Efficiency of C4 photosynthesis in Atriplex lentiformis under salinity stress

Photosynthetic gas exchange, carboxylase activities, and leaf tissue carbon isotope discrimination (Delta) were measured in Atriplex lentiformis (Torr.) Wats. (saltbush) plants grown in a glasshouse at five levels of salinity ranging from 0 to 600 mM NaCl. The net CO2 assimilation rate decreased by 64% from the lowest to the highest level of salinity imposed. The quantum yield for CO2 uptake was maximal in plants grown at 50 mM NaCl, and decreased sharply above and below this salinity level. The ratio of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity to that of phosphoenolpyruvate carboxylase (PEPC) decreased from 0.96 in plants grown at 0 mM NaCl to 0.37 in plants grown at 600 mM NaCl because PEPC activity on a leaf area basis increased linearly with increasing salinity, while Rubisco activity remained relatively constant. Compensatory changes in the leaf area/dry weight ratio and area-based leaf N content with increasing salinity suggested that the linear increase in PEPC activity was a passive response to increasing area-based leaf N content, whereas Rubisco activity on a leaf N basis actually dropped sharply. Relative leakage of CO2 from the bundle sheath, calculated from measurements of gas exchange and foliar Delta values, increased with increasing salinity in parallel with the decrease in the ratio of C-3 cycle activity relative to C-4 cycle activity. These results suggest that salinity stress diminished the inherent efficiency of the C-4 CO2 concentrating mechanism in addition to reducing net CO2 uptake.