Cotton has a high salt tolerance. However, due to the high salt content and low K⁺/Na⁺ ratio in saline soils, cotton yield and fiber quality are difficult to improve. To investigate the effects of potassium (K) on cotton fiber length under salt stress, a two-year bucket-based field experiment was conducted using two different cultivars (CCRI 79, salt tolerant, and Simian 3, salt sensitive). Three K rates (K0, 0 kg K₂O ha⁻¹; K150, 150 kg K₂O ha⁻¹; and K300, 300 kg K₂O ha⁻¹) were applied at low, middle, and high soil electrical conductivities (S1, 1.7–1.8 dS m⁻¹; S2, 6.4–6.9 dS m⁻¹; and S3, 10.6–11.8 dS m⁻¹) to investigate the absorption, transport, and distribution characteristics of K⁺ and Na⁺ in the boll-leaf system (including the leaf subtending the cotton boll (LSCB), fruiting branch, boll shell, and fiber) of both cotton cultivars, as well as the relationship with fiber length. The results showed that K application (K150 and K300) significantly increased the cotton fiber length under salt stress, with the largest fiber length alleviation coefficients (AC) in the middle fruiting branches. The AC decreased with an increase in salt stress and was greater in CCRI 79 than in Simian 3. The K150 treatment (soil K⁺/Na⁺ = 1/13) completely mitigated the reduction in fiber length caused by S2 salt stress in CCRI 79, whereas the K300 treatment (soil K⁺/Na⁺ = 1/10) completely eased the reduction in fiber length caused by S2 salt stress in Simian 3. An application of K under salt stress increased the K⁺ content and K⁺/Na⁺ ratio in the soil and the organs of the boll-leaf system, regulated the K⁺/Na⁺ homeostasis in the boll-leaf system, enhanced the K⁺-selective transport coefficient (S_K-Na) in the LSCB, maintained a high K⁺/Na⁺ ratio in the fiber, and mitigated the fiber length reduction. In conclusion, the fiber length reduction in salt-tolerant cultivars was completely mitigated by K150 (i.e., soil K⁺/Na⁺ = 1/13) under moderate salt stress; however, it was not completely mitigated by K application under high salt stress.

In China, due to labor shortages and increasing labor costs, manual topping is gradually being replaced by chemical capping with mepiquat chloride (DPC). External chemicals can adjust plant growth by affecting endogenous hormones. Based on the hormone changes combined with the development of the main stem of cotton plants, a comparative experiment was conducted in 2019 and 2020 to determine the regulatory mechanism of the growth of the cotton main stem after chemical capping. In the experiment, two treatment times (T1: 12 July, T2: 18 July) and two treatment agents (CA [chemical capping agent] and DPC) were set, the hormone (auxin IAA, abscisic acid ABA, Gibberellin GA₃ and Zeatin ZR) concentrations at the top of main stem (0–5 and 5–10 cm) were continuously measured and the main stem development situation was observed and recorded. The results showed that after chemical capping, the IAA concentration decreased firstly and increased later, lower than that of CK. ABA concentration increased significantly and GA₃ concentration decreased significantly compared with CK. ZR concentrations fluctuated obviously at T1 and gently at T2. In terms of main stem growth, the plant height, number of fruit branches and average length of upper internode (fifth and above) were decreased compared with CK, while the CA treatment was inhibited more strongly than the DPC treatment. To conclude, chemical capping operation affected the hormone concentration at the plant apex significantly and effectively regulated plant development. In comparison with DPC treatment, CA regulated hormones effectively, which is favorable for conducive reasonable plant shaping.