Ammonium (15N) metabolism in cotton under salt stress

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Abstract
Plant growth and metabolism is impaired under stress conditions, resulting in decreased crop yields. The purpose of this investigation was to evaluate the NaCl stress effects on NH4+ metabolism in cotton plants at vegetative and reproductive stages of growth. Cotton (Gossypium hirsutum L.) plants grown in normal (control) and NaCl treated Hoagland solutions were analyzed for distribution of 15N in NH4+ plus amide-N, free .alpha.-amino-N, total soluble-N and protein-N after the plants were provided 15NH4NO3 in nutrient solutions for 6, 12 and 24 h. The concentration of protein-15N was enhanced under a low level of NaCl (-0.4 MPa osmotic potential) at the vegetative growth stage. The difference between the protein-15N concentration of the moderately salinized (-0.8 MPa) plants and the controls was not significant. A high level of NaCl (-1.2 MPa) significantly decreased protein-15N content of plants compared with the controls and any other level of salinity. The NaCl increased accumulation of NH4+ plus amide-N, free .alpha.-amino-N, and total soluble-N in cotton shoots, at both stages of growth. Low osmotic potential (high osmotic pressure) of the nutrient solution induced by excessive amounts of NaCl in nutrient solution inhibited NH4+ metabolism and decreased protein synthesis, thus resulting in accumulation of soluble N-compounds. The ionic effect probably contributed also to inhibition of protein synthesis.