River mangrove Aegiceras corniculatum maintains certain levels of Na+ and Cl− in leaves to adapt seasonal freshwater

Abstract

AbstractMangrove environments are often characterized by large fluctuations in salinity, ranging from freshwater to hyper‐saline conditions. Most reports have focused on the mechanisms by which mangroves adapt to high salinity. However, how mangroves cope with seasonal freshwater habitats has seldom been studied. To address this question, we surveyed the river salinity and leaf traits (chlorophyll fluorescence, ion concentrations, carbon isotope ratios and osmolality) of Aegiceras corniculatum (L.) Blanco (river mangrove) along a freshwater‐dominated river. A. corniculatum at the upstream site was subjected low salinity, being in fresh water for a long period (up to 310 h) in the wet season, which drastically diminished during the dry season. At both sites, the actual photosystem II efficiency (ФPSII) and electron transport rates (ETR) of the leaves were higher in the wet season than in the dry season, and the carbon isotope (δ13C), ions (Na+ and Cl−) concentrations and osmolality showed no seasonal variations. However, the maximal quantum yield of PSII photochemistry (Fv/Fm), the proportion for closed PSII reaction centres (1 − qP) and non‐photochemical quenching (NPQ) of the leaves at the upstream site maintained high levels in the wet season. This quenching analysis indicated that there was a down‐regulation of photoprotection at the upstream site during wet season. An explanation for this is that long‐term freshwater is a stressful environment for A. corniculatum and maintaining certain level Na+ and Cl− in saline water episode is necessary for A. corniculatum to maintain low water potentials to take up water.