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

We have investigated the importance of the long wavelength absorbing spectr al forms (red forms) of Photosystem I in photosynthetic light harvesting by leaves. To this end leaf spectra were simulated by using a linear combinat ion of absorption (OD) spectra of purified Photosystem I, Photosystem II an d LHC II, multiplied by an empirical multiple scattering chloroplast/leaf c onversion 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 l ight conditions of `shadelight', this relative increase in the absorption c ross section of PS I can be understood by considering the increased synthes is of the major PS II antenna complex, LHC II, known to occur in plants gro wing under these light conditions. It is demonstrated that for plants in a moderate to deep `shadelight' regime the PS II cross section needs to incre ase by 50% to 100% via LHC II synthesis to balance the increased PS I absor ption by the red forms. The possibility that under `shade light' conditions the increased PS I cross section may serve in cyclic phosphorylation is al so discussed.