Greenhouse-grown soybean (Glycine max L.) plants were subjected to water stress and diurnal changes in the contents of free amino acids were determined. In non-stressed plants with full water supply, the major parts of the free amino acids showed a similar diurnal variation, and the maximum contents were obtained between 11 a.m. and 2 p.m. except for aspartate. The maximum content of aspartate was obtained during the night. Glycine, serine and alanine, in particular, showed a light-sensitive response. In water-stressed plants, the water potential of the leaves fell from -12.8 bars at 7 a.m. to -22.2 bars at 7 p.m. and returned to -19.0 bars after sunset. When the leaf water potential fell, the total protein decreased with increase in the total free amino acids. Water stress induced a remarkable accumulation of free amino acids: histidine, valine, isoleucine, leucine, tyrosine, phenylalanine, and especially proline and asparagine. The maximum contents of proline and asparagine were 31 and 24% of the total free amino acid nitrogen, respectively. The contents of lysine, arginine, aspartate, glutamate, glutamine, glycine and alanine did not show a large increase. These results indicate a difference in turnover rate of amino acids after liberation by proteolysis. The ammonia content of the water-stressed plants was consistently low during 24 hr.
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Fukutoku et al. (1981) studied this question.
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