Optimizing cotton yield through appropriate irrigation water salinity: Coordinating above- and below-ground growth and enhancing photosynthetic capacity

The employment of brackish water for irrigation holds considerable potential for addressing the shortfall of freshwater resources in arid regions. It is crucial to discern the impact of salinity on agricultural output and its mechanisms when irrigating with brackish water. Here, a two-year field experiment (2021–2022) was undertaken in Xinjiang, China to assess the effects of water salinity (0.85 g L−1, CK; 3 g L−1, S1; 5 g L−1, S2; and 8 g L−1, S3) on the growth, photosynthesis, carbon accumulation, and cotton yield under mulched drip irrigation. Results showed that, compared to CK, the S1 treatment led to increased biomass, the boll capacity of the root system (BCR), and a reduced root-to-shoot ratio (R/S). In addition, the S1 treatment enhanced the net photosynthetic rate (Pn) and chlorophyll content (Chl) of cotton, enhancing the total carbon accumulation of boll (Cboll) and consequently, the seed cotton yield. However, the higher salinity treatments, S2 and S3, demonstrated a detrimental impact on these indicators. In comparison to CK, the S1 treatment showed a yield increase of 5.92% and 4.76% in the respective two years, yet the S2 and S3 treatments showed a yield decrease of 5.55–15.77% and 5.06–16.74%. The application of a structural equation model (SEM) elucidated a direct effect of photosynthetic parameters (Pn and Chl) and biomass characteristics (total biomass, R/S, and BCR) on yield. These factors also indirectly modulated seed cotton yield through the Cboll/Croot ratio. These results suggest that irrigation with brackish water of 3 g L−1 optimizes cotton production by coordinating crop growth of aboveground and underground parts. This study offers practical strategies for the cotton industry to enhance yields under brackish water irrigation.