Enhancing Soybean Growth with Phytohormone-enriched PGPR Bioinoculants and Biofertilizers in Plant Stress Physiology

Background: The present investigation aimed to evaluate the effects of plant growth-promoting rhizobacteria (PGPR) bioinoculants and their carrier-based biofertilizers on the physiology of soybean. Methods: Seeds of two soybean varieties (cv. NARC-1 and cv. William-82) were either soaked in broth cultures of two PGPR strains, Planomicrobium chinense (MF616408) and Pseudomonas putida (KX574857), containing 106 cells/mL, or coated with biofertilizers. Three biofertilizers, namely biostimulant, biozote and biofert, were used. Result: Significant increases in root and shoot weight were observed with both liquid bioinoculants and carrier-based biofertilizers compared to the control. The consortium of P. chinense and P. putida resulted in the highest proline content and superoxide dismutase (SOD) activity. Biostimulant-treated plants exhibited higher catalase (CAT) activity and phenolics content. Indole-3-acetic acid (IAA) and gibberellic acid (GA) levels were enhanced in plants treated with PGPR and biofertilizers. Specifically, P. chinense and the biostimulant significantly promoted IAA content in cv. William-82, while biozote and P. chinense treatments increased GA content in both varieties. The consortium of P. chinense and P. putida showed the lowest ABA levels. The highest IAA/ABA and GA/ABA ratios were observed in plants treated with P. chinense. Liquid bioinoculants had a more pronounced effect on plant growth than carrier-based biofertilizers, though biofertilizers had stronger residual effects on rhizosphere soil organic matter. The combined effects of phytohormones, sugar, protein and proline production and antioxidant enzyme activity influenced plant growth and development.