Amino Acid Metabolism in Young Pea Seedlings

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Abstract
Roots, shoots, and cotyledons of Pisum seedlings, 0-25 days old, were analyzed for starch, protein, and soluble nitrogenous components. The gain of soluble N, fa shoots, roots, and cotyledons, plus the gain fa protein N in the root and shoot, could easily be accounted for by the loss of protein N from the cotyledon. In 7-day-old shoots, homoserine (HS) accounted for 70% of the free amino acids. Labeled aspartate, glutamate, homoserine, leucine, and glucose were supplied to cotyledons of young seedlings. Three-fourths of the label from L-glutamate U-C14 and L-aspartate-U-C14 supplied to 2-day-old seedlings was recovered in CO2 and HS after 24 hours. While the labeled HS level in the cotyledons had begun to decrease at this time it was increasing in the root and shoot. Labeled HS supplied to 2- and 10-day-old cotyledons was translocated to the root and shoot but not catabolized. L-aspartic acid U-C14 supplied to 10-day-old cotyledons was respired as CO2 with very little recovery of label in HS. These data indicate a low HS level in older seedlings is a result of decreased synthesis rather than greater utilization of HS. Major portions of glucose-C14 and L-leucine-C14 supplied to cotyledons were converted to CO2, starch, and protein, but the largest single repository of C14 in the soluble fraction was HS.