SCREENING MARKERS FOR DROUGHT TOLERANCE IN COCOA HYBRIDS

The genetic variability of agro-morphological, physiological, biochemical and molecular characters in relation to drought tolerance were studied in cocoa hybrids and their parents of Malaysian and Nigerian origin both in greenhouse and field conditions. The hybrid varieties were subjected to water deficit regime in the greenhouse. The first treatment, with daily watering, the water content of the pot was maintained at its field capacity, the second treatment kept at moderate stress, with an addition of water corresponding to 50% of the field capacity and the third treatment was under severe stress, with an addition of water only to 20% of the field capacity. Agro-morphological traits (vegetative vigor, leaf emergence, leaf area, relative plant water content, biomass), physiological parameters (mortality, plant water potential, epicuticular wax, chlorophyll fluorescence, stomatal conductance and resistance, photosynthetic activity, and transpiration rate) and biochemical parameters (proline content, total soluble sugars, reducing sugars, proteins, enzyme activity: peroxidases, catalases and superoxide dismutase) were assessed under greenhouse. A significant effect was observed for the water treatment x cocoa variety interaction (p=0.024) indicating the influence of treatments on morphological traits. Significant differences were also observed among varieties for vegetative vigor (p=0.0001), biomass (p=0.0001), total leaf area (p=0.0001), leaf emergence (p=0.0001) and mortality rate (p=0.0016) as well as for physiological parameters. The multivariate principal component analysis (PCA) of morphophysiological traits observed under induced water deficit stress and the hierarchical ascending classification made it possible to distinguish two large groups of hybrids according to their sensitivity. The first group, composed of hybrids VTLCP-22, VTLCP-26, VTLCP-25 and VTLCH-4, characterized by their vegetative vigor at a young age correlated to its biomass with a strong stomatal resistance and a good photosynthetic activity. The second group constituted by hybrids VTLCP-29, VTLCP-27, VTLCP-11; VTLCP-28, VTLCH-3, VTLCP-24 characterized by their mortality rate, strong transpiration correlated to strong stomatal conductance. The second part of this work, based on the results of the first, consisted of a comparative study of genetic markers associated with tolerance to water deficit stress. The identification of biochemical markers related to the growth of hybrid cocoa varieties under induced water deficit stress shows that the hybrid varieties constituting group 1, which are said to be tolerant to water stress in a controlled environment, have a higher cytoplasmic accumulation of proline, free amino acids, proteins and a higher activity of peroxidases (POD) and superoxide dismutase (SOD) than those of group 2. The morpho-physiological and biochemical markers identified will contribute to the early selection of cocoa tree genotypes against water deficit and drought in cocoa breeding programmes.

Keywords: cacao, drought, markers, morpho-physiological, biochemical