A manifold response of forage rape to combined heat wave and drought under current and elevated CO2

Intensity and frequency of heat waves (HW) accompanied by drought are increasing, however, still little is known about its effect on forage plants. A pot experiment was carried out in growth chambers in order to elucidate the multiple response of forage rape to combined HW and drought (HD) under current and elevated CO2 (HD + CO2). Forage rape was proved to be relatively HD-resistant plant, with no significant and long-term effects on shoot growth and photosynthetic performance, and effective stimulation of antioxidative system, especially in the case of HD + CO2 exposure. Activities of enzymes of ascorbate-glutathione cycle were further stimulated by elevated CO2 even after the termination of HD treatment. Nutritive value of forage rape was also positively affected, as was shown by elevated content of water-soluble carbohydrates after exposure and starch after the period of recovery (up to 80% and 92%, respectively). Concentrations of most macro- and microelements were increased, and stabilized higher or equalized to control level after the period of recovery in a similar pattern for HD and HD + CO2 exposure. However, content of Ca was reduced and remained significantly lower (similar to 20%). One of the most important negative and lagged HD effects was reduction of dry root biomass (similar to 30%), most possibly induced by accumulation of non-structural carbohydrates in shoot and was not mitigated by elevated CO2. To conclude, forage rape was proved to be a promising forage crop for the future climate with increased frequency of HWs; however, a special attention should be paid to the lagged effects, such as inhibition of root growth.