Maximum Quantum Yields of O2 Evolution in C4 Plants under High CO2

The quantum yields of photosynthetic O₂ evolution were measuredin 15 species of C₄ plants belonging to three different decarboxylationtypes (NADP-ME type, NAD-ME type and PEP-CK type) and 5 speciesof C₃ plants and evaluated relative to the maximum theoreticalvalue of 0.125 mol oxygen quanta^-1. At 25°C and 1% CO₂,the quantum yield in C₄ plants averaged 0.079 (differences betweensubgroups not significant) which was significantly lower thanthe quantum yield in C₃ plants (average of 0.105 for 5 species).This lower quantum yield in C₄ plants is thought to reflectthe requirement of energy in the C₄ cycle. For the C₄ NADP-MEtype plant Z. mays and NAD-ME type plant P. miliaceum, quantumyields were also measured over a range of CO₂ levels between1 and 20%. In both species maximum quantum yields were obtainedunder 10% CO₂ (0.105 O₂ quanta_-1 in Z. mays and 0.097 O₂ quanta_-1in P. miliaceum) indicating that at this CO₂ concentration thequantum yields are similar to those obtained in C₃ plants underCO₂ saturation. The high quantum yield values in C₄ plants undervery high CO₂ may be accomplished by direct diffusion of atmosphericCO₂ to bundle sheath cells, its fixation in the C₃ pathway,and feedback inhibition of the C₄ cycle by inorganic carbon. (Received June 6, 1995; Accepted August 15, 1995)