of changes in endogenous ABA levels, may contribute to the occurrence of the small fruit. The seed coat represents an import link between the seed, the mesocarp and the parental plant tissues. Loss of seed coat and endosperm integrity accompanied fruit maturation and a reduction in the movement of solutes into the seed. An increase in polyphenolics in the seed coat tissue seemed critical in this reduced movement. Both the small and ABA-treated fruit were characterised by early senescence of the seed coat, which was accompanied by both a loss of transport into and out of the seed and premature maturation of the fruit. This premature seed coat senescence appeared similar to programmed cell death in tissues exposed to stress or elevated reactive oxygen species, stimuli that are often accompanied by elevated ABA levels. Callose was localised to the plasmodesmata and is proposed to play a role in the gating of, and hence movement through, these pores. Small fruit were characterised by a loss of symplastic continuity, as represented by fewer plasmodesmata, and reduced callose degradation. Comparison of callose content and rates of synthesis suggest that ABA-treatment, similarly, reduces callose catabolism. The association of ABA with both the premature senescence of the seed coat and a reduction in symplastic continuity, and, hence, a reduction in solute transport, further cements the potential role of ABA in the occurrence of the small fruit phenotype.
Plant hormone homeostasis and the control of 'hass' avocado fruit size.
(2002) Taylor, Nicolette Jane.; Cairns, Andrew Lawrence Patrick.; Cowan, Ashton Keith.; Van Staden, Johannes.
The 'Hass' avocado produces two distinct phenotypically different populations of fruit, i.e. normal and small fruit. The small fruit variant is characterized by early seed coat senescence that results in arrested growth, due to dramatically reduced cell cycle activity. This system has been used to study the metabolic control of fruit growth for two reasons. Firstly, the 'Hass' avocado is a major export crop in South Africa and unmarketable small fruit cost the industry millions of rands per season. Secondly, in the absence of evergreen tree-crop mutants with which to dissect controlling mechanisms contributing to the control of final fruit size, the 'Hass' avocado and its small fruit variant provides an ideal system to investigate the physiology, biochemistry and molecular biology of fruit growth in subtropical species. A detailed study was conducted to probe the contribution of hormones in the control of final fruit size by comparing and contrasting tissue distribution and content of hormones in developing 'Hass' avocado and its small fruit variant. In addition the proposal that changes in hormone homeostasis occur as a result of differences in the allocation of the molybdenum cofactor (MoCo) and changes in the activity of xanthine dehydrogenase (XDH) and the aldehyde oxidases (AO) involved in abscisic acid (ABA) and indole-3-acetic acid (IAA) metabolism was evaluated. Activity of XDH, xanthoxal (XAN) oxidase, indole acetaldehyde (IA-aid) oxidase and cytokinin oxidase (CKOX) was related to tissue content and composition of IAA and ABA. Comparisons between normal and small fruit revealed that under conditions where CKOX is elevated, the increased adenine produced inhibits XDH activity, which leads to elevated activity of the AOs involved in ABA and possibly IAA biosynthesis as a result of increased MoCo allocation to these enzymes. Further analyses revealed that both cytokinin (CK) and auxin elevates CKOX activity and that adenine and CK do indeed inhibit XDH activity, which leads to increased AO activity. In addition, application of CK to normal fruit increased IAA in mesocarp tissue but reduced IAA content of seed tissue and reduced ABA in mesocarp tissue but had no effect on ABA in seed tissue. Cytokinin oxidase therefore contributes to the regulation of ABA and IAA metabolism during plant organ growth by modulating the activity of XDH. Low XDH and lA-aid oxidase activity together with high XAN oxidase and CKOX activity early in fruit development combine to reduce both elongation and radial growth, which results in the appearance of the 'Hass' small fruit phenotype. This event was associated with high ABA and low IAA in seed tissue of small fruit, but high ABA and IAA in seed coat and mesocarp tissue of these fruit. Thus, whilst low IAA in seed tissue is associated with reduced growth the . reverse is true in seed coat and mesocarp tissue where high IAA retards tissue growth. Calculation of CK/ABA and CK/IAA ratios revealed that a decrease in these ratios was found in mesocarp tissue of small fruit. However, in seed tissue of small fruit both IAA and ABA were decreased relative to CK. The maintenance of the correct hormonal balance in avocado fruit thus ensures the continuation of cell division cycle activity, with any changes responsible for the high incidence of a small fruit variant in the 'Hass' avocado.
Gladiolus scabridus - the road to conservation and commercialisation.
(2005) Campbell, Tracey Barbara.; Bower, John Patrick.; Van Niekerk, Renate.
