MAXIMUM QUANTUM YIELDS OF O-2 EVOLUTION IN C-4 PLANTS UNDER HIGH CO2

The quantum yields of photosynthetic O-2 evolution were measured in 15 species of C-4 plants belonging to three different decarboxylation ty pes (NADP-ME type, NAD-ME type and PEP-CK type) and 5 species of C-3 p lants and evaluated relative to the maximum theoretical value of 0.125 mol oxygen quanta(-1). At 25 degrees C and 1% CO2, the quantum yield in C-4 plants averaged 0.079 (differences between subgroups not signif icant) which was significantly lower than the quantum yield in C-3 pla nts (average of 0.105 for 5 species). This lower quantum yield in C-4 plants is thought to reflect the requirement of energy in the C-4 cycl e. For the C-4 NADP-ME type plant Z. mays and NAD-ME type plant P. mil iaceum, quantum yields were also measured over a range of CO2 levels b etween 1 and 20%. In both species maximum quantum yields were obtained under 10% CO2 (0.105 O-2 quanta(-1) in Z. mays and 0.097 O-2 quanta(- 1) in P. miliaceum) indicating that at this CO2 concentration the quan tum yields are similar to those obtained in C-3 plants under CO2 satur ation. The high quantum yield values in C-4 plants under very high CO2 may be accomplished by direct diffusion of atmospheric CO2 to bundle sheath cells, its fixation in the C-3 pathway, and feedback inhibition of the C-4 cycle by inorganic carbon.