Elevated carbon dioxide and warming impact silicon and phenolic‐based defences differently in native and exotic grasses

Abstract

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper-accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO ) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO (eCO ) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (-19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (-10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade-off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth-defence trade-off. Grasses responded in a species-specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.

Climate warming and plant biomechanical defences: Silicon addition contributes to herbivore suppression in a pasture grass

“…Phalaris aquatica originated in Southern Europe and the Caucasus and while now naturalized, it is exotic to Australia (Popay, ). The observed declines in Si at warmer temperatures observed in the present study are therefore consistent with patterns for exotic species tested by Johnson and Hartley (). We should caution, however, that comparatively few grass species have been investigated in terms of global warming and Si uptake, and most of these (like the current study) were conducted over relatively short time periods.…”

“…In a glasshouse study using eight grass species, elevated temperatures were reported not to affect Si concentrations overall, but some species showed significant responses (Johnson & Hartley, ). In particular, grass species native to Australia contained higher concentrations of Si, which rose in response to warmer conditions ( c .…”

“…−10%) in Si concentrations. It remains speculative, but having the capacity to absorb large amounts of Si might be a selective advantage to plants evolving in low‐nutrient soils and seasonally arid climates such as Australia (Johnson & Hartley, ). Phalaris aquatica originated in Southern Europe and the Caucasus and while now naturalized, it is exotic to Australia (Popay, ).…”

“…Depending on emissions of greenhouse gases, global mean air temperatures are predicted to increase by 0.3–1.7°C (low emissions), 1.1–3.1°C (moderate emissions) and 2.6–4.8°C (high emissions) by 2081–2100 (IPCC, ). Given the important role played by Si in plant stress alleviation, it is surprising that so little is known about how global warming will affect Si uptake in plants (Cooke et al, ; Johnson & Hartley, ). Warming often increases transpiration rates in plants which may increase Si uptake, as could higher metabolically driven uptake of nutrients in general.…”

Do silicon and nitric oxide induce resistance to Mahanarva spectabilis (Hemiptera: Cercopidae) in forage grasses?

“…An increasing number of studies published during recent decades has shown that silicon increases the transcription levels of genes related to plant defense and the activities of enzymes involved in the biosynthesis of phenolic compounds . However, these results contrast with those of Coskun et al , which suggested that, in the absence of stress, silicon supplementation has little or no effect, those of Cooke and Leishman, and those of Johnson and Hartley, who mentioned that foliar silicon concentration was significantly negatively correlated with total phenols.…”