Long-term straw return with appropriate nitrogen (N) fertilization increases seedcotton yield and fiber quality, and the root system plays an important role in cotton production. However, under straw return and N fertilization, the relationship between the cotton boll-loading capacity of the root system and seedcotton yield remains unclear. In this study, a ten years of long-term field experiment was conducted in a wheat–cotton rotation system. The effects of straw treatments (straw return and straw removal) and N rates (N0, N75, N150 and N300 representing 0, 75, 150 and 300 kg N ha⁻¹, respectively) on cotton root activity, boll-loading capacity of the root system and their relationship to seedcotton yield from 2019 to 2022 were quantified. The results showed that straw return with an appropriate N fertilization of N150 increased root biomass, the rate and components of root-bleeding sap, as well as boll-loading capacity of the root system and seedcotton yield, but decreased the ratio of root to shoot biomass. Furthermore, the root-bleeding sap rate reached the maximum at 30 d post anthesis (DPA) during the peak boll setting stage. However, the contents of nitrate-N, free amino acids and soluble sugar in root-bleeding sap decreased from 10 DPA. Notably, in 2021 and at 30 DPA, the highest contents of nitrate-N (4.8 μg mL⁻¹) and free amino acids (8.3 μg mL⁻¹), as well as soluble sugar (3.4 μg mL⁻¹) were observed at N150 under straw return. The increase in seedcotton yield is positively correlated to the soluble sugar content. Straw return significantly increased the boll-loading capacity of the root system, which first increased but then decreased with the increase in N fertilization. Under straw return with N150, the maximum seecotton yield (3455–4544 kg ha⁻¹) was recorded, and the largest boll loading (49–54 boll 100 g⁻¹) and boll capacity (242–292 g 100 g⁻¹) of root system at the boll opening stage were observed. Therefore, straw return with appropriate N fertilization improved root activity and the boll-loading capacity of the root system, thereby increasing seedcotton yield. This study provides new insights into improving seedcotton yield from the perspective of coordinating cotton growth.

As the main byproduct of cotton production, cottonseed yields edible vegetable oil, ruminant feed, and industrial products. We evaluated the individual and interactive effects of elevated air temperature and soil drought on cottonseed yield and nutritional quality using two cotton cultivars, Sumian 15 (heat-susceptible) and PHY370WR (heat-tolerant). The experiment was conducted under three levels of soil relative water content (SRWC): (75 ± 5)%, (60 ± 5)% and (45 ± 5)% and two temperature regimes: ambient temperature (AT, 31.0/26.4 ℃, mean daytime/night temperature) and elevated temperature (ET, 33.4/28.9 ℃). Cottonseed yield, boll number, seed number, and single-seed weight were lower under combined ET and SRWC(45 ± 5)% than either individual stress or combined stresses in comparison with the control treatment (SRWC(75 ± 5)% under AT). Drought tended to increase oil content and reduce protein content, whereas ET showed almost the opposite effects. Under the combination of ET and soil drought, oil content was still higher than under control, although ET weakened the beneficial effects of drought. For protein, ET offset the negative impacts of mild drought on protein content, but protein content was not increased under SRWC(45 ± 5)%. Each stress or combined stress reduced oil and protein yields under all treatments, owing to declines in cottonseed yields. The combined stress reduced unsaturated fatty acid (UFA)/saturated fatty acid (SFA) and essential amino acid (EAA)/non-essential amino acid (NAA). Compared with PHY370WR, the sensitivity of Sumian 15 to the combined factors was evidenced in the following ways: (1) seed yield, yield components, oil and protein yields were decreased more for Sumian 15 than PHY370WR compared with the control treatment; (2) the combined stresses caused lower oil content, UFA, and UFA/SFA in Sumian 15 than PHY370WR; (3) interaction effects of the combined factors on protein content and EAA/NAA were detected only in Sumian 15.