Respostas fisiológicas e metabólicas mediadas pela fonte de nitrogênio: papel do NH4+ na tolerância de plantas de sorgo sob salinidade

Abstract

Salinity is an abiotic factor that most limits plant growth and development, causing metabolic, morphological and molecular disorders, it is photosynthesis one of the physiological processes most affected by stress. Recent studies report the beneficial action of nitrogen sources, especially NH4+, reducing these deleterious effects through induced acclimatization mechanisms that confer greater stress tolerance. However, their effects on photosynthetic efficiency and metabolite regulation are still poorly understood. Therefore, this study aimed to test the hypothesis that acclimatization to NH4+ nutrition triggers defense mechanisms and causes metabolic changes that confer greater salt tolerance. Herein, sorghum seedlings, cv. CSF 20, were grown in a modified Hoagland solution to contain 5 mM N in the form of sole NO3- or NH4+, or in the equimolar mixed-N (NO3-:NH4+) and, after 12 days of acclimatization, the plants were subjected to saline stress (75 mM NaCl) for more 12 days. Salinity severely reduced plant growth for all forms or N regimes nutrition, but this inhibitory effect was lower in NH4+- fed plants, which had higher leaf area and shoot dry mass. Besides, NH4+-fed plants had lower osmotic potential and electrolyte leakage and higher K+/Na+ ratio compared to other N nutrition under saline conditions. Although NH4+ nutrition induced high basal H2O2 content, no damage to chloroplast integrity was observed, even after saline stress. Additionally, NH4+ nutrition promoted better photosynthetic performance compared to other N forms under salinity. This was related to higher CO2 uptake rate and rubisco carboxylation efficiency (A/Ci), as well as maintenance of photosystem II efficiency. Salinity caused several changes in the metabolic profile of sorghum leaves. However, the degree of variation, as well as the profile of altered metabolites was dependent on the N source. Amino acids, organic acids, sugars, polyols, and other groups of metabolites were significantly increased or decreased by salinity. Plants fed with mixed N showed increases in several amino acids, which may be associated with protein and chlorophyll degradation, which can be evidenced by the loss of chloroplast integrity and significant reductions in CO2 assimilation and photochemical efficiency. The increase of proline content was observed under saline conditions in plants fed with mixed N and sole NO3- nutrition, while other metabolites (sucrose, trehalose, maltitol) were responsible for osmotic adjustment in plants grown with NH4+ nutrition. Asparagine had an important contribution to the separation of metabolic profiles among different N sources, as well as was responsive to salt stress in all N nutrition. Under NH4+ nutrition, salinity increased the content of ascorbic acid and dehydroascorbic acid, which may have a significant effect on cellular redox homeostasis and the protection of chloroplasts against oxidative damage. Overall, NH4+-fed plants exhibited few changes in metabolites compared to other N sources after salt stress. This may be related to previous changes during acclimatization to NH4+, allowing quick responses to secondary stresses, such as salinity. Therefore, NH4+ nutrition was able to activate mechanisms involved in the maintenance of photosynthetic efficiency and regulation of important metabolites, which attenuated the deleterious effects of salinity on sorghum plants.