XANTHOPHYLL CYCLE-DEPENDENT ENERGY-DISSIPATION AND FLEXIBLE PHOTOSYSTEM-II EFFICIENCY IN PLANTS ACCLIMATED TO LIGHT STRESS

The effect of an acclimation to light stress during the growth of leav es on their response to high photon flux densities (PFDs) was characte rised by quantifying changes in photosystem II (PSII) characteristics and carotenoid composition. During brief experimental exposures to hig h PFDs sun leaves exhibited: (a) much higher levels of antheraxanthin + zeaxanthin than shade leaves, (b) a greater extent of energy dissipa tion in the light-harvesting antennae, and (c) a greater decrease of i ntrinsic PSII efficiency that was rapidly reversible. During longer ex perimental exposures to high PFD, deep-shade leaves but not the sun le aves showed slowly developing secondary decreases in intrinsic PSII ef ficiency. Recovery of these secondary responses was also slow and inhi bited by Lincomycin, an inhibitor of chloroplast-encoded protein synth esis. In contrast, under field conditions all changes in intrinsic PSI I efficiency in open sun-exposed habitats as well as understory sites with intense sunflecks appeared to be caused by xanthophyll cycle-depe ndent energy dissipation. Furthermore, comparison of leaves with diffe rent maximal rates of electron transport revealed that all leaves comp ensated fully for these differences by dissipating very different amou nts of absorbed light via xanthophyll cycle-dependent energy dissipati on, thereby all maintaining a similarly low PSII reduction state. It i s our conclusion that an increased capacity for xanthophyll cycle-depe ndent energy dissipation is a key component of the acclimation of leav es to a variety of different forms of light stress, and that the respo nse of leaves to excess light experienced in the growth environment is thus likely to be qualitatively different from that to sudden experim ental exposures to PFDs exceeding the growth PFD.