Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, mu mol CO2 m(-2) s(-1)) declined during the dry season from 11.9+/-1.8 to 7.0+/-1.5 and from 9/-.6+2.0 to 4.7+/-2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO2 mol(-1) Chl) were from 31.6+/-4.7 to 20.5+/-4.3 and from 21.9+/-3.7 to 15.2+/-8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chi nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO2 diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration.