Growth and photosynthetic responses in Brassica napus differ during stress and recovery periods when exposed to combined heat, drought and elevated CO2.

This study was intended to investigate how an agronomically important crop Brassica napus will be able to cope with the combined impact of a heatwave (21/14 °C vs. 33/26 °C day/night) and drought under ambient or elevated CO2 (800 vs. 400 μmol mol-1) and to what degree their recovery will be ensured after the stress, when additional CO2 is also removed. The obtained results revealed that, in the presence of an adequate water supply, B. napus performed well under heatwave conditions. However, drought fully negated all the advantages gained from hotter climate and led to a slower and incomplete recovery of gas exchange and retarded growth after the stress, regardless mitigating the effect of elevated CO2 during the stress. The mechanism by which the elevated CO2 diminished the adverse effect of a combined heat and drought stress on photosynthetic rate at saturating light (Asat) was attributed to the improved plant water relations. However, it had little effect on the recovery of Asat. In contrast, the mechanism by which photosynthesis was more impaired under the combination of heatwave and drought, compared to single drought treatment, was attributed mainly to the faster soil drying as well as faster and sharper decrease in stomatal conductance and subsequent in Ci/Ca. Keeping in mind that photosynthesis can acclimatize by downregulation to higher CO2, the results of this study, showing a weak memory of mitigating the effect of elevated CO2, highlight a potential risk of more intense and frequent heatwaves and droughts on B. napus.