Maximum quantum yields of O2 evolution in C4 plants under high CO2

The quantum yields of photosynthetic 0 2 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. Key words: Cj — C4 — NADP-malic enzyme — NAD-malic enzyme — PEP-carboxykinase — Quantum yield of O2 evolution. The quantum yield of photosynthesis (#J may be de-fined as the moles O2 evolved or CO2 assimilated per mole of photons absorbed by the photosynthesis apparatus. At low PPFD, where the rate of photosynthesis is a linear func-tion of light intensity, <f>t is maximal, and depicts the max-imum efficiency with which light is converted to stable pho-tosynthetic products. Measurements of maximum quantum yields on leaves of higher plants have been reported for a considerable num