The Effect of Salinity upon Photosynthesis in Rice (Oryza sativa L.): Gas Exchange by Individual Leaves in relation to their Salt Content

Abstract

The effect of salinity upon net photosynthesis and transpiration by individual leaves of rice has been investigated by gas exchange measurements in seedlings at the five to six leaf stage. Salinity did not, initially, reduce net photosynthesis in the whole plant but only in the older leaves in which sodium accumulated. Analysis of the course of events in leaf four following salinization of the medium showed that net photosynthesis was inversely correlated with the sodium concentration in the leaf tissue. There was no evidence of a threshold effect; net photosynthesis declined linearly with increasing leaf sodium concentration and was reduced by 50% at only 05 mmol sodium per gram dry weight. The relationship between transpiration rate and leaf sodium concentration closely paralleled that for photosynthesis; there was no effect of leaf sodium concentration on the carbon dioxide concentration in the intercellular spaces, showing that sodium accumulation in the leaf affected stomatal aperture and carbon dioxide fixation simultaneously. Photosynthesis was reduced by half at a sodium concentration in the leaf which did not reduce the concentration of chlorophyll. The nature of the effect of salinity upon leaf gas exchange is discussed.