Doctoral Degrees (Horticulture)

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Browsing Doctoral Degrees (Horticulture) by Subject "Apples -- Development -- Effect of temperature on"

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    Physiological dynamics of dormancy in apple buds grown in areas with insufficient winter chill
    (Stellenbosch : Stellenbosch University, 2020-04) Inamahoro, Micheline; Louw, Esme D.; Steyn, Wiehann J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Apple trees, that do not fulfil their winter chill requirement, undergo inadequate dormancy release resulting in poor budbreak with irregular and delayed flowering that impact negatively on fruit production and tree architecture. Under mild winter conditions, such as most production sites in the Western Cape region of South Africa, the application of chemical rest breaking agents is standard practise to artificially release dormancy and synchronise budbreak in spring to ensure sustainable and profitable apple production. However, rest breaking chemicals can be harmful to the buds and may result in phytotoxic damage if not applied optimally, therefore a good understanding of the dormancy status of the buds is valuable and necessary to mitigate this risk when applying a rest breaking agent. Furthermore, little information is available on the physiological and biochemical dynamics of commercially produced apple buds when under these milder conditions and no information is known for the ‘Cripps Pink’ variety when grown under climatically contrasting South African conditions. The main aim of this study was to investigate the effect of inadequate winter chill and the application of a blend of hydrogen cyanamide and mineral oil (HCo) as a rest breaking agent on the respiration rate and lipid composition of the terminal buds of full bearing ‘Cripps Pink’ apple trees. Dormant apple buds from two climatic regions, Elgin (insufficient winter chill) and the Koue Bokkeveld (sufficient winter chill) were compared in terms of their respiration rate and lipid composition. Buds from the Elgin region were also tested and compared after receiving a commercial HCo treatment at budswell. Shoots were excised and given a specified amount of artificial chill in the laboratory to act as a point of reference. Apple buds exposed to sufficient chill (orchard and laboratory) showed an early dormancy entrance, high maximum dormancy level and an early release, while buds from the milder area generally showed atypical dormancy behaviour. During winter, the total respiration rate and the rate of the main respiratory pathways (tricarboxylic acid cycle (TCA) and cytochrome C (CYT)) were reduced with the decline in temperature. Their levels increased again in the cold area at the beginning of spring to provide energy for growth resumption. In contrast, the main respiration levels remained low in the warm area and the pentose phosphate pathway (PPP) and alternative pathway (ALT) tested higher suggesting an attempt by the plant to compensate for the deficiency in energy production. The HCo treatment induced hypoxia in the buds and immediately decreased the total respiration as well as the main respiratory pathways (TCA and CYT). After this initial response the treated buds showed a significant increase in respiration and reached high levels towards budbreak and green-tip stage. This increase was not observed in the untreated buds from the mild winter climate, in contrast, these buds maintained a higher use of the PPP and ALT pathways. It was also seen that the HCo treatment followed by warmer spring temperatures hastened the occurrence of the green-tip stage. Both cold and warm winter temperatures also affected the lipid composition in the apple buds at the end of the winter period towards the onset of spring. Warmer winter conditions reduced the desaturase process preventing the desaturation of linoleic acid to linolenic acid, a polyunsaturated fatty acid with three double bonds. Lower free phospholipids and higher free sterols content were also detected in buds from the warmer area and is thought to reduce the membrane fluidity and permeability hampering budbreak and growth resumption even under favourable spring conditions. The high saturated and monounsaturated fatty acids in buds from the mild winter area confirmed the results and again pointed towards less fluid and permeable membranes. The HCo treated buds from a mild winter area showed results similar to buds that received sufficient winter chill, characterised by induced desaturation and higher linolenic acid levels enabling earlier budbreak. It is suggested that the artificial rest breaking of terminal apple buds, via the use of HCo, targets similar biological pathways and physiological mechanisms as the natural accumulation of sufficient chill when inducing growth resumption. In general, the physiological aspects under investigation in this study provided evidence that treatment with HCo, in the area with insufficient winter chill, act very similar to sufficient winter chill. It enhanced the main respiration pathways and stimulated the production of polyunsaturated fatty acid, which in turn, provides more fluid and permeable membranes that increase energy production needed for growth resumption in spring. Results from this study provide evidence that future development of artificial ways to target these same systems may improve apple cultivation in marginal production areas, especially in the Western Cape of South Africa.