The influence of vapour pressure deficit on leaf water relations of Cocos nucifera in northeast Brazil.

Daily courses of leaf gas exchange and leaf water potential (Ψ leaf ) of green dwarf coconut palm ( Cocos nucifera ) were measured in irrigated plantations on the wet coastal plateau and in a dry semi-arid area of northeast Brazil. At both sites, significant correlations were obtained between stomatal conductance (gs) and vapour pressure deficit (VPD air ), Ψ leaf and VPD air , leaf transpiration (E) and gs, and E-Ψ leaf . Despite these similar relationships between sites, stronger correlations involving gs-VPD air and E-Ψ leaf were found at the semi-arid site, where whole-plant hydraulic conductance (gp) was correlated significantly with VPD air . In addition, at the semi-arid site, only, the net photosynthesis (P N ) was not correlated with E and Ψ leaf , and the intrinsic water use efficiency (WUEi) was disconnected from VPD air and Ψ leaf . The different behaviour of leaf gas exchange and Ψ leaf between sites was probably caused by low gs in response to high VPD air at the semi-arid site. Our results indicate potential for significant alterations in the pattern of leaf gas exchange during future climatic changes with increasing temperature and concomitant increases in VPD air . The atmospheric water stress will probably reinforce the strength of connection among water relation variables (E, Ψ leaf , gs, gp, and VPD air ), but it will disrupt the linear relationship between net CO 2 assimilation and leaf water relations such as P N -E, P N -Ψ leaf , WUEi-VPD air and WUEi-Ψ leaf .