Sources of Proline-nitrogen in Water-stressed Soybean (Glycine maxL.) I. Protein Metabolism and Proline Accumulation

摘要

Water stress was imposed upon soybean plants (Glycine maxL.) grown in a greenhouse by withholding irrigation for 10 days after 5 weeks of growth, and the changes under stress in the levels of free amino acids, free ammonia and protein were determined in detail. With a decrease in the leaf water potential, the protein content gradually decreased, whereas the free ammonia content was relatively constant. Water stress induced an increase in the levels of free amino acids normally present in protein such as isoleucine, leucine, valine, phenylalanine, glutamine and histidine, indicating that protein hydrolysis occurs under stress. Proline accumulated only under severe stress (below−1.5 MPa) and attained 0.86of the dry weight on day 10 (−2.6 MPa). Asparagine also accumulated only under severe stress (below−2.0 MPa). The concentration of glutamic acid, alanine, aspartic acid, serine, glycine and arginine remained virtually unchanged during the stress period. Total proline (protein-bound+free) first decreased during mild to moderate stress, and then increased over that of the well-irrigated control plants at severe stress due to a remarkable accumulation of free proline. These findings indicate that some de novo synthesis of proline occurs under severe stress and that the nitrogen source for this proline synthesis may be pr