MSc

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.