Individual and combined effects of heat and drought and subsequent recovery on winter wheat (Triticum aestivum L.) photosynthesis, nitrogen metabolism, cell osmoregulation, and yield formation.

Extreme temperatures and droughts are considered as the two main factors that limit wheat growth and production. Although responses of wheat plants to heat and drought stress have been extensively investigated, little is known about the extent to which wheat plants can recover after stress relief. In this study, a winter wheat pot experiment was conducted to evaluate the growth, physiological activities, and yield formation responses of wheat to stress and recovery periods under heat stress (36 °C, daily maximum temperature), drought (45-55% of soil water holding capacity), and combined stress conditions. Heat and drought stress significantly reduced photosynthesis, leaf relative water content (LRWC), leaf water potential (LWP), and nitrogen metabolism enzyme activities and increased electrolyte leakage. These parameters showed significant interactions between heat and drought stress. Beneficial osmoregulation of membrane stability was observed in stressed plants because of the accumulation of proline and soluble sugars. Within a range of stresses, the abovementioned physiological processes of individual heat- and drought-stressed plants recovered to levels comparable to those of the control. The recovery capacities of the physiological traits decreased gradually with increasing stress duration, particularly under combined stress. The recovery of LWP and LRWC contributed to the improved photosynthetic performance after stress relief. The combined stress caused greater yield losses than individual heat and drought stress, which was mainly attributed to low levels of thousand grain weight (TGW), the number of grains per ear, and the grain filling rate. After stress relief, the recovery of proline content, glutamine synthetase activity, photosynthetic rate, and LRWC were closely associated with grain yield and thousand grain weight. Collectively, these findings contribute to a better understanding of the coordinated responses of winter wheat during the combined heat and drought stress and recovery periods.