Responses of photosystem I compared with photosystem II to high-light stress in tropical shade and sun leaves

Sun and shade leaves of several plant species from a neotropical forest were exposed to excessive light to evaluate the responses of photosystem I in comparison to those of photosystem II. Potential photosystem I activity was determined by means of the maximum P700 absorbance change around 810 nm (DeltaA(810max)) in saturating far-red light. Leaf absorbance changes in dependence of increasing far-red light fluence rates were used to calculate a 'saturation constant', K-s, representing the far-red irradiance at which half of the maximal absorbance change (DeltaA(810max)/2) was reached in the steady state. Photosystem II efficiency was assessed by measuring the ratio of variable to maximum chlorophyll fluorescence, F-v/F-m, in dark-adapted leaf samples. Strong illumination caused a high degree of photo-inhibition of photosystem II in all leaves, particularly in shade leaves. Exposure to 1800-2000 mu mol photons m(-2) s(-1) for 75 min did not substantially affect the potential activity of photosystem I in all species tested, but caused a more than 40-fold increase of K-s in shade leaves, and a three-fold increase of K-s in sun leaves. The increase in K-s was reversible during recovery under low light, and the recovery process was much faster in sun than in shade leaves. The novel effect of high-light stress on the light saturation of P700 oxidation described here may represent a complex reversible mechanism within photosystem I that regulates light-energy dissipation and thus protects photosystem I from photo-oxidative damage. Moreover, we show that under high-light stress a high proportion of P700 accumulates in the oxidized state, P700(+). Presumably, conversion of excitation energy to heat by this cation radical may efficiently contribute to photoprotection.