Salinity effects on CO 2 assimilation and diffusive conductance of cowpea leaves

The effect of NaCl salinity at concentrations of 43–173 mM in nutrient solution on net gas exchange of attached cowpea [Vigna unguiculata (L.) Walp cv. California Black‐eye No. 5 (CB5)] leaves was investigated under both greenhouse and growth chamber conditions. There was a marked decrease in leaf conductance to water vapor after exposure to low salinity levels and a slighter decrease when salinity levels were higher. The decrease in net assimilation was much more gradual throughout the entire salinity range. The altered responses of net assimilation and leaf conductance to salinity were more evident at a high light intensity. A decrease in intercellular partial CO 2 pressure [p(CO 2 )] was found at the low and intermediate salinity levels but not at the high level. These findings suggest that CO, assimilation was mainly controlled by stomatal conductance and the fixation of CO, might have been increased due to stimulated biochemical activity or to higher chlorophyll concentration per unit leaf area. A decrease in assimilation was already found one day after salinization and pro‐ceeded up to 4 days when it was inhibited by 50% at 43 mM NaCl and up to 85% at 173 mM. The decrease in transpiration was larger than the decrease in net assimila‐tion, and both were attributed to osmotic stress. Partial recovery was found thereaf‐ter and new steady‐state rates, in the range of 55 to 100% of the control, were then obtained for salinity levels between 43 and 130 mM. Inhibition of net CO, assimila‐tion at this stage was attributed partly to a specific sodium effect and partly to plant water status. A linear relationship between leaf sodium content and net photosynthe‐sis was also evident at this stage. Net CO, assimilation recovered more completely than transpiration when salt stress was removed, but at 173 mM NaCl recovery was neglible.