Morpho-physiological Responses of Cotton Shoot Apex to the Chemical Topping with Fortified Mepiquat Chloride.

The plant growth regulator mepiquat chloride (1,1-dimethyl piperidinium chloride, DPC) has been successfully and worldwide used in cotton production. Fortified mepiquat chloride is a type of aqueous formulation containing 25% DPC (referred to DPC⁺ hereafter), which can slightly damage young tissues of epidermis. DPC⁺ has shown potential in cotton chemical topping in China, and may replace the conventional manual topping in future. In order to investigate the mechanism of cotton chemical topping with DPC⁺, this field study was conducted in 2015. DPC⁺ (1125 mL ha^-1) was applied after peak blooming stage on 24 July, with water as a control (CK). DPC⁺ application significantly decreased plant height and reduced the nodes above the last white flower (NAWF) as compared with CK. After three days of DPC⁺ treatment, cotton shoot apical meristem (SAM) became flatter than CK, and the ratio of height/length of SAM was significantly less than that of CK. With respect to redox status at shoot apex, O₂^- production rate, H₂O₂ generation and MDA content were significantly increased at six hours after DPC⁺ application. In addition, the expression of GhSPL3 (a SPL transcription factor, which might play an important role in bud differentiation, the transition of growth phase and flower formation), GhV1 (a B3-domain containing transcription factor, which potentially involved in floral initiation), and GhREV3 (a class III homeodomain-leucine zipper transcription factors, which has key roles in meristem and organ development) were down-regulated by DPC⁺ also at six hours after application. In conclusion, DPC⁺ application during later flowering period can implement cotton chemical topping by inducing short-time oxidative stress at cotton apex, down-regulating genes involved in SAM development, flower bud differentiation, and reducing cotton shoot growth. [ABSTRACT FROM AUTHOR]