Quantum Yields for CO2 Uptake in C3 and C4 Plants

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Abstract
The quantum yields of C3 [Atriplex glabriuscula, A. heterosperma, A. hortensis, A. triangularis, Plantago lanceolata, Encelia californica and E. farinosa] and C4 [Tidestromia oblongifolia, Atriplex argentea, A. rosea, A. sabulosa and A. seranana] plants from a number of genera and families as well as from ecologically diverse habitats [coastal strand, coastal sage, grassland and desert] were measured in normal air of 21% O2 and in 2% O2. At 30.degree. C, the quantum yields of C3 plants averaged 0.0524 .+-. 0.0014 mol CO2/absorbed einstein and 0.0733 .+-. 0.0008 mol CO2/absorbed einstein under 21 and 2% O2. At 30.degree. C, the quantum yields of C4 plants averaged 0.0534 .+-. 0.0009 mol CO2/absorbed einstein and 0.0538 .+-. 0.0011 mol CO2/absorbed einstein under 21 and 2% O2. At 21% O2, the quantum yield of a C3 plant is strongly dependent on both the intercellular CO2 concentration and leaf temperature. The quantum yield of a C4 plant, which is independent of the intercellular CO2 concentration, is independent of leaf temperature over the ranges measured. The changes in the quantum yields of C3 plants are due to changes in the O2 inhibition. The evolutionary significance of the CO2 dependence of the quantum yield in C3 plants and the ecological significance of the temperature effects on the quantum yields of C3 and C4 plants are discussed.