Gas exchange, water relations, and ion concentrations of salt-stressed tomato and melon plants

Tomato and melon plants were grown in a greenhouse and irrigated with nutri ent solution having an EC of 2 dS m(-1) (control treatment) and 4, 6, and 8 dS m(-1), produced by adding NaCl to the control nutrient solution. After 84 days, leaf water relations, gas exchange parameters, and ion concentrati ons, as well as plant growth, were measured. Melon plants showed a greater reduction in shoot weight and leaf area than tomato at the two highest sali nity levels used (6 and 8 dS m(-1)). Net photosynthesis (Pn) in melon plant s tended to be lower than in tomato, for all saline treatments tested. Pn w as reduced by 32% in melon plants grown in nutrient solution having an EC o f 4 dS m(-1) relative to control plants, and no further decline occurred at higher EC levels. In tomato plants, the Pn decline occurred at EC of 6 dS m(-1), and no further reduction was detected at EC of 8 dS m(-1). The signi ficant reductions in Pn corresponded to similar leaf Cl- concentrations (ar ound 409 mmol kg(-1) dry weight) in both plant species. Net Pn and stomatal conductance were linearly correlated in both tomato and melon plants, Pn b eing more sensitive to changes in stomatal conductance (g(s)) in melon than in tomato leaves. The decline in the growth parameters caused by salinity in melon and tomato plants was influenced by other factors in addition to r eduction in Pn rates. Melon leaves accumulated larger amounts of Cl- than t omato, which caused a greater reduction in growth and a reduction in Pn at lower salinity levels than in tomato plants. These facts indicate that toma to is more salt-tolerant than melon.