Soil salinity effects on transpiration and net photosynthetic rates, stomatal conductance and Na+ and Cl- contents in durum wheat

Leaf gas exchange, plant growth and leaf ion content were measured in wheat (Triticum durum L. cv. HD 4502) exposed to steady-state salinities (1.6, 12.0 and 16.0 dS m(-1)) for 8 weeks. Salinity reduced leaf area and number of tillers, and increased Na+ and Cl- concentrations in leaves. Leaf-to-leaf gradients of these ions were observed. The oldest leaf contained 6 to 8 times more Na+ and Cl- than the flag leaf. Net photosynthetic rate (P-N), transpiration rate (E) and stomatal conductance (g(s)) were the highest in flag leaf, declined in the middle and fully expanded leaves, and were minimum in the oldest leaves. These processes were reduced by salinity with similar leaf-to-leaf gradients. Intercellular CO2 concentrations in the older leaves were higher than in the flag leaf in non-saline plants, and increased similarly with salinity. Leaf age was the major factor in reducing P-N, and senescence processes were promoted by salinity.