The low cost of plant bioregulators can help improve plant defense systems under stress conditions. In this study, the potential role of thiourea to ameliorate the adverse effects of heat stress in canola was evaluated. The 2-year study comprised of (i) exogenous thiourea applications (1000 mg L−1); no applications, hydropriming, water spray, thiourea priming, and thiourea spray (TS), and (ii) high-temperature stress; 35/25 °C (HS1) vs. 25/18 °C (HS0; control). High-temperature stress caused a significant reduction in plant biomass, photosynthetic pigments, and gas exchange attributes due to overaccumulation of oxygen radicles, thiobarbituric acid reactive substances, and electrolyte leakage compared with control. Elevated temperature caused a reduction in photosynthetic rate and stomatal conductance by 46.1 and 31.5%, respectively, which lead to a significant reduction in seed yield and seed oil concentration, by 47.1 and 23.4%, respectively. However, exogenous application of thiourea increased the growth and yield due to the accumulation of total soluble proteins, sugars, osmolytes, and antioxidant enzyme activities. In addition, thiourea-induced decrease in the levels of free oxygen radicles and thiobarbituric acid reactive substances helped improve the cell membrane stability. Thiourea applications improved the stomatal conductance and CO2 assimilation rate by 27.2 and 26.5%, respectively, which lead to improving the seed yield and seed oil concentrations. Nevertheless, thiourea applied as the foliar spray was more promising to alleviate heat-induced oxidative stress as compared to thiourea seed priming. Foliar applications of thiourea were the most effective treatment as it helped to activate the antioxidant enzymes and improved total soluble proteins, reducing and non-reducing sugars resulting in osmoregulation and improvement in plant growth, seed yield, and seed oil concentration under high-temperature stress.
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