Influence of inorganic nitrogen sources on K+/Na+ homeostasis and salt tolerance in sorghum plants

This work aimed to study the regulation of K+/Na+ homeostasis and the physiological responses of salt-treated sorghum plants [Sorghum bicolor (L.) Moench] grown with different inorganic nitrogen (N) sources. Four days after sowing (DAS), the plants were transferred to complete nutrient solutions containing 0.75 mM K+ and 5 mM N, supplied as either NO3− or NH4+. Twelve DAS, the plants were subjected to salt stress with 75 mM NaCl, which was applied in two doses of 37.5 mM. The plants were harvested on the third and seventh days after the exposure to NaCl. Under the salt stress conditions, the reduction of K+ concentrations in the shoot and roots was higher in the culture with NO3− than with NH4+. However, the more conspicuous effect of N was on the Na+ accumulation, which was severely limited in the presence of NH4+. This ionic regulation had a positive influence on the K+/Na+ ratio and the selective absorption and transport of K+ in the plants grown with NH4+. Under control and salt stress conditions, higher accumulation of free amino acids and soluble proteins was promoted in NH4+ grown roots than NO3− grown roots at both harvesting time, whereas higher accumulation of soluble sugars was observed only at 7 days of salt stress exposure. Unlike the NH4+ grown plants, the gas exchanges of the NO3− grown plants were reduced after 7 days of salt stress. These results suggest that external NH4+ may limit Na+ accumulation in sorghum, which could contribute to improving its physiological and metabolic responses to salt stress.