Abstract
Grasses (Poaceae) lack the complex biochemical pathways and structural defenses employed by other plant families; instead they deposit microscopic silica (SiO2) granules in their leaf blades (i.e., phytoliths) as a putative defense strategy. Silica accumulation in grasses has generally been considered an inducible defense; other research suggests silica accumulation occurs by passive diffusion and should therefore be closely coupled with whole plant transpiration. We tested the hypothesis that grasses increase leaf silica concentration in response to artificial defoliation in a common garden study in the Serengeti ecosystem of East Africa. Additionally, a watering treatment tested the alternative hypothesis that leaf silica was largely driven by plant water status. Leaf silica content of two dominant C4 Serengeti grass species, Themeda triandra and Digitaria macroblephara, was quantified after a 10-month clipping × water experiment in which defoliation occurred approximately every 2 months and supplementary water was added every 2 weeks. Themeda had greater silica content than Digitaria, and Themeda also varied in foliar silica content according to collection site. Clipping had no significant effect on leaf silica in either species and watering significantly increased silica content of the dominant tall grass species, Themeda, but not the lawn species, Digitaria. Our data, and those collected as part of a supplementary literature review, suggest that silicon induction responses are contingent upon a combination of plant identity (i.e., species, genotype, life history limitations) and environmental factors (i.e., precipitation, soil nutrients, grazing intensity). Specifically, we propose that an interaction between plant functional type and water balance plays an especially important role in determining silica uptake and accumulation.
Highlights
Plants have two general and non-mutually exclusive adaptive strategies to cope with herbivory: tolerance and resistance (Mauricio et al, 1997)
There was a significant species by watering interaction effect (p
This result arose because individuals of T. triandra that were watered had a higher leaf silica concentration (4.4 ± 0.3%) compared to those T. triandra plants that were maintained at ambient soil moisture levels (3.3 ± 0.3%)
Summary
Plants have two general and non-mutually exclusive adaptive strategies to cope with herbivory: tolerance and resistance (Mauricio et al, 1997). Some plant groups face relatively intense or frequent herbivory and utilize both tolerance and direct resistance strategies. Grasses utilize both secondary chemicals, e.g., phenolics (Schaller et al, 2012), and structural components, e.g., microscopic deposits of solid silica termed phytoliths (Ma and Yamaji, 2006), to deter herbivores. Accumulation of silica phytoliths has been considered the main defensive strategy of grasses (Coughenour, 1985), as they can amass relatively large amounts of silica and lack chemical defenses as compared to dicotyledonous plants. Other than grasses, silica accumulation occurs primarily in ancient plant groups such as mosses, ferns, and horsetail (Hodson et al, 2005), and this fact, along with the observation that tooth enamel is considerably harder than phytoliths (Sanson et al, 2007) raises questions about the efficacy of silica as a deterrent of large-bodied mammalian grazers. Poales (the group containing grasses) are the principal silica accumulators, with wetland Gramineae accumulating up to 15% dry www.frontiersin.org
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