Can elevated CO2 buffer the effects of heat waves on wheat in a dryland cropping system

Abstract Increasing atmospheric CO 2 concentration [CO 2 ] drives the rise in global temperatures, with predictions of an increased frequency of heat waves (short periods of high temperatures). Both, CO 2 and high temperature, have profound effects on wheat growth and productivity. We tested whether elevated [CO 2 ] (eCO 2 ) has a potential to ameliorate the effects of simulated heat waves (HT) on wheat in a dryland cropping system. Wheat was field-grown at the Australian Grains Free Air CO 2 Enrichment (AGFACE) facility under ambient [CO 2 ] (∼390 ppm) or eCO 2 (∼550 ppm) for two growing seasons, one with ample water supply and one of severe drought. Using heated chambers, heat waves (3-day periods of high temperatures) were imposed at critical growth stages before anthesis (HT1) or post-anthesis (HT2, HT3). Gas exchange, chlorophyll content and concentration of nitrogen (N) in mainstem flag leaves, as well as concentrations of stem water-soluble carbohydrates (WSC) in mainstems were monitored throughout the season. Yield, biomass and thousand kernel weights (TKW) were measured at maturity. Elevated [CO 2 ] moderated the effect on net CO 2 assimilation rates of pre-anthesis (HT1), but not of post-anthesis heat waves (HT2, HT3). Growth under eCO 2 increased stem WSC both, with and without experimental heat waves, but remobilisation decreased significantly under heat indicating that a greater WSC pool does not necessarily translate into greater remobilisation into the grain. Grain yield (g m −2 ) was greater under eCO 2 and especially pre-anthesis heat stress decreased grain yield in the wetter season, and this decrease was stronger under eCO 2 (up to 20%) than under aCO 2 (up to 10%). Grain N decreased under eCO 2 , but less so under heat stress. We conclude that eCO 2 may moderate some effects of heat stress in wheat but such effects strongly depend on seasonal conditions and timing of heat stress.