There is at present a growing concern and awareness of the endangered status of many indigenous South African plants in the wild, a number of which have potential for commercial production. One such example is Gladiolus scabridus, a vulnerable species endemic to the mountains of northern KwaZulu-Natal and southern Swaziland. It has considerable potential due to its floral characteristics. However, little is known about its horticultural requirements. Thus propagation and cultural practices were investigated with the aim of both conservation and commercialisation of the species. The ideal conditions for G. scabridus seed germination were determined. The presence or absence of light had no significant effect. Optimum germination was achieved at 20 QC of both winged and dewinged seeds. Higher temperatures appear to have a negative effect on germination and none of the winged seeds germinated at 30 QC. Under ideal conditions, fresh seed showed significantly higher vigour and viability than stored seed although the germination of stored seed was hampered by a higher internal fungal content. Successful tissue culture protocol was established for G. scabridus axillary bud and cormel halve explants. During the initial shoot initiation and proliferation stages (stage I and 11), the significantly higher shoot numbers occurred in the absence of growth regulators in both explant types. However, shoots with 1.0 mg 1 -1 6-benzyl-amino-purine (BAP) and 0.5 mg 1 -1 1-naphthalene-acetic-acid (NAA) were healthier in appearance. Higher levels of 5.0 mg 1 -1 NAA inhibited shoot production and encouraged root development in cormel halve explants. During stage 11, axillary bud explants showed root and cormlet development. More roots were initiated without growth regulators, whilst 5.0 mg 1 -1 NAA resulted in significantly better cormlet development. Shoot and cormlet growth of cormel halve explants during stage II was not significantly affected by the presence or absence of NAA and BAP. Significantly more roots were produced with 5.0 mg 1 -1 NAA. During stage III (rooting), the presence of activated charcoal (AC) was essential for the initiation and development of roots in vitro. Root and cormlet development in all explant types was significantly affected by the interaction between the previous treatments from stages I and 11 and the new treatments. During stage IV (hardening off) , most explants died down in the hardening off media leaving resting cormlets. There was a significant interaction between 1.0 mg 1 -1 NAA and 0.3 % AC from stage III, resulting in significantly more cormlets in both axillary bud and cormel halve explants. There was successful cormlet growth after cold storage which is advantageous in reducing the need for acclimatization. G. scabridus corms were successfully forced out of their normal flowering period. Temperatures of 10,5.5 and 2 QC showed successful corm initiation although the corms need to be stored for longer at warmer temperatures. It is suggested that a treatment of 2 QC for 6 weeks is the optimal condition for forcing G. scabridus corms. Although originating from a stressful environment, G. scabridus appears to show capacities for improved growth under controlled conditions. Fertilizer applications enhanced growth and reduced the time to flowering. Nitrogen (N) was found to be important for vegetative growth, flowering and daughter corm development, whereas potassium (K) influenced cormel production. Fertilizer with higher N and lower K is appropriate for the beginning and middle of the growing season and then adjusted to a lower N and higher K fertilizer to promote cormel formation. G. scabridus was found to produce prolific numbers of cormels which is an important source of plant material. Mineral leaf analysis showed that optimum levels for wild species are lower than those for hybrid gladioli with the optimum levels affected by physiological corm maturity and subsequent plant growth. Norms for postharvest handling of cut G. scabridus spikes have been developed. Spikes held in 2 % sucrose had a longer vase life and better floret opening and quality than those kept in distilled water, Prolong, Chrysal, 2 % sucrose and 2 % ethanol, 1 % sucrose, 4 % sucrose, 1 % sucrose and 0.5 % JIK, 2 % sucrose and 0.5 % JIK and 4 % sucrose and 0.5 % JIK, 2 % fructose and 2 % glucose. The use of commercially available solutions should be used with caution. Florets produced a climacteric-like CO2 peak, but levels of ethylene were unmeasurable. Cold storage and the use of polypropylene sleeves delays senescence. G. scabridus spikes secrete droplets of a sticky substance which was confirmed to be extra floral nectar through HPLC analysis. Market research revealed a positive response to the species from consumers and retailers alike with potential for cultivation as a cut flower and bedding plant. However, the cost will determine supply and demand. A field study conducted at Bivane Dam, northern KwaZulu-Natal, confirmed that G. scabridus colonies prevail in rocky, quartzite outcrops where they become wedged between the rocks. Plants were found at different stages of development with populations of not more than 108 plants per colony. Soil data of G. scabridus sites was compared to that of two sites nearby. It was found that G. scabridus soils are higher in phosphorus (P), zinc (Zn) and organic carbon. Leaf analysis confirmed that they have adapted their growth to low nutrient levels. The G. scabridus studies have clearly shown that the species can be successfully moved from a wild plant to a commercially viable one and in so doing its conservation status can also be improved.
Factors affecting mesocarp discolouration severity in 'Pinkerton' avocados (Persea americana MILL.)
(2005) Van Rooyen, Zelda.; Bower, John Patrick.
