Moderate heat stress prevented the observed biomass and yield stimulation caused by elevated CO₂ in two well-watered...

Heat stress (HS) under well-watered conditions was not detrimental to leaf photosynthesis or yield but modified the elevated CO₂ response of photosynthesis and yield in two contrasting wheat cultivars. Climate change is increasing the frequency of extreme events such as heat waves, adversely affecting crop productivity. While positive impacts of elevated carbon dioxide (eCO₂) on crop productivity are evident, the interactive effects of eCO₂ and environmental stresses are still unclear. To investigate the interactive effects of elevated CO₂ and heat stress (HS), we grew two contrasting wheat cultivars, early-maturing Scout and high-tillering Yitpi, under non-limiting water and nutrients at ambient (aCO₂, 450 ppm) or elevated (eCO₂, 650 ppm) CO₂ and 22 °C in the glasshouse. Plants were exposed to two 3-day HS cycles at the vegetative (38.1 °C) and/or flowering (33.5 °C) stage. At aCO₂, both wheat cultivars showed similar responses of photosynthesis and mesophyll conductance to temperature and produced similar grain yield. Relative to aCO₂, eCO₂ enhanced photosynthesis rate and reduced stomatal conductance and maximal carboxylation rate (V_cmax). During HS, high temperature stimulated photosynthesis at eCO₂ in both cultivars, while eCO₂ stimulated photosynthesis in Scout. Electron transport rate (J_max) was unaffected by any treatment. eCO₂ equally enhanced biomass and grain yield of both cultivars in control, but not HS, plants. HS reduced biomass and yield of Scout at eCO₂. Yitpi, the cultivar with higher grain nitrogen, underwent a trade-off between grain yield and nitrogen. In conclusion, eCO₂ improved photosynthesis of control and HS wheat, and improved biomass and grain yield of control plants only. Under well-watered conditions, HS was not detrimental to photosynthesis or growth but precluded a yield response to eCO₂.