Abstract:
Genetic studies in cocoa (Theobroma cacao L.) using various
population viz., clones, biclonal crosses, biclonal pair crosses and inbreds
were undertaken in College of Horticulture, Kerala Agricultural University,
Thrissur, during 1994-97. The study assessed variability, association of
yield attributes and relative importance of these attributes for yield
variation using multiple regression and path analysis. Selection indices,
inbreeding depression and limits to genetic diversity for heterosis were
also worked out.
Wide spectrum of variability existed in the populations studied. The
highest variability was observed in yield of dry beans/tree and precocity of
bearing (pods/tree - 5 years from planting), whereas, traits pod width
and bean width showed low variability. Clones GI 10.2, S 27.16, GI 14.17
of Clonal base population, hybrids H'8 (GI 5.9 x GII 23.3), H2 (M 13.12 x
GI 5.9) of Series IV biclonal crosses and hybrids H4 (M 16.9 x GII 19:5),
H3 (M 16.9 x GII 20.4) of Series 11 biclonal crosses, exhibited superior yield
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performance. Yield however, recorded only moderate heritability and
therefore moderate expected genetic advance.
Strong correlation was observed for yield with pod weight, dry
weight/bean and efficiency index but not with precocity of bearing,
chlorophyll contents and net photosynthesis rate. Multiple regression and
path analysis revealed the importance of dry weight/bean, bean size and
efficiency index in determining the yield in cocoa. High correlation exhibited
by many traits was due to indirect effects. Selection index for yield, based
on the above traits viz., dry weight/bean, bean length, bean thickness
and efficiency index recorded a relative efficiency of 150.35 per cent
over direct selection.
Inbreeding depression was observed for growth traits. The rate of
depression was more in the second inbred generation as compared to that
in the first inbred generation. Number of hybrids exhibiting relative
heterosis in desirable direction ranged from one (eg. pod width, bean length
and thickness) to seventeen (number of beans/pod). Considerable genetic
divergence among clonal genotypes was evident. Results indicated that the
chances for occurrence of a high frequency of heterotic crosses and high
values of heterosis are more, when the parental divergence is moderate.
