Effects of salinity on the growth, photosynthesis and mineral constituents of the mangrove Rhizophora apiculata L. seedlings

Abstract

The effects of salinity on growth, chlorophyll content, photosynthetic rates, as measured by leaf stomatal conductance and leaf chlorophyll fluorescence induction, and ion accumulation in the mangrove plant Rhizophora apiculata were determined. The following questions were addressed:                (1) What effect does salinity have on growth responses at different ages? (2) Is R. apiculata an ion accumulator? (3) Does R. apiculata accumulate chlorophyll, net photosynthesis and chlorophyll fluorescence in response to salinity? Rhizophora apiculata  plants were grown in pots at 0, 25, 50, 75 and 100 % in sand culture in a plant growth chamber and plants were harvested after 30 and 90 d. Plant total fresh and dry weight and moisture content was significantly inhibited at 75 per cent seawater. Seawater salinity stimulated the chlorophyll contents and they were increased upto 75 per cent seawater.  The net photosynthesis increased with increasing salinity upto the optimal level and the CO₂ uptake rate was identical in this species at various salt concentrations. Even at extreme salinity, the CO₂ uptake was comparable to that non-saline control plants and CO₂ uptake could be correlated with the chlorophyll content. The photochemical activities such as PSI and PSII of the isolated cells increased upto the optimal salinity. The result of the chlorophyll fluorescence kinetics provided with additional proof to the finding of CO₂ exchange rate and photochemical activities.  The Na⁺ and Cl^{−} content in both shoots and roots increased with increases in salinity. Increased treatment levels of NaCl induced decreases in Ca⁺, P, K⁺, Mg^+   and N in plants.