Modulatory Effects of Mepiquat Chloride on Physiological, Agronomic, and Fiber Quality Parameters of Cotton (Gossypium hirsutum L.) under Varying Application Timings

Background A two-year field experiment elucidates the interactive effects of genotype and Mepiquat Chloride (MC) management to enhance cotton growth and productivity using a split–split–plot design. Plant growth regulators (PGRs) play a pivotal role in modulating physiological processes, thereby enhancing plant health, architecture, yield potential, and fiber quality. In this context, the study assessed the impact of foliar-applied MC at varying rates on two conventional cotton cultivars, Selin and Sezener. Mepiquat Chloride treatments were administered at key phenological stages: at squaring, 8 weeks after planting (WAP), and boll-setting (11 WAP) across two consecutive growing seasons to determine their effects on growth regulation, yield stability, and overall crop performance. Results Significant genotype × environment × management interactions were observed in cotton growth, yield, and fiber quality responses MC application. Plant height was significantly affected by year × variety interaction (p < 0.01), with the tallest plants in Selin (2023) and Sezener (2022). Leaf area was higher in 2022, while 800 cc. ha ^− 1 enhanced leaf expansion. Monopodial branches were significantly greater in 2023 (9.59 plant⁻¹), with early MC application favoring branch initiation. Seed cotton yield was peak in Selin (3623 kg ha⁻¹), 25% greater than Sezener, with split application (400 + 400 cc. ha ^− 1 ) yielding best results. Boll number and individual fiber weight were higher in Selin and Sezener, respectively, with the latter showing a 48% increase under 400 cc. ha ^− 1 . Seed index was significantly higher in Sezener, and fiber moisture increased in 2023 (8.94%). Fiber quality traits revealed pronounced year, genotype, and application timing effects: fiber length peaked in Sezener (30.33 mm), elongation reached 5.80%, and micronaire increased to 4.89 µg inch⁻¹ under split MC application. Fiber strength increased by 13.07% in 2023, and while uniformity (85.62%) maximized with later application. Short fiber content decreased by 14.81% in 2023. Conclusions These findings highlight that MC concentration, timing, and genotype interactions significantly modulate morphophysiological traits, yield components, and fiber quality, with split or moderate-dose applications enhancing productivity and fiber properties under Mediterranean conditions.