Abstract
Grasses accumulate large amounts of silicon (Si) which is deposited in trichomes, specialised silica cells and cell walls. This may increase leaf toughness and reduce cell rupture, palatability and digestion. Few studies have measured leaf mechanical traits in response to Si, thus the effect of Si on herbivores can be difficult to disentangle from Si-induced changes in leaf surface morphology. We assessed the effects of Si on Brachypodium distachyon mechanical traits (specific leaf area (SLA), thickness, leaf dry matter content (LDMC), relative electrolyte leakage (REL)) and leaf surface morphology (macrohairs, prickle, silica and epidermal cells) and determined the effects of Si on the growth of two generalist insect herbivores (Helicoverpa armigera and Acheta domesticus). Si had no effect on leaf mechanical traits; however, Si changed leaf surface morphology: silica and prickle cells were on average 127% and 36% larger in Si supplemented plants, respectively. Prickle cell density was significantly reduced by Si, while macrohair density remained unchanged. Caterpillars were more negatively affected by Si compared to crickets, possibly due to the latter having a thicker and thus more protective gut lining. Our data show that Si acts as a direct defence against leaf-chewing insects by changing the morphology of specialised defence structures without altering leaf mechanical traits.
Highlights
Many grass species have the ability to hyperaccumulate silicon, which can make up to 10% dry weight in some species [1,2]
Mean Si concentration in Si+ plants was 1.54% (± 0.14) while Si- plants were below the limit of quantification (
Si had no significant effect on specific leaf area (SLA) or leaf thickness (Figure 1C and Table 1a)
Summary
Many grass species have the ability to hyperaccumulate silicon, which can make up to 10% dry weight in some species [1,2]. Grasses can deposit Si as amorphous solid silica in various ways: inside cell walls and intercellular spaces, in silicified external structures such as trichomes and in specialised silica cells [8]. These Si-induced changes in leaf surface morphology are effective against insect herbivores, reducing leaf consumption and hindering digestion [9,10]. Si can be deposited in other surface structures such as trichomes, increasing their length and density [11,13]. Trichomes and leaf hairs are often sharp with rigid spikes, and have been shown to significantly damage herbivore digestive tracts [14]. When reinforced with Si, these surface structures cause even greater damage to the insect gut epithelium when ingested [15]
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