Synergistic improvement of cotton yield and fiber quality in heat-deficient regions: Selecting varieties sensitive to mepiquat chloride and increasing planting density

Cotton ( Gossypium hirsutum L.) is a thermophilic and photophilic crop with indeterminate growth, with cotton yield and fiber quality significantly influenced by heat resources during growth. In heat-limited regions, high-density planting boosts cotton yield. Chemical regulation with mepiquat chloride (DPC) further enhances this effect. However, the regulatory effects of DPC on cotton varieties with different sensitivity levels under high-density planting remain unclear. In this study, a split-plot factorial field experiment (AB+C design) was implemented to assess the influence of cotton variety, planting density, and topping method, along with their interactions on canopy architectural traits, canopy photosynthetic rate (CAP), and vertical distribution of boll weight and fiber quality across canopy levels. The main plot A contained cotton varieties displaying varying sensitivity to DPC (insensitive and sensitive); the main plot B contained a gradient of planting densities (high, 55 plants m −2 ; medium, 28 plants m −2 ; and low, 14 plants m −2 , respectively); and subplots contained three topping treatments (no topping; manual topping, removal of the primary stem's growing point; and chemical topping, mixed liquor consisted of 180 g ha −1 DPC combined with 150 ml ha −1 special additives). The leaf mean tilt angle (MTA) of the whole canopy increased by 8.9 %–23.5 % under chemical topping in both insensitive and sensitive varieties, compared with no topping. The MTA in the upper canopy of sensitive varieties increased by 8.3 %–19.8 % under chemical topping, and MTA showed a significant positive correlation with CAP. The upper and middle canopy boll weight, lint percentage, and micronaire of insensitive varieties decreased with increasing planting density. Conversely, sensitive varieties exhibited better adaptability to high-density planting systems, which had minimal density-dependent effects on yield and fiber quality. Compared with manual topping, chemical topping increased the top canopy boll weight of both insensitive and sensitive varieties, while fiber strength and upper half mean length increased by 1.4 %–4.9 % and 7.9 %–12.2 % in the top and upper canopy bolls, respectively. The adoption of cultivars with larger MTA combined with chemical topping enhanced the CAP and boll weight of the top canopy. Overall, the findings show that the optimal integration of DPC-sensitive varieties with 28–55 plants m −2 , and precision DPC application (low concentration chemical topping) effectively modulates canopy architecture, promotes concentrated boll setting, and increases the proportion of premium-quality bolls per unit area, thereby synergistically enhancing cotton yield and fiber quality. The graphical abstract is shown in Figure 1 below. Fig. 1 The graphical abstract in our manuscript. • Chemical topping increased boll weight in the top canopy. • Mepiquat chloride-sensitive varieties exhibit adaptability to high-density planting. • Sensitive varieties with high planting density modulates canopy architecture. • High planting density of sensitive varieties improves cotton yield and fiber quality.