Evaluation of chlorophyll fluorescence parameters and proline content in tomato seedlings grown under different salt stress conditions

Chlorophyll fluorescence (CF) parameters, plant growth, and chlorophyll and proline content were investigated in the seedlings of four tomato (Solanum lycopersicum L.) cultivars ('Dafnis', 'Maxifort', 'BKO', and 'B-blocking') exposed to 0, 50, 100, 150, 200, 300, and 400 mM NaCl concentrations for 15 days. The maximum quantum yield (Fv/Fm) showed progressively decreased at 300 and 400 mM salt concentrations in all the cultivars, with the highest decrement in Dafnis. In contrast, the quantum yield of nonregulated energy dissipation in photosystem II (PSII) [Y(NO)] showed a reverse accumulation pattern at higher salt concentrations as the treatment time increased, regardless of the genotype. Other CF parameters, such as the coefficient of nonphotochemical quenching, excitation transfer efficiency from antenna pigments to the reaction center of PSII in the light-adapted state, effective quantum yield of photochemical energy conversion in PSII, nonphotochemical quenching, and the ratio of chlorophyll fluorescence decrease were lower at higher salt concentrations; however, some inconsistencies were observed at 5 and 10 days after the treatment time depending upon the cultivar and salt concentration. Growth parameters such as plant height, stem diameter, number of nodes, and shoot fresh and dry weight decreased by about 2.2-3.2, 1.1-1.6, 1.7-2.0, 3.1-6.8, and 1.8-3.2 times, respectively, in the salt-stressed cultivars. In the seedlings, the proline content significantly increased, with the highest increment observed in the seedlings treated with 100-200 mM salt concentration. Both chlorophyll a and b decreased as the salt concentration increased. Among the four cultivars, Dafnis showed the highest response to the salt concentrations with respect to all the parameters. In conclusion, our results suggest that the CF parameters Fv/Fm and Y(NO) can be used as indices for screening tomato genotypes with salt tolerance.