278 Chorbadjian et al.: Defensive Chemistry and Herbivore Resistance of Austrian Pine and Paper Birch Arboriculture & Urban Forestry 2011. 37(6): 278–287 Effect of the Growth Regulator Paclobutrazol and Fertilization on Defensive Chemistry and Herbivore Resistance of Austrian Pine (Pinus nigra) and Paper Birch (Betula papyrifera) Rodrigo A. Chorbadjian, Pierluigi Bonello, and Daniel A. Herms 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 paclobutra- zol slowed the growth of paper birch (Betula papyrifera) and Austrian pine (Pinus nigra) with no effect on photosynthesis. In response, fo- liar 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 consis- tent 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. Key Words. Allocation Trade-offs; Growth/Differentiation Balance Hypothesis; Growth Regulator; Insect Resistance; Plant Defense Theory. Secondary metabolites are important defenses of plants to in- sects and pathogens, and their concentration can be mediated by resource-based allocation trade-offs between growth and other functions (Herms and Mattson 1992; Herms and Mattson 1997; Herms 2002). The Growth/Differentiation Balance Hy- pothesis predicts that environmental factors that reduce plant growth with little effect on photosynthesis should increase the availability of resources allocated to the production of second- ary metabolites, thereby increasing resistance to insects and other herbivores (Herms and Mattson 1992; Glynn et al. 2007). The plant growth regulator paclobutrazol reduces plant growth, which may reduce the need for mechanical pruning of shade trees in urban settings and utility rights-of-way (Mann et al. 1995), although tree species can vary in their sensitiv- ity to this growth regulator (e.g., Bai et al. 2004). Paclobutra- zol decreases plant growth by inhibiting gibberellin biosynthe- sis (Rademacher 2000) without diminishing photosynthetic rate (Wieland and Wample 1985; Archbold and Houtz 1988; Vu and Yelenosky 1988; Yim et al. 1997). It reduces plant growth with- out directly interfering with secondary metabolite biosynthetic pathways, as it inhibits gibberellin synthesis downstream in the chain of reactions leading to the production of secondary me- tabolites, i.e., the biosynthesis of tannins, phenolic compounds, and terpenoids (Rademacher 2000). Hence, in the context of the Growth/Differentiation Balance Hypothesis, it can be hypoth- esized that paclobutrazol will increase availability of photosyn- thate for allocation to secondary metabolism and thus increase plant resistance to herbivores. Indeed, some studies have found ©2011 International Society of Arboriculture that paclobutrazol can enhance the resistance of plants to insect pests (Raese and Burts 1983; Campbell et al. 1989; Coffelt et al. 1993), pathogens (Blaedow et al. 2006; DeStefano et al. 2007; Collins and Rawnsley 2008; but see Hartman et al. 2009) and water stress (Marshall et al. 2000; Percival and AlBalushi 2007). Conversely, nitrogen fertilization generally increases growth with less effect on photosynthesis, which has been associated with reduced insect resistance (Herms 2002). Fertilization increases growth rate because trees are stimulated to allocate a greater pro- portion of their carbon to production of new leaf area. However, root growth is proportionally decreased, as are concentrations of storage and defensive compounds (e.g., phenolics and other secondary metabolites) (Herms and Mattson 1997; Herms 2002; Glynn et al. 2007). Fast-growing plants typically have lower mass per leaf area (specific leaf mass), which is generally associated with lower levels of plant defensive chemicals and increased leaf nitrogen (N) content, both factors being beneficial to insect growth (Herms and Mattson 1992; Herms and Mattson 1997). The objective of this study was to quantify the effects of paclobutrazol and fertilization, separately and in combina- tion, on growth, photosynthesis, phytochemistry, and leaf morphology of paper birch (Betula papyrifera) and Austrian pine (Pinus nigra), as well as their resistance to key defoliat- ing insect pests. Specifically, the researchers quantified growth and survival of gypsy moth (Lymantria dispar), forest tent cat- erpillar (Malacosoma disstria), and white-marked tussock moth (Orgyia leucostigma) larvae on paper birch, and Euro- pean pine sawfly (Neodiprion sertifer) larvae on Austrian pine.
November 2011
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