O2-Insensitive Photosynthesis in C3 Plants

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Abstract
Leaves of C3 plants which exhibit a normal O2 inhibition of CO2 fixation at less than saturating light intensity exhibit O2-insensitive photosynthesis at high light. This behavior was observed in Phaseolus vulgaris L., Xanthium strumarium L., and Scrophularia deserotorum (Shawn.) Munz. O2-insensitive photosynthesis was reported in 9 other C3 species and usually occurred when the intercellular CO2 pressure was about double the normal pressure. A lack O2 inhibition of photosynthesis was always accompanied by a failure of increased CO2 pressure to stimulate photosynthesis to the expected degree. O2-insensitive photosynthesis also occurred after plants were water stressed. Under such conditions photosynthesis became O2 and CO2 insensitive at physiological CO2 pressures. Postillumination CO2 exchange kinetics showed that O2 and CO2 insensitivity was not the result of elimination of photorespiration. O2 and CO2 insensitivity occurs when the concentration of PO43- in the chloroplast stroma cannot be both high enough to allow photophosphorylation and low enough to allow starch and sucrose synthesis at the rates required by the rest of the photosynthetic component processes. Under these conditions, the energy diverted to photorespiration does not adversely affect the potential for CO2 assimilation.