Background: Cotton production in China is challenged by high labor input including manual topping (MT). Recently, to replace MT in the Xinjiang cotton region of China, mepiquat chloride (MC) was applied once more than the traditional multiple-application; this was designated as chemical topping (CT), but it is unclear whether the amount of irrigation needs to be adjusted to accommodate CT. Results: The main plots were assigned to three drip irrigation amounts [300 (I₁), 480 (I₂), and 660 (I₃) mm], and the subplots were assigned to the CT treatments [450 (MC₁), 750 (MC₂), and 1 050 (MC₃) mL·hm⁻² 25% MC] with MT as a control that was performed after early bloom. The optimum drip irrigation amount for CT was explored based on leaf photosynthesis, chlorophyll fluorescence, biomass accumulation, and yield. There were significant influences of drip irrigation, topping treatments and their interaction on chlorophyll fluorescence characteristics, gas exchange parameters and biomass accumulation characteristics as well as yield. The combination of I₂ and MC₂ (I₂MC₂) performed best. Compared with I₂MT, the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and photochemical quenching coefficient (qP) of I₂MC₂ significantly increased by 4.0%∼7.2%, 6.8%∼17.1%, 5.2%∼17.6%, and 4.8%∼9.6%, respectively, from the peak flowering to boll opening stages. Moreover, I₂MC₂ showed fast reproductive organ biomass accumulation and the highest seed cotton yield; the latter was 6.6%∼12.8% higher than that of I₂MT. Further analysis revealed that a 25% MC emulsion in water (MC_EW) application resulted in yield improvement by increasing Pn, φPSII, and qP to promote biomass accumulation and transport to reproductive organs. Conclusion: The results showed that the 480 mm drip irrigation combined with 750 mL·hm⁻² MC increased the rate of dry matter accumulation in reproductive organs by increasing Pn, φPSII, and qP to improve photosynthetic performance, thus achieving higher yield.

(1) Background: Nitric oxide (NO) serves as a crucial signaling molecule in plant abiotic stress responses. Although its role in enhancing drought resistance in cotton has been recognized, the specific mechanisms underlying this physiological and molecular regulation remain largely unexplored. This study aims to elucidate the multi-layered mechanisms by which NO modulates drought resistance in cotton; (2) Methods: Cotton seedlings were subjected to drought stress with the application of the NO donor sodium nitroprusside (SNP). A combination of confocal laser scanning microscopy, transcriptional expression analysis, biochemical assay of enzyme activity, virus-induced gene silencing (VIGS), and in vitro protein modification assays was applied to characterize the effects of NO on the drought stress response in cotton; (3) Results: Exogenous NO significantly reinforced drought resistance in cotton seedlings by improving leaf water retention capacity and photosynthetic efficiency, eliminating excessive drought-induced reactive oxygen species (ROS), upregulating the transcription and enzymatic activity of antioxidant enzymes, and promoting stomatal closure. Mechanistically, NO triggered S-nitrosylation of the plasma membrane H⁺-ATPase isoform GhHA2, thereby enhancing its protein stability; (4) Conclusions: These findings reveal that exogenous NO orchestrates cotton drought tolerance via multiple interconnected physiological and molecular pathways, in which the activation of the antioxidant defense system and the modulation of stomatal closure serve as central regulatory mechanisms.

Chemical harvest aids facilitate mechanical cotton harvesting, but their potential carryover effects on succeeding wheat remain uncertain. Two independent site-year field experiments were conducted in Beijing during 2021–2022 and in Hejian, Hebei Province, during 2022–2023. Experiment 1 compared a freshwater control with preceding Xinsaili applications at 1500 and 3000 g ha⁻¹, whereas Experiment 2 evaluated five biochar treatments under preceding freshwater or Xinsaili application at 1575 g ha⁻¹. In Experiment 1, Xinsaili reduced early aboveground biomass and soil bacterial, fungal, and actinomycete abundance. At 3000 g ha⁻¹, grain number per spike and grain yield decreased by 12.2% and 24.0%, respectively, relative to the water control. In Experiment 2, Xinsaili reduced mean and maximum grain-filling rates by 3.2% and 4.3%, respectively. For the biochar main effect, maize-straw biochar at 750 and 1500 kg ha⁻¹ and rice-husk biochar at 1500 kg ha⁻¹ increased the initial grain-filling potential from 0.25 mg grain⁻¹ without biochar to 0.38–0.42 mg grain⁻¹, with high-rate maize-straw biochar producing the greatest increase. Although Xinsaili × biochar interactions were significant for both grain-filling rates, no biochar treatment significantly improved either rate under Xinsaili application. The grain-filling responses were not consistent between the two independent experiments, potentially reflecting differences in site, year, wheat cultivar, Xinsaili application rate, and experimental design. Thidiazuron and ethephon residues were not quantified in soil or plant tissues; therefore, the observed responses represent indirect field evidence and do not demonstrate either residue-mediated effects or biochar-mediated remediation. Overall, preceding Xinsaili application was associated with treatment- and site-year-dependent wheat responses, while the capacity of biochar to improve final grain yield was not confirmed.