Floral scent production in Clarkia (Onagraceae): I. Localization and developmental modulation of monoterpene emission and linalool synthase activity

Abstract

The flowers of many plants emit volatile compounds as a means of attracting pollinators. We have previously shown that the strong, sweet fragrance of Clarkia breweri (Onagraceae), an annual plant native to California, consists of approximately 8 to 12 volatile compounds - three monoterpenes and nine benzoate derivatives (R.A. Raguso and E. Pichersky [1994] Plant Syst Evol [in press]). Here we report that the monoterpene alcohol linalool is synthesized and emitted mostly by petals but to a lesser extent also by the pistil and stamens. Two linalool oxides are produced and emitted almost exclusively by the pistil. These three monoterpenes are first discernible in mature unopened buds, and their tissue levels are highest during the first 2 to 3 d after anthesis. Levels of emission by the different floral parts throughout the life span of the flower were correlated with levels of these monoterpenes in the respective tissues, suggesting that these monoterpenes are emitted soon after their synthesis. Activity of linalool synthase, an enzyme that converts the ubiquitous C10 isoprenoid intermediate geranyl pyrophosphate to linalool, was highest in petals, the organ that emits most of the linalool. However, linalool synthase activity on a fresh weight basis was highest in stigma and style (i.e. the pistil). Most of the linalool produced in the pistil is apparently converted into linalool oxides. Lower levels (0.1%) of monoterpene emission and linalool synthase activity are found in the stigma of Clarkia concinna, a nonscented relative of C. breweri, suggesting that monoterpenes may have other functions in the flower in addition to attracting pollinators.