Photosynthetic and Stomatal Responses of Spinach Leaves to Salt Stress

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Abstract
The gas exchange of spinach plants, salt-stressed by adding NaCl to the nutrient solution in increments of 25 millimolar per day to a final concentration of 200 millimolar, was studied 3 weeks after starting NaCl treatment. Photosynthesis became light saturated at 1100 to 1400 micromoles per square meter per second in salt-treated plants and at approximately 2000 micromoles per square meter per second in control plants. Photosynthetic capacity of the mesophyll measured as a function of intercellular partial pressure of CO2 at the light intensity prevailing during growth and at light saturation were both decreased in the salttreated plants. The CO2 compensation points and relative enhancements of photosynthesis at low O2 were not affected by salinity. The lower photosynthetic rates in salt-treated leaves at 450 micromoles per square meter per second were associated with a 70% reduction in stomatal conductance and low intercellular CO2 (219 microbars; cf. 285 microbars for controls). Increasing photon flux density to light saturation extended the linear portions of the CO2 response curves, increased stomatal conductances, increased intercellular CO2 in the salt-treated plants, but lowered it in controls, and accentuated differences in photosynthetic rate (area basis) between the treatments.

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