Delaying initial irrigation is feasible because the upper canopy has significant advantages in the distribution of photosynthetic products in cotton bolls

The production of cotton and other crops must be efficient and sustainable at a global scale. Deep drip irrigation and dense planting with equal row spacing are innovative cotton planting strategies designed to address persistent high production costs. We hypothesize that adjusting the irrigation amount and frequency in this approach will strongly benefit yield, particularly at the population level. Therefore, a field experiment spanning three years (2021–2023) was conducted; the split-plot design featured two different irrigation amounts (m 3 ha −1 ; I1: 3177; I2: 3840) for the main plots and 9 (F1), 8 (F2), and 7 (F3) irrigation frequencies for the subplots. The irrigation frequency was varied by adjusting the timing of the initial and final irrigation events. The I2 treatment increased the leaf area index (LAI) in the upper plant parts and the chlorophyll (Chl) content to varying degrees. These physiological changes increased the canopy apparent photosynthesis (CAP) by 18.8–29.5 % and canopy respiration (CR) by 17.6–78.6 %. Additionally, the ratio of upper CR to total CAP (CR/TCP) increased by 26.2–34.4 % from 105 to 120 days after emergence (DAE). Concurrently, the I2 treatment substantially increased the 13 C content ( 13 Ci) in cotton roots, stems, leaves, and bolls in particular, 13 C allocation rate to bolls increased by 4.8–5.5 % from 90–120 DAE ultimately leading to increases in boll number, boll weight, and yield. In the I2 treatment group, the F2 treatment specifically amplified these effects during the late growth period. The I2F2 treatment led to increases in the upper canopy LAI and Chl content, consequently increasing the upper CAP and CR/TCP. This further promoted significant increases in the 13 Ci and 13 C distributions to bolls, resulting in a substantial increase in boll biomass of 19.1–33.2 %. This ultimately resulted in a greater boll number (3.0–13.0 % increase) and seed yield (3.9–20.5 % increase). However, in the I1 treatment group, the F1 treatment resulted in greater distribution of Chl a+b, CAP, CR, CR/TCP, 13 Ci and 13 C to bolls, with increases of 2.1–11.5 %, 8.5–32.1 %, 4.3–22.7 %, 2.1–7.8 %, 33.6–63.0 %, and 3.3–6.3 %, respectively. Consequently, boll biomass increased by 7.8–11.1 %, resulting in 6.4 % and 7.7 % increases in seed and lint yields, respectively. Moreover, correlation analysis revealed that higher levels of cotton LAI, Chl content, CAP, CR, biomass, and 13 Ci in leaves and bolls, along with a higher allocation rate of 13 C to bolls, collectively contributed to greater yields. Under sufficient irrigation conditions (3840 m 3 ha −1 ), delaying the first irrigation (F2) can result in the optimal yield; however, at a reduced irrigation amount (3177 m 3 ha −1 ), the lint yield under conventional-frequency irrigation (F1) is still much higher than the average yield in the local region. This study offers recommendations for suitable irrigation frequencies under deep drip irrigation and variable moisture regimes, enhancing the environmental friendliness and effective resource utilization of the cotton production. • Optimized irrigation enhanced upper canopy photosynthesis by boosting chlorophyll content and leaf area index. • Delayed initial irrigation improved 13 C allocation to bolls, enhancing boll biomass. • Early termination of irrigation negatively impacts cotton yield stability. • The planting system achieved the objectives of reducing production cost and maintaining yield.