Aust. J. Plant Physiol., 1995, 22, 249-60 Xanthophyll Cycle-dependent Energy Dissipation and Flexible Photosystem I1 Efficiency in Plants Acclimated to Light Stress Barbara ~emrnig~dams~~, William W. Adams llIA, Barry A. ~ o~an~, and Amy S. verhoevenA A~epartment of Environmental, Population, and Organismic Biology, University of Colorado, Bouldel; CO 80309-0334, USA. B~orresponding authol; email: demmigad@spot. Colorado.edu Abstract. The effect of an acclimation to light stress during the growth of leaves on their response to high photon flux densities (PFDs) was characterised by quantifying changes in photosystem I1 (PSII) characteristics and carotenoid composition. During brief experimental exposures to high PFDs sun leaves exhibited: (a) much higher levels of antheraxanthin + zeaxanthin than shade leaves, (b) a greater extent of energy dissipation in the light-harvesting antennae, and (c) a greater decrease of intrinsic PSII efficiency that was rapidly reversible. During longer experimental exposures to high PFD, deep-shade leaves but not the sun leaves showed slowly developing secondary decreases in intrinsic PSII efficiency. Recovery of these secondary responses was also slow and inhibited by lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In contrast, under field conditions all changes in intrinsic PSII efficiency in open sun-exposed habitats as well as understory sites with intense sunflecks appeared to be caused by xanthophyll cycle-dependent energy dissipation. Furthermore, comparison of leaves with different maximal rates of electron transport revealed that all leaves compensated fully for these differences by dissipating very different amounts of absorbed light via xanthophyll cycle-dependent energy dissipation, thereby all maintaining a similarly low PSII reduction state. It is our conclusion that an increased capacity for xanthophyll cycle-dependent energy dissipation is a key component of the acclimation of leaves to a variety of different forms of light stress, and that the response of leaves to excess light experienced in the growth environment is thus likely to be qualitatively different from that to sudden experimental exposures to PFDs exceeding the growth PFD. Introduction The response of plants to a variable and stressful environment has long been the focal point of ecophysiology. While considerable interest has focused on possible adverse effects of high light stress on the photosynthetic apparatus, pre-emptive avoidance mechanisms that afford protection from damage have more recently been characterised. These include processes employed by some plant species such as changes in leaf angle and optical properties that can prevent the absorption of excess light in the first place (for a review see Bjorkman and Demmig-Adams 1994). At the biochemical level accumulation of excess absorbed light is counteracted, firstly, by an ubiquitous process that dissipates excess excitation energy harmlessly directly within the light- harvesting chlorophyll (Chl)-carotenoid antenna complexes of photosystem I and photosystem I1 (PSII)* (for recent reviews see Demmig-Adams and Adams 1992a, 1993; Horton et al. 1994). This process requires a high proton concentration within the thylakoid membrane and is also dependent on the presence of the de-epoxidised components of the xanthophyll cycle (see below). In addition, a suite of scavenger systems serves in the detoxification of reactive oxygen species in the chloroplast (Alscher and Hess 1993; Foyer and Mullineaux 1994). While mechanistic aspects of the various photoprotective processes continue to be pursued, more attention must now be directed at the interaction and relative roles of the various possible responses under ecologically meaningful stress "Abbreviations used: A, antheraxanthin; Chl, chlorophyll; Fo and Fm,minimal yield (at open PSII reaction centres) and maximal yield (at closed centres) of Chl fluorescence in darkness; Fo' and F,', minimal yield (at open centres) and maximal yield (at closed centres) of Chl fluorescence during or (for F,,') immediately after illumination with photosynthetically active radiation; FVIFm, intrinsic efficiency of PSII in darkness; FV1IFm', intrinsic efficiency of the open PSII centres during illumination with photosynthetically active radiation; NPQ, non-photochemical quenching of Chl fluorescence (=FmIFmf-1); PFD, photon flux density; PSII, photosystem 11; V, violaxanthin; Z, zeaxanthin.