The importance of PSI chlorophyll red forms in light-harvesting by leaves

We have investigated the importance of the long wavelength absorbing spectral forms (red forms) of Photosystem I in photosynthetic light harvesting by leaves. To this end leaf spectra were simulated by using a linear combination of absorption (OD) spectra of purified Photosystem I, Photosystem II and LHC II, multiplied by an empirical multiple scattering chloroplast/leaf conversion function. In this way it is demonstrated that while the PS I red forms account for only about 4-5% of light absorption in a normal `daylight' environment, in different `shadelight' environments these long wavelength pigments may be responsible for up to 40% of total photon capture. In the context of maximising the photosynthetic quantum efficiency under the low light conditions of `shadelight', this relative increase in the absorption cross section of PS I can be understood by considering the increased synthesis of the major PS II antenna complex, LHC II, known to occur in plants growing under these light conditions. It is demonstrated that for plants in a moderate to deep `shadelight' regime the PS II cross section needs to increase by 50% to 100% via LHC II synthesis to balance the increased PS I absorption by the red forms. The possibility that under `shade light' conditions the increased PS I cross section may serve in cyclic phosphorylation is also discussed.