Thermotolerance of Leaf Discs from Four Isoprene-Emitting Species Is Not Enhanced by Exposure to Exogenous Isoprene1

Abstract

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...

An ecological and evolutionary analysis of photosynthetic thermotolerance using the temperature-dependent increase in fluorescence

“…Stems with several healthy leaves were collected early in the morning and kept with their cut-end in water and acclimated to very low light (<10 µmol m -2 s -1 ) for 1-7 h. Our measurements concur with previous reports that the duration of low light accli mation has no effect on the inflection temperature of the T-F o curve (data not presented; Logan and Monson 1999). Immediately prior to measurement a single entire leaf was collected and placed on damp filter paper on top of the ceramic surface of a 4×4-cm peltier thermoelectric heater (Melcor, Trenton, N.J., USA).…”

Differential Accumulation of Dimethylallyl Diphosphate in Leaves and Needles of Isoprene- and Methylbutenol-Emitting and Nonemitting Species

“…Several experiments have provided support for the hypothesis that volatile isoprenoids (isoprene, monoterpenes, and other short-chain hydrocarbons) protect leaves from short high-temperature bursts (Sharkey and Yeh, 2001). However, evidence for increased thermotolerance due to isoprene was not found in at least two studies (Logan and Monson, 1999;Logan et al, 1999). Other experiments demonstrate that isoprene protects leaves exposed to high-ozone episodes and therefore isoprene may play an important antioxidative role in plant leaves (Loreto et al, 2001).…”

Seasonal variations and acclimation potential of the thermostability of photochemistry in four Mediterranean conifers

“…Thermostability of photosynthesis may be related to thylakoid membrane fluidity [2,29], that might be partly controlled by the concentration of free zeaxanthin and therefore the inter-conversion process between violaxanthin and zeaxanthin [18]. Isoprene emitted by several species has been suggested to contribute to the thylakoid stability [34,35] although this point is still debated [26]. Low molecular weight heat shock proteins might also play the role of chaperonine protecting protein complexes of the chloroplastic electron transport chain against heat denaturation [19].…”