The susceptibility of the 'Pinkerton' avocado cultivar to mesocarp discolouration, after storage, has seriously threatened its export from South Africa. This disorder has proven to be complex, requiring a better understanding of the fruit's physiology. The purposes of this study were to identify the role of pre- and postharvest factors, or their interactions, in the development of the problem. This was done by obtaining fruit from several production areas of varying mesocarp discolouration histories (referred to as "high", "medium" or "Iow risk" areas) during the 2000 and 2001 seasons. Fruit were stored at 8, 5.5 and 2°C for 30 days, as well as ambient (20°C). Evaluations of fruit quality were made before and after storage, as well as after softening. Once removed from storage the weight loss (during storage) was determined, and fruit firmness and carbon dioxide (C02) production rates monitored daily. It was found that temperatures below the recommended shipping temperature of 5.5°C, i.e. 2°C, produced the best internal fruit quality. This was supported by the membrane integrity studies that showed less membrane stability at the warmer storage temperature of 8°C. Furthermore, remained hard during storage and subsequently had an extended shelf life. Fruit origin was also found to play a major role in browning potential, with discolouration being consistently more severe in fruit from "high risk" areas and increasing in severity as the season progressed. The rate of CO2 production was found to follow a similar trend, with rates increasing as the season progressed, and also being slightly higher in fruit from "high risk" areas. The higher CO2 production rates were thought to be related to a decrease in membrane integrity as the season progressed. While storage temperature was not found to have a significant effect on the rate of CO2 production after storage, it did affect the time taken to reach the maximum rate, with fruit stored at 2°C taking longer. Biochemical analyses to determine the concentration of total phenolics and the activity of the enzyme polyphenol oxidase (PPO) also showed that the potential for browning was initiated by preharvest conditions. While no significant differences were found between growers with regards to total phenol concentrations, the PPO activity was found to be higher in fruit from poor quality areas, and subsequently browning potential was expected to be higher in these fruit. It was, however, found that the potential for browning could be reduced by storing fruit at 2°C, as this decreased the total phenolics concentration. This evidence further emphasized the idea that storage at 2°C could be highly advantageous. Fruit mineral analysis showed that certain key elements played a significant role in the severity of mesocarp discolouration, with excessive fruit nitrogen and decreasing copper and manganese concentrations appearing to play major roles. The high fruit nitrogen concentrations were suspected to reflect fruit grown on very vigorous trees, resulting in shoots competing with fruit for available reserves. It is suggested that 'Pinkerton' of a quality acceptable to the market, can be produced by manipulating source:sink relationships, particularly through decreasing the availability of nitrogen, followed by low temperature (24° C) shipping. Future work should concentrate on manipulation of source:sink relationships, to take account of both climatic conditions and leaf to fruit ratios. The evaluation of chlorophyll fluorescence as a tool for predicting mesocarp discolouration potential in 'Pinkerton' proved to be unsuccessful in this study and future studies may require modifications to the current technique. It is suspected that differences in chlorophyll content, for example, between fruit from different origins, will have to be taken into account when interpreting results. The success of using 2°C storage to improve the internal quality on 'Pinkerton' fruit prompted further studies, during 2004, to ensure that the development of external chilling injury would not decrease the marketability of the cultivar. Low temperature conditioning treatments, prior to storage, proved to be highly successful in reducing the development of external chilling injury, thus further improving fruit quality as a whole. Preconditioning treatments consisted of fruit that were kept at either 10°C, 15°C or 20°C for 1 or 2 days before being placed into storage for 30 days at 2°C or 5.5°C. All preconditioning treatments were compared to fruit that were placed directly into storage. The effect of fruit packaging on moisture loss (as determined by weight loss) and chilling injury was also investigated using unwaxed fruit, commercially waxed and unwaxed fruit individually sealed in micro-perforated polypropylene bags with an anti-mist coating on the inside (polybags). Holding 'Pinkerton' fruit, regardless of packaging treatment, at 10°C for 2 days prior to storage at 2°C or 5.5°C significantly decreased the severity of external chilling injury. The use of polybags during preconditioning and storage showed potential in further reducing the development of external chilling injury, although the higher incidence of fungal infections in these fruit needs to be addressed. The determination of proline concentrations in fruit exocarp tissue after storage was helpful in determining the level of stress experienced by fruit that were subjected to different packaging and preconditioning treatments. In this study waxed fruit subjected to 1 d preconditioning at 10°C, 15°C or 20°C or placed directly into storage at 2°C showed very high proline concentrations and also displayed more severe external chilling injury, despite unwaxed fruit losing more weight during these treatments. The role of moisture loss thus needs further investigation. The thickness and method of wax application was thought to play an important role in the higher external chilling injury ratings in this study as waxed fruit often developed chilling injury symptoms around the lenticels and it was suspected that either the lenticels were damaged by the brushes used to apply the wax or that the lenticels became clogged thus resulting in reduced gaseous exchange. Nevertheless, the success of low temperature conditioning in reducing external chilling injury, while maintaining sound internal quality, may enable storage temperatures to be dropped even further, thus enabling South Africa to export avocados to countries that require a cold disinfestation period prior to entry to eliminate quarantine pests (e.g. fruit fly).
