Sources of Carbon for the Synthesis of Protein Amino Acids in Attached Photosynthesizing Wheat Leaves

Attached, rapidly growing wheat leaves were allowed to photoassimilate C 14 O 2 and C 12 O 2 alternately. Samples of leaves were collected after each period of photosynthesis and were analyzed for amounts and total activities of soluble sugars and amino acids, and protein amino acids. The leaves were also analyzed for protein nitrogen and amounts and total activities of respired carbon. Samples of roots were also collected and the amounts, total activities, and specific activities of their soluble compounds were determined. It was possible to calculate from these data the proportions of carbon entering some protein amino acids which came either from soluble amino acid pools or by a direct route from photosynthate, bypassing the soluble pools. More than half of the carbon entering protein-bound serine and glycine was derived from newly assimilated CO 2 , while protein glutamic acid, aspartic acid, and alanine derived more of their carbon from the soluble amino acid pools. Analysis of the data from roots indicated that a considerable portion of the carbon translocated from the leaves was derived from newly assimilated CO 2 . There was some indication that protein turnover took place, but it was concluded that proteins could not have contributed significantly as substrates for respiration in these leaves.