In the present study we explored the possibility of assessing the allocation of photons absorbed by photosystem II (PSII) antennae to thermal energy dissipation and photosynthetic electron transport in leaves of several plant species under field conditions. Changes in chlorophyll fluorescence parameters were determined in situ over the course of an entire day in the field in sun‐exposed leaves of two species with different maximal rates of photosynthesis, Helianthus annuus (sunflower) and Vinca major. Leaves of Vinca minor (periwinkle) growing in a deeply shaded location were also monitored. We propose using diurnal changes in the efficiency of open PSII centers (F′v/F′m) in these sun and shade leaves to (a) assess diurnal changes in the allocation of absorbed light to photochemistry and thermal energy dissipation and, furthermore, (b) make an estimate of changes in the rate of thermal energy dissipation, an analogous expression to the rate of photochemistry. The fraction of light absorbed in PSII antennae that is dissipated thermally (D) is proposed to be estimated from D = 1‐F′v/F′m, in analogy to the widely used estimation of the fraction of light absorbed in PSII antennae (P) that is utilized in PSII photochemistry from P = F′v/F′m× qP (where qP is the coefficient for photochemical quenching; Genty, B., Briantais, J.‐M. & Baker, N. R. 1989. Biochim. Biophys. Acta 990: 87‐92). The rate of thermal dissipation is consequently given by D × PFD (photon flux density), again in analogy to the rate of photochemistry P × PFD, both assuming a matching behavior of photosystems I and II. Characterization of energy dissipation from the efficiency of open PSII centers allows an assessment from a single set of measurements at any time of day; this is particularly useful under field conditions where the fully relaxed reference values of variable or maximal fluorescence needed for the computation of nonphotochemical quenching may not be available. The usefulness of the assessment described above is compared with other currently used parameters to quantify nonphotochemical and photochemical chlorophyll fluorescence quenching.

SummaryEvergreens are faced with the challenge, during winter, of low temperatures in combination with light exposure, resulting in an imbalance between light absorption and its utilization via photosynthetic carbon reduction. To cope with excess light, evergreens increase their use of thermal energy dissipation, which occurs in a sustained form during winter. There are two forms of sustained thermal dissipation that occur in winter-stressed evergreens, characterized by their rate of reversal upon warming. A rapid form reverses within minutes to hours upon warming, while a slower form reverses over the course of days. The light environment and the severity of winter conditions both play a role in determining the relative amount of each type of sustained energy dissipation throughout the winter. It is suggested that the two forms of sustained dissipation observed in winter-stressed evergreens correspond to sustained forms of the two mechanisms of thermal energy dissipation proposed by Holzwarth and colleagues, with the rapidly reversible component corresponding to a sustained form of the energy-dependent form of thermal energy dissipation (qE) and the slowly reversible component corresponding to a sustained form of the zeaxanthin-dependent mechanism (qZ). Additional outstanding questions and future directions are discussed.

Sustained decreases in intrinsic photosystem II efficiency (i.e. Fv/Fm) in response to high light and chilling temperatures were examined in eight species, and were found to be accompanied by the retention of zeaxanthin (Z) and antheraxanthin (A) overnight. The quantitative relationship between changes in Fv/Fm and the A + Z level during these sustained changes on cold days was similar to that obtained for rapidly reversible changes on warm days. Furthermore, upon removal of leaves from the field, recovery from 'photoinhibition' (the reversal of the depression of Fv/Fm) matched the timecourse of the epoxidation of Z and A to violaxanthin (V). These findings suggest that the 'photoinhibition' occuring in these species might be due to the sustained engagement of these de-epoxidised components of the xanthophyll cycle in photoprotective energy dissipation. When examined over the course of several days during the winter, the predawn conversion state of the xanthophyll cycle responded to the daily changes in minimum air (and leaf) temperature, such that the xanthophyll cycle was largely de-epoxidised prior to sunrise on cold nights and was present predominantly as V after nights when the nocturnal temperatures were above freezing. In addition, in some of the species examined, there was a large acclimation of the xanthophyll cycle pool size to the level of excessive light, with a much larger pool present in the leaves examined during the winter and that pool being de-epoxidised to Z and A to a much greater degree at midday than from similar leaves examined during the summer. The xanthophyll cycle, and the photoprotective energy dissipation process associated with it, would thus appear to provide plants the flexibility required to deal with the excessive levels of light absorbed by chlorophyll under a wide range of climatic conditions, and can quite possibly account for the 'photoinhibition' observed during winter stress.

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 II (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.

The involvement of the xanthophyll cycle in photoprotection of N-deficient spinach (Spinacia oleracea 1. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0.5 mM (low N) fertilizer, and grown under both high-and low-light conditions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid and chlorophyll analysis. Compared with the high-N plants, the low-N plants showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem l i photosynthetic electron transport and a corresponding increase in thermal energy dissipation activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cycle pool as zeaxanthin and antheraxanthin at midday, and an increase in the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the lightcollecting system and the photosynthetic rate decrease. However, the increased dissipation of excess energy shows that there is excess light absorbed at midday. We conclude that spinach responds to N limitation by a combination of decreased light collection and increased thermal dissipation involving the xanthophyll cycle.