Developing a breeding strategy for butternut squash (Cucurbita moschata) in South Africa
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Date
2021-11
Authors
Swanepoel, Jacobus Francois
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
Knowledge about inheritance mechanisms of economically important traits and the influence of environmental factors on their expression are crucial for the formulation of an appropriate breeding strategy in any crop. Currently limited information is available on this subject in butternut and even more so with regard to internal fruit quality characteristics. The aim of this study was to design an effective breeding approach to improve butternut internal fruit quality, without sacrificing yield. The aim was extended with the objectives to quantify phenotypic and genotypic variability in 42 genotypes for characteristics across environments and confirm stability in high-performing individuals, and to identify characteristics showing the greatest potential for improvement through estimating the genetic parameters and interrelations between these characteristics. From heterosis studies, it could be confirmed which characteristics should be improved through hybridisation. From the combined analysis of variance on 15 morpho-agronomic and internal fruit quality characteristics over three locations and two seasons, highly significant differences (p≤0.001) were observed between genotypes for all traits including leaf chlorophyll content (CHL), green-red (Leaf a*) and yellow-blue colour contribution in the leaf canopy (Leaf b*), leaf width (LW), petiole length (PL), average fruit mass (AFM), dry matter yield (DMY), fruit number (FN), uniformity, yield, total soluble solids (TSS), dry matter content (DMC), green-red colour contribution in the fruit mesocarp (Fruit a*), internal fruit breakdown (IBD) and penetrometer readings as an indication of mesocarp firmness (PEN). With the exception of CHL and LW, all characteristics displayed significant mean square differences for genotype x location x season interactions, suggesting differential ranking of genotypes across environments. Phenotypic variability attributed to genetic variation ranged from 14% to 33% in plant morphological characteristics, 16% to 62% in yield and yield-dependent traits and 50% to 67% in fruit quality characteristics, supporting the existence of immense inherent variability within the population. The additive main effect and multiplicative interaction analyses across six environments indicated internal fruit quality characteristics to be more stable across environments than AFM, FN and yield. None of the genotypes was stable for all characteristics. Based on stability and performance, G11 and G13 were identified as the most desirable genotypes for the processing and small-fruited market segments respectively. Similarly, both G16 and G17 were most desirable for the fresh market segment. Based on the genetic components estimated using 27 F1 genotypes, AFM, FN, TSS, DMC, Fruit a* and PEN were observed to be under additive genetic control, implying selection in early generations would be effective for their improvement. In contrast, heterosis breeding could be more effective for the improvement of Leaf a*, DMY, uniformity and yield. Moderately high broad-sense heritability with lower genetic gain as a percentage of the mean was recorded for CHL, Leaf b*, LW, PL and IBD, suggesting additive genetic control although the environment plays a larger role in the expression of the phenotypes. A strong negative association was observed between AFM and FN as well as a strong positive correlation between DMC and TSS. CHL had moderate correlations with AFM and FN. Weak negative correlations were also estimated between yield and internal fruit quality characteristics and more specifically TSS, DMC and Fruit a*. Findings from the line x tester analysis, involving four lines and four testers, showed none of the parents to be consistently good general combiners for all characteristics. Leaf a*, Leaf b*, FN, TSS, DMC and PEN revealed nonsignificant line x tester interaction mean squares. LW, PL, AFM, Fruit a* and IBD were found to be predominantly under additive genetic control. CHL, DMY, uniformity and yield were demonstrated to be mostly under non-additive genetic control. Significant mid-parent heterosis was estimated for all characteristics with heterosis percentages above 45% for AFM, DMY, FN and yield. Using the current germplasm collection, the most feasible strategy for the improvement of butternut genotypes would be through selection in early segregating generations. For yield, uniformity and DMY more desirable results will be achieved through heterosis breeding. Focussing on yield, FN and AFM, in combination with DMC and TSS, will be the most effective approach to develop high-performing, stable and desirable hybrids.
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Keywords
Thesis (Ph.D. (Plant Breeding))--University of the Free State, 2021, Butternut, Combining ability, Correlation, Genetic advance, Genotype x, Environment interaction, Heritability, Heterosis, Line x tester analysis, Phenotypic diversity, Stability