Effect of increased salinity on CO2 assimilation, O 2 evolution and the δ (13)C values of leaves of Plantago maritima L. developed at low and high NaCl levels.

The effect of increased salinity on photosynthesis was studied in leaves of Plantago maritima L. that developed while plants were at low and high NaCl levels. In leaves that developed while plants were grown at 50 mol·m(-3), exposure to 200 and 350 mol·m(-3) NaCl resulted in reductions in net CO2 assimilation and stomatal conductance. The decline in CO2 assimilation in plants at 200 and 350 mol·m(-3) NaCl occurred almost exclusively at high intercellular CO2 concentrations. The initial slope of the CO2 assimilation-intercellular CO2 (A-C i) curve, determined after salinity was increased, was identical or very similar to that measured initially. In contrast to the reductions observed in CO2 assimilation, there were no significant differences in O2 evolution rates measured at 5% CO2 among leaves from plants exposed to higher salinity and plants remaining at low salinity.Leaves that developed while plants were at increased salinity levels also had significantly lower net CO2 assimilation rates than plants remaining at 50 mol·m(-3) NaCl. The lower CO2 assimilation rates in plants grown at 200 and 350 mol·m(-3) NaCl were a result of reduced stomatal conductance and low intercellular CO2 concentration. There were no significant differences among treatments for O2 evolution rates measured at high CO2 levels. The increased stomatal limitation of photosynthesis was confirmed by measurements of the (13)C/(12)C composition of leaf tissue. Water-use efficiency was increased in the plants grown at high salinity.