High-Irradiance Stress in Higher Plants and Interaction with other Stress Factors

Abstract

INTRODUCTION Exposure of leaves to light levels in excess of what can be utilized in photosynthesis often results in a decline in photosynthetic activity (1). This high-l ight effect is especially evident after return of the leaf to a low light level as a reduction in the photon yield of photosynthetic O 2 evolution or CO 2 uptake. In common usage the term photoinhibition includes any sustained reduction in photosynthetic activity induced by excessive light, irrespective of mechanistic considerations, but does not include transient reductions that are rapidly reversible and likely to reflect short-term regulation. In my talk today I will attempt to distinguish between two kinds of high-light-induced reduction in the efficiency of photochemistry of PSII: 1) an increase in the rate constant for dissipation of excitation energy in the antenna and 2) a decrease in the rate constant for the photo-chemistry of PSII which is likely to be caused by damage to the PSII reaction centers. RELATIONSHIP BETWEEN PHOTOCHEMISTRY AND CHLOROPHYLLFLUORESCENCE Much of our information on the nature of the response of leaves to excessive light comes from studies in which ch lorophyll fluorescence is used as an intrinsic probe of photochemical events. The light energy absorbed by the PSII antenna chlorophyll molecules can be dissipated 1) as fluorescence (F); 2) as heat by non-radiative dissipation (D); 3) by transfer to PSI (T); and 4) in photochemical act ivity by PSII (P). The corresponding rate constants for these dissipation mechan isms will be named K F , K D , K T and K p. In a leaf photosynthesizing in weak light, the PSII reaction centers are reoxidized fast enough for the traps to remain open, and most of the energy is dissipated via P; only a very small fraction is dissipated as F. In a leaf photosynthesizing at satu-rating light levels, a high proportion of the reaction centers are reduced (the traps are closed) and a greater fraction (although still small) of the excitation energy is diss ipated via fluorescence. It is noteworthy, however, that the fluorescence yield does not increase as much as would be expected when photosynthesis becomes light-saturated. This can be explained by an increase in the fraction of the excitation energy that is dissipated via D. One kind of increase in non-radiative dissipation is associated with a build-up of a proton grad ient across the thylakoid membrane (energy or lIpH-dependent quench ing). This mechanism of energy regulation (2), presumably helps to prevent the reaction centers from becoming fully reduced , thereby decreasing the probability of over-excitation and consequent photoinhibitory damage to the reaction centers. The ind uction and relaxation of lIpH-dependent energy quenching is very fast (2) so it seems unlikely that this type of non-radiative energy diss ipation would reduce the photon yield of photo-synthes is observed after return to rate-limiting light levels. However, there also exists another type of fluorescence quenching with much longer induction and relaxation times and which indicates the operation of a mechanism that can cause a sustained increase in the fraction of exc itation energy that is diss ipated in the PSII antenna, and hence also to a sustained lowered efficiency of the photon yield of photosynthesis (3,4; also see 5). This response has caused me to modify my earlier views of the interactions that exist between high-irradiance and other stress factors.