Photosynthetic induction in a C4 dicot, Flaveria trinervia. II. Metabolism of products of 14CO2 fixation after different illumination times

The metabolism of fixed 14CO2 and the utilization of the C-4 carboxyl of malate and aspartate were examined during photosynthetic induction in Flaveria trinervia, a C2 dicot of the NADP-malic enzyme subgroup. Pulse/chase experiments indicated that both malate and aspartate appeared to function directly in the C4 cycle at all times during the induction period (examined after 30 seconds, 5 minutes and 20 minutes illumination). However, the rate of loss of 14C-label from the C-4 position of malate plus aspartate was relatively slow after 30 seconds of illumination, compared to treatments after 5 or 20 minutes of illumination. Similarly, the appearance of label in other photosynthetic products (e.g. 3-phosphoglycerate, sugar phosphates, alanine) during the chase periods was generally slower after only 30 seconds of leaf illumination, compared to that after 5 of 20 minutes illumination. This may be due to the lower rate of photosynthesis after 30 seconds of leaf illumination, compared to that after 5 of 20 minutes illumination. The appearance of label in carbons 1→3 of each C4 acid during the chase periods was relatively slow after either 30 seconds or 5 minutes illumination, while there was a relatively rapid accumulation of label in carbons 1→3 of both C4 acids after 20 minutes illumination. Thus, while the turnover rate of the 14C-4 label in both C4 acids increased only during the first 5 minutes of the induction period, only later during induction is there an increased rate of appearance of label in other carbon atoms of the C4 acids. The implied source of 14C for labeling of the 1→3 positions of the C4 acids is an apparent carbon flux from 3-phosphoglycerate of the reductive pentose phosphate pathway to phospoenolpyruvate of the C4 cycle