The mechanisms underlying cotton cell damage and leaf sticking induced by the defoliant thidiazuron.

Chemical defoliation is essential for mechanized cotton harvesting, but excessive thidiazuron (TDZ), frequently induces a "leaf sticking" phenotype characterized by wilting without abscission, which severely compromises harvest efficiency. We integrated field and greenhouse experiments with anatomical, physiological, and transcriptomic analyses to investigate this abnormal response. TDZ (0.45 and 2.27 mM) was rapidly absorbed by leaf blades within 3-6 h, and accumulation increased with concentration. Early cellular damage (3-24 h) featured membrane integrity loss, and cell death, occurring prior to substantial ROS accumulation or senescence/PCD-related gene activation, suggesting direct membrane perturbation rather than ROS- or transcription-mediated injury. At later stages (48-96 h), excessive ROS and upregulated senescence/PCD transcripts further aggravated cellular damage. The optimal concentration (0.45 mM) promoted abscission zone (AZ) cell separation and complete abscission by 96 h, an excessive concentration (2.27 mM) initiated AZ separation as early as 24 h but halted its progression, causing persistent adhesion and leaf sticking. Localized application revealed that TDZ application to the blade alone was sufficient to induce petiole wilting, confirming that leaves retain the ability to generate signals even under severe injury. Direct AZ application neither induced abscission nor visibly damaged AZ cells. These findings indicate that abscission failure under excessive TDZ is not due to impaired blade-to-AZ signaling or direct killing of AZ cells, but rather to disrupted coordination of blade-derived abscission cues. Collectively, excessive TDZ uncouples leaf damage from AZ differentiation by impairing signal coordination, providing a mechanistic framework for understanding defoliation failure and optimizing TDZ application strategies in cotton production.