Optimal row spacing configuration to improve cotton yield or quality is regulated by plant density and irrigation rate

Conventional high-density planting of machine-picked cotton in Xinjiang is associated with poor light distribution and low fiber quality. The objective of this study was to assess how different row spacing configurations, irrigation rates, and planting densities impact cotton photosynthetic production and fiber quality. We conducted a 3-year field experiment in Shihezi, Xinjiang, China, to evaluate the impact of different row spacing configurations and irrigation amounts on cotton photosynthesis and fiber quality. The four row spacing configurations used were: RS66+10H (high-density with 66 +10 cm spacing), RS66+10L (low-density with 66 +10 cm spacing), RS76H (high-density with 76 cm spacing), and RS76L (low-density with 76 cm spacing). Additionally, two irrigation amounts were tested: MD-T3 (mulch drip irrigation with three tapes on one film) and MD-T2 (mulch drip irrigation with two tapes on one film). We measured and compared the distribution of cotton canopy leaves, light, canopy apparent photosynthesis, reproductive organs and fiber quality among these groups. The results showed that RS66+10H had a higher lint yield than RS76L under MD-T2, with an increase of 5.4–7.1%. This can be attributed to the higher fraction of photosynthetically active radiation, canopy apparent photosynthesis (CAP), and boll biomass in RS66+10H between the full boll and boll opening stages. The maximum lint yield (3227–3580 kg ha−1) was obtained in RS66+10H under MD-T3. Interestingly, RS76L had a similar lint yield to RS66+10H under MD-T3, mainly due to the middle and lower canopy having higher leaf area index (LAI) values ranging from 1.8 to 2.1. This resulted in increased CAP and cotton boll biomass of the middle and lower canopy compared to RS66+10H during the full boll to boll opening stage. Under MD-T3, RS76L also exhibited longer fiber length in the middle and lower canopy (30.2–32.0 mm and 31.6–33.3 mm, respectively) compared to RS66+10H, with a difference of 3.2–3.8% and 5.6–6.5%. Furthermore, RS76L demonstrated higher fiber strength in the middle and lower canopy (31.4–32.7 cN tex−1 and 33.2–33.9 cN tex−1, respectively) compared to RS66+10H, with a difference of 2.6–6.2% and 5.9–6.8%. Maintaining an LAI of around 2.0 during the transition from full boll to boll opening stage optimized canopy architecture and enhanced photosynthetic productivity. Under adequate irrigation, RS76L could be a suitable replacement for RS66+10H in order to improve cotton fiber quality by optimizing leaf-boll spatial distribution. This study provides guidance for selecting the appropriate row spacing configuration under different water conditions. RS66+10H is recommended for increasing yield under water-deficient conditions, while RS76L is suitable for improving fiber quality under adequate irrigation.