Interactive effects of salinity and irradiance on photoprotection in acclimated seedlings of two sympatric mangroves

The synergistic effects of irradiance and salinity on leaf angle, the photochemical efficiency of photosystem II and photosynthetic pigment composition of mangroves were studied in a factorial experiment. Seedlings of Aegiceras corniculatum (L.) Blanco (Myrsinaceae) and Avicennia marina (Forstk.) Vierh var. australasica (Walp.) Moldenke (Avicenniaceae) were grown under salinity treatments (0, 5, 25, 50, 75, and 100% artificial seawater), in full sunlight or under shade cloth (transmitting 30 or 70% sunlight), during summer and autumn. Significant species differences and effects of salinity and growth irradiance were found for key measures. Depressions in F_v/F_m due to salinity and growth irradiance were chronic, they were least in 25% seawater and in 30% sunlight, and greater in low and high salinity, and higher irradiance. A diurnal depression of F_v/F_m was superimposed on the chronic depression, and was greater for Ae. corniculatum than Av. marina. Increases in leaf angle; and increases in the size, and de-epoxidation state of the xanthophyll cycle pigment pool afforded protection from adverse effects of excess excitation energy. Adverse effects of the highest salinities on ,-carotene and ,-carotene biosynthetic pathways were suggested, particularly in Ae. corniculatum. The ecological significance of differences in species extent and temporal patterns of response are discussed.