CO-2 DONATION BY MALATE AND ASPARTATE REDUCES PHOTO-RESPIRATION IN PANICUM-MILIOIDES, A C3-C4 INTERMEDIATE SPECIES

Oxygen inhibition of leaf slice photosynthesis in Panicum milioides increased from 20% to 30% at 21% O2 in the presence of maleate, a phosphoenolpyruvate carboxylase inhibitor. The increased O2 sensitivity was completely reversed by the addition of malate and aspartate, the stable products of the phosphoenolpyruvate carboxylase reaction. The C4 acids, malate and aspartate, also reduced O2 inhibition of photosynthesis by isolated bundle sheath strands, but not mesophyll protoplasts. Similarly, only bundle sheath strands exhibited an active C4 acid-dependent O2 evolution. Compartmentation of C4 cycle enzymes, with pyruvate, Pi dikinase in the mesophyll and NAD-malic enzyme in the bundle sheath, was demonstrated. It is concluded that reduced photorespiration in P. milioides is due to a limited potential for C4 photosynthesis permitting an increase in pCO2 at the site of bundle sheath ribulosebisphosphate carboxylase. © 1978.