Comparative characterization and adaptive mechanisms of salt tolerance of different wheat genotypes

The study of adaptive mechanisms of salt tolerance for the identification and selection of resistant wheat genotypes remains an urgent task since the area of lands with high salinity is constantly increasing worldwide. This work was focused on the processes of accumulation and excretion of toxic ions from the roots and leaves of different wheat genotypes and the effect of these ions on the state of plant tissues. It was shown that in the studied varieties of durum wheat, with an increase in salinity, the size of the root system decreased and, as a result, the absorption of toxic Na+ ions decreased. In soft wheat, ionic conductivity increased and the excretion of Na+ ions increased too. We compared the manifestations of salt stress caused by high concentrations of NaCl, and the mechanisms of salt tolerance and adaptation to its toxic effect in varieties of different types of wheat in the conditions of the Orenburg region in the field. The initial assessment of salt tolerance of wheat varieties was carried out according to the degree of growth inhibition by sodium chloride. Based on the initial assessment, wheat varieties differing in salt tolerance were selected, i.e., two durum wheat ( Triticum durum Desf.) varieties Zolotaya and Orenburgskaya 10 and two soft wheat ( Triticum aestivum L.) varieties Ulyanovskaia 105 and Orenburgskaya 22. The final assessment of the salt tolerance of the selected varieties was carried out in 2022, growing plants in small-scale (1.8 m2 plots) field trials in 3 repetitions of each variant. To create salt stress, sodium chloride was applied for root watering after emergence and before the tillering phase in the form of a solution (200 mM NaCl). Plants grown without the addition of salts served as control. The adaptive mechanisms of resistance of different wheat genotypes to sodium chloride were studied using biochemical, molecular genetic and light-optical methods of analysis. The defense mechanisms of plants against the action of salt stress include blocking and excretion of Na+ and Cl- from the cell cytoplasm, activation of the antioxidant defense system, and an increase in plant tolerance due to various mechanisms of regulation of gene activity. Our results show that the Ulyanovska 105 and Orenburgskaya 22 wheat cultivars retain the selectivity of K+ ions with respect to Na+ ions in the roots and maintain a higher K+/Na+ ratio (4.12 and 4.18, respectively) under stress compared to with varieties Zolotaya and Orenburgskaya 10 (1.26 and 3.75, respectively). The regulation of ion flows is provided by ion transporters. Increased activity of the genes of two classes of НKT transporters in the Orenburgskaya 22 variety contributes to a greater excretion of Cl- and Na+, and, conversely, in the Zolotaya variety, accumulation of toxic ions occurred, which leads to a decrease in the content of chlorophylls a and b (Chl a and Chl b). The lower Chl a/Chl b ratio in Orenburgskaya 22 than in Zolotaya 22 (2.03 vs. 2.43) indicates a high content of Chl b. This expands the light absorption spectrum, which contributes to an increase in photosynthesis. The difference in the accumulation of reactive oxygen species (ROS) indicates the initiation of different mechanisms of antioxidant defense in different wheat genotypes. Accumulation of ROS products in the roots of Ulyanovskaia 105 and Orenburgskaya 22 cultivars during salinization occurs more intensively in the zones of the cap and meristem, which is 1.3 times lower compared to the Zolotaya and Orenburgskaya 10 cultivars. In these wheat varieties, ROS products accumulate in all the studied zones, however, in the area of the cap is less than in the other zones. NaCl-sensitive wheat genotypes showed damage in root cells, while damage was minimal in resistant wheat genotypes. Salt stress in sensitive wheat varieties can lead to programmed cell death. The degradation of nucleic acids is a marker of the plant death. According to the degree of DNA degradation, the Zolotaya variety turned out to be the most unstable. The integrated approach used by us in this study made it possible to identify various mechanisms of resistance to salt stress in wheat genotypes. The obtained results can be in demand by breeders and specialists of the agrarian complex.