MSc

Drought is considered to be one of the most limiting factors for cocoa production. Preliminary efforts have been made in Kerala Agricultural University to identify drought tolerant cocoa genotypes. In continuation of the same, the present study was proposed to develop superior hybrids tolerant to drought in cocoa. The study was conducted at Department of Plant Breeding and Genetics, College of Horticulture (CoH), Cocoa Research Centre and College of Forestry, KAU. Four genotypes viz. M 13.12, G I 5.9, G II 19.5 and G VI 55 identified as drought tolerant in previous study were taken as parent materials. They were hand pollinated in all possible combinations. Hybrid pods were obtained in all the crosses except in G VI 55 x M 13.12 and successful crosses showed more than 85 per cent germination. Since initial vigour was found to be correlated with final yield, the seedlings were screened at the third month stage for their Height x Diameter² (HD²) value. Based on the seedling vigour, 1505 hybrids were screened representing all the crosses. During the fifth month, they were screened for drought tolerance by maintaining the soil at 40 per cent field capacity for two weeks and a total of 120 hybrids were selected. Based on the percentage of leaves retained on the hybrids, the plants were classified into four categories: highly tolerant (more than 70 % leaves retained), tolerant (40-70 % leaves retained), susceptible (10-40 % leaves retained) and highly susceptible (less than 10 % leaves retained). Hybrids maintained under full irrigation were taken as control. Various biochemical and physiological parameters related to drought were observed. Proline, glycine betaine and superoxide dismutase were found to be high in tolerant and highly tolerant hybrids and was low in susceptible hybrids. The control was found to have very less amount of proline, glycine betaine and SOD. However, under drought conditions, plants showed reduced nitrate reductase activity. The control plants showed highest nitrate reductase activity. In case of physiological parameters, high chlorophyll stability index, membrane stability, relative water content, photosynthetic rate and chlorophyll content were recorded in all tolerant and highly tolerant hybrids whereas these parameters were comparatively low in susceptible hybrids. The control plants showed high value for all these characters. However, in case of transpiration rate, a reverse trend was observed. The highly tolerant and tolerant hybrids showed low transpirational rate, whereas the susceptible hybrids had high transpirational rate. The control also had high transpirational rate. It indicated the ability of tolerant hybrids to conserve water during drought stress. The leaf temperature did not show any variation among the hybrids. Correlation studies showed that all physiological and biochemical characters except transpiration rate and leaf temperature have positive correlation with percentage of leaves retained. Effect of these characters on dependent variable i.e., percentage of leaves retained was estimated by path analysis. Characters having direct effect on leaf retention were proline, nitrate reductase activity, membrane stability, photosynthetic rate and relative water content. These characters were analysed for their genetic parameters. It was found that except membrane stability, all other charcters were having high heritability and genetic gain. Among the four characters considered, proline, nitrate reductase activity and relative water content were having additive gene action and hence, were suitable as selection parameters for population improvement and heterosis breeding. Photosynthetic rate which was having comparatively less additive gene action, can be exploited in heterosis breeding. Based on these four characters, a selection criteria was designed (Proline, > 410 µg/g, NRA, > 6 mmol nitrate/g/hr, RWC, > 40% and Photosynthetic rate, > 0.700 µmol CO2 m-2 s-1). Three characters which expressed high additive gene action, were subjected to binomial regression analysis and was found that if nitrate reductase activity when used as a selection criteria, population will experience 87.48 per cent improvement over base population. Similarly, 51.87 per cent and 51.55 per cent improvement will be there with the characters relative water content and proline respectively. Combining ability studies indicated that the best general combiners were G VI 55 and G II 19.5 and the best specific crosses were M 13.12 x G II 19.5, G I 5.9 x G VI 55 and G II 19.5 x G VI 55. The 120 hybrids used in the study are now field planted. They have to be evaluated at their various stages of growth by subjecting to drought stress. For resistant breeding studies, 120 hybrids will not form substantial volume. Hence, more hybrids have to be evolved by using selected best combiners.