Warmer winters and extreme climate fluctuations induced by increasing global temperatures result in freeze-thaw cycle stress. Helping winter wheat cope with this stress is a key to maintaining stable agricultural development. The goal of this study is to reveal the role of different fertilizer applications in enhancing winter wheat tolerance to freeze-thaw as evidenced by physiological and biochemical parameters.
In late fall, winter wheat was sown in pots and grown for 7 days, then fertilized with either nitrogen (60 mmol/L NH₄CO₃), calcium (100 mmol/L CaCl₂) or salt (100 mmol/L NaCl) for 30 days or given no fertilizer (control). After 30 days, seedlings were transferred to growth-chambers and subjected to a freeze-thaw cycle using a regimen of 3 hours at each of 15℃, 0℃, -15℃, 0℃, and 15℃. When the temperature decreased from 15℃ to -15℃,the leaves froze and membrane permeability increased by 62.3% and lipid peroxidation productd by 40%. When the temperature was increased from -15℃ to 15℃, the leaves thawed and membrane permeability and lipid peroxidation declined. The changes in membrane permeability and lipid peroxidation of winter wheat leaves exhibited a significant negative correlation (R =-0.89^{* *}, R =-0.85^{* *}) with thaw-freeze stress. The activity of catabalase (CAT) increased with the temperature drop from 15℃ to -15℃,but the activities of superoxide dismutase (SOD) and peroxidase (POD) were higher at 0℃.
However, compared to the controls, the membrane permeability and MDA content in the leaves of winter wheat treated with different fertilizers and salt were lower, but the activities of SOD, CAT and POD were higher, indicating that fertilizer application reduces membrane lipid peroxidation and enhances membrane antioxidant capacity. There were differences between nitrogen and calcium fertilizers and salt in regulation of osmolytes in the leaves of winter wheat under thaw-freeze-thaw stress. During thaw-freeze-thaw stress (temperature drop from 15℃ to -15℃ and increase from -15℃ to 15℃), the leaves of winter wheat with N fertilizer had 44.3% higher proline and 23.6% higher soluble protein content but 17.3% lower soluble sugar content than controls. The leaves with calcium fertilizer had 57.5% higher soluble sugar content and leaves with NaCl had 37.1% higher soluble sugar content than controls. This indicates that nitrogen fertilizer may be increasing proline and soluble protein content through improved nitrogen metabolism, and the increase in soluble sugar content with calcium fertilizer and salt may be through improved glucose metabolism.
This study suggests it may be efficient and cost-effective to provide winter wheat seedlings both kinds of fertilizers in winter. These fertilizers can not only increase several types of osmolytes to prevent formation of ice within plant cells but also enhance the activities of antioxidant enzymes to prevent membrane lipid peroxidation in response to freeze-thaw stress. The application of fertilizers in winter thus make seedlings more tolerant to freeze-thaw stress and strengthen growth the next spring.