Special carbohydrates of avocado : their function as 'sources of energy' and 'anti-oxidants'.
(2009) Tesfay, Samson Zeray.; Bertling, Isa.
There is increasing interest in special heptose carbohydrates, their multifunctional roles from a plant physiological view point in fruit growth and development as well as in the whole plant in general due to their potential in mitigating photo-oxidative injury to the whole plant system and the image of avocado as ‘health fruit’. Studies have been carried out to investigate the role of avocado heptoses, rare carbohydrates predominantly produced in avocado. Several authors have documented various research findings and speculated on multifunctional roles of avocado special sugars. However, few reports have made an attempt to elucidate the multifunctional roles of avocado heptose carbohydrates as: ‘sources of energy’, storage and phloem-mobile transport sugars, and precursors for formation of antioxidants. Assessing the avocado carbohydrates over the plant growth and development during ontogeny may, therefore, offer clues to better understand whole plant behaviour. Plant sampling was carried out over different developmental stages. Using plants grown in the light versus etiolated seedlings; sugar determinations were also done to determine what sugar is produced from which storage organs. The sugars were extracted and analysed by isocratic HPLC/RID. The embryo had 47.11 % hexose and 52.96 % heptose sugars. The seed, however, also released significant amounts of D-mannoheptulose (7.09 ± 1.44 mg g-1 d. wt) and perseitol (5.36 ± 0.61 mg g-1 d. wt). Similarly fruit and leaf tissues had significant amounts of heptoses relative to hexoses at specific phenological stages. In postharvest ‘readyto-eat’ fruit the following carbohydrate concentrations were as follows:exocarp heptoses 13 ± 0.8; hexoses 4.37 ± 1.6 mg g-1 d. wt, mesocarp heptoses 8 ± 0.2; hexoses 3.55 ± 0.12 mg g-1 d. wt), seed heptoses (only perseitol) 13 ± 1.1; hexoses 5.79 ± 0.53 mg g-1 d. wt. The results of this experiment was the first to demonstrate that the heptoses D-mannoheptulose, and its polyol form, perseitol, are found in all tissues/organs at various phenological stages of avocado growth and development. Secondly, heptoses, as well as starch are carbohydrate reserves that are found in avocado. The heptoses, beyond being abundantly produced in the avocado plant, are also found in phloem and xylem saps as mobile sugars. The study also presents data on the interconversion of the C7 sugars Dmannoheptulose and perseitol. It is deduced that D-mannoheptulose can be reduced to perseitol, and perseitol can also be oxidized to D-mannoheptulose by enzymes present in a protein extract of the mesocarp. The potential catalyzing enzyme is proposed to be an aldolase, as electrophoretic determinations prove the presence of such an enzyme during various stages of development in various plant organs. Avocado heptoses play an important role in plant growth and development and in fruit in particular. Moreover, they are reported as sources of anti-oxidants, and contribute significantly to fruit physiology if they function in coordination with other anti-oxidants in fruit tissues. To evaluate the presence of anti-oxidant systems throughout avocado fruit development, various tissues were analysed for their total and specific anti-oxidant compositions. Total anti-oxidant levels were found to be higher in the exocarp and in seed tissue than in the mesocarp. While seed tissues contained predominantly ascorbic acid (AsA) and total phenolics (TP), the anti-oxidant composition of the mesocarp was characterised by the C7 sugar, D-mannoheptulose. Among the anti-oxidant enzymes assayed, peroxidase (POX) and catalase (CAT) were present in higher concentrations than superoxide dismutase (SOD) in mesocarp tissue. Different anti-oxidant systems seem to be dominant within the various fruit tissues. Carbohydrates are the universal source of carbon for cell metabolism and provide the precursors for the biosynthesis of secondary metabolites, for example via the shikimic acid pathway for phenols. The preharvest free and membrane-bound phenols, catechin and epicatechin, are distributed differently in the various fruit tissues. Membrane-bound and free phenols also play a role as anti-oxidants, with free ones being more important. KSil (potassium silicate) application to fruit as postharvest treatment was used to facilitate the release of conjugates to free phenols via lysis. This treatment improved fruit shelf life. Western blotting also revealed that postharvest Si treatment affects the expression of enzymatic anti-oxidant-catalase (CAT). Overall the thesis results revealed that C7 sugars have anti-oxidant properties and that D-mannoheptulose is the important anti-oxidant in the edible portion of the avocado fruit. Dmannoheptulose is furthermore of paramount importance as a transport sugar. Perseitol on the other hand acts as the storage product of D-mannoheptulose, which can be easily converted into D-mannoheptulose.
