Optimizing cotton growth in saline soil: Compound microbial agent modulates indigenous bacteria to enhance photosynthesis and vegetative-reproductive balance

Abstract

Salt-tolerant composite plant growth-promoting rhizobacteria (STC-PGPR) can alleviate salt stress, though its complexity makes its stability and mechanisms in indeterminate-growing crops unclear. We tested STC-PGPR on two cotton (Gossypium hirsutum L.) varieties (G1: Xinluzao 72, G2: Zhongmiansuo 49), with two microbial treatments (B1: none, B2: with STC-PGPR) and three salinity levels (S1: 0 g NaCl kg⁻¹, S2: 4 g NaCl kg⁻¹, S3: 8 g NaCl kg⁻¹). The pot experiment used a three-factor completely randomized design in a greenhouse, with NaCl mixed into the soil before transplanting. STC-PGPR (Paenibacillus polymyxa SC2, Bacillus velezensis DSYZ, Lactobacillus casei DY-3, and Saccharomyces cerevisiae DY-4) was applied 14 days post-transplant and during flowering and boll development. STC-PGPR was diluted with water and applied to the rhizosphere. Results showed that under S1, STC-PGPR increased G2’s seed cotton dry matter accumulation (Sc) by 25.1 %. Under S2, STC-PGPR increased G1’s boll dry matter accumulation (Bo) from 0.21 g to 7.7 g per pot. Other STC-PGPR treatments were less effective due to overdeveloped vegetative organs, highlighting the need for balanced vegetative and reproductive growth. High salinity reduced STC-PGPR effectiveness under S3. Random forest analysis identified crucial indicators regulated by STC-PGPR that significantly affected reproductive organ dry matter accumulation (P<0.001). Bioinformatics and correlation analyses showed biomarkers enriched by STC-PGPR influenced these indicators, not STC-PGPR directly. In S1, G2B2’s biomarkers enhanced stomatal conductance, increasing the net photosynthetic rate (Pn) and promoting dry matter accumulation. They also increased stem diameter and inhibited vegetative growth, enhancing transport and channeling energy to reproductive organs. In S2, G1B2’s biomarkers reduced leaf thickness, increased chlorophyll content to enhance Pn, and extended the maturation period. This enabled cotton to accumulate more photosynthetic products and allocate them to reproductive organs. This study demonstrates that STC-PGPR enhances productivity under salt stress, providing insights for optimizing saline soil cultivation.