The effects of atmospheric CO2 on silicon accumulation, plant defensive traits and herbivore attack

Abstract

The general aim of this thesis is to examine effects of atmospheric change, particularly different CO2 concentrations, on Si uptake and accumulation in the Poaceae family (e.g. grasses, wheat) and the consequences for a polyphagous insect herbivore, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). A model grass Brachypodium distachyon was the main study species used in this work due to its phylogenetic and anatomical similarities with important cereal crops such as wheat, maize and barley as well as its short generation time, simple growth requirements and comparable levels of shoot Si to most grass crops. This makes this species useful for studying the transport, accumulation and functional properties of Si in crop plants. Tall fescue (Festuca arundinacea Schreb. and wheat (Triticum aestivum L.) were also used in some chapters (chapters 4 and 6). Helicoverpa armigera was used as a model of an aboveground insect herbivore since it is a global pest of many economically important agricultural crops, particularly in Africa, America, Asia, Australia, and Europe which causes crop losses estimated at US$7 billion every year. This insect can damage at least 172 plant species due to its polyphagy and wide host range, and long persistence in cropping areas. Additionally, this insect has developed broad-spectrum pesticide resistance which makes it difficult to control using conventional techniques and thus developing alternative control strategies is necessary. Taken together, this PhD research highlights the contrasting effect of Miocene and Anthropocene CO2 levels on Si uptake, and accumulation as well as Si-based plant defence responses against the generalist aboveground insect herbivores and interactions between herbivores. This provides novel insights into the evolutionary basis for grasses utilising Si as an anti-herbivore defence and how Si defences may change under future conditions. Overall, this work provides evidence that Si accumulation plays an important role in plant biology and ecology in terms of providing plant resistance by altering plant physical, biochemical and secondary metabolite defence responses. Further, the contrasting effect of Miocene and Anthropocene CO2 levels on Si accumulation suggests that some grasses may become more susceptible to insect herbivores due to declines in Si-based defences against insect herbivory under projected climate change scenarios. Avenues for future research and the limitations of the current work are discussed.

Date of Award	2023
Original language	English