Effects of sulphur nutrition on nitrogen and carbon metabolism in lucerne (Medicago sativa L.)

Abstract

Studies were conducted with one‐year‐old plants of the perennial legume, lucerne (Medicago sativa L.), to determine the effects of various levels of S (0.3, 1.5, and 7.5 mM) on N2‐fixation, photosynthesis, herbage free amino acid pools and protein amino acids, levels of protein, and activities of key enzymes of leaf nitrogen and carbon metabolism. Sulphur deficiencies in the 0.3 mM S treatment, as determined by N:S ratios, did not appear until the second growth cycle. This treatment was severely S‐deficient by the end of the third growth cycle. Sulphur deficiencies did not occur in the other two treatments over three cycles of growth. By the middle of the second growth cycle acetylene reduction rates of plants grown with 0.3 mM S were lower than those of other treatments. At the end of the third growth cycle acetylene reduction rates of the 0.3 mM S treatment were ca. 30–35% that of the other treatments. There was no effect of S‐deficiency on photosynthesis, levels of leaf NADP‐isocitrate dehydrogenase and glutamine synthetase activity, or concentrations of leaf protein over the course of the experiments. Levels of NAD‐glutamate dehydrogenase, NAD‐malate dehydrogenase, and glutamate oxaloacetate trans‐aminase activity and concentrations of free methionine, arginine, urea, and ammonia increased, with S‐deficiency. Boron concentrations were very high (ca. 300 μg/g dry wt.) in S‐deficient plants. The mole percent methiomne in leaf protein decreased slightly and chlorophyll concentration decreased markedly with S‐deficiency. This study suggests that N2‐fixation is affected early during S‐deprivation of lucerne and that subsequent effects of S‐deficiency may be due to loss of fixed nitrogen.