Effects of salt stress and zinc fertilization on some biochemical changes and growth parameters in tomato

Tomato plants (Lycopersicon esculentum Mill.) were grown in potted soils exposed to six salinity levels (0, 3, 5, 7, 9 and 11 dS/m), four salt compositions (C-1= 100% NaCl, C-2=60% NaCl and 40% Na2SO4, C-3=40% NaCl and 60% Na2SO4 and C-4=100% Na2SO4 on chemical equivalent basis) and four zinc (Zn) levels (0, 5, 10, and 20 mg/kg as ZnSO4, 7H(2)O). Proline did not start to accumulate in tomato leaves up to the soil salinity of 7 dS/m, and above this threshold, it increased linearly with increasing salinity. However, plants exposed to C, treatment contained more proline than those subjected to the other salt compositions. Furthermore, proline did not accumulate until concentration of Na in tomato leaves reached a threshold values of 0.40, 0.38 and 0.25% for C-1, C-2 and C-3 salinity regimes, respectively. The results of the present study indicate that sugar concentration de creased by increasing soil salinity and increased with, Zn application. The harmful effect of Cl and Na accumulation on sugar metabolism and/or a rise in the plant respiration might have been responsible for such salt stress behavior. Although, salt treatment lowered transpiration rate and leaf area, a given increment of soil salinity produced larger decrement in leaf size in Q-treated plants than in tomato seedlings exposed to C-4 salinity regime. Finally, our results indicated that Cl accumulation was more toxic to tomato growth than that of Na.