WATER RELATIONS AND GAS-EXCHANGE IN YOUNG COCONUT PALM (COCOS-NUCIFERA L) AS INFLUENCED BY WATER-DEFICIT

Drought is the main climatic limitation to coconut palm (Cocos nucifer a L.) production. To identify early screening parameters for drought r esistance, physiological responses to water deficit were investigated for the first time using containerized young plants grown outside. Thr ee varieties were studied: 'West Coast Tall' (drought avoiding), 'Mala yan Yellow Dwarf' (drought susceptible), and their progeny, the hybrid 'PB 121' (drought resistant). Leaf water status (relative water conte nt, leaf water potential) and leaf gas exchange parameters (stomatal c onductance to water vapor, net photosynthetic rate, and leaf internal CO2 concentration) were measured throughout a drying cycle induced by withholding watering. On fully hydrated plants, the sensitivity of sto mata to atmospheric water vapor deficit was also investigated. In the three varieties, stomatal conductance to water vapor declined before l eaf water status parameters were affected. The existence of a root-to- shoot communication system was proposed. Net photosynthetic rate was h ighly dependent on stomatal aperture, but nonstomatal factors also par ticipated in the reduction of CO2 fixation. Since gas exchange rates w ere equally sensitive to drought in the three varieties, they could no t be used as screening parameters for drought resistance. Under severe drought stress, both relative water content and leaf water potential differed significantly among populations. 'West Coast Tall' maintained a higher leaf water status than 'Malayan Yellow Dwarf'. Remarkably, t he water status parameters of 'PB 121' were intermediate between those of the two parents. Similar ranking was obtained in experiments with excised leaflets. The high reproductibility of the results suggests th at leaf water status parameters might be useful as early selection cri teria for drought resistance in coconut palm.