Foliar Application of Nano Carbon Dots and Melatonin Improves Rice Lodging Resistance and Yield

Excessive nitrogen application in rice production leads to low translocation efficiency of photosynthates in stems and lodging, resulting in inadequate grain filling and yield loss. The application of nanotechnology and growth regulators provides a feasible way to solve these problems. In this study, field experiments were conducted to explore the effects of foliar spraying of different concentrations of carbon dots and melatonin combinations on rice growth and development, photosynthesis, accumulation and translocation of non-structural carbohydrates (NSC) in stems and leaves, stem strength and yield under reduced nitrogen input. The results showed that the combination of carbon dots and melatonin improved rice yield and stem lodging resistance under the condition of 20% nitrogen reduction. Specifically, application of 0.3 mg mL^-1 carbon dots and 100 μmol mL^-1 melatonin increased the grain yield by 7.5%-26.8%. Foliar spraying of carbon dots and melatonin increased the photosynthetic rate and leaf area index of rice leaves, thereby increasing the content of stem NSC and plant biomass by 37.2%-153.8% and 4.5%-23.5%, respectively, and promoted NSC translocation from stems (51.9%-127.1% apparent transferred mass) and leaves (66.7%-161.6% apparent transferred mass) to grains by increasing the number and cross-sectional area of large and small vascular bundles of peduncle, the activities of α-amylase, β-amylase and SPS in stems, and upregulating the expression of sugar translocation related genes in stems, leaves and grains, and consequently improved the grain filling percentage and yield. In addition, foliar spraying of carbon dots and melatonin increased cellulose, hemicellulose and lignin contents in the basal stem by 10.8%-34.3%, 10.8%-42.1%, 17.2%-71.9%, and increased the basal internode stem diameter (+5.1%-10.8%), stem wall thickness (+6.3%-21.4%) and breaking resistance (+8.7%-66.7%), thus improving the lodging resistance of rice plants.