Plant Physiol, November 2002, Vol. 130, pp. 1371-1385

Stomatal Constraints May Affect Emission of Oxygenated Monoterpenoids from the Foliage of Pinus pinea^1,[w]

Department of Plant Physiology, Institute of Molecular and Cell Biology, University of Tartu, Riia 23, EE 51010 Tartu, Estonia (Ü.N.); Department of Plant Ecology, University of Bayreuth, D-95440 Bayreuth, Germany (M.R., J.D.T.); and Joint Research Centre of the European Commission, Environment Institute, 21020 Ispra (Va), Italy (M.S., G.S.)

Dependence of monoterpenoid emission and fractional composition on stomatal conductance (G_V) was studied in Mediterranean conifer Pinus pinea, which primarily emits limonene and trans--ocimene but also large fractions of oxygenated monoterpenoids linalool and 1,8-cineole. Strong decreases in G_V attributable to diurnal water stress were accompanied by a significant reduction in total monoterpenoid emission rate in midday. However, various monoterpenoids responded differently to the reduction in G_V, with the emission rates of limonene and trans--ocimene being unaffected but those of linalool and 1,8-cineole closely following diurnal variability in G_V. A dynamic emission model indicated that stomatal sensitivity of emissions was associated with monoterpenoid Henry's law constant (H, gas/liquid phase partition coefficient). Monoterpenoids with a large H such as trans--ocimene sustain higher intercellular partial pressure for a certain liquid phase concentration, and stomatal closure is balanced by a nearly immediate increase in monoterpene diffusion gradient from intercellular air-space to ambient air. The partial pressure rises also in compounds with a low H, but more than 1,000-fold higher liquid phase concentrations of linalool and 1,8-cineole are necessary to increase intercellular partial pressure high enough to balance stomatal closure. The system response is accordingly slower, and the emission rates may be transiently suppressed by low G_V. Simulations further suggested that linalool and 1,8-cineole synthesis rates also decreased with decreasing G_V, possibly as the result of selective inhibition of various monoterpene synthases by stomata. We conclude that physicochemical characteristics of volatiles not only affect total emission but also alter the fractional composition of emitted monoterpenoids.