Evaluation of salinity tolerance in rice: Photosynthesis of excised leaves in relation to sodium accumulation

Salinity tolerance at the seedling stage of rice (Oryza sativa L.) is related to the ability to maintain photosynthesis under salinity. Sensitivity of photosynthesis to sodium accumulation in leaf blade was evaluated in 13 rice varieties differing in salinity tolerance at the seedling stage. Leaf blades were excised at the leaf base, the cut end immersed in 50 mM sodium chloride solution, and the rates of photosynthesis and transpiration were measured. Photosynthetic rate decreased in parallel to stomatal closure until leaf sodium content reached 3 to 5 mg g^-1. However, in the leaves with sodium content of over 3 to 5 mg g^-1, the transpiration rate was even, while photosynthetic rate decreased further due to another factor related to sodium accumulation in the leaf blade. Vulnerability of photosynthesis to sodium accumulation measured using excised leaves varied with the variety, similarly to that measured using the leaves of intact plants. These results indicated that salinity tolerance at the seedling stage in rice could be evaluated by measuring photosynthetic rate in excised leaf blades.