Promoting Effects on Watermelon and Fermentation Optimization of Plantibacter sp. WZW03

Plantibacter sp. WZW03 can effectively improve the utilization efficiency of soil potassium by plants. However, high production cost of Plantibacter sp. WZW03 has limited its utilization. The study assessed the plant growth-promoting ability of Plantibacter sp. WZW03 on watermelon (Citrullus lanatus L. cv. Jingxin No. 1) by pot experiment and investigated the fermentation process using different substrate by response surface analysis. The results showed Plantibacter sp. WZW03 exhibited the maximum indole-3-acetic acid (IAA) secretion of 89.41 mg/L at 96 h and siderophore activity of 30.68% at 48 h in Medium-K basalt medium. Application of Plantibacter sp. WZW03 to watermelon effectively promoted root growth and increased root weight, length, and volume. The optimal fermentation substrate of Plantibacter sp. WZW03 was 65.7% chicken manure, 32.9% straw, 0.9% starch, and 0.5% Fe(OH)₃. The optimal conditions included the culture time on 20.36 h, culture temperature at 29.87 °C and C/N at 19:1. The maximum biomass was 1.47 × 10⁹ cfu/mL based on the optimum conditions, in agreement with the predicted value of the response surface. Compared with the silicate bacteria medium, the culture time was shortened by 12 h, the biomass was improved by 95.52%, and the cost of the fermentation process was lowered by 89.04%. Plantibacter sp. WZW03 could promote root growth and optimize root structure in watermelon and present the strong capacity to secrete IAA and produce siderophores. The developed fermentation process significantly enhanced the bacterial biomass, shortened the fermentation period, and lowered the production cost, thus providing theoretical and technical support for applying Plantibacter sp. WZW03 to promote the growth of watermelon.