Effectiveness of Mepiquat Chloride Application Timing on Physiological, Agronomic, and Fiber Quality Traits of Cotton (Gossypium spp.) Varieties

A two-year field study underscores the importance of integrating genotype selection with optimized Mepiquat Chloride (MC) management to enhance cotton performance under a split–split–plot design. 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<sup>-1</sup> enhanced leaf expansion. Monopodial branches were significantly greater in 2023 (9.59 plant<sup>-1</sup>), with early MC application favoring branch initiation. Seed cotton yield was peak in Selin (3623 kg ha<sup>-1</sup>), 25% greater than Sezener, with split application (400 + 400 cc. ha<sup>-1</sup>) 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<sup>-1</sup>. 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<sup>-1</sup> 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. 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.