M. Keinanen et al., Trade-offs in phenolic metabolism of silver birch: Effects of fertilization, defoliation, and genotype, ECOLOGY, 80(6), 1999, pp. 1970-1986
We examined the chemical responses of 10 silver birch (Betula pendula) clon
es to fertilization and defoliation in a held experiment. In defoliation, e
very second leaf was removed from the saplings. Three days later, two undam
aged short-shoot leaves were collected, air-dried, and analyzed for condens
ed tannins and 34 nontannin phenolic compounds by high-performance liquid c
hromatography. The clones showed substantial variation in phenolic composit
ion of the leaves and in chemical responses to fertilization and defoliatio
n. A cluster analysis by UPGMA indicated that the phenolic profiles of birc
h leaves were affected more by genotype than fertilization or defoliation,
and the clones could thus be distinguished from each other. In addition, on
the basis of their overall phenolic composition, the clones were clustered
loosely in three clone groups.
The leaves of fertilized saplings contained lower levels of condensed tanni
ns than controls, as predicted by carbon/nutrient balance (CNB) hypothesis.
However, fertilization had no effect on the total amount of nontannin phen
olics. The concentrations of (+)-catechin, 3,4'-dihydroxypropiophenone 3-gl
ucoside (DHPPG), 3-cinnamoylquinic acids, and flavone aglycones were lower
in fertilized saplings, whereas the opposite was true for 5-cinnamoylquinic
acids and the total amount of flavonol glycosides. Although our results pr
ovide support for the CNB hypothesis, they also show that the accumulation
of phenolic compounds in birch leaves is strongly coordinated. Different br
anches of the biosynthetic pathway of phenolic compounds may compete for su
bstrates, and such internal metabolic trade-offs may explain the differenti
al accumulation of the compounds. In fertilized saplings, the concentration
of condensed tannins was also negatively correlated with the amount of tri
terpenoid resin droplets measured from the same saplings, We suggest that a
linkage via malonyl-CoA between the biosynthetic routes to terpenoids and
flavonoid derivatives, such as condensed tannins, may explain the different
responses to fertilization reported for terpenoids and phenolics.
Undamaged leaves of partially defoliated saplings contained more DHPPG and
flavone aglycones and less cinnamic acid derivatives and (+)-catechin than
did leaves of control saplings. The induction of DHPPG and flavonoid aglyco
nes was significantly and negatively correlated with the concentration of m
yricetin glycosides in fertilized saplings, which may indicate a trade-off
between induced and constitutive defense. Moreover, in fertilized saplings,
the three clone groups formed by UPGMA clustering differed significantly i
n the magnitude of induction of DHPPG and flavone aglycones. Different birc
h genotypes may thus have different modes of chemical defense, and the magn
itude of chemical response of a genotype may partly depend on resource avai
lability. In general, our results show that new insights in the theory of c
hemical defense can be gained by accomplishing studies on plant-herbivore i
nteraction with high chemical resolution.