Ammonium improves tolerance to salinity stress in Sorghum bicolor plants

Sorghum bicolor plants were grown in five combinations of NO₃ ⁻:NH₄ ⁺ nutrition (100:0, 75:25, 50:50, 25:75 and 0:100) to test the hypothesis that nitrogen nutrition with ammonium (NH₄ ⁺) increases tolerance to salinity. Analyses were performed after 10 days of 75 mM NaCl stress. The K⁺ contents in the shoots and roots were reduced under salinity stress; however, salt-stressed plants from NO₃ ⁻:NH₄ ⁺ treatments of 100:0 and 0:100 showed higher K⁺ concentrations in the shoots. Under salinity, Na⁺ accumulation was severely limited in the presence of NH₄ ⁺ (0:100), which positively influenced K⁺/Na⁺ homeostasis. In parallel, NH₄ ⁺-fed plants displayed a substantial accumulation of N-rich amino acids (primarily glutamine and asparagine) in both tissues, which might be fundamental for alleviating NH₄ ⁺ toxicity. These responses were reflected in favorable gas exchange and growth. Whereas the photosynthetic rate (A) of plants treated with 100:0, 75:25, 50:50 and 25:75 ratios were unaltered or reduced under salinity stress, plants treated with a ratio 0:100 showed increases in A; this phenomenon was strongly correlated with the higher instantaneous carboxylation efficiency of Rubisco (A/Ci) and total N content in the shoots. As a result, although the leaf area and shoot and root dry mass from all nitrogen treatments were severely reduced through stress, the NH₄ ⁺ supply was effective to alleviate the harmful effects of salinity. The results clearly demonstrate that ammonium nutrition is more advantageous for the growth of S. bicolor under salinity than nitrate nutrition. Our findings suggest that improved plant acclimation induced through NH₄ ⁺ resulted from increased photosynthesis performance, reduced Na⁺ toxicity effects and major N-compound content.