HERBIVORE RESISTANCE IN<i>BETULA PENDULA</i>: EFFECT OF FERTILIZATION, DEFOLIATION, AND PLANT GENOTYPE

Abstract

Plant resistance to herbivores is affected both by genetic and environmental factors. The carbon–nutrient balance hypothesis (CNB) explains environmentally 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 nutrients (e.g., fertilization) decreases both constitutive and induced resistance against herbivores. We tested the relative roles of plant genotype, defoliation, and soil fertility in determining herbivore resistance of cloned silver birch Betula pendula Roth saplings. As indicators of insect and mammalian resistance we conducted bioassays with a geometrid moth, Epirrita autumnata (Borkhausen), and counted the resin droplets on the shoot of the saplings, respectively. In addition, we measured rapid induced resistance (RIR) against the insect herbivore. Finally, we analyzed leaf secondary chemistry to investigate the correlations of secondary chemicals with the level of resistance measured using the performance of E. autumnata. With respect to the constitutive resistance against an insect herbivore, our results support the CNB hypothesis; the larvae of E. autumnata had a higher relative growth rate and pupal mass on fertilized saplings compared to nonfertilized 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 saplings. 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 resin droplets are not in accordance with the CNB hypothesis. The concentration of condensed tannins correlated negatively with E. autumnata growth rate and pupal mass in both fertilization levels, whereas the concentration of total nontannin phenolics correlated positively with the E. autumnata growth rate in nonfertilized saplings. In addition, the concentration of myricetin glycosides correlated negatively with the pupal mass of E. autumnata, whereas the correlations between E. autumnata performance indices and other groups of flavonol glycosides were either significantly positive (kaempferol glycosides) or nonsignificant (quercetin glycosides). Further, the concentration of 3,4′-dihydroxypropiophenone 3-glucoside (DHPPG) correlated 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. autumnata performance indices suggest that the constitutive level of resistance of B. pendula against E. autumnata is mainly determined by the concentration of condensed tannins, whereas the induced resistance is determined by the concentration of nontannin phenolics, such as flavonol glycosides and DHPPG. We observed significant differences among the clones in their insect and mammalian resistance (i.e., genetic basis for the resistance), which indicates that resistance can evolve as a response to herbivory. However, fertilization explained a higher proportion of variance in insect performance indices than the genotype of the plant, whereas the opposite was true for the amount of resin droplets, which we used as an indicator of mammalian resistance.