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Plant and Cell Physiology, 1995, Vol. 36, No. 7 1311-1317
© 1995

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

Archana Lal and Gerald E. Edwards

Department of Botany, Washington State University Pullman, WA 99164-4238, U.S.A.

The quantum yields of photosynthetic O2 evolution were measured in 15 species of C4 plants belonging to three different decarboxylation types (NADP-ME type, NAD-ME type and PEP-CK type) and 5 species of C3 plants and evaluated relative to the maximum theoretical value of 0.125 mol oxygen quanta-1. At 25°C and 1% CO2, the quantum yield in C4 plants averaged 0.079 (differences between subgroups not significant) which was significantly lower than the quantum yield in C3 plants (average of 0.105 for 5 species). This lower quantum yield in C4 plants is thought to reflect the requirement of energy in the C4 cycle. For the C4 NADP-ME type plant Z. mays and NAD-ME type plant P. miliaceum, quantum yields were also measured over a range of CO2 levels between 1 and 20%. In both species maximum quantum yields were obtained under 10% CO2 (0.105 O2 quanta-1 in Z. mays and 0.097 O2 quanta-1 in P. miliaceum) indicating that at this CO2 concentration the quantum yields are similar to those obtained in C3 plants under CO2 saturation. The high quantum yield values in C4 plants under very high CO2 may be accomplished by direct diffusion of atmospheric CO2 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)
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