RESPONSES OF PHOTOSYNTHESIS, LEAF CONDUCTANCE AND GROWTH TO DIFFERENT SALINITIES IN 3 COASTAL DUNE PLANTS

The effects of soil-water salinity on growth and photosynthesis of three coastal dune plants were examined by salt-treatment in order to clarify the causal relationship between salinity and plant distribution in a dune habitat. Plants were cultivated hydroponically at three salinity levels: 0, 10 and 100 mM NaCl. With the 100 mM salt treatment, Calystegia soldanella (C3 species) had the highest relative growth rate (RGR) (0.085 g g-1 d-1), followed by Carex kobomugi (C3) (0.066), and Ischaemum anthephoroides (C4) (0.060). This order coincides with the distribution pattern of the three species on coastal dunes; Calystegia soldanella is generally distributed in more seaward areas whereas I. anthephoroides occurs further inland. The order of RGR was determined exclusively by leaf area ratio (LAR) among the three species. Due to its C4 pathway, I. anthephoroides had higher net photosynthetic rate (Pn) and net assimilation rate (NAR) than the two C3 plants at all NaCl concentrations, despite its low RGR. This apparent discrepancy is explainable by differences of LAR among the three species; LAR of I. anthephoroides was lowest, and about half that of Calystegia soldanella. These results suggest that LAR is one of the main determinants of salt tolerance based on RGR, whereas Pn or NAR may not be significant.