Contribution of proline and inorganic solutes to osmotic adjustment in cotton under salt stress

Physiological responses to salt stress were investigated in two cotton (Gossypium hirsutum L.) cultivars (Pora and Guazuncho) grown hydroponically under various concentrations of NaCl. Dry matter partitioning, plant water relations, mineral composition and proline content were studied. Proline and inorganic solutes were measured to determine their relative contribution to osmotic adjustment. Both leaf water potential (Psi (w)) and osmotic potential (Psi (s)) decreased in response to NaCl levels. Although Psi (w) and Psi (s), decreased during salt stress, pressure potential (Psi (p)) remained between 0.5 to 0.7 MPa in control and all NaCl treatments, even under 200 mol m(-3) NaCl. Increased NaCl levels resulted in a significant decrease in root, shoot and leaf growth biomass. Root/shoot ratio increased in response to salt stress. The responses of both cultivars to NaCl stress were similar. Increasing salinity levels increased plant Na+ and Cl-. Potassium level remained stable in the leaves and decreased in the roots with increasing salinity. Salinity decreased Ca2+ and Mg2+ concentrations in leaves but did not affect the root levels of these nutrients. The K/Na selectivity ratio was much greater in the saline treated plants than in the control plants. Osmotic adjustment of roots and leaves was predominantly due to Nat and Cl- accumulation; the contribution of proline to the osmotic adjustment seemed to be less important in these cotton cultivars.