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ENVIRONMENTAL STRESS AND ADAPTATION

The Capacity for Thermal Protection of Photosynthetic Electron Transport Varies for Different Monoterpenes in Quercus ilex¹

Department of Plant Physiology, Institute of Molecular and Cell Biology, University of Tartu, Tartu 51010, Estonia (L.O.C., Ü.N.); and Unitat Ecofisiologia CSIC-CEAB-CREAF, Center for Ecological Research and Forestry Applications, Edifici C, Universitat Autònoma de Barcelona, 08193 Barcelona, Spain (I.F., J.L., J.P.)

Heat stress resistance of foliar photosynthetic apparatus was investigated in the Mediterranean monoterpene-emitting evergreen sclerophyll species Quercus ilex. Leaf feeding with fosmidomycin, which is a specific inhibitor of the chloroplastic isoprenoid synthesis pathway, essentially stopped monoterpene emission and resulted in the decrease of the optimum temperature of photosynthetic electron transport from approximately 38°C to approximately 30°C. The heat stress resistance was partly restored by fumigation with 4 to 5 nmol mol^–1 air concentrations of monoterpene -pinene but not with fumigations with monoterpene alcohol -terpineol. Analyses of monoterpene physicochemical characteristics demonstrated that -pinene was primarily distributed to leaf gas and lipid phases, while -terpineol was primarily distributed to leaf aqueous phase. Thus, for a common monoterpene uptake rate, -terpineol is less efficient in stabilizing membrane liquid-crystalline structure and as an antioxidant in plant membranes. Furthermore, -terpineol uptake rate (U) strongly decreased with increasing temperature, while the uptake rates of -pinene increased with increasing temperature, providing a further explanation of the lower efficiency of thermal protection by -terpineol. The temperature-dependent decrease of -terpineol uptake was both due to decreases in stomatal conductance, g_w, and increased volatility of -terpineol at higher temperature that decreased the monoterpene diffusion gradient between the ambient air (F_A) and leaf (F_I; U = g_w[F_A – F_I]). Model analyses suggested that -pinene reacted within the leaf at higher temperatures, possibly within the lipid phase, thereby avoiding the decrease in diffusion gradient, F_A – F_I. Thus, these data contribute to the hypothesis of the antioxidative protection of leaf membranes during heat stress by monoterpenes. These data further suggest that fumigation with the relatively low atmospheric concentrations of monoterpenes that are occasionally observed during warm windless days in the Mediterranean canopies may significantly improve the heat tolerance of nonemitting vegetation that grows intermixed with emitting species.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.065995.

^* Corresponding author; e-mail ylon{at}ut.ee; fax 003727366050.

Received May 20, 2005; returned for revision June 2, 2005; accepted July 6, 2005.

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