The Genetic Basis for Variation in the Concentration of Phenolic Glycosides in Salix sericea" Clonal Variation and Sex-based Differences

-Many members of the Salicaceae produce phenolic glycosides, and the concentrations of these are known to vary among plants. We determined whether the concentration of two phenolic glycosides, salicortin and 2'-cinnamoyl salicortin, varied among clones of Salix sericea and whether this variation was due to the sex of the plant. We tested 16 clones, seven male and nine female, for variation in glycoside concentration and several plant growth rate parameters. The concentrations of salicortin and 2'-cinnamoyl salicortin varied extensively between clones and exhibited broad-sense heritability. Clones also varied in growth rate. Interestingly, none of these clone effects were due to differences between male and female plants. There was no correlation between shoot length and chemistry, although it appears that faster growing clones often produce lower concentrations of the two phenolic glycosides. We discuss these findings in relation to current theory concerning plant allocation to growth and defense. Introduction Willows and other Salicaceous plant species often produce phenolic glycosides (Julkunen-Tiitto, 1986, 1989; Lindroth et al., 1987a). The concentrations of the phenolic glycosides may vary among individuals and in some cases influence the susceptibility of plants to both insect and mammalian herbivores (Basey et al., 1988; Clausen et al., 1989; Lindroth and Peterson, 1988; Lindroth et a/., 1988; RowelI-Rahier, 1984; Tahvanainen et al., 1985). Recently researchers have investigated the causes of variation among willow plants. In some willow species, mate plants have lower concentrations of phenolic glycosides than females (Boecklen et al., 1990; Palo, 1984; Price etal., 1989; Elmqvist eta/., 1981), which may provide an explanation for why male plants are often more susceptible to herbivores (Danell eta/. , 1985; Elmqvist et al., 1988) and pathogens (Verwijst, 1990). These sex-based differences in chemistry and susceptibility may be due to male plants investing a greater percentage of their resources to growth (Boecklen eta/., 1990; Jing and Coley, 1990). Although sex-based differences appear common, non-sex related genetic differences may be important as well. A couple of studies have provided evidence indicating that genetic differences may occur in members of the Salicaceae family (Lindroth et al., 1987b; Price et aL, 1987), yet neither study appeared to assess the role of sex-based differences. Our study species, Salix sericea Marshall, produces phenolic glycosides. The two major phenolic glycosides are salicortin (1) and a recently reported compound, 2'-cinnamoyl salicortin (2) (Fig. 1) (Nichols-Orians et al., 1992). Compounds with the salicortin core structure have been shown to be biologically active against a variety of insect herbivores (Clausen et al., 1989; Lindroth, 1992; Lindroth and Peterson, 1988; Lindroth et al., 1988). Products from the enzymatic or chemical breakdown of 1 are tAuthor to whom correspondence should be addressed at Department of Biology, Williams College, Williamstown, MA 01267, U.S.A. (Received 1 December 1992)