Combined effects of elevated temperature and CO2 enhance threat from low temperature hazard to winter wheat growth in North China.

We examined the growth and yield of winter wheat (Triticum aestivum) in response to the predicted elevated CO2 concentration and temperature to determine the mechanism of the combined impacts in North China Plain. An elevated treatment (CO2: 600 µmol mol-1, temperature: +2.5~3.0 °C, ECTI) and a control treatment (ambient CO2 and temperature, CK) were conducted in open-top chambers from October 2013 to June 2016. Post-winter growth stages of winter wheat largely advanced and shifted to a cooler period of nature season under combined impact of elevated CO2 and temperature during the entire growing season. The mean temperature and accumulated photosynthetic active radiations (PAR) over the post-winter growing period in ECTI decreased by 0.8-1.5 °C and 10-13%, respectively compared with that in CK, negatively impacted winter wheat growth. As a result, winter wheat in ECTI suffered from low temperature hazards during critical period of floret development and anthesis and grain number per ear was reduced by 10-31% in the three years. Although 1000-kernel weight in ECTI increased by 8-9% mainly due to elevated CO2, increasing CO2 concentration from 400 to 600 µmol mol-1 throughout the growth stage was not able to offset the adverse effect of warming on winter wheat growth and yield.