Effects of Salinity on the Growth and Survival of the Seedlings of Mangrove, Rhizophora stylosa

Aims: There are contradictory views on whether mangroves are obligate halophytes or facultative halophytes. In this study, we examined: (1) if seedlings of a mangrove, Rhizophora stylosa, require NaCl for their survival and (2) whether the growth response of R. stylosa seedlings under varying salinities was explained from the context of carbon economics.
Methodology: Seedlings of R. stylosa were hydroponically grown under varying salinities (0 - 480 mMNaCl) and their growth, mortality, photosynthetic and respiration rates were analyzed.
Results: Most of the seedlings grown under NaCl-free condition died during the 34-week culture, demonstrating their salt dependency. The best growth was accomplished under moderate salinity (240 mMNaCl) with highest stem elongation, maximum biomass gain and lowest leaf mortality. Whole-plant photosynthetic production was highest under the moderate salinity and declined towards high and low salinity ranges whereas whole-plant respiration did not increase towards high and low salinity ranges. The lower photosynthetic production under high salinity involved reductions in both leaf area and photosynthetic potential per area while the lower photosynthetic production under low-salinity involved reduction in leaf area only.
Conclusion: Rhizophora stylosa appeared to require salt for survival. The maximal growth under moderate salinity might be explained by reduced photosynthetic production under low and high salinity ranges. The reduced photosynthetic potential and leaf area causing lower photosynthetic production under high salinity might be ultimately explained by the accumulation of excessive salt in leaf cells. In contrast, the ultimate causes for the reduced leaf area and increased mortality under low salinity remained unclear. Several possible mechanisms are discussed in relation to ion metabolism.