Spraying KH2PO4 Alleviates the Damage of Spring Low-Temperature Stress by Improving the Physiological Characteristics of Wheat Flag Leaves

The low-temperature stress (LTS) in spring results in tremendous yield loss in wheat production, and the application of potassium dihydrogen phosphate (KH₂PO₄) can alleviate stress-induced damage. However, the underlying effect of spraying KH₂PO₄ on the physiological characteristics of wheat flag leaves under spring LTS remains unclear. In this study, we investigated the effect of spraying KH₂PO₄ on flag leaf physiological traits and yield under spring LTS, including treatments at 15 °C and spraying H₂O (CK), treatment at -4 °C and spraying H₂O (LT1), and treatment at -4 °C and spraying KH₂PO₄ (LT2). The results showed that spraying KH₂PO₄ significantly increased the activities of the superoxide dismutase (SOD), the peroxidase (POD), and the catalase (CAT), and reduced malondialdehyde (MDA) content in the flag leaves. Compared to LT1, the SOD, POD, and CAT activities in the flag leaves of the Yangnong19 (YN19) and Xinmai26 (XM26) via LT2 increased by 5.5%, 10.9%, and 3.9%, and 5.4%, 9.2%, and 4.4%, respectively, and the MDA content of the YN19 and XM26 decreased by 10.5% and 9.1%, respectively, at 0-12 d after low temperature treatment (DALTT). Spraying KH₂PO₄ appreciably alleviated damage to the leaf cell morphology and tissue integrity, and increased the accumulation of proline and soluble protein, the chlorophyll content, and the activities of Ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxykinase. The net photosynthetic rate in the flag leaves of the YN19 and XM26 via LT2 increased by 37.9% and 35.9%, respectively, at 0-12 DALTT, compared to LT1. Moreover, spraying KH₂PO₄ reduced the yield loss rate of the YN19 and XM26 by 13.06% and 16.72%, respectively. The present study demonstrates that spraying KH₂PO₄ can enhance wheat resistance to spring LTS and maintain the photosynthetic capacity of flag leaves, alleviating the negative effects of LTS on grain yield.