Elevated CO2 alleviates the negative impact of heat stress on wheat physiology but not on grain yield

Hot days are becoming hotter and more frequent, threatening wheat yields worldwide. Developing wheat varieties ready for future climates calls for improved understanding of how elevated CO₂ (eCO₂) and heat stress (HS) interactively impact wheat yields. We grew a modern, high-yielding wheat cultivar (Scout) at ambient CO₂ (aCO₂, 419 μl l -1) or eCO₂ (654 μl l-1) in a glasshouse maintained at 22/15 °C (day/night). Half of the plants were exposed to HS (40/24 °C) for 5 d at anthesis. In non-HS plants, eCO₂ enhanced (+36%) CO₂ assimilation rates (A) measured at growth CO₂ despite down-regulation of photosynthetic capacity. HS reduced Asat (-42%) in aCO₂- but not in eCO₂-grown plants because eCO₂ protected photosynthesis by increasing ribulose bisphosphate regeneration capacity and reducing photochemical damage under HS. eCO₂ stimulated biomass (+35%) of all plants and grain yield (+30%) of non-HS plants only. Plant biomass initially decreased following HS but recovered at maturity due to late tillering. HS equally reduced grain yield (-40%) in aCO₂- and eCO₂-grown plants due to grain abortion and reduced grain filling. While eCO₂ mitigated the negative impacts of HS at anthesis on wheat photosynthesis and biomass, grain yield was reduced by HS in both CO₂ treatments