The Dependence of Quantum Yield on Wavelength and Growth Irradiance

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Abstract
The wavelength dependence of photosynthetic quantum yield was measured in a leaf disc oxygen electrode using narrow-band interference filters. Photorespiration was suppressed by measuring oxygen evolution in air containing .apprx. 1% CO2. Rates of oxygen evolution were determined as a function of absorbed irradiances between 0 and 100 .mu.mol quanta m-2s-1 and the slope was taken as the quantum yield. The wavelength-dependence previously observed in many species was confirmed for pea and spinach leaves. The maximum quantum yields obtained here and by others are close to 0.111 mol O2 mol-1 quanta for red light. Given the wavelength dependence of quantum yield and the spectral distribution of light, the quantum yields for various white lights were calculated to be around 85% of the red maximum. The quantum yields in white light were the same for plants grown at different irradiances. It is argued that the wavelength dependence of quantum yield reflects the distribution of quanta between the two photosystems with the quantum yield dropping at wavelengths strongly absorbed by chlorophyll b as this is mainly associated with photosystem II.