[Changes in the growth and photosynthesis of cotton seedlings under progressive drought after saltwater irrigation].

Pot grown cotton plants were watered with saltwater (NaCl solutions of different concentrations), followed by a duration of progressive drought stress. The changes in plantlet growth, photosynthetic rate, chlorophyll fluorescence were measured, and the water status of the plantlets, such as relative water content, water potential, osmotic potential, the Na+ and K+ contents in leaves during drought were measured and analyzed, in order to get an insight into the role of Na+ played in the adaptation of cotton to drought stress. The results showed that the growth of the plantlets was significantly inhibited, the net photosynthetic rates were remarkably lowered by the drought stress, but the plant height, biomass, net photosynthetic rate and Fv/Fm values in the cotton plants watered with 25-100 mmol x L(-1) x NaCl solution under drought stress were significantly higher than those watered with water under the same intensity of drought stress. Meanwhile, the soil and leaf relative water content, cell turgor, Na+ concentration in plants watered with 25-100 mmol x L(-1) NaCl solution were all significantly higher compared with that of plants watered with water, but the plant water potential and tissue osmotic potential were significantly lower with the decrease in tissue osmotic potential significantly correlated with the Na+ content. These results indicated that the presence of a moderate amount of Na+ in the soil could improve the water status of both the soil and the cotton plants, accelerate the absorption and accumulation of Na+ in the roots, lower the tissue osmotic potential, thereby enhancing the suction force of water in plant for maintaining a high cell turgor to maintain a relatively higher photosynthetic rate and growth rate. In this sense, it was suggested that the existence of a certain amount of NaCl in soil could effectively alleviate the adverse effects of drought on cotton.