Is salinity tolerance related to Na accumulation in Upland cotton (Gossypium hirsutum) seedlings?

Physiological responses to salt stress were studied in two cotton cultivars previously selected on the basis of growth under salinity. Plants were grown in nutrient solutions under controlled conditions. In the first experiment, the genotypes were grown at different salt concentrations (0, 100 and 200 mM NaCl) and growth rates, water contents and ion accumulation were determined. In a second experiment, both genotypes were grown at the same salt concentration (200 mM NaCl). Dry matter partitioning in individual leaves, stem and roots, water contents, specific leaf area (SLA), ion accumulation (K+, Na+, Cl-) and leaf water potentials were measured. Finally, an experiment with low salt levels (2.7 and 27 mM NaCl) was run to compare K and Na+ uptake and distribution. There were no differences in growth between the cultivars in the absence of salt stress, whereas under stress genotype Z407 had higher leaf area and dry matter accumulation than P792. Leaf water potential and leaf water content were lower in cv P792 than in cv Z407. There were no significant differences in the levels of Cl- accumulation between genotypes. The main feature of the tolerant genotype (Z407) was a higher accumulation of Naf in leaves and an apparent capacity for K+ redistribution to younger leaves. We postulate that the higher tolerance in Z407 is the result of several traits such as a higher Na+ uptake and water content. Adaptation through adequate, but tightly controlled ion uptake, typical of some halophytes, matched with efficient ion compartmentation and redistribution, would result in an improved water uptake capacity under salt stress and lead to maintenance of higher growth rates.