Effects of O2 and CO2 Concentrations on Quantum Yields of Photosystems I and II in Tobacco Leaf Tissue

The interactive effects of irradiance and O(2) and CO(2) levels on the quantum yields of photosystems I and II have been studied under steady-state conditions at 25 degrees C in leaf tissue of tobacco (Nicotiana tabacum). Assessment of radiant energy utilization in photosystem II was based on changes in chlorophyll fluorescence yield excited by a weak measuring beam of modulated red light. Independent estimates of photosystem I quantum yield were based on the light-dark in vivo absorbance change at 830 nanometers, the absorption band of P700(+). Normal (i.e. 20.5%, v/v) levels of O(2) generally enhanced photosystem II quantum yield relative to that measured under 1.6% O(2) as the irradiance approached saturation. Photorespiration is suspected to mediate such positive effects of O(2) through increases in the availability of CO(2) and recycling of orthophosphate. Conversely, at low intercellular CO(2) concentrations, 41.2% O(2) was associated with lower photosystem II quantum yield compared with that observed at 20.5% O(2). Inhibitory effects of 41.2% O(2) may occur in response to negative feedback on photosystem II arising from a build-up in the thylakoid proton gradient during electron transport to O(2). Covariation between quantum yields of photosystems I and II was not affected by concentrations of either O(2) or CO(2). The dependence of quantum yield of electron transport to CO(2) measured by gas exchange upon photosystem II quantum yield as determined by fluorescence was unaffected by CO(2) concentration.