Management of avocado postharvest physiology.
(2011) Blakey, Robert John.; Bower, John Patrick.; Bertling, Isa.
Avocados are an important horticultural crop in South Africa, especially in the provinces of KwaZulu-Natal, Mpumalanga and Limpopo. The distance to traditional export markets, phytosanitary restrictions to lucrative markets such as China, the USA and Japan and increased competition in the European market have challenged the South African avocado industry. The industry has responded with improved logistics and shipping, a co-ordinated market access program and a global system to co-ordinate exports of avocados to the European market. To remain competitive on the global market, further improvements and innovations are required to improve the efficiency of postharvest operations. These improvements and innovations should be guided by a greater understanding of postharvest physiology. Avocados are a relatively new export crop, so there is still much to be learnt about avocado postharvest physiology and the optimisation of postharvest management. In this regard, reduced temperature storage (1°C) and modified humidity packaging (MHP) were investigated for their effect on fruit physiology and quality, the effect of a water- and ABA-infusion on ripening was examined and the effect of a cold chain break on fruit physiology and quality determined; near-infrared spectroscopy was also examined for its potential for its use in the avocado industry. As an initial study, the relationships between individual sugars, protein and oil were studied to understand the changes in avocado fruit during ripening. It was found that mannoheptulose and perseitol were the predominant sugars at harvest, but declined to very low levels during the first 10 days postharvest. The concentrations of glucose and fructose increased, while sucrose declined slightly during ripening. The concentration of protein increased sigmoidally during ripening, reflecting the increase in the ripening enzymes, particularly cellulase and polygalacturonase. The oil content fluctuated slightly during ripening. It is suggested that mannoheptulose and perseitol are important carbon and energy sources during ripening. Glucose concentration was also found to increase earlier in fast ripening fruit compared to slow ripening fruit, which is related to increased cellulase activity and may be related to the ABA functioning. Thereafter, storage and ripening trials in two consecutive seasons showed that 1°C storage and the use of MHP for 28 days reduced mass loss, water loss, ethylene production, respiration, softening and heptose consumption, without appreciably affecting fungal rots, physiological disorders or external chilling injury, compared to fruit stored at 5.5°C and regular atmosphere respectively. Also, the storage of fruit in MHP delayed the rise in the activity of cellulase during ripening, compared to fruit not stored in MHP, but there was no significant difference in the peak activity of cellulase, polygalacturonase or pectin methylesterase. In a separate experiment, fruit ripening was significantly affected by the infusion of ABA in an aqueous solution. Water slightly reduced the variation in ripening while ABA reduced the time to ripening and the variation; it is suggested that water stress and ABA are intrinsically involved in the ripening processes and may act as a ripening trigger. The water concentration in fruit was measured non-destructively using reflectance NIR; this model was used to determine the maturity of fruit and the loss of water during cold storage. In the cold chain break experiment, it was found that although fruit recovered after a cold chain break, in terms of ethylene production and respiration, there was a loss in quality because of severe shrivelling as a result of increased water loss. Fruit that were stored at 1°C were generally of a better quality at ripeness, if the cold chain was broken, compared to fruit stored at 5.5°C. In a follow-up experiment, it was found that significant changes occurred in avocado physiology over a 6h period. The respiration rate of fruit significantly increased after 4h at room temperature and mannoheptulose declined by 32% in control fruit and by 16% in ethephon-treated fruit after 6h. This demonstrates the potential for quality loss in a short amount of time. Furthermore, a model of avocado ripening is proposed, outlining the role of water, ABA, ethylene, respiration, ripening enzymes and individual sugars. This study has contributed to the understanding of avocado postharvest physiology and should aid in better management of avocados for improved fruit quality and consumer satisfaction.
The cascade of physiological events leading to chilling injury : the effect of post-harvest hot water and molybdenum applications to lemon (citrus limon) fruit.
(2012) Mathaba, Nhlanhla.; Bertling, Isa.; Bower, John Patrick.
