Physiological responses for moisture stress and development of an index for screening coconut (Cocos nucifera L.) genotypes for drought

The effect of moisture stress on leaf water potential [Ψ], relative water content [RWC], stomatal conductance [gs], transpiration [E], rate of net photosynthesis [A], leaf tem-perature [Tleaf] and ratio of intercellular and atmospheric CO2 concentration [Ci/Ca] of four coconut genotypes (Clovis [CL], Dwarf Green [DG], Dwarf Brown [DB] and Cam-eron Red Dwarf [CRD]) were studied, to identify factors that contribute to drought tolerance of some coconut genotypes and to develop an index for screening drought tolerant genotypes. All palms were about 15 years of age, grown in IL1 Agro-Ecological Region and were exposed to 80-day natural drought. RWC of leaves showed that the three dwarf genotypes dehydrated faster than CL. The reduction in gs in response to moisture stress was observed in all genotypes, even though the Ψ was not significantly reduced. This suggests that there is a signal from roots that induces a reduction in gs. CL and DB appeared more drought tolerant by maintaining high A even under low soil moisture conditions and by having low gs, while, DB responded quickly to subsequent rains by regaining its initial rates of photosynthesis and gs. Moreover, CL maintained lowest Tleaf and highest Ci/Ca ratio during dry weather. CRD showed highest rate of reduction of gs with the inception of dry spell, indicating its highest sensitivity to mois-ture stress conditions. Based on the index of stomatal performance, calculated by using gs and Ψ CRD was selected as the most drought sensitive genotype, and DB as the most drought tolerant genotype.