Inhibition of the Capacity and Efficiency of Photosynthesis in Bean Leaflets Illuminated in a CO2-Free Atmosphere at Low Oxygen: A Possible Role for Photorespiration

Mature, intact, attached leaflets of Phaseolus vulgaris illuminated at 2000 μE m-2 s-1 in CO2-free N2 containing 1% O2 for a 3-h treatment period showed an 80% reduction in the light-saturated CO2 assimilation rate. Young bean leaflets showed photoinhibition but exhibited a partial recovery of CO2 assimilation rate within 2 h of treatment whereas mature leaflets did not. Photoinhibition by illumination under conditions which prevent photorespiration was markedly dependent upon the exposure time. Photoinhibition was not observed following illumination in the presence of CO2-free N2 containing above 7% O2 or in N2 containing 1% O2 and an intercellular CO2 partial pressure equivalent to the CO2 compensation point in 21% O2 (62 μbar, 6.2 Pa). Photoinhibition involved concomitant reduction in the light-saturated and the light-limited rate of CO2 assimilation; apparent quantum yield was reduced following treatment. These experiments show that internal generation of CO2 by the photorespiratory carbon oxidation cycle is sufficient to prevent the photoinhibition observed when photorespiration is prevented. Possible mechanisms and implications of these data are discussed.