Impact of PGPR-enriched tuff on biochemical characteristics, antioxidant enzyme activities and hormonal responses in walnut saplings

Abstract

Walnut (Juglans regia L.) is one of the most economically important tree nuts worldwide due to its nutritional and health-promoting properties. The use of plant growth-promoting rhizobacteria (PGPR) in agriculture is increasing attention as a sustainable alternative to intensively used chemical inputs; however, their application to walnut seedlings and their effects on changes in plant hormones, antioxidant enzyme activities and biochemical metabolites are not yet fully understood. Therefore, this study aims to investigate the effects of PGPR-enriched tuff on the hormonal responses and metabolite profiles of walnut seedlings. The experiment was conducted for two consecutive years in a greenhouse using the ‘Şebin’ cultivar. Medium-sized tuff pieces (8 × 8 × 11 mm) were inoculated with approximately 2.9 × 109 colony-forming units (cfu) g−1 PGPR applied directly to the root zone of each seedling. The results of the experiment showed that the application of tuff enriched with PGPR significantly improved physiological and biochemical properties compared to the control group. Significant increases were recorded in chlorophyll content, stomatal conductance, membrane permeability, and photosynthetic rate. Antioxidant enzyme activities and concentrations of key plant hormones including indole acetic acid (IAA), gibberellic acid (GA), and salicylic acid (SA) reached their maximum levels under PGPR treatment, while abscisic acid (ABA) showed the highest accumulation in the control. Amino acid and organic acid contents of walnut leaves generally increased with PGPR application. Most nutrient element concentrations were enhanced. PGPR-inoculated tuff demonstrated strong potential as a sustainable soil amendment to improve walnut seedling nutrient uptake, physiological performance, and stress tolerance.