A balance between the rate of synthesis and rate of loss controls the accumulation of any compound. In the plant's economy, it is important whether there is rapid metabolic turnover of secondary compounds, whether some may be catabolized to primary metabolites late in the life of a plant organ, or whether some may be released into the environment. A rapid rate of turnover (including biosynthetic interconversion, conjugation reactions, and polymerization) could increase the cost of maintaining a given concentration of defensive compounds and is often thought to be a major component of the cost of plant chemical defense (Gershenzon 1994a, b). Traditionally, terpenoid secondary chemicals have been viewed as stable end products of metabolism, although Burbott and Loomis (1969) demonstrated rapid turnover of monoterpenes in leaves of detached stem cuttings of peppermint (Mentha). This report was much cited, especially by ecologists. On the other hand, when Mihaliak et al. (1991) repeated the experiments using rooted, intact plants, either low rates or no turnover was detected, suggesting that short-term turnover of mono-terpenes does not occur normally in mint leaves but is an artifact seen only in cuttings. In further experiments to test various parameters that could affect turnover in intact plants, Gershenzon et al. (1993) were unable to detect significant turnover in developing leaves of species from a range of taxonomically distant terpene-accumulating families that synthesize mono-, sesqui-, and diterpenes and that store the products in various kinds of secretory structures.
In contrast to the lack of evidence for rapid or short-term turnover of monoterpenes, it is well known that various mono-, sesqui-, di-, and triter-penes (some of which occur in resin) may be lost from leaves late in their development. Some monoterpenes in mature leaves of several mint species are mobilized prior to senescence, when they no longer serve defensive roles (Gershenzon 1994a, b). These terpenes can be catabolized to water-soluble glycosides, which apparently are exported to the root and oxidatively degraded to acetyl coenzyme A (Croteau and Martinkus 1979, Croteau and Sood 1985, Croteau 1988). Thus, apparently, the fixed carbon of some terpenes can be recycled into usable primary metabolites for biosynthesis of new materials (Figure 1-1). Evidence further suggests that synthesis, storage, and catabolism of terpenes may be partially controlled by a balance of photosynthesis and use of the photosynthate through growth and differentiation into various structures and compounds (Loomis and Croteau 1973, Gershenzon
1994b). Although Gershenzon et al. (2000) found no evidence for monoter-pene catabolism in peppermint, they suggested that large variances in mono-terpene incorporation after pulse labeling may have prevented its detection. Alternatively, they hypothesized that the degradation enzyme in mints may detoxify monoterpenes that have come into contact with living cells following damage to the secretory structures. Although catabolism of terpenes may have considerable physiological and ecological significance, the data are fragmentary and little is known about the process or even if such catabolism occurs in the complex mixture constituting resin.
Studies of Mentha have shown that the rate of monoterpene biosynthesis, determined by 14CO2 incorporation, closely correlates with monoterpene accumulation and appears to be the principal factor controlling the mono-terpene level of peppermint leaves (Gershenzon et al. 2000). In addition to lack of detection of catabolic losses through leaf development, volatilization occurred at a low rate, which on a monthly basis represented less than 1% of the total pool of stored monoterpenes. Composition of the volatilized monoterpenes was sufficiently different from the total plant monoterpene pool that Gershenzon and coworkers suggested that the volatilized products may arise from a separate secretory system, as inferred from previous studies using other plant species (Chapter 3). It is not known if monoterpenes in a resin respond differently when they are formed in different secretory structures, especially with the evidence of terpenoid volatilization from conifers and its role in tropospheric chemistry (Chapter 5).
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