Abstract
Understanding interactions between grasses and their herbivores is central to the conservation of species-rich grasslands and the protection of our most important crops against pests. Grasses employ a range of defenses against their natural enemies; silicon-based defenses have been shown to be one of the most effective. Silicon (Si) is laid down on the leaf surface as spines and other sharp bodies, known as phytoliths, making grasses abrasive and their foliage indigestible to herbivores. Previous studies on Si defenses found that closely related species may have similar levels of Si in the leaves but differ markedly in abrasiveness. Here we show how the number, shape and distribution of Si-rich phytoliths and spines differ within and between different grass species and demonstrate that species also differ in their ability to change the deposition and distribution of these defenses in response to damage or increases in Si supply. Specifically, we tested the response of two genotypes of Festuca arundinacea known to differ in their surface texture and three different grass species (F. ovina, F. rubra, and Deschampsia cespitosa) differing in their abrasiveness to combined manipulation of leaf damage and Si supply. F. arundinacea plants with a harsh leaf surface had higher Si content and more spines on their leaf surface than soft varieties. F. ovina and D. cespitosa plants increased their leaf Si concentration and produced an increase in the number of leaf spines and phytoliths on the leaf surface in response to Si addition. F rubra also increased leaf Si content in response to treatments, particularly in damaged leaves, but did not deposit this in the form of spines or increased densities of phytoliths. We discuss how the form in which grasses deposit Si may affect their anti-herbivore characteristics and consider the ecological and agricultural implications of the differences in allocation to Si-based defenses between grass species.
Highlights
Grasslands including managed rangelands and pastures cover ∼40% of the earth’s surface and grasses are an important plant family agriculturally, economically and ecologically (Strömberg, 2005; Gibson, 2009)
INTRASPECIFIC DIFFERENCES The F. arundinacea variety with a harsh leaf surface texture had significantly higher leaf Si concentration than the soft texture variety (F1,19 = 8.586, P < 0.01), but there was no significant interaction between Si addition and variety (F1,19 = 0.282, P > 0.5; Figure 1)
For the F. arundinacea experiment, samples were taken from the harsh variety with added Si and the soft variety with no added Si (Si addition had no effect on Si levels in this experiment – see below)
Summary
Grasslands including managed rangelands and pastures cover ∼40% of the earth’s surface and grasses are an important plant family agriculturally, economically and ecologically (Strömberg, 2005; Gibson, 2009). Are our most widely grown and consumed food crops domesticated grass species, but grasses provide grazing for both wild and domesticated animals In their long co-evolution with grazers (Coughenour et al, 1985), grasses have developed a number of defensive strategies to both tolerate and repel herbivory (Vicari and Bazely, 1993), including rapid regrowth ability from their basal meristems ( an adaptation to fire and trampling common in these ecosystems) and a combination of both chemical defenses (including those provided by endophyte mutualists; Hartley and Gange, 2009) and physical defenses (McNaughton and Tarrants, 1983). The amount of mandible wear feeding imposed on African armyworm (Spodoptera exempta Walker), and the reduction in their ability to extract nitrogen from their food, is correlated with the Si levels of the foliage they consume
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