Masters Degrees (Horticultural Science)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6628

Browse

Now showing 1 - 5 of 5

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.
The biochemical basis of colour as an aesthetic quality in Citrus sinensis.
(2001) Oberholster, Renate.; Cowan, Ashton Keith.
The development of fruit colour in the sweet orange (Citrus sinensis) is a cultivar characteristic affected by climate and environment. Although external colour is not always an indication of internal quality or maturity, it is probably the most important factor determining consumer acceptance. In the present investigation, efforts were made to determine the biochemical basis of colour as an aesthetic quality in C. sinensis 'Navel' and 'Valencia'. Furthermore, the changes in pigment content and composition during the period of colour development were recorded. Finally, in an attempt to manipulate citrus colour, the effect of dehydrating agents (alcohols), plant hormones, micro-nutrients and low temperature on flavedo carotenoid content was determined. Saponification of the two major colour-imparting components resolved by thin layer chromatography, followed by reversed-phase high performance liquid chromatography revealed that the principal colour-imparting pigments in C. sinensis flavedo are the yellow-coloured xanthophyll 9-Z-violaxanthin and the red C(30) apocarotenoid β-citraurin. Both pigments occur in the flavedo in esterified form. Identification of the chromophores was based on co-chromatography and online spectral analysis. The colour quality of mature fruit was dependant on the content and relative amounts of 9-Z-violaxanthin and Pcitraurin. Quantitative results revealed that increased colour intensity of citrus flavedo was associated with a decline in the 9-Z-violaxanthin : β-citraurin ratio from greater than 50 to below 10, and an increase in 9-Z-violaxanthin and β-citraurin content. Measurement of the mass and ratio of these pigments can be used to accurately colour-grade orange fruit for local and export markets. These parameters will also aid in the evaluation of colour manipulatory techniques. Visual colour break in C. sinensis appears to be associated with a minimum in total pigment as well as total carotenoid content. The period prior to colour break is characterised by a reduction in chlorophylls, carotenes and free xanthophylls usually associated with photosynthetic activity. Following colour break, a massive increase in xanthophyll acyl esters (particularly 9-Z-violaxanthin) is observed. Efforts were made to manipulate carotenoid content of citrus flavedo in vivo, with a view to manipulate fruit colour commercially. It was shown that the micro-nutrients tungsten and molybdenum, and the plant hormones abscisic acid and jasmonic acid increased carotenoid content of flavedo discs; whereas the plant hormone gibberellic acid decreased carotenoid levels. The dehydrating agents ethanol and butanol increased carotenoid content in whole fruit flavedo and flavedo discs. Optimum concentrations were shown to be 20-30% (v/v) for ethanol and 5-10% (v/v) for butanol.
Biosynthetic origin of abscisic acid in ripening avocado fruit.
(2000) Guillaume Maurel, J. C.; Cowan, Ashton Keith.
Mesocarp of ripening avocado fruit incorporated label from [2-(14)C]mevalonolactone, [1-(14)C] acetic acid, [1-(14)C] glucose and [1 -(14)C] pyruvate into ABA, although incorporation from mevalonolactone was significantly higher. Inhibition of the mevalonate pathway at the HMGR level using mevastatin reduced incorporation from acetate and MVL, while increasing incorporation from pyruvate and glucose. The carotenoid biosynthesis inhibitors AMO 1618 (inhibitor of lycopene cyclase) and fluridone (inhibitor of phytoene desaturase) both decreased incorporation of MVL into ABA, while the plant growth regulators ancymidol (inhibitor of GA synthesis and cytochrome P450) and jasmonic acid (senescence stimulator reducing the carotenoid content of plants) both increased incorporation of MVL into ABA. Tungstate was found to reduce incorporation from all four substrates into ABA, although more significantly from MVL and acetate. Further investigation revealed that the tungstate induced decrease in MVL incorporation into ABA occurred concomitantly with increased label incorporation into XAN. Cobalt, an inhibitor of ACC oxidase and therefore of ethylene production, increased incorporation of MVL into ABA. Nickel had a similar effect. Analysis of the methyl ester of ABA extracted from avocado mesocarp supplied with either [1-(13)C] acetic acid or [1-(13)C] glucose revealed incorporation of label from acetate consistent with formation of ABA via the acetate/mevalonate pathway whereas glucose was incorporated via the triose phosphate pathway of isopentenyl pyrophosphate formation. Methane, positive ion-chemical ionisation-mass spectrometry of the cis, trans and all- trans isomers of ABA indicated more intense labelling of trans, trans-ABA, irrespective of substrate used. These results indicate that trans, trans- and cis, trans-ABA are derived by different pathways and that ABA is formed in avocado by both the mevalonate and non-mevalonate pathways of isopentenyl diphosphate synthesis.
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.
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.

Browse

Browsing Masters Degrees (Horticultural Science) by Author "Cowan, Ashton Keith."