Published absorption spectra for SDS-solubilised chlorophyll-protein complexes were found to have similar mean extinction coefficients in the 400-720 nm wavelength range. Three spectral distributions of light were chosen to represent the extremes experienced by terrestrial plants: sunlight on a clear day, shadelight at ground level in a tropical rainforest and sunfleck within a wheat crop canopy. The distributions of chlorophyll between the chlorophyll-protein complexes were selected to represent the adaptation to irradiance during growth by peas and an obligate shade species Alocasia macrorrhiza. The proportion of light absorbed by each photosystem was then calculated for each light spectrum. The following conclusions were drawn: 1) Adaptation to low light by altering the relative proportions of the chlorophyll-protein complexes does not enable the chloroplast to capture shadelight more efficiently. 2) The spectrum of the ratio of light absorbed by Photosystem II (PS II) to that by Photosystem I (PS I) is qualitatively similar to spectra of the Emerson enhancement effect. 3) PS II absorbs more light than PS I and this proportion increases with adaptation to shade. Phosphorylation of the light-harvesting complex can redress the imbalance of light distribution between the two photosystems equally well in sunlight or shadelight. 4) Packaging of PS II chlorophyll in grana stacks scarcely reduces the amount of light captured by a PS II chlorophyll relative to a PS I chlorophyll. Explanations for the similarity of quantum yields for sun- and shade-adapted leaves are discussed.
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John R. Evans (1986) studied this question.
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