Genotype-by-environment interaction analysis on bud break dynamics, yield potential and fruit quality of diverse apple (Malus domestica) trees in contrasting environments
Date
2024-12
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Stellenbosch University
Abstract
The apple industry plays a pivotal role in the global economy and holds a noticeable position in South Africa’s economic landscape. This thesis includes three research papers that explore the influence of environmental conditions as well as inherent genetics of different genotypes on their bud break patterns, thinning requirements, yield potential, fruit size and fruit quality. A multi-environmental trail (MET) was conducted which consisted of three climatically contrasting environments. Each environment included a complete randomised block design which consisted of ten genotypes (3rd and 4th leaf) with varying chilling requirements (low, medium and high). The impact of a rest-breaking agent (RBA) treatment was assessed using a split-plot design. The trees were trained to a central leader system with M7 rootstocks. Data was collected for an abundant number of traits with the focus on vegetative and reproductive bud break dynamics across two seasons and yield potential and fruit quality for a single season. The data was analysed using an ANOVA, DA, AMMI, and GGE biplot analysis to assess adaptability (performance and stability) across different environments and RBA treatment. Many interactions were present, however the main contributors towards the observed variability for most of the traits were either the inherent genetics of the genotypes and/or the environment. RBA treatment had some effect, but to a lesser extent. Patterns of adaptability were observed amongst the genotypes, where the medium and high chill genotypes had a higher amount of bud break and shorter bud break period in the cold and intermediate environments. An environmental effect was observed where the number of vegetative buds decreased, and reproductive buds increased with increasing winter temperatures. The low chill genotypes showed broad adaptability, with prolonged bud break periods in cooler environments due to an ecodormancy effect. RBA treatment resulted in accelerated bud break periods and higher bud break for some genotypes in the warmer environments. The thinning traits were mainly determined by the genotype; however, the high chill genotypes required more thinning in the cooler environment and larger variation in fruitlet size was observed in the warmer environment. The low chill genotypes again showed broader adaptability. Fruit size at harvest, yield and yield efficiency were greatly influenced by the environment, where a higher number of smaller fruit was harvested for both low- and high chill genotypes in the warmer environment, and a higher yield efficiency observed in the cooler environments. The fruit quality and storability traits showed that many of the internal characteristics such as TA and TSS were largely related to genotype genetics, however it was also influenced by the environment and RBA. Fruit firmness was also determined by genetics, but smaller fruits tended to have higher fruit firmness. The high chill genotypes showed good storability, whereas the low chill genotypes did not. This study emphasised the importance of selecting well adapted genotypes during orchard establishment to suit specific micro-climates and illustrated the effect of both the inherent genetics of the genotypes as well as the environment and RBA application on genotype adaptability.