The Photosystem Stoichiometry in Thylakoids of Some Australian Shade-adapted Plant Species

The concentrations of functional photosystem I1 (PSII) reaction centres in leaves and photosystem I reaction centres (P700) in thylakoids isolated from comparable leaves of Australian shade-adapted plant species of diverse taxa, life-forms and habitats were compared. The concentrations of PSII were determined directly in leaves by the oxygen yield per single-turnover flash in the presence of far-red background illumination. The concentrations of P700 were determined by the light-induced absorbance change of thylakoid membranes at 703 nm. On a chlorophyll basis, the amounts of both functional PSII and P700 were lower in shade species than in sun species. The PSII/PSI reaction centre stoichiometries of the shade species ranged from 1.2 to 1.9 indicating that (i) shade-adapted species do not have a fixed 1: 1 ratio; and (ii) their PSWPSI ratios are usually lower than those of sun species (1.7-1.8). We conclude that shade plants display variable photosystem stoichiometry. The results are discussed in terms of the interplay between the adjustment of photosystem stoichiometry and that of the light-harvesting chlorophyll antenna size of each photosystem in the thylakoid membrane of shade species.