Sources of proline-nitrogen in water-stressed soybean (Glycine max). II. Fate of 15N-labelled protein

The absorption of nitrate, protein metabolism and the source of nitrogen for proline synthesis were studied in soybean ( Glycine max L. cv. Akisengoku) with ¹⁵N tracer technique under water stress conditions. The absorption of nitrate was sensitive to water stress and the flow of nitrate into the leaves completely ceased under severe stress conditions. Net protein loss from the water-stressed leaves was attributable to both a decrease in synthetic activity and a stimulation of protein degradation. Proline and asparagine accumulated extensively in the severely water-stressed plant tissues, especially in the younger green leaves. Fifty four % of the loss of leaf protein-¹⁵N during the stress period was balanced by a gain in ¹⁵N in the free amino acids, 41% being found in proline and asparagine. The increase in ¹⁵N content of the free proline was 3 times greater than the decrease in ¹⁵N content of the protein-bound proline in the leaf. The results indicate that the accumulation of proline in response to water stress was caused by enhanced synthesis and that the nitrogen source for this proline is the leaf protein. The possible association of these findings with stress tolerance is discussed.     

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

Water stress was imposed upon soybean plants (Glycine max L.)grown in a greenhouse by withholding irrigation for 10 daysafter 5 weeks of growth, and the changes under stress in thelevels of free amino acids, free ammonia and protein were determinedin detail. With a decrease in the leaf water potential, theprotein content gradually decreased, whereas the free ammoniacontent was relatively constant. Water stress induced an increasein the levels of free amino acids normally present in proteinsuch as isoleucine, leucine, valine, phenylalanine, glutamineand histidine, indicating that protein hydrolysis occurs understress. Proline accumulated only under severe stress (below–1.5 MPa) and attained 0.86% of the dry weight on day10 (–2.6 MPa). Asparagine also accumulated only undersevere stress (below –2.0 MPa). The concentration of glutamicacid, alanine, aspartic acid, serine, glycine and arginine remainedvirtually unchanged during the stress period. Total proline(protein-bound+free) first decreased during mild to moderatestress, and then increased over that of the well-irrigated controlplants at severe stress due to a remarkable accumulation offree proline. These findings indicate that some de novo synthesisof proline occurs under severe stress and that the nitrogensource for this proline synthesis may be protein. (Received July 4, 1981; Accepted September 11, 1981)