A study of leaf gas exchanges during the dry season and rainy season confirmed the importance of the Cl- ion in coconut. In the dry season, compared to a control, chloride deficiency led to lower stomatal conductance in coconut, hence a reduction in leaf gas exchanges. This resulted in less intense net photosynthesis and transpiration, which accounted for the poorer drought tolerance of Cl- deficient palms. During the dry season, even if not very intense, coconut palms that were not Cl- deficient maintained and even increased their stomatal conductance. This enabled them to increase leaf transpiration, hence responding to the increase in evaporative demand and maintaining a high level of net assimilation. Conversely, deficient palms reduced their stomatal conductance, leading to a drop in transpiration and in net photosynthesis. Observations during the rainy season failed to reveal any effect of chloride deficiency on intrinsic photosynthesis. The Cl- ion therefore seems to act primarily during the dry season, and in stomatal regulation.