Effects of foliar application of potassium dihydrogen phosphate on the physiological responses of rice seedlings under high temperature stress

This study investigated the alleviating effects and physiological responses to foliar-applied potassium dihydrogen phosphate (KDP) on rice seedlings under high-temperature (HT) stress. An early indica hybrid rice, YLY17 (high-temperature-sensitive), was used as the planting material. Four treatment groups were set up: (a) NT - normal temperature; (b) NT + KDP - normal temperature with foliar application of different KDP concentrations (0.1, 0.2, 0.3, and 0.4%); (c) HT - high temperature treatment without foliar application of KDP, and (d) HT + KDP - high temperature with foliar application of different KDP concentrations. At the three-leaf stage, rice seedlings were subjected to simulated HT stress (32~38 °C during the day and 26~32 °C at night) for 10 days. Growth indicators, photosynthetic parameters, antioxidant characteristics, osmotic adjustment substances, and related metabolic enzymatic activities of young rice seedlings were quantified, and the alleviating effect of KDP was comprehensively evaluated by principal component analysis (PCA). The results showed that HT stress significantly reduced plant height, fresh weight, and dry weight, decreased chlorophyll content and SPAD value, and decreased the net photosynthetic rate (Pn), stomatal conductance (gs), and transpiration rate (Tr), while increasing intercellular carbon dioxide (CO2) concentration (ci). At the same time, it led to the accumulation of superoxide anion (O2-), hydrogen peroxide (H2O2), and malondialdehyde (MDA), and induced increases in antioxidant enzyme and osmotic adjustment-related enzyme activities. Foliar spraying of KDP could effectively alleviate the above damage caused by HT stress, with 0.3% KDP being the most effective treatment. Compared with HT treatment, 0.3% KDP treatment significantly increased plant height, fresh weight and dry weight by 7.6, 10.6 and 10.2%, respectively, improved chlorophyll content and photosynthetic parameters, enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX), reduced the accumulation of reactive oxygen species (ROS) and MDA, and promoted the accumulation of osmotic adjustment substances such as soluble protein (SP), proline (Pro), soluble sugar (SS) and sucrose (SUC), as well as increased the activities of nitrate reductase (NR), glutamine synthetase (GS), sucrose synthase (SUS) and sucrose phosphate synthase (SPS). The PCA results showed that the order of comprehensive physiological activity index was NT + KDP > NT > KDP + HT > HT, indicating that KDP enhances heat tolerance by coordinately regulating photosynthesis, antioxidant defence, and osmotic adjustments. This study provides a theoretical basis and technical reference for using KDP to alleviate HT stress in rice seedlings.