Leaf silicification provides herbivore defence regardless of the extensive impacts of water stress

Altered precipitation patterns due to climate change are likely to impose
water-deficit stress in plants resulting in changes to specific leaf mass,
leaf water content and chemical defences that may impact herbivorous
arthropods. Grasses, in particular, accumulate large concentrations of
silicon (Si) which provides physical defence against herbivores. Although
Si uptake by plants may be affected by water availability, very few
studies have investigated the combined effect of water-deficit stress and
Si on insect herbivore performance. We grew tall fescue (Festuca
arundinacea Schreb.) hydroponically, with and without Si, and half of the
plants were treated with 20% polyethylene glycol (PEG) to impose osmotic
stress. Eleven leaf traits (physiological, chemical and structural) were
measured, silicified phytoliths on the leaf surface were visualised using
scanning electron microscopy (SEM) in conjunction with X-ray mapping, and
plants were exposed to a chewing insect herbivore (Helicoverpa armigera
Hübner (Lepidoptera: Noctuidae)). Although osmotic stress was associated
with changes to leaf physiological and chemical traits, including
increased specific leaf mass, decreased leaf relative water content and
increased leaf nitrogen (N), there was no significant effect on H.
armigera relative growth rate (RGR). However, Si reduced RGR of H.
armigera by 80-98%, while generating few changes to physiological and
chemical leaf traits. Instead, the decline in RGR with Si was associated
with changes to leaf structural traits, in particular, a greater density
of silicified phytoliths on the leaf surface. Comparison of effect sizes
indicated that leaf traits were primarily affected by osmotic stress but
not Si, and that herbivore RGR was strongly negatively affected by Si but
not osmotic stress. There was no interactive effect between the osmotic
stress and Si treatments on H. armigera RGR or plant traits except for
leaf nitrogen and phenolic concentrations. This study provides further
support that Si may prove to be beneficial to plants against chewing
insect pests and remains robust regardless of water-deficit stress
conditions.