New emerging markets such as Japan and the United States require cold sterilisation of South African citrus fruit as a phytosanitary standard against fruit fly. However, citrus fruit are chilling susceptible, with lemons being the second-most chilling susceptible after grapefruit. Chilling injury is a physiological rind disorder; the occurrence of which is despite its prevalence in horticultural commodities, not well understood. Therefore, the aim of this study was to investigate physiological compounds regulating chilling susceptibility or resistance in citrus fruit, with special emphasis on lemons. Furthermore, the potential of hot water dips or “molybdenum soaks” to maintain a certain level of physiological compounds which determine manifestation of chilling injury symptoms in citrus fruit was investigated. Moreover, it was attempted to create an understanding of the order in which physiological compounds mitigate chilling injury. Lemon fruit from different farms known to be chilling susceptible or resistant were obtained during the 2007 and 2008 harvest season. Thereafter, fruit were treated by soaking for 30 min in 1μM NaMo04.2H20 solution followed by a 2 min HWD 47 or 53°C. Treated fruit were waxed, weighed and stored at -0.5°C for up to 28 days and sampled for chilling injury evaluation 7, 14, 21, or 28 days into cold storage. A second evaluation was carried out five days after withdrawal from cold storage to allow development of chilling injury symptoms as a shelf-life simulation. After the second evaluation fruit were peeled, peel freeze-dried, milled using mortar and pestle and stored at -21°C for further physiological analysis. Freeze-dried peel was analysed for soluble sugars (glucose, fructose, sucrose), vitamin C (ascorbic acid), vitamin E (α-tocopherol), β-carotene, polyamines (putrescine, spermine, spermidine), specific flavanones (naringin and hesperidin) using HPLC-UV-Vis detector and proline, total antioxidant assays (FRAP, ABTS, DPPH), total phenolics, total flavonoids, lipid peroxidation using spectrophotometry, as well as for the heat shock protein (HSP70) using electrophoresis and silver-staining. Chilling susceptibility of lemon fruit varied with fruit source; those sourced from Ukulinga and Eston Estates were chilling resistant, while fruit from Sun Valley Estates showed chilling injury symptoms after 28 days of cold storage plus five days shelf-life. Furthermore, hot water dips (HW) 53°C, 1 μM Molybdenum (Mo) and 10 μM Mo plus HW 53°C significantly reduced chilling injury symptoms compared with the control and HW 47°C. In addition, Sun Valley Estates fruit also showed higher fruit weight loss compared with non-chilling resistant lemons. The alignment of higher fruit weight loss during storage with chilling susceptibility ascertains the use of weight loss as a non-destructive parameter for chilling susceptibility. With respect to flavedo sugars, glucose was found to be the dominant soluble sugar with multi-functional roles during cold storage. This plays a significant role in mitigating cellular stress. Chilling susceptible lemons from Sun Valley Estates had low flavedo glucose concentrations and, therefore, little conversion of glucose to ascorbic acid was possible resulting in a low antioxidant capacity. However, treatments with HW 53°C and Mo soaks seemed to enhance the enzymatic conversion of glucose to ascorbic acid leading to a higher antioxidant capacity in the flavedo of such treated fruit. Furthermore, glucose also feeds into the pentose phosphate pathway which is coupled with the shikimate pathway synthesizing secondary metabolites, especially of the phenolics group. The decrease in glucose was aligned to the levels of total phenolics, but not to that of β-carotene, naringin and hesperidin through 28 days into cold storage period. Moreover, as glucose also feeds into shikimate pathway, simultaneously an increase in proline flavedo concentration was observed. Proline is an antioxidant synthesized from glutamate; as cellular glucose decreases so does the total antioxidant capacity during cold storage. Ascorbic acid is a dominant and potent antioxidant in lemon flavedo as proven with the FRAP, ABTS and DPPH assays. Chilling resistant fruit have significantly higher ascorbic acid conversion. Furthermore, ascorbic acid also acts to generate the α-tocopheroxy radical to further important membrane-bound antioxidant, vitamin E (α-tocopherol equivalent). Furthermore, the DPPH assay was found to be effective in quantifying total antioxidants in lemon flavedo since it detects both lipophilic and hydrophilic antioxidants compared with the ABTS and FRAP assays which are bias to the estimation of liphophilic or hydrophilic antioxidants, respectively. The hot water and molybdenum treatments increased total antioxidants (DPPH assay) with reduced lipid peroxidation 7 days into cold storage and therefore, reduced chilling symptoms in fruit from Sun Valley Estates. The capacity of antioxidant to scavenge reactive oxygen species (ROS) was increased during cold storage and membrane stability significantly improved. Furthermore, putrescine as low valency polyamine was reduced as such compound acted as precursor to the synthesis of the high valency polyamines, spermine and spermidine. Chilling susceptible lemons from Sun Valley Estates showed increased soluble-conjugated polyamines as a response to stress. Furthermore, HW 53°C, 1 μM Mo and 10 μM Mo plus HW 53°C significantly increased the protein concentration and, therefore, likely also the occurrence of proteins with 70kDa (as estimator of HSP70). Additionally, the concentration of conjugated high valency polyamines was also increased, resulting in reduced chilling injury symptoms. The effect of ROS has only been viewed as damaging, while recently their role has also been viewed as stress acclamatory signalling compounds when produced concentrations below critical damaging threshold. Therefore, hot water dips seems to signals synthesis of total protein which include HSPs which then act throughout cold stress to protect other protein and channel other damaged proteins towards proteolysis. While molybdenum increased ROS production below damaging critical threshold, with ROS signalling stress acclimation by further signalling production of bioactive compound with antioxidant properties.
