284 Chorbadjian et al.: Defensive Chemistry and Herbivore Resistance of Austrian Pine and Paper Birch sistent with a well-documented pattern that slower growing trees are more resis- tant to defoliating insects (Herms 2002). Given that paclobutrazol substantially Figure 4. Linear correlation analyses between leaf chemical composition and annual diameter trunk growth of paper birch. Results for 2003 show a correlation between sec- ondary chemicals of leaves sampled on June 24, 2003, and trunk growth during 2003. Results for 2004 show a correlation between secondary chemicals of leaves sampled on June 14, 2004, and trunk growth during 2004. resources (Gross 1972; Tuomi et al. 1982). Other studies have also observed paclobutrazol to increase fruit production in woody plants (e.g., Struve et al. 1989; Huang et al. 1995). The Growth/Differentiation Balance Hypothesis predicts that when growth decreases proportionally more than photosynthe- sis, the availability of carbon to support other processes, such as the production of secondary metabolites, will increase, thereby increasing insect resistance (Herms and Mattson 1992; Herms and Mattson 1997; Herms 2002). The responses observed in this study were only partially consistent with these predictions. As predicted, paclobutrazol increased foliar concentrations of con- densed tannins, but not total phenolics, of paper birch in 2003 (in the growing season after the treatment was applied the pre- vious autumn). However, the treatment persisted for only one year, with no effect observed in 2004. The negative correlation between growth and secondary metabolites observed in paper birch across all treatments in 2003 and 2004 is also consis- tent with growth-defense allocation tradeoff predicted by the Growth/Differentiation Balance Hypothesis (Herms and Matt- son 1992; Herms and Mattson 1997). In one of the few studies to examine effects on secondary metabolism, Wang and Stef- fens (1987) also observed paclobutrazol to increase foliar phe- nolics in apple trees. Several studies with deciduous trees have observed paclobutrazol to increase the accumulation of storage compounds (Steffens et al. 1985; Wieland and Wample 1985; Wang et al. 1986; Yim et al. 1997), which is often associated with increased biosynthesis of secondary metabolites (Herms 2002). Effect of paclobutrazol on insect growth did not corre- spond closely with effects on secondary chemistry. It had no effect on larval growth of gypsy moth and forest tent caterpillar in 2003, the year that paclobutrazol increased foliar concentrations of condensed tannins. However, pa- clobutrazol did increase paper birch resistance to gypsy moth and white-marked tussock moth in 2004, which is con- ©2011 International Society of Arboriculture decreased growth of Austrian pine with no effect of photosynthesis, the lack of any ef- fect on condensed tannins, total phenolics, and terpenes, as well as the lack of nega- tive correlations between tree growth and secondary metabolism, are inconsistent with the prediction of allocation theory of a physiological tradeoff between growth and defense in Austrian pine (Herms and Mattson 1992). Paclobutrazol also had no effect on growth of European pine sawfly larvae, but this is not surprising given the lack of effect on foliar concentrations of nitrogen or secondary metabolites. Hart- man et al. (2009) also found that the regu- lator decreased growth of Austrian pine, but with no effect on resistance to diplo- dia tip blight pathogen, Diplodia pinea. Other studies also have observed vari- able effects of paclobutrazol on plant re- sistance to insects and mites. For example, the regulator decreased populations of twospotted spider mite (Tetranychus urti- cae) on ageratum (Ageratum houstonianum) after four weeks, while populations of thrips (Frankliniella occidentalis) and green peach aphid (Myzus persicae) were not affected (Lat- imer and Oetting 1999). In the same study, paclobutrazol had no effect on pest resistance of other herbaceous species includ- ing columbine (Aquilegia × hybrida), New Guinea impatiens (Impatients hawkeri), and marigold (Tagetes erecta). Drench application of paclobutrazol reduced the number of pear rust mites (Epitrimerus pyri) on pear (Raese and Burts 1983), while foliar sprays reduced the population level of pear rust mite and pear psyllid (Cacopsylla pyricola) (Campbell et al. 1989). Interestingly, the foliar spray applications had no direct con- tact insecticidal activity, suggesting that effects on pests were mediated by physiological responses of the host plant to pa- clobutrazol. Recommended rates of paclobutrazol applied as a soil drench did not consistently affect the survival or devel- opmental time of orangestriped oakworm (Anisota senato- ria) on willow oak, although higher rates (4× recommended) did increase tree resistance to this insect (Coffelt et al. 1993). In this study, fertilization had no effect on growth of paper birch or Austrian pine, which suggests that the native soils were inherently fertile enough to fully meet the nutrient re- quirements of both species. This pattern has also been ob- served frequently in other fertilization experiments with trees (reviewed in Struve 2002). Fertilization also had no effect on secondary metabolism of paper birch or Austrian pine, which is consistent with the Growth/Differentiation Balance Hypoth- esis, given that fertilization also had no effect on tree growth (Herms 2002). The increased growth of gypsy moth on fertil- ized birch trees in 2004 is consistent with numerous studies that have found fertilization to decrease insect resistance of trees (Herms 2002). Contrary to conventional wisdom (e.g., Smith 1978; Iles 2000; Smiley 2007), researchers in this study
November 2011
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
