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

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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 (Mucunasp.), quaking aspen (Populus tremuloidesMichx.), and pussy willow (Salix discolor Muhl). Isoprene, supplied to the leaves at either 18 μL L−1 in compressed air or 21 μL L−1 in N2, 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 μmol photons m−2s−1 in an N2 atmosphere, 21 μL 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 μL L−1 isoprene. Therefore, there were no discernible effects of isoprene on the occurrence of symptoms of high-temperature damage to thylakoid membranes. Our data do not support the hypothesis that isoprene enhances leaf thermotolerance.