Effects of thidiazuron and methyl jasmonate tank mixes on cotton defoliation and the underlying phytohormonal mechanisms

Abstract Background Thidiazuron (TDZ) is a widely used chemical defoliant in commercial cotton production and is often combined with the herbicide Diuron to form the commercial defoliant mixture known as TDZ·Diuron (T·D, 540 g·L -1 suspension). However, due to increasing concerns about the environmental and biological risks posed by Diuron, there is an urgent need to develop safer and more effective alternatives. Jasmonic acid (JA) and its derivatives are key phytohormones in organ senescence and abscission. Results Greenhouse experiments at the seedling stage revealed that Me-JA (0.8 mmol·L -1 ) alone did not induce defoliation. However, its co-application with TDZ (0.45 mmol·L -1 ) at concentrations of 0.6, 0.8, and 1.0 mmol·L -1 significantly enhanced defoliation efficacy. The most effective combination—TDZ with 0.8 mmol·L -1 Me-JA—achieved a 100% defoliation rate at 5 days after treatment (DAT), 23.7 percentage points higher than TDZ alone, and comparable to the commercial TDZ·Diuron formulation with equivalent TDZ content. Field trials conducted in Beijing (Shangzhuang), Hebei (Hejian), and Xinjiang (Shihezi) confirmed that the combination of 0.6 mmol·L -1 Me-JA with 1.70 mmol·L -1 TDZ provided optimal defoliation performance. At 21 DAT, the defoliation rate increased by 13.5–16.3 percentage points compared with TDZ alone. Furthermore, boll opening rates improved by 5.7–12.7 percentage points relative to TDZ-only treatments. Phytohormonal analyses from the Shangzhuang site showed that the combined treatment significantly altered hormone levels in both leaves and petioles. Compared with TDZ alone, the mixture reduced concentrations of auxin (IAA), cytokinins (Z + ZR, iP + iPA, DHZ + DHZR), and gibberellic acid (GA₃), while increasing levels of JA, abscisic acid (ABA), and brassinosteroids (BR). These hormonal shifts may underlie the enhanced defoliation observed with the combined treatment. Importantly, the TDZ-Me-JA combination did not adversely affect cotton yield, yield components, or fiber quality. Conclusion The combination of Me-JA and TDZ has a good defoliation effect without affecting crop yield or fiber quality. And it provides a promising foundation for the development of novel, environmentally friendly cotton defoliants.