Effects of salinity on nitrate, total nitrogen, soluble protein and free amino add levels in tomato plants

Abstract

The effects of salinity on nitrogen compounds were studied in three tomato (L. esc- ulentum Mill.) genotypes of different salt tolerance. The plants were grown under controlled conditions, and the salt treatments (0, 70 and 140 raM NaCl) were applied for three (Harvest 1) and ten (Harvest 2) weeks. The effects of salinity on total N and particularly N03 concentrations depended partly on the NaCl level and duration of the stress, but mainly on the different degrees of salt tolerance of the genotypes. In Harvest 1, the most tolerant genotype (GC-72) showed the highest N03 increase in the roots and no decreases in stem and leaf with increasing salinity; the intermediate-tolerant genotype (P-73) showed a similar response to that of GC-72 only at 70 mM NaCl. However, the most sensitive genotype (Volgogradskij) showed the greatest reductions in stem and leaf N03 concentrations with salinity. With longer durations of stress the different responses between the more tolerant genotypes were less evident; only Volgogradskij continued to show the highest decreases in stem and leaf N03 concentrations. A restriction of N03 transport from the root to the shoot was noted in the plants of the more tolerant genotypes treated for three weeks. This capacity for retention disappeared when the salinity induced reductions in the total N03 contents in the plants, as in the harvest 2. There was an inverse relation between N03 and Cl accumulations in shoots of all genotypes and its slope decreased with the salt sensitivity of the genotype. There was no relation between the leaf protein content and the salt tolerance of the tomato genotypes. In Harvest 1, a higher accumulation of amino acids, especially proline, was found in the leaves of the more tolerant genotypes at 140 mM NaCl. However, in Harvest 2, a similar accumulation of leaf proline was found in all genotypes, independent of their salt tolerances, and the other amino acid contents remained similar or else decreased with salinity.