Herbivore resistance in Betula pendula: Effect of fertilization, defoliation, and plant genotype

Plant resistance to herbivores is affected both by genetic and environmenta l factors. The carbon-nutrient balance hypothesis (CNB) explains environmen tally induced variation in both constitutive and delayed herbivore-induced resistance (DIR) in terms of variation in soil fertility and light regime. The CNB hypothesis predicts that an increase in the availability of nutrien ts (e.g., fertilization) decreases both constitutive and induced resistance against herbivores. We tested the relative roles of plant genotype, defoli ation, and soil fertility in determining herbivore resistance of cloned sil ver birch Betula pendula Roth saplings. As indicators of insect and mammali an resistance we conducted bioassays with a geometrid moth, Epirrita autumn ata (Borkhausen), and counted the resin droplets on the shoot of the saplin gs, respectively. In addition, we measured rapid induced resistance (RIR) a gainst the insect herbivore. Finally, we analyzed leaf secondary chemistry to investigate the correlations of secondary chemicals with the level of re sistance measured using the performance of E. autumnata. With respect to the constitutive resistance against an insect herbivore, ou r results support the CNB hypothesis; the larvae of E. autumnata had a high er relative growth rate and pupal mass on fertilized saplings compared to n onfertilized saplings, i.e., the fertilized saplings had a lower resistance level. However, the relative growth rate of E. autumnata was significantly decreased by defoliation only when the larvae were grown on fertilized sap lings. The number of resin droplets increased due to fertilization and, in fertilized saplings, following defoliation, but these responses were highly determined by the genotype of the sapling. Altogether, the results on resi n droplets are not in accordance with the CNB hypothesis. The concentration of condensed tannins correlated negatively with E. autumn ata growth rate and pupal mass in both fertilization levels, whereas the co ncentration of total nontannin phenolics correlated positively with the E. autumnata growth rate in nonfertilized saplings. In addition, the concentra tion of myricetin glycosides correlated negatively with the pupal mass of E . autumnata, whereas the correlations between E. autumnata performance indi ces and other groups of flavonol glycosides were either significantly posit ive (kaempferol glycosides) or nonsignificant (quercetin glycosides). Furth er, the concentration of 3,4'-dihydroxypropiophenone 3-glucoside (DHPPG) co rrelated positively with the magnitude of induction in E. autumnata growth rate and pupal mass in fertilized saplings, where the significant induction in resistance occurred. The correlations of secondary chemistry and E, aut umnata performance indices suggest that the constitutive level of resistanc e of B. pendula against E. autumnata is mainly determined by the concentrat ion of condensed tannins, whereas the induced resistance is determined by t he concentration of nontannin phenolics, such as flavonol glycosides and DH PPG. We observed significant differences among the clones in their insect and ma mmalian resistance (i.e., genetic basis for the resistance), which indicate s that resistance can evolve as a response to herbivory. However, fertiliza tion explained a higher proportion of variance in insect performance indice s than the genotype of the plant, whereas the opposite was true for the amo unt of resin droplets, which we used as an indicator of mammalian resistanc e.