Varietal differences in the toxicity of sodium ions in rice leaves

Visual damage is commonly used as the criterion for assessment of salinity resistance in rice breeding programmes. The use of other indicators, such as sodium uptake, is being evaluated: a correlation between initial sodium uptake to the third leaf and varietal survival was established and the physiological basis of this examined. Chlorophyll was used as an indicator of metabolic status and the relationship between leaf sodium and chlorophyll concentrations investigated for nine varieties differing in their resistance to salinity. By sampling a population of leaves having a wide range of salt concentrations, inverse relationships between chlorophyll and sodium concentrations were established. The salt concentration in the leaf tissue which causes equivalent toxicity (50% loss of chlorophyll) differed 3‐fold amongst these varieties. Varieties showing the greatest tolerance to salt within the leaves were not necessarily those showing the greatest overall phenotypic resistance to salinity. The enzymes malate dehydrogenase and nitrate reductase were activated equally by sodium and potassium ions at 60–80 mol m −3 in both control and saline grown plants and severely inhibited at higher concentrations. If all the salt in the tissue of leaves with 50% chlorophyll remaining was in solution and uniformly distributed the concentration would be 135–500 mol m −3 . This is improbable and some level of compartmentation is likely. It is concluded that while initial sodium uptake has predictive value for salinity resistance of varieties, there are other characteristics which are masked by excess salt entry and require independent assessment; no single factor confers resistance.