P. Mutikainen et al., Herbivore resistance in Betula pendula: Effect of fertilization, defoliation, and plant genotype, ECOLOGY, 81(1), 2000, pp. 49-65
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.