The purpose of the present study was to test the possible plant thermotolerance role of isoprene and to study its relationship with non-enzymatic antioxidants and terpene emissions. The gas exchange, chlorophyll fluorescence, extent of photo-and oxidative stress, leaf damage, mecha-nisms of photo-and antioxidant protection, and terpene emission were measured in leaves of Quercus ilex seedlings exposed to a ramp of temperatures of 5 ∞ ∞ ∞ ∞ C steps from 25 to 50 ∞ ∞ ∞ ∞ C growing with and without isoprene (10 m m m m L L ----1) fumi-gation. The results showed that isoprene actually conferred thermotolerance (shifted the decrease of net photosyn-thetic rates from 35 to 45 ∞ ∞ ∞ ∞ C, increased F v / F m at 50 ∞ ∞ ∞ ∞ C from 0.38 to 0.65, and decreased the leaf area damaged from 27 to 15%), that it precluded or delayed the enhancement of the antioxidant non-enzymatic defence conferred by a a a a -tocopherol, ascorbic acid or b b b b -carotene consumption in response to increasing temperatures, and that it decreased by approximately 70% the emissions of monoterpenes at the highest temperatures. This suggests that there are inducible mechanisms triggered by the initial stages of ther-mal damage that up-regulate these antioxidant compounds at high temperatures and that these mechanisms are some-how suppressed in the presence of exogenous isoprene, which seems to already exert an antioxidant-like behaviour.
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