Induced resistance in the indeterminate growth of aspen (Populus tremuloides)

Studies of induction in trees have examined rapid induced resistance (RIR) or delayed induced resistance (DIR), but have not examined induction that occurs in leaves produced by indeterminately growing trees subsequent to, but in the same season as, damage. We refer to induction that occurs during this time period as intermediate-delayed induced resistance (IDIR). We assessed the influences of genetic and environmental factors, and their interactions, on temporal and spatial variation in induction and on tradeoffs between induced and constitutive levels of resistance in indeterminately growing saplings of aspen (Populus tremuloides). We utilized a common garden of 12 aspen genotypes experiencing two levels of defoliation and two levels of soil nutrients. We assessed concentrations of phenolic glycosides and condensed tannins in damaged leaf remnants collected 1 week after defoliation to examine rapid and local induction, and in undamaged leaves produced 8 weeks after defoliation to assess intermediate-delayed and systemic induction. In general, tannins showed RIR, while phenolic glycosides expressed IDIR. For both classes of allelochemicals, we found high estimates of broad-sense heritability and genetic variation in both induced and constitutive levels. Genetic variation may be maintained by both direct costs of allelochemicals and by costs of inducibility (phenotypic plasticity). Such costs may drive the tradeoff exhibited between induced and constitutive levels of phenolic glycosides. IDIR may be important in reducing total-season tissue loss by providing augmented resistance against late summer herbivores in trees that have experienced damage earlier in the season. Herbivore-resistant compensatory growth is especially beneficial to young trees growing in competitive environments.