Synergistic regulation of cadmium stress, nutrient homeostasis and fiber quality in cotton by caffeic acid, ethylene and PGPR

Cadmium (Cd) contamination in agricultural soils poses a severe global threat to crop productivity and food safety. This study evaluated the synergistic potential of caffeic acid (CA), ethylene (ET) and plant growth-promoting rhizobacteria (PGPR, Bacillus subtilis) in mitigating Cd-induced phytotoxicity in cotton (Gossypium hirsutum L.). A loamy soil was spiked with 10 mg Cd kg⁻¹ and 17 treatments, including individual and combined applications of CA (30/60 µM), ET (100/200 µM) and PGPR (10¹¹ CFU kg⁻¹), were tested in a completely randomized design (CRD) with four replicates over 90 days. Cd stress alone significantly decreased root length (21.79%), shoot length (17.12%), fiber strength (12.30%), chlorophyll content (60.91%), photosynthetic rate (46.43%) and micronutrient uptake (Fe by 25.80% and Zn by 24.23%) and increased oxidative markers (MDA by 40.76%) and Cd accumulation in tissues (up to 17,900% in roots). In contrast, the combined application of CA 60 µM + ET 200 µM + PGPR (T17) markedly reversed these effects, increasing vegetative growth (up to 55.31%), gas exchange attributes (photosynthesis by 93.41%), fiber quality (length by 9.65%, strength by 14.93%), antioxidant defense suppression (SOD activity decreased by 17.43%) and micronutrient content (Fe in shoots by 31.33%, Mn in roots by 58.33%), while decreasing Cd concentration in shoots by 34.54% and in leaves by 32.39%. Soil health also improved, with DTPA-extractable Cd reducing by 65% and organic matter increasing by 8.3%. These findings demonstrate that the triple amendment strategy is a highly effective and eco-friendly approach for sustaining cotton productivity in Cd-contaminated agroecosystems. This study demonstrates a novel, field-applicable “triple-amendment recipe,” 60 µM caffeic acid + 200 µM ethylene + Bacillus subtilis, that simultaneously improve cotton fiber quality, nutrient homeostasis and soil health in Cd-contaminated agroecosystems.