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

Abstract

ABSTRACTSun 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 (ΔA810max) in saturating far‐red light. Leaf absorbance changes in dependence of increasing far‐red light fluence rates were used to calculate a ‘saturation constant’, Ks, representing the far‐red irradiance at which half of the maximal absorbance change (ΔA810max/2) was reached in the steady state. Photosystem II efficiency was assessed by measuring the ratio of variable to maximum chlorophyll fluorescence, Fv/Fm, 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 μmol 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 Ks in shade leaves, and a three‐fold increase of Ks in sun leaves. The increase in Ks 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.