Leaf characteristics and gas exchange of the mangrove Laguncularia racemosa as affected by salinity

Under constant salinity we analysed the leaf characteristics of Laguncularia racemosa (L.) Gaertn. in combination with gas exchange and carbon isotopic composition to estimate leaf water-use efficiency (WUE) and potential nitrogen-use efficiency (NUE). NaCl was not added to the control plants and the others were maintained at salinities of 15 and 30 parts per thousand (S-0, S-15, and S-30, respectively). Leaf succulence, sodium (Na), nitrogen (N), and chlorophyll (Chl) contents increased under salinity. Salinity had a negative impact on net photosynthetic rate P-N) and stomatal conductance (g(s)) at high and moderated irradiances. Potential NUE declined significantly (p<0.05) with salinity by 37 and 58 % at S-15 and S-30, respectively, compared to S-0 plants. Conversely, compared to S-0 plants, P-N/g(s) increased under saline conditions by 12% (S-15) and 50 % (S-30). Thus, VXE inferred from P-N/g(s) was consistent with salinity improved short-term WUE. Long-term leaf WUE was also enhanced by salinity as suggested by significantly increased leaf delta C-13 with salinity. Improved WUE under salinity explains the eco-physiological success of mangrove species under increasing salinity. Conversely, decline in NUE may pose a problem for L. racemosa under hyper-saline environments regardless of N availability.