Methyl jasmonate and salicylic acid enhance chilling tolerance in lemon (citrus limon) fruit.
(2013) Siboza, Xolani Irvin.; Bertling, Isa.; Odindo, Alfred Oduor.
South African ‘Eureka’ lemon fruit must be exposed to chilling temperatures (± 0.6°C) as a mandatory quarantine treatment against insect pests for all its overseas markets. Chilling lemon fruit at such temperatures may develop chilling injury (CI) symptoms on the flavedo. This negative effect on fruit quality reduces fruit marketability. This study evaluated postharvest factors influencing physiological, biochemical and ultra-structural mechanisms involved in alleviating CI in lemon fruit. It was hypothesised that treatment with methyl jasmonate (MJ) and salicylic acid (SA) may enhance chilling tolerance in lemon fruit by maintaining cellular integrity and inducing synthesis of enzymatic and non-enzymatic antioxidants. Furthermore, fruit susceptibility to CI was associated with the source of fruit. Lemon fruit were harvested from three locations representative of moderate subtropical, warm temperate and cool subtropical environments. Harvested fruit were treated either with 10 μM MJ, 2 mM SA or 10 μM MJ plus 2 mM SA, stored either at -0.5, 2 or 4.5°C for 0, 7, 14, 21, or 28 days and afterwards transferred to 23°C for a week as shelf-life simulation. Thereafter, fruit were evaluated for alterations in physiological, biochemical and ultra-structural features involved in the manifestation of CI symptoms. Chilling damage was more severe in untreated lemon fruit than in treated lemon fruit. Storing lemon fruit at 4.5°C accelerated the manifestation of CI symptoms more so than at 2°C while storage at -0.5°C delayed the manifestation of CI symptoms. Lemon fruit of moderate subtropical origin were more chilling-tolerant than lemon fruit of warm temperate and cool subtropical origin. Treatment with 10 μM MJ plus 2 mM SA significantly (P < 0.05) improved chilling tolerance in lemon fruit. This treatment effectively maintained membrane integrity, thereby retarding electrolyte leakage and membrane lipid peroxidation as well as mass loss and respiration rate. Treatment with 10 μM MJ plus 2 mM SA was also effective in enhancing the antioxidant concentrations of vitamin E and carotenoids. The production of these antioxidants could have been part of a defence system against chilling damage, reducing CI and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA enhanced the concentration of compounds involved in chilling resistance, such as proline, soluble sugars, ascorbic acid and total phenolics as well as the enzyme phenylalanine ammonia-lyase (PAL). The enhancement of the defence mechanisms may have played a role in enhancing chilling tolerance in lemon fruit. The treatment also inhibited certain enzymes involved in tissue browning, such as peroxidase (POD) which might have contributed to delaying manifestation of symptoms. Polyphenol oxidase (PPO) was found to not be a good biochemical marker of the occurrence of CI. Treatment with 10 μM MJ plus 2 mM SA appeared to be able to enhance chilling tolerance in lemon fruit by maintaining the ultra-structure of the cuticle, cell wall integrity, cell membrane of parenchyma cells of the flavedo. This treatment also preserved the mineral nutrients of the flavedo (carbon, oxygen, phosphorus, potassium, calcium, magnesium, sulphur, sodium, silicon and aluminium) during cold storage. This could have played a role in protecting the fruit against chilling stress and maintaining fruit quality. Treatment with 10 μM MJ plus 2 mM SA reduced ROS production, while the activity of enzymatic antioxidants such as catalyse (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), and accumulation of essential proteins was enhanced. This increase in activity of enzymatic antioxidants and the presence of stress-responsive proteins in the lemon flavedo could have been directly involved in enhancing chilling tolerance. The CI symptoms were accompanied by an increase in membrane permeability, membrane lipid peroxidation as well as phospholipase D (PLD) and lipoxygenase (LOX) activity; however, treatment with 10 μM MJ plus 2 mM SA effectively reduced the membrane permeability, membrane lipid peroxidation, and PLD and LOX activity induced by the cold treatment. This could have contributed to the efficacy of 10 μM MJ plus 2 mM SA in inhibiting the manifestation of CI symptoms. Treatment with 10 μM MJ plus 2 mM SA enhanced flavedo total antioxidant capacity measured by ferric reducing ability of plasma; 2,2-diphenyl-1-picrylhydrazyl; 2,2-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) and the oxygen radical absorption capacity assays. The enhancement of antioxidant capacity in lemon flavedo could have contributed to the fruit’s chilling tolerance. Therefore, the effect of 10 μM MJ plus 2 mM SA treatment, enhancing chilling tolerance, may be attributed to its ability to enhance enzymatic and non-enzymatic antioxidants; activate essential proteins and mitigate the effect of ROS accumulation. With the use of 10 μM MJ plus 2 mM SA treatments, the South African citrus industry will be able to meet the quarantine temperature requirements for exportation of lemon fruit whilst reducing economic losses, depending on the preharvest conditions experienced by the fruit in each shipment.
