Differential leaf gas exchange responses to salinity and drought in the mangrove tree Avicecennia germinans (Avicenniaceae).

Leaf gas exchange was assessed in Avicennia germinans L. grown under different NaCl concentrations (0-40 per thousand), after salt-relief, and then during drought. Stomatal conductance (gs) and net photosynthetic rate (Pn) decreased with increasing NaCl concentration, and intrinsic water use efficiency (Pn / gs) increased. Under desalinization Pn / gs declined. Thus, gs did not change in plants grown at low NaCl concentration (10 per thousand), but increased up to 30-32% at higher NaCl concentration (20-40 per thousand). However, Pn was only slightly enhanced (10-15%). Under drought, Pn decreased by as much as 46% in plants grown at low NaCl concentration (10 per thousand) and by 22% at high NaCl concentration (40 per thousand). Thus, Pn / gs decreased and water use efficiency was lower during drought compared to estimates prior to salt-relief.