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

Abstract

Abstract Shifts in †13C of the graminaceous C3 halophyte Puccinellia nuttalliana (Schultes) Hitch. can be induced by salinization. To investigate this phenomenon, three approaches were taken: assay of carboxylases, CO2‐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 C3 plants. When plants were grown at four NaCl concentrations under atmospheres of 310 and 1300 cm3 m−3 CO2, the CO2‐enrichment enhanced the effects of salinity on †13C. This is consistent with a biophysical explanation for salt‐induced shifts in †13C, whereby there is a steepening of the CO2 diffusion gradient into the leaf. Gas exchange analysis indicated that intercellular CO2 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.