Analysis of bean characters in cocoa (Theobroma cacao L.) hybrids bred for bold beans
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Date
2015
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Department of Plant Breeding and Genetics, College of Horticulture, Vellanikkara
Abstract
Cocoa is a tropical tree crop cultivated for its beans which form the main source of chocolate. Many of the cocoa varieties cultivated by the farmers, even though high yielders, are having small beans. However, the standard for cocoa bean in the international market is one gram or above. The size of bean is significantly influenced by various environmental conditions especially temperature, prevailing during the pod formation stage. Hence, the cocoa beans produced in summer months may fail to keep the international standards. To overcome this, it is necessary to evolve varieties with large beans of more than 2g so that even if reduction occurs during summer months the beans will have enough size to meet the international standards. The present study which forms part of an ongoing project at Cocoa Research Centre (CRC), KAU, Vellanikkara was hence, taken up during 2012-14 to evaluate the inheritance of various bean characters and to assess the threshold bean size as well as the heterotic effect for various bean characters. Forty hybrid progenies derived from four crosses involving five parents, which are in full bearing stage and maintained at CRC, Vellanikkara formed the material for the study. These hybrids were subjected to morphological and biochemical evaluation along with their parents. The morphological evaluation based on eight qualitative and 14 quantitative characters was done using the descriptor developed by Bekele and Butler (2000). Fat and polyphenol contents were estimated following standard procedures for biochemical characterization. Variability was observed among the hybrid progenies for all the qualitative characters evaluated. Variations expressed by the hybrid progenies in terms of pod and bean quantitative characters were also significant, indicating the heterogeneity among them.
Analysis of bean characters revealed that the hybrids 13.7, 16.8, 14.6 and 12.5 expressed higher values with respect to wet weight of unpeeled bean (460g), length of peeled bean (17.78mm), breadth of peeled bean (10.81mm) and peeling ratio 82.4% respectively. The highest wet weight of peeled bean (1.78g) was recorded by hybrids 12.5 and 12.6. The dry weight of peeled bean (1.31g) and thickness of peeled bean (4.98mm) were the highest in hybrid 12.6. The fat content ranged from 33.3% in hybrid 7.8 to 58.5% in hybrid 10.6 and polyphenol content ranged from 0.15% in hybrid 16.8 to 0.56 % in hybrid 5.7. Among the pod and bean characters evaluated wet weight of peeled bean exhibited high GCV (510%), high heritability and high genetic gain (1163%). The bean characters varied significantly over seasons in hybrids as well as parents. Relatively higher bean weights were observed in September- November period harvests in contrast to the lower bean weights during June to August period.
Assessment of performance of hybrids based on year round observation on bean characters revealed that the major factors contributing significantly to boldness of bean were weight, breadth and thickness of beans. Based on these parameters a descriptor was developed confirming the international standards, for deciding the threshold bean size. The descriptor scales were prescribed based on the best subset values in the post hoc tests. Hybrids 12.6, 13.7 and 14.6 satisfied the developed scales Significant positive heterotic effect (average heterosis as well as heterobeltiosis) for various bean characters was expressed by hybrids 13.7 and 12.6. Among the hybrid progenies evaluated hybrid 13.7 was found to be a desirable one with low pod index value (PI value), high pod breadth, pod weight and wet weight of unpeeled bean coupled with significant positive heterotic effect for various bean character followed by hybrids 12.6, 16.5 and 14.6.
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