Responses of a drought-stressed pasture grass to silicon supplementation, fungal endophytes and simulated grazing

Globally, approximately 50% of grasslands experience degradation due to drought and anthropogenic disturbances. Reports suggest that the negative impacts of drought in grasses can be alleviated by accumulating biogenic silicon or forming beneficial associations with Epichloë fungal endophytes. The restorative impact of these drivers in pasture grasses is unclear, however, and their combined effect is unknown. Moreover, silicon accumulation and endophytic associations may be more costly to plant growth than previously thought, especially under drought conditions. Using rain exclusion shelters, we investigated how drought-stressed tall fescue (Lolium arundinaceum) responded to silicon supplementation in the presence or absence of endophytes, also determining how these treatments affected plant resistance to a global insect pest (Helicoverpa armigera). We applied simulated vertebrate grazing to determine its influence on silicon accumulation. Drought reduced plant growth by up to 33% for shoot mass and 45% for groundcover, adversely impacting physiological responses such as leaf gas exchange. Silicon supplementation and endophytes had no ameliorative impact, with plant growth generally worst in endophytic plants under drought. Drought caused plants to accumulate around 21% less silicon than well-watered plants. However, simulated grazing prior to drought helped plants sustain silicon uptake to levels akin to well-watered plants. Insect development was unaffected by endophytes but decreased by 92% when feeding on silicon-supplemented plants. Our results suggest that silicon supplementation and Epichloë endophytes do not mitigate adverse impacts of drought on tall fescue. Grazed plants sustained silicon uptake under drought, potentially as a survival strategy to resist herbivory when experiencing drought-stress.