Integrated analysis of physiological, endogenous phytohormones, and transcriptomics reveals the effects of exogenous CPPPU on sweet potato agronomic traits

N-(2-chloro-4-pyridyl)-N'-phenylurea (Forchlorfenuron, CPPU) is widely used in crop production to promote fruit growth. However, the mechanism of action in root and tuber crops remains unclear. Sweet potatoes are the third most important root crop, and the application of exogenous CPPU in the field production of sweet potatoes is of great significance. In this study, we characterised the effects of different concentrations of CPPU applied 30 days after planting on the agronomic traits of sweet potatoes, as well as the effects of applying 9 mg/L CPPU on endogenous hormones and gene expression in the stem tips and young roots of sweet potatoes. The results showed that spraying 9 mg/L CPPU on sweet potato cuttings 30 d after planting increased the total yield of sweet potato tubers, marketable tuber yield, tuber dry matter content, and soluble sugar content. Furthermore, the foliar application of 9 mg/L CPPU reduced the levels of GAs, auxins, and ABA in stem tips and young roots, while increasing CKs and SA content in stem tips and decreasing SA content in young roots. Transcriptome data analysis revealed that CPPU mainly regulates the growth and development of sweet potato tubers by promoting lignin synthesis, affecting the phenylpropanoid biosynthetic pathway, and regulating key genes in the carotenoid biosynthetic pathway. RT-qPCR results indicated that exogenous CPPU treatment induced an upregulation in the expression levels of most key genes and transcription factors. This study offers a new reference for enhancing the practical production of sweet potatoes, sheds new light on the impact of CPPU on the growth and development of root crops, and provides fresh insights into future research on sweet potato root tuber enlargement.