Casey R. Hall, Meena Mikhael, Susan E. Hartley and Scott N. Johnson

Many grasses take up large amounts of silicon (Si) which acts as a potent physical defence against insect herbivores. Silicon is taken up by grasses and embedded into the leaves where it forms tiny silica particles. To an insect herbivore this is the equivalent to eating with a mouth full of sand.

Silicon supplementation has also been linked to the plant defence signalling hormone, jasmonic acid (JA). Both silicon and JA defences are reduced under elevated CO2 (eCO2), potentially increasing plant susceptibility to pest insects. To date, only a few studies have investigated the effect of eCO2 on Si uptake. The majority of crops grown throughout the world are grasses, making this particularly relevant for global food security, pest management and species interactions under climate change.

We grew the grass Brachypodium distachyon with and without silicon in both ambient and elevated CO2 conditions. We found significantly lower silicon in plants grown in eCO2. In addition, both eCO2 and silicon lowered the defence hormone response in this grass. Silicon supplemented grasses significantly reduced the growth of caterpillars of the cotton bollworm, yet the reduced silicon in plants grown under eCO2 did not cause an increase in caterpillar growth. Our results show that even with reduced uptake, Si remained an effective defence against pest insect herbivory under eCO2.

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