Effect of the Growth Regulator Paclobutrazol and Fertilization on Defensive Chemistry and Herbivore Resistance of Austrian Pine (Pinus nigra) and Paper Birch (Betula papyrifera)

Abstract

The Growth/Differentiation Balance Hypothesis predicts that environmental factors that limit growth of plants more than their rate of photosynthesis should increase secondary metabolism and resistance to insects. Soil drench application of the plant growth regulator paclobutrazol slowed the growth of paper birch (Betula papyrifera) and Austrian pine (Pinus nigra) with no effect on photosynthesis. In response, foliar concentrations of condensed tannins (but not total phenolics) in birch increased as predicted, which increased birch resistance to gypsy moth (Lymantria dispar) and whitemarked tussock moth (Orgyia leucostigma), but only during the second season after treatment. In both years, there was a negative correlation between foliar concentrations of total phenolic and condensed tannins and growth of paper birch, which is consistent with the predicted trade-off between growth and defense. Conversely, in Austrian pine, paclobutrazol and fertilization did not have an effect on foliar concentration of tannins, phenolics, and terpenes, nor did the treatments have any effect on resistance to European pine sawfly (Neodiprion sertifer). Hence, the effects of paclobutrazol on tree growth, defensive chemistry, and insect resistance were species-specific and time sensitive.