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Influence of nutrient availability on the mechanisms of tolerance to herbivory in an annual grass, Avena barbata (Poaceae)¹

Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1

ABSTRACT

Tolerance, or the capacity of a genotype to survive and reproduce following herbivore damage, varies widely across the plant kingdom. One proximate cause of this variation is resource availability, which can influence tolerance through mechanisms such as growth rate and photosynthesis. We examined the effect of high and low soil nutrient levels on the relationship between tolerance and two of its underlying mechanisms, biomass regrowth and photosynthetic upregulation, among genotypes of the Mediterranean annual grass Avena barbata. Although defoliated plants did not reach the same biomass as controls, biomass regrowth was higher at high nutrients. However, increased seed abortion at high nutrients caused tolerance to be the same in both nutrient treatments. Increased seed abortion also uncoupled biomass regrowth from tolerance at high nutrients. We found no evidence for photosynthetic upregulation in defoliated compared to control plants in either nutrient treatment. However, tolerance was positively correlated with predefoliation photosynthetic efficiency at high nutrients. Thus, constitutive photosynthetic efficiency may be a better predictor of tolerance than photosynthetic responses following herbivory in A. barbata. More generally, our results highlight the possibility that the mechanisms of tolerance can differ across resource environments even if tolerance is the same.

Key Words: Avena barbata • biomass regrowth • chlorophyll fluorescence • fitness • herbivory • nutrients • seed abortion

Received for publication 8 August 2007. Accepted for publication 14 December 2007.

FOOTNOTES

¹ The authors thank R. G. Latta for providing seeds of the Avena barbata RILs and S. Campbell, C. M. Caruso, S. M. Louda, A. Parachnowitsch, D. Pilson, M. Sherrard, and three anonymous reviewers for helpful comments on previous drafts of this manuscript. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the Ontario Innovation Trust.

² Current address: School of Biological Sciences, University of Nebraska, Lincoln, NE 68588 USA; e-mail: tsuwa{at}unlserve.unl.edu

³ Author for correspondence (e-mail: maherali{at}uoguelph.ca)

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