Herbicides act as restrictive factors in rice-watermelon rotation

•Herbicides used in rice season may cause malformed root of afterculture watermelon.•Watermelon is more sensitive to PXL and BSM than squash and bottle gourd.•PXL and BSM negatively affect root architecture by disturbing cell ultrastructure’.•Auxin and cytokinin signaling play roles in root response to herbicide stress. Paddy-upland rotation, as a popular cultivation mode in many Asian countries, could largely increase land use efficiency and its productivity. In recent years, watermelon production is badly impaired by unknown factors that result in watermelon malformed roots in rice-watermelon rotation area. In this study, we examined the effects of several herbicides, which are often used for the weed control in rice season, on the growth of succeeding culture of watermelon. The effects of four different herbicides (penoxsulam, bensulfuron-methyl, chylapof-butyl and fhiroxypyr) with different concentrations at 1, 10, and 100 ppb, were investigated on three cucurbitaceae crops of watermelon, squash, and bottle gourd. All species were severely affected in plant growth by penoxsulam and bensulfuron-methyl herbicides at as low as 10 ppb after one week, whereas there was no obvious toxic effect of chylapof-butyl and fhiroxypyr treatments on plant growth even after two weeks. Among the three cucurbitaceae crops, watermelon showed the highest sensitivity to herbicide exposure, and the root tip ultrastructure micrograph further conformed the sensitivity of watermelon to herbicides. Moreover, the expression of auxin and cytokinin response factors suggested that plant hormones, auxin and cytokinin, play important roles in root growth and development under herbicide stress. In conclusion, we postulated that the residues of herbicides from application in rice production is one of the major barriers for successful production of watermelon crop. The present study provides new insights for avoiding the detrimental effects of herbicide residues in soil and improving watermelon production in sustainable rice-watermelon rotation cultivation mode.