Yield and quality parameters of tomato cultivars as affected by different soilless production systems and beneficial micro-organisms.
(2013) Maboko, Martin Makgose.; Bertling, Isa.; Du Plooy, Christian Phillipus.
Most tomato cultivars used for commercial food production are imported into South Africa. Optimal growing conditions for these specific cultivars need to be determined, as wrong cultivar choices can lead to great financial losses. Lack of information on selecting well-performing cultivars may lead to lower yield or unacceptable fruit quality. Information on the performance of tomato cultivars under South African conditions, utilizing plastic tunnels or shadenet structures under soilless cultivation is still very limited. Soilless cultivation of vegetables is becoming a preferable over in-soil cultivation due to the improved yield and quality of produce, efficient water and nutrients usage by the crop; furthermore, the grower can regulate nutrient solution, electrical conductivity and pH of the nutrient solution. To identify the optimal system for growing tomatoes hydroponically, the performance of four tomato cultivars (‘FA593’, ‘Miramar’, ‘FiveOFive’ and ‘Malory’) under different growing conditions was evaluated: directly planted in soil under 40% shadenet with drip irrigation, a closed hydroponic system under 40% shadenet, an open bag system under 40% shadenet, or an open-bag system in a temperature controlled as well as a non-temperature controlled tunnel. The study revealed that ‘Miramar’ performed better than the other cultivars in all production systems, with the exception of soil cultivation where there were no differences amongst the four cultivars. Fruit cracking was found to be directly correlated with fruit size, as the large-sized cultivars ‘Malory’ and ‘FA593’ were more susceptible than the other two cultivars. Plants grown under shadenet were prone to fruit cracking and raincheck as well as early blight. Higher yields were obtained when plants were produced in the open bag system under temperature controlled conditions and in the closed system under shadenet. Growing tomatoes in the non-temperature controlled tunnel resulted in high incidences of fruit cracking, poor yield and pre-mature fruit ripening probably due to high and fluctuating temperatures under such conditions. The average marketable yield was 88% and 59% of the total yield in the temperature controlled and non-temperature controlled tunnels, respectively. A further experiment was carried out to improve yield and quality of tunnel tomatoes using beneficial micro-organisms, i.e., arbuscular mycorrhiza fungi (AMF) at different nutrient concentrations. Tomato seedlings were treated with Mycoroot™ containing four mycorrhiza species (Glomus etunicatum, Paraglomus occultum, Glomus clarum and Glomus mossea) at transplanting and subsequently transferred to either a temperature controlled or a non-temperature controlled tunnel under the recommended (100%) or reduced (75 and 50%) nutrient concentrations. Sawdust was used as a growing medium in this experiment. Application of AMF neither enhanced plant growth, yield, nor fruit mineral nutrient concentrations; although fruit Mn and Zn concentrations in the temperature controlled tunnel increased significantly following AMF application. Plants grown in the non-temperature controlled tunnel had significantly poorer plant growth, and lower yield and lower fruit mineral concentrations, compared with fruit from plants in the temperature controlled tunnel. Tomato plants in the non-temperature controlled tunnel had higher levels of micro-elements in leaf tissue, compared with those in the temperature controlled tunnel. The highest yields were obtained from plants fertigated with 75% of the recommended nutrient concentration, as compared with the 100 and 50% nutrient concentrations. When coir was subsequently used as the growing medium, Mycoroot™ applied at seeding and transplanting did not enhance mycorrhizal colonization or fruit quality. Growing tomatoes under reduced nutrient supply reduced the total soluble solids in the juice of the fruit, but improved total and marketable yield, as well as the number of marketable fruit. This effect was more substantial in the temperature controlled than in the non-temperature controlled tunnel. Fruit firmness and leaf chlorophyll concentrations were significantly higher in plants grown in the temperature controlled tunnel. Growing tomatoes in sawdust improved the leaf Mn and Ca concentration over that of tomato plants grown in coir. Mycorrhizal colonisation did not have a beneficial effect on tomato yield and quality. The study indicated that cultivar selection was important in obtaining the highest yield and quality of tomato using the closed hydroponic system under shadenet and the open bag hydroponic system in the temperature controlled tunnel. Temperature controlled tunnels with a pad–and-fan cooling system are still an effective way of cooling the tunnel environment which resulted in high yield and high quality of tomatoes with a higher fruit mineral content than that obtained under non-temperature controlled conditions where only natural ventilation is relied on. Results also demonstrated that mycorrhizal colonization in soilless condition has limited beneficial effects in allowing for better nutrient uptake and thereby for improved yield and quality of tomatoes. Further studies, including different media, nutrient composition and concentrations, need to be carried out to investigate the possible causes of AMF failure to improve yield, despite good AMF root colonization.
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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