Development of a pre-selection index for cider apple (Malus x domestica Borkh.) genotypes by phenotyping a low-chill requiring red-fleshed breeding progeny
ENGLISH ABSTRACT: The rapid growth of the cider industry in South Africa, coupled with wine grape producers in warmer regions seeking diversification options, led to the concept of developing low-chill, red-fleshed cider apples to support both industries. The low-chill trait allows apples to be cultivated in warmer areas traditionally associated with wine grape production, while the red-fleshed characteristic offers novelty to the final product by adding red colouration and enhancing fruit quality traits associated with cider-making suitability. This innovation could lead to the creation of a unique red cider product. To achieve this, a cross was made between ‘Anna’ (a low-chill genotype) and ‘Geneva’ (a red-fleshed genotype). The progeny was screened for red leaf colouration, an indicator of Type 1 red-fleshed apples, allowing the selection and planting of only red-fleshed genotypes. These selected genotypes with red foliage were grafted onto M.7 rootstocks and planted in a low-chill environment. Since traditional breeding processes are time-consuming, the phenotypic variation within this progeny for reproductive, vegetative and fruit quality was quantified to establish a pre-selection index that could accelerate the development of low-chill, red-fleshed cider apple genotypes. The index was constructed by identifying the best-performing genotypes based on traits related to reproduction, vegetative growth, and fruit quality, serving as a baseline for comparison. A random decision forest model was employed to assign importance to these traits, and in conjunction with the baseline, a pre-selection index was developed to identify promising genotypes. The sensory attributes and biochemical composition of this unique low-chill, red-fleshed progeny were also analysed. The indexing results suggested that sweet cider production is the most viable outcome, as most top-performing genotypes were classified as sweet cider apples, which aligned well with the South African market’s preference for sweeter products. Among the progeny, Genotype 10 showed exceptional potential as a low-chill, red-fleshed sweet cider apple. Several other genotypes were also identified as viable options for cultivation in warmer areas, with additional future crosses recommended to further advance the development of low-chill, red-fleshed apples. Sensory analyses on apple juice revealed that the selected genotypes displayed sweet berry and fruity flavours, complemented by varying levels of bitterness and astringency. Notably, Genotype 100 demonstrated a biochemical composition that could enable colour retention and stability during processing, making it invaluable in the development of a red cider product. Grounded in horticulture and genetics, this thesis lays the framework for identifying low-chill, red-fleshed cider apple genotypes suitable for production in non-traditional apple-growing regions, addressing the needs of both producers and the cider industry.
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