Incorporation of (14)CO 2 into C 4 acids by leaves of C 3-C 4 intermediate and C 3 species of Moricandia and Panicum at the CO 2 compensation concentration.

Comparative (14)CO2 pulse-(12)CO2 chase studies performed at CO2 compensation (Γ)-versus air-concentrations of CO2 demonstrated a four-to eightfold increase in assimilation of (14)CO2 into the C4 acids malate and aspartate by leaves of the C3-C4 intermediate species Panicum milioides Nees ex Trin., P. decipiens Nees ex Trin., Moricandia arvensis (L.) DC., and M. spinosa Pomel at Γ. Specifically, the distribution of (14)C in malate and aspartate following a 10-s pulse with (14)CO2 increases from 2% to 17% (P. milioides) and 4% to 16% (M. arvensis) when leaves are illuminated at the CO2 compensation concentration (20 μl CO2/l, 21% O2) versus air (340 μl CO2/l, 21% O2). Chasing recently incorporated (14)C for up to 5 min with (12)CO2 failed to show any substantial turnover of label in the C4 acids or in carbon-4 of malate. The C4-acid labeling patterns of leaves of the closely related C3 species, P. laxum Sw. and M. moricandioides (Boiss.) Heywood, were found to be relatively unresponsive to changes in pCO2 from air to Γ. These data demonstrate that the C3-C4 intermediate species of Panicum and Moricandia possess an inherently greater capacity for CO2 assimilation via phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) at the CO2 compensation concentration than closely related C3 species. However, even at Γ, CO2 fixation by PEP carboxylase is minor compared to that via ribulosebisphosphate carboxylase (EC 4.1.1.39) and the C3 cycle, and it is, therefore, unlikely to contribute in a major way to the mechanism(s) facilitating reduced photorespiration in the C3-C4 intermediate species of Panicum and Moricandia.