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Airborne Ethylene May Alter Antioxidant Protection and Reduce Tolerance of Holm Oak to Heat and Drought Stress¹

Departament de Biologia Vegetal, Universitat de Barcelona, Facultat de Biologia, 08028 Barcelona, Spain (S.M.-B.); and Unitat d'Ecofisiologia CSIC-CEAB-CREAF, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain (J.P., D.A., J.L.)

Plant-emitted ethylene has received considerable attention as a stress hormone and is considered to play a major role at low concentrations in the tolerance of several species to biotic and abiotic stresses. However, airborne ethylene at high concentrations, such as those found in polluted areas (20–100 nL L^–1) for several days, has received far less attention in studies of plant stress tolerance, though it has been shown to alter photosynthesis and reproductive stages (seed germination, flowering, and fruit ripening) in some species. To assess the potential effects of airborne ethylene on plant stress tolerance in polluted areas, the extent of oxidative stress, photo- and antioxidant protection, and visual leaf area damage were evaluated in ethylene-treated (approximately 100 nL L^–1 in air) and control (without ethylene fumigation) holm oak (Quercus ilex) plants exposed to heat stress or to a combination of heat and drought stress. Control plants displayed tolerance to temperatures as high as 50°C, which might be attributed, at least in part, to enhanced xanthophyll de-epoxidation and 2-fold increases in -tocopherol, and they suffered oxidative stress only when water deficit was superimposed on temperatures above 45°C. By contrast, ethylene-treated plants showed symptoms of oxidative stress at lower temperatures (35°C) than the controls in drought, as indicated by enhanced malondialdehyde levels, lower -tocopherol and ascorbate concentrations, and a shift of the redox state of ascorbate to its oxidized form. In addition, ethylene-treated plants showed higher visual leaf area damage and greater reductions in the maximum efficiency of the PSII photochemistry than controls in response to heat stress or to a combination of heat and drought stress. These results demonstrate for the first time that airborne ethylene at concentrations similar to those found in polluted areas may reduce plant stress tolerance by altering, among other possible mechanisms, antioxidant defenses.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.050005.

^* Corresponding author; e-mail smunne{at}ub.edu; fax 34–934112842.

Received July 16, 2004; returned for revision August 6, 2004; accepted August 11, 2004.

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