Photosynthesis in response to salinity and submergence in two Rhizophoraceae mangroves adapted to different tidal elevations

Abstract

Abstract Mangrove ecosystems are vulnerable to rising sea levels. When the sea level rises, the plants are exposed to increased salinity and tidal submergence. In Taiwan, the mangrove species Kandelia obovata, Liu & Yong and Rhizophora stylosa, Griffith grow in different habitats and at different elevations. To understand the response of photosynthesis to salinity and submergence in mangroves adapted to different tidal elevations, gas-exchange and chlorophyll fluorescence parameters were measured in K. obovata and R. stylosa under different salinity (20‰ and 40‰) and submergence treatments. The period of light induction of photosynthesis for the two mangrove species was greater than 60 min. In the induction process, the increase in photosystem efficiency was faster than the increase in stomatal opening, but CO2 fixation efficiency was restricted by stomatal conductance. The constraint of stomatal opening speed is related to the conservative water-use strategy developed in response to mangrove environments. Submergence increased the photosynthetic rate of K. obovata, but not that of R. stylosa. Although R. stylosa was more salt tolerant than K. obovata, R. stylosa was not submergence tolerant in a high salinity environment, which may be the reason for the higher intertidal elevations observed for R. stylosa in comparison to K. obovata. The photosynthetic rate and energy-dependent quenching (qE) of the two mangroves presented a negative relationship with photoinhibition, and high-salt treatment simultaneously reduced photosynthetic rate and qE. A decrease in the photosynthetic rate increased excess energy, whereas a decrease in qE decreased photoprotection; both of which increased photoinhibition. As the degree of photoinhibition can be easily measured in the field, it is a useful ecological monitoring index that provides a suitable reference for mangrove restoration, habitat construction, and ecological monitoring.