Identification and development of drought-tolerant cocoa hybrids: physiological insights for enhanced water use...

Water stress affects the most important determinants of yield-canopy architecture, photosynthesis, and partitioning of assimilates. Being a perennial crop, the water requirement in cocoa is fairly high. Therefore, identifying drought-tolerant plant varieties within cocoa species is critically important, especially in the context of ongoing climate variability attributed to climate change. This research aimed to develop cultivars that efficiently utilize available/minimal water resources and maintain optimal yields despite environmental stressors. Hence, efforts were taken to identify drought-tolerant genotypes, taking physiological characteristics as the main parameters. Drought-tolerant parents (M 13.12, G I 5.9, G II 19.5, and G VI 55) identified in previous studies were crossed in all possible combinations to attain drought-tolerant hybrids. All the crosses were obtained except for GV1 55 x M 13.12, which may be due to an incompatibility reaction between these genotypes, which is a common mechanism in cocoa. The hybrids were then subjected to drought stress (under 40% field capacity) and were analyzed for various physiological parameters such as chlorophyll stability index, membrane stability, relative water content, photosynthetic rate, transpiration rate, leaf temperature and chlorophyll content.

The highly tolerant and tolerant hybrids in almost all the crosses studied had relatively high chlorophyll stability index (highest being 86.73% in highly tolerant hybrid of cross M 13.12 x G I 5.9), membrane stability (highest value of 86.36% observed in tolerant hybrids of cross M 13.12 x G I 5.9), relative water content (highest value being 79.43% observed in the highly tolerant hybrid of cross G II 19.5 x G VI 55 while the lowest value of 23.51% being shown by the susceptible hybrids of cross G VI 55 x G II 19.5), photosynthetic rates (highest being 1.627 µmol CO₂ m^- 2s^- 1 observed in cross M 13.12 x G I 5.9) and chlorophyll content (highest being 41.27 SPAD units observed in the highly tolerant hybrids of G II 19.5 x G VI 55) as compared to susceptible hybrids. Tolerant hybrids had lower transpiration rates (lowest being 0.306 mmol H₂O m^- 2 s^- 1 in cross G VI 55 x G II 19.5) than susceptible hybrids (highest being observed in the susceptible hybrids of cross M 13.12 x G I 5.9 having the value of 2.067 mmol H₂O m^- 2 s^- 1) indicating their efficiency in handling water stress. However, all the tolerant, susceptible and fully irrigated hybrids showed comparable values ranging between 30 and 33⁰ C for the leaf temperature indicating the efficiency of cocoa plants in regulating the water stress even during drought. Correlation and path analysis revealed that relative water content and photosynthetic rate were positively associated with the dependent variable, the number of leaves retained. However, the transpiration rate showed a negative correlation with several parameters such as cell membrane stability (-0.550), relative water content (-0.528) and chlorophyll stability index (-0.319). Binary regression analysis indicated that relative water content and photosynthetic rate will show 51.87 and 66.29% improvement over the base population, respectively, if used as selection criteria for a new population in future drought breeding programmes.

This experiment indicated how the plants were able to regulate various physiological mechanisms under drought stress and how these parameters can be utilized in distinguishing between drought tolerant and susceptible plants. Ranking based on the values of physiological parameters revealed that crosses M 13.12 x G I 5.9, G II 19.5 x G VI 55, and G II 19.5 x G I 5.9 produced hybrids with favourable responses to drought tolerance. It indicated that these crosses could be further utilized in developing drought-tolerant genotypes. Further, the binary regression studies indicated that relative water content and photosynthetic rate can be used as selection parameters to identify drought-tolerant hybrids more efficiently in the future cocoa stress breeding programmes.