Yield Formation of Winter Wheat Influenced By CO2 Concentration, Warming and Heat Stress Events: Measurements and Simulation.

Increasing atmospheric CO₂ concentration ([CO₂]) and changing air temperature (T_air) are major climate change impacts on yield formation of winter wheat. Canopy temperatures (T_c) increase under elevated [CO₂] because transpiration rates and cooling decrease. Under elevated [CO₂], T_c may reach critical thresholds earlier. This has rarely been investigated under field conditions. Nevertheless, crop models have already been parametrized on basis of data gained under controlled environments. However, the field validation of these results is still outstanding.

In the growing seasons 2013/2014 and 2014/2015, a FACE experiment with winter wheat was conducted in Braunschweig, Germany to investigate the interactions of elevated [CO₂], warming and heat events on yield formation. Elevated [CO₂] was applied throughout the growing season (600 µmol mol^-1), whereas T_air and T_c manipulation took place during selected developmental stages ([i] short-term heat stress during EC 55 & 65; [ii] long-term warming by +2°C and +4°C past EC 71). Irrigation was used to prevent drought stress. During heat stress treatments, maximum T_c of > 30°C were measured while T_air reached up to 34 °C. The mean T_c difference between ambient and FACE was below 1°C.

Elevated [CO₂] resulted in 14% yield increase by simultaneously increasing the grain number and thousand kernel weight (TKW). Heat stress treatments during EC 55 & 65 had no effect on yield although the grain number per chaff dry weight decreased by up to 5%. This was compensated by an equivalent rise in TKW. The +4°C long-term warming resulted in lower TKW (-5%) with little effects on yield. At +2°C, yield remained unaffected. Depending on post anthesis radiation conditions, warming increased the T_c sum based grain filling duration.

In our study, transpiration cooling led to an avoidance of extreme temperatures. Hence, physiological plant reactions seem to compensate yield depression caused by temperature stress to a certain extent. A solid quantification of compensating and avoiding strategies may only be shown under field conditions. Approaches to implement these strategies for the evaluation of T_c into existing crop models and their benefits for modelling performances will be discussed.