Effects of plant growth regulators on resistance to brown planthopper Nilaparvata lugens（Stål） and secondary metabolites in seedling stage of rice

【Objective】This study aimed to elucidate the effects of different plant growth regulators on brown planthopper Nilaparvata lugens（Stål） resistance in rice at seedling stage, identify plant growth regulators capable of enhancing resistance of rice against brown planthopper at seedling stage, and analyze the relationship between resistance changes and the defensive secondary metabolites, the total phenolic content and flavonoid content, in rice. These findings would provide theoretical support for the targeted regulation of rice pest resistance by plant growth regulators and the reduction of pesticide application. 【Method】The resistance phenotype of the brown planthopper susceptible rice variety TN1 under different plant growth regulator treatments and application methods（seed soaking and foliar spraying） was evaluated to assess their effects on brown planthopper resistance of rice at the seedling stage. Based on the screening results, the highly resistant plant growth regulators and the optimal application methods were selected, the resistance and sustained resistance of TN1 were evaluated under different concentrations of selected plant growth regulators. The changes of total phenol content and total flavonoid content in rice plants after treatment with pesticides of higher resistance level were determined by the Folin-phenol method and enzyme-linked immunosorbent assay respectively. The effects of plant growth regulators on the changes of total phenol and total flavonoid contents in rice and their relationship with brown planthopper resistance were revealed. 【Result】No effects on brown planthopper resistance in TN1 at seedling stage were observed via seed soaking and foliar spraying with 0.667 mL/L of 3% gibberellic acid EC, 0.500 mL/L of 0.01% 28-epibrassinolide SL,1000 mL/L of 1% amino-oligosaccharide AS, or 2 mL/L 1.2% indole-3-butyric acid AS. By contrast, treatment with 0.1000 mL/L of 0.136% gibberellin·indoleacetic acid·brassinosteroid WP via both seed soaking and foliar spraying increased the average resistance rating of TN1 seedlings to brown planthopper from highly susceptible to moderately susceptible, with foliar spraying proving more effective than seed soaking. Further studies showed that application of 0.136% gibberellin·indoleacetic acid·brassinosteroid WP at 0.2000-0.4000 mL/L elevated TN1 resistance to brown planthopper from highly susceptible to moderately resistant, with the highest resistance enhancement observed at 0.2000 mL/L. This concentration resulted in a mean damage rating of 5.20 and a resistance persistence period of 6.20 d. BPH infestation was conducted after application, and 10 d after application, the 0.2000 mL/L concentration treatment, which exhibited the highest total phenolic content of 1.003 mg/g, corresponded to the highest resistance level and sustained resistance grade in TN1. The top 3 total phenolic content treatments, 0.2000, 0.1000, and 0.4000 mL/L were the only ones achieving moderate sustained resistance among the gradient concentrations. For all the brown planthoppers infestation treatments after spraying 0.136% gibberellin·indoleacetic acid·brassinosteroid WP, the total flavonoid content of TN1 rice stems was significantly higher than that of brown planthoppers infestation CK（P＜0.05, the same below）, and also significantly higher than that of the same concentration without brown planthoppers infestation treatment. The total flavonoid content of rice stems increased with the increase of the agent concentration, and the maximum was 0.4000 mL/L for brown planthoppers infestation treatment. The total flavonoid content reached 0.138 mg/g. 【Conclusion】The 0.136% gibberellin·indoleacetic acid·brassinosteroid WP has a promoting effect on the synthesis of total phenols and flavonoids, which are secondary metabolic defense substances in rice, thereby improving brown planthopper resistance in susceptible rice variety TN1 at the seedling stage. It is recommended to apply 0.136% gibberellin·indoleacetic acid·brassinosteroid WP during the rice seedling stage as a supplementary measure for preventing brown planthopper damage, thereby achieving the goal of reducing pesticide application and enhancing efficacy simultaneously.