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.

We investigated differences between summer and winter in photosynthesis, xanthophyll cycle-dependent energy dissipation, and antioxidant systems in populations of Mahonia repens (Lindley) Don growing in the eastern foothills of the Colorado Rocky Mountains in deep shade, full exposure, and under a single-layered canopy of Pinus ponderosa (partially shaded). In summer, increasing growth irradiance (from deep shade to partial shade to full exposure) was associated with increased xanthophyll cycle-dependent energy dissipation in PSII and an increased capacity to detoxify reactive reduced oxygen species, as measured by increases in the activities of ascorbate peroxidase, superoxide scavenging, glutathione reductase, and monodehydroascorbate reductase, as well as increases in leaf ascorbate and glutathione content. Leaves of exposed and partially shaded plants exhibited decreased capacities for photosynthetic O evolution in winter compared to summer, while in the deeply shaded plants this parameter did not differ seasonally. Seasonal differences in the levels of antioxidants generally exhibited an inverse response to photosynthesis, being higher in winter compared to summer in the exposed and partially shaded populations, but remaining unchanged in the deeply shaded population. In addition, total pool size and conversion state of the xanthophyll cycle were higher in winter than in summer in all populations. These trends suggest that both xanthophyll cycle-dependent energy dissipation in PSII and the capacity to detoxify reactive reduced oxygen species responded to the level of excess light absorption.

Since 2006, six satellites measuring solar‐induced chlorophyll fluorescence (SIF) have been launched to better constrain terrestrial gross primary productivity (GPP). The promise of the SIF signal as a proxy for photosynthesis with a strong relationship to GPP has been widely cited in carbon cycling studies. However, chlorophyll fluorescence originates from dynamic energy partitioning at the leaf level and does not exhibit a uniformly linear relationship with photosynthesis at finer scales. We induced stomatal closure in deciduous woody tree branches and measured SIF at a proximal scale, alongside leaf‐level gas exchange, pulse amplitude modulated (PAM) fluorescence, and leaf pigment content. We found no change in SIF or steady‐state PAM fluorescence, despite clear reductions in stomatal conductance, carbon assimilation, and light‐use efficiency in treated leaves. These findings suggest that equating SIF and photosynthesis is an oversimplification that may undermine the utility of SIF as a biophysical parameter in GPP models.

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The effects of exogenously supplied isoprene on chlorophyll fluorescence characteristics were examined in leaf discs of four isoprene-emitting plant species, kudzu (Pueraria lobata [Willd.] Ohwi.), velvet bean (Mucuna sp.), quaking aspen (Populus tremuloides Michx.), and pussy willow (Salix discolor Muhl). Isoprene, supplied to the leaves at either 18 L L ؊1 in compressed air or 21 L L ؊1 in N 2 , had no effect on the temperature at which minimal fluorescence exhibited an upward inflection during controlled increases in leaf-disc temperature. During exposure to 1008 mol photons m ؊2 s ؊1 in an N 2 atmosphere, 21 L L ؊1 isoprene had no effect on the thermally induced inflection of steady-state fluorescence. The maximum quantum efficiency of photosystem II photochemistry decreased sharply as leaf-disc temperature was increased; however, this decrease was unaffected by exposure of leaf discs to 21 L L ؊1 isoprene. Therefore, there were no discernible effects of isoprene on the occurrence of symptoms of hightemperature damage to thylakoid membranes. Our data do not support the hypothesis that isoprene enhances leaf thermotolerance.More than 4 decades ago emission of isoprene (2-methyl-1,3-butadiene) from leaves of higher plants was first described (Sanadze, 1957). Since that time understanding of the biochemistry and environmental controls of isoprene emission has grown considerably, along with an appreciation for the role that phytogenic isoprene plays in critical oxidative atmospheric processes (for reviews, see Sharkey et al., 1991; Sharkey, 1996; Lerdau et al., 1997). However, a function for isoprene in leaves remained elusive until Sharkey and Singsaas (1995) reported evidence from kudzu (Pueraria lobata [Willd.] Ohwi.) that isoprene protected thylakoid membranes against damage induced by high leaf temperatures. Under conditions that suppressed endogenous isoprene synthesis, isoprene supplied exogenously at physiologically realistic concentrations resulted in an increase in the temperature at which chlorophyll fluorescence emission exhibited a distinct upward inflection (Sharkey and Singsaas, 1995; Singsaas et al., 1997). Furthermore, it was reported that a linear relationship exists between the concentration of supplied isoprene and the extent of its effect on leaf thermotolerance (Singsaas et al., 1997). Subsequently, it was reported that certain monoterpenes, another class of phytogenic hydrocarbons, protect the photosynthetic apparatus of Quercus ilex L. from thermal damage (Loreto et al., 1998).High temperature-induced inflection of chlorophyll fluorescence has been used widely as an indicator of thermal damage and correlates with the temperature at which leaves experience significant tissue necrosis (Bilger et al., 1984). Dislocation between the light-harvesting complexes and PSII reaction centers due to excessive membrane fluidity is thought to underlie this phenomenon (Armond et al., 1980), although Yamane et al. (1997) suggested that denaturation of PSII reaction center proteins may be involved as...