14CO2 incorporation into C4 acids by leaves of the C3-C4 intermediate species, Moricandia arvensis and Panicum milioides, at Gamma-levels of CO2

The crucifer M. arvensis is a species with photorespiratory characteristics intermediate between C3 and C4 plants. Although the precise mechanism promoting decreased photorespiration has yet to be elucidated, evidence to date disfavors participation of C4 metabolism. In order to conclusively assess the possible contribution, if any, of limited C4 metabolism to the reduction of photorespiration, detached leaves of M. arvensis and M. moricandioides (C3) were labeled with 14CO2. When M. arvensis was illuminated and pulsed for 10 s the amount of label in malate increased from 4.5% of total soluble dpm fixed at 21% O2 and 340 μl/l CO2 to 10.3% at 20 μl/l. A two-fold increase was also evident for aspartate (from 2.9% to 6.5%). The C3 Moricandia species failed to show a similar response to decreased CO2. The physiological significance of the increase in C4 acid-labeling which occurred in the intermediate species will be investigated by pulse-chase analyses of C4 acids and glycine. Similar data for Panicum milioides (C3-C4) will also be presented