Airborne limonene confers limited thermotolerance to Quercus ilex

The purpose of the study was to test the possible and controversial thermotolerance role of monoterpene production and emission and the related responses of antioxidants. Quercus ilex seedlings were exposed to a ramp of temperatures of 5degreesC steps from 25 to 50degreesC growing with and without limonene fumigation (7.5 mul l(-1)). Net photosynthetic rates, maximal photochemical efficiency of PSII (F(v)/F(m)), oxidation state of ascorbic acid, and lipid peroxidation estimated by malondialdehyde concentrations of limonene-fumigated (LF) plants did not significantly differ from control (C) plants. No consistent changes in emissions of the other monoterpenes, alpha-pinene, beta-phellandrene, beta-pinene or beta-myrcene were found. However, slight differences were found in the concentration of antioxidants. The amounts of alpha-tocopherol did not change or even tended to decrease at high temperatures in LF plants whereas they tended to increase by approximately 60% at 45 and 50degreesC relative to 25degreesC in C plants. Ascorbic acid reached its maximum concentration only at 45degreesC in LF plants whereas it reached its maximum at 35degreesC in C plants. beta-Carotene did not decrease at high temperatures in LF plants whereas it decreased by approximately 15% at 45-50degreesC in C plants. Brown pigment index (BPI), an optical indicator of tissue oxidative processes, was lower in LF plants than in C plants. The photochemical reflectance index (PRI), an optical indicator of photosynthetic light use efficiency, was higher for LF plants than for C plants at elevated temperatures. Visual leaf damage (browning) tended to be less in LF plants than in C plans although not significantly (26.5 +/- 8.5 versus 16.2 +/- 4.8%). These results show that limonene does not confer clear and strong thermotolerance but might have some minor role. These results are in agreement with previous indications of weaker thermotolerance effect of monoterpenes than of isoprene.