Impact of salinity induced by high concentration of NaCl or by high concentration of nutrients on tomato plants

Salinity is a constantly aggravating agricultural problem due to the increasing demand for irrigation water and the concomitant overexploitation of water resources. Large amounts of saline waters worldwide are rich in Ca; however, in coastal regions the problem arises mainly from high concentration of NaCl in the available water resources. In order to evaluate the extent the salt speciation affects plant growth, photosynthesis and production of tomato, a greenhouse experiment was conducted. Plants were supplied with three different nutrient solutions: (1) a standard nutrient solution (SNS) with an electrical conductivity (EC) of 2.5 dS m-1, (2) a saline nutrient solution (12.5 dS m-1), which was obtained by adding 100 mM of NaCl to the SNS and (3) a highly concentrated nutrient solution (12.5 dS m-1). The results confirmed the negative impact of salinity on growth characteristics and production. Moreover, tomato plants that were grown under the higher EC values (12.5 dS m-1) caused by excess amounts of macronutrients showed a 20% less decrease in yield than those grown under NaCl-induced salinity. Although stomatal conductance, transpiration rate and intercellular CO2 decreased significantly during the exposure of plants to salinity, the rate of photosynthesis was maintained at the same levels as the control. In addition, the total chlorophyll content of leaves per unit of leaf area increased due to the reduction of the leaf area. In conclusion, it seems that the growth and production of tomato are influenced by salinity, but at high EC levels, salt speciation is of major importance. The lower susceptibility of tomato plants to nutrient-induced salinity in comparison to equally high EC levels caused by NaCl is ascribed to differences in osmotic pressure in combination with the occurrence of specific ion toxicity in the case of NaCl-induced salinity.