Regulation of the Negative Effects of Lime-Induced Stress on Plant Hormone Balance in Blackberry by Plant Growth-Promoting Rhizobacteria

Approximately 30% of the global agricultural area consists of calcareous soil. Horticultural plants, especially fruit trees, are sensitive to calcareous soil. People frequently use lime-resistant rootstocks and varieties in regions where they grow fruit in calcareous soils. While the use of rootstocks and varieties is not a solution in some cases, the use of plant growth-promoting rhizobacteria (PGPR) has emerged. People are increasingly using PGPR to enhance plant growth conditions, particularly when addressing suboptimal soil quality through soil remediation. This study investigated the impact of four distinct species of PGPR—Alcaligenes faecalis 637Ca, Microbacterium esteraromaticum SY48, Rhizobium radiobacter SY55, and Kocuria rhizophila SK63—on the levels of phytohormones, including abscisic acid (ABA), indole 3‑acetic acid (IAA), gibberellin (GA), jasmonic acid (JA), salicylic acid (SA), and zeatin, in the ‘Chester’ and ‘Jumbo’ blackberry cultivars. The research was conducted under high calcareous soil (41%) conditions. The research findings indicate that treatments T1 and T8 exhibited the lowest levels of ABA content, with values of 5020.24 ng g−1 dry tissue (DT) and 836.84 ng g−1 DT, respectively. Conversely, treatment T6 demonstrated the highest levels of GA (17.02 ng g−1 DT), zeatin (5.67 ng g−1 DT), and JA (62.19 ng g−1 DT) content. Additionally, treatment T3 resulted in the highest levels of SA (21.26 ng g−1 DT) and IAA (9.61 ng g−1 DT) content. The findings suggest that a combination of the 637Ca, SY48, and SK63 bacterial strains may be a suitable recommendation for fruit cultivation in calcareous soil conditions.