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Perception of salt stress in plant cells induces a change in the free cytosolic Ca 2+ , Ca 2+ cyt , which transfers downstream reactions toward salt tolerance. Changes in cytosolic H + concentration, H + cyt , are closely linked to the Ca 2+ cyt dynamics under various stress signals. In this study, salt‐induced changes in Ca 2+ cyt , and H + cyt and vacuolar H + concentrations were monitored in single protoplasts of rice ( Oryza sativa L. indica cvs. Pokkali and BRRI Dhan29) by fluorescence microscopy. Changes in cytosolic Ca 2+ and H + were detected by use of the fluorescent dyes acetoxy methyl ester of calcium‐binding benzofuran and acetoxy methyl ester of 2′, 7′‐bis‐(2‐carboxyethyl)‐5‐(and‐6) carboxyfluorescein, respectively, and for vacuolar pH, fluorescent 6‐carboxyfluorescein and confocal microscopy were used. Addition of NaCl induced a higher increase in Ca 2+ cyt in the salt‐tolerant cv. Pokkali than in the salt‐sensitive cv. BRRI Dhan29. From inhibitor studies, we conclude that the internal stores appear to be the major source for Ca 2+ cyt increase in Pokkali, although the apoplast is more important in BRRI Dhan29. The Ca 2+ cyt measurements in rice also suggest that Na + should be sensed inside the cytosol, before any increase in Ca 2+ cyt occurs. Moreover, our results with individual mesophyll protoplasts suggest that ionic stress causes an increase in Ca 2+ cyt and that osmotic stress sharply decreases Ca 2+ cyt in rice. The pH cyt was differently shifted in the two rice cultivars in response to salt stress and may be coupled to different activities of the H + ‐ATPases. The changes in vacuolar pH were correlated with the expressional analysis of rice vacuolar H + ‐ATPase in these two rice cultivars.
Kader et al. (2007) studied this question.