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.

气候变化引发的降水格局改变，很可能会使植物遭受水分亏缺胁迫，进而改变植物的比叶质量（specific leaf mass）、叶片含水量（leaf water content）以及化学防御（chemical defences）相关特征，这些变化可能会对植食性节肢动物（herbivorous arthropods）产生影响。尤以禾本科植物为例，其会积累高浓度的硅（silicon, Si），以此构建针对植食动物的物理防御屏障。尽管植物对硅的吸收可能会受到水分有效性的影响，但目前极少有研究探讨水分亏缺胁迫与硅共同作用对昆虫植食者生长表现的影响。

本研究采用水培法种植高羊茅（Festuca arundinacea Schreb.），设置施加硅与不施加硅两组处理，并利用20%浓度的聚乙二醇（polyethylene glycol, PEG）对半数植株施加渗透胁迫（osmotic stress）。研究人员测定了11项叶片性状（涵盖生理、化学与结构性状），结合X射线成像（X-ray mapping）技术，利用扫描电子显微镜（scanning electron microscopy, SEM）可视化观察了叶片表面的硅化植硅体（silicified phytoliths）；同时将植株暴露于咀嚼式植食昆虫（chewing insect herbivore）——棉铃虫（Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)）。

尽管渗透胁迫会改变叶片的生理与化学性状，具体表现为比叶质量升高、叶片相对含水量（leaf relative water content）下降以及叶片氮（nitrogen, N）含量上升，但并未对棉铃虫的相对生长率（relative growth rate, RGR）产生显著影响。然而，硅可使棉铃虫的相对生长率降低80%~98%，且仅对叶片生理与化学性状产生微弱影响。与之相反，硅处理下棉铃虫相对生长率的下降，与叶片结构性状的改变密切相关，尤其是叶片表面硅化植硅体的密度显著升高。

效应量对比分析结果显示，叶片性状主要受渗透胁迫的影响，而非硅处理；而昆虫植食者的相对生长率则显著受到硅的负向调控，与渗透胁迫无关。渗透胁迫与硅处理之间，除对叶片氮与酚类物质含量（phenolic concentrations）存在交互效应外，并未对棉铃虫相对生长率或其他植株性状产生交互影响。

本研究进一步证实，硅可帮助植物抵御咀嚼式昆虫害虫，且在水分亏缺胁迫条件下仍能保持良好的防御效果。