Changes in the Water Balance and Photosynthesis of Onion, Bean and Cotton Plants under Saline Conditions

The osmotic concentration (osmotic potential) of onion leaf sap did not adjust to chloride salinity, and consequently water potential, turgor, stomatal aperture and transpiration were reduced. Although osmotic concentration of bean and cotton leaf sap did adjust to a saline root medium and turgor was no less in the salinized plants than in the controls, stomata of the salinized plants remained only partly open and transpiration was reduced. Net photosynthesis of onion plants was reduced by salinity (this effect being much enhanced in a hot dry atmosphere) but it could be rapidly raised to the level of the controls by inducing elevated leaf turgor. Stomatal closure was initially responsible for most of the ～30 % reduction in photosynthesis of salinized beans. This was due to interference with CO₂ diffusion and could be overcome by raising the CO₂ concentration in the air. At a later stage of growth, salinity affected the light reaction of bean photosynthesis, and elevation of the air CO₂ had little effect. Closure of stomata of salinized cotton plants had only a relatively small effect on net photosynthesis. Light intensity and CO₂ concentration experiments showed that salinity was reducing the photosynthesis of cotton leaves mainly by affecting the light reaction of photosynthesis. It is concluded that chloride salinity does affect the water balance and rate of photosynthesis of plants and that the nature and degree of the effects will depend upon climatic conditions and may be very different between plant species and in the same species at different periods of growth.