Photosynthesis and the influence of CO₂‐enrichment on δ¹³C values in a C₃ halophyte

Shifts in † 13 C of the graminaceous C 3 halophyte Puccinellia nuttalliana (Schultes) Hitch. can be induced by salinization. To investigate this phenomenon, three approaches were taken: assay of carboxylases, CO 2 ‐enrichment studies, and gas exchange analysis. Although ribulose‐1,5‐bisphosphate carboxylase activity decreased with salinity, phosphoenolpyruvate carboxylase activity did not increase and its levels were not atypical of C 3 plants. When plants were grown at four NaCl concentrations under atmospheres of 310 and 1300 cm 3 m −3 CO 2 , the CO 2 ‐enrichment enhanced the effects of salinity on † 13 C. This is consistent with a biophysical explanation for salt‐induced shifts in † 13 C, whereby there is a steepening of the CO 2 diffusion gradient into the leaf. Gas exchange analysis indicated that intercellular CO 2 concentrations were depressed in the leaves of salt‐affected plants. This resulted from a greatly decreased stomatal conductance coupled with only small effects on intrinsic photosynthetic capacity. Water‐use efficiency was enhanced.