Salicylic Acid Reduces Wheat Yield Loss Caused by High Temperature Stress by Enhancing the Photosynthetic Performance of the Flag Leaves

High temperature stress during grain filling substantially decreases wheat productivity: thus, to ensure food security, heat tolerance in wheat must be developed. It remains unclear whether exogenous salicylic acid (SA) can induce tolerance to high temperatures in wheat at the grain-filling stage. In this study, a two-year pot culture experiment using the wheat cultivar &lsquo:Yangmai 18&rsquo: was conducted from 2018 to 2020. The plants were pre-sprayed with SA from the heading stage (SAH), anthesis stage (SAA), 5 days after anthesis (DAA: SA5), and 10 DAA (SA10). After that, the wheat plants were subjected to high temperature stress (G) simulated using a passive warming method during the period between 15 and 19 DAA. The results showed that, compared with the normal temperature control group (NN), high temperature stress at the grain-filling stage significantly reduced the yield and photosynthetic capacity of wheat. The application of SA at different stages reduced the yield loss and the damage to the photosynthetic capacity caused by high temperature stress: the effectiveness of the treatments in descending order was SAAG &gt: SA5G &gt: SA10G &gt: SAHG. Exogenous SA treatment increased the amount and proportion of dry matter distributed in the stem sheaths and leaves and grains, and decreased the amount and proportion of dry matter distributed in the rachises and glumes at the maturity stage, thereby reducing the yield loss under high temperature stress. The application of SA significantly increased the leaf area, stomatal density, chlorophyll content, soluble protein content, maximum photochemical efficiency (Fv/Fm), actual photochemical efficiency (&Phi:PSII), and activity of sucrose phosphate synthase (SPS) of the wheat flag leaves under high temperature stress at the grain-filling stage, thereby improving the photosynthetic performance of the flag leaves under stress. In summary, exogenous SA significantly restored the photosynthetic capacity of wheat flag leaves injured by post-anthesis high temperature stress, which effectively alleviated the inhibition of wheat growth caused by the stress and ultimately reduced the yield loss. Spraying SA at the anthesis stage had the greatest effect abating the loss of yield and reduced photosynthetic performance under high temperature stress.