Role of sodium in the ABA-mediated long-term growth response of bean to salt stress

Long-term salt effects on plant growth have often been related to direct ion toxicity due to the accumulation of high ion concentrations in plant tissue. This work examines the relative importance of endogenous ABA, as well as Na+ and Cl- toxicity, in the inhibition of leaf growth and photosynthesis, in bean plants grown at 1, 25, 50 and 75 mM NaCl until the fruit-bearing stage. All salt-treated plants showed very high leaf Cl- concentrations, with little difference between plants exposed to 50 or 75 mM NaCl. The 25 and 50 mM salt-treated plants were able to successfully exclude Na+ From their leaves, and only suffered an initial decline in the rate of leaf growth. Plants exposed to 75 mM NaCl showed an increase in Na+ leaf concentrations with an accompanying decrease in growth and photosynthesis as salt exposure progressed. A high correlation was found between leaf Na+ and leaf growth. Leaf ABA significantly increased with salt supply, and was highly correlated with both leaf Na+ and leaf growth. Our results suggest that in bean plants under long-term salt stress, leaf ABA may participate in the regulation of leaf growth, and leaf Na+ would be at least partly responsible for increased ABA levels.