Morphological and physiological responses of Pistacia rootstocks to salinity stress and commercial microbial formulation

Salinity is a major constraint to pistachio establishment in arid and semi-arid regions. The use of plant growth-promoting rhizobacteria (PGPR) may enhance the salinity tolerance of different pistachio rootstocks. This study assessed the morphological and physiological responses of seven pistachio rootstocks [Pistacia khinjuk (type A and B), P. atlantica, P. mutica, P. terebinthus, and P. vera (Siirt and Kırmızı cultivars)] to salinity and a commercial microbial formulation (Lifebac S Plus - Chlorobium limcola, C. vibrioforme, Nitrosomonas spp, Nitrobacter spp, Pseudomonas vulgaris with a cumulative concentration of 1 × 106 cfu/ml). Two-year-old seedlings of all rootstocks were subjected to 0, 100, and 200 mM NaCl with or without Lifebac S Plus (hereafter PGPR) inoculation at 15, 30, and 45 days after initiation of salinity treatments. Increasing salinity significantly reduced biomass production, leaf development, relative water content, chlorophyll index, stomatal conductance, and the K⁺/Na⁺ ratio, while increased Na⁺ accumulation. The PGPR caused genotype-dependent responses, which were evident under moderate salinity (100 mM NaCl). Inoculation partially maintained shoot and root growth, tissue hydration, stomatal activity, and ionic balance in P. atlantica, P. mutica, and Siirt rootstocks under moderate salinity. These advantages were limited in non-saline conditions, and PGPR proved unable to mitigate severe stress (200 mM NaCl). Principal component analysis revealed clear differentiation of treatments along a salt-induced gradient characterized by Na⁺ accumulation and biomass reduction, with PGPR-treated plants at moderate salinity exhibiting improved physiological and ionic conditions. Salinity was the primary factor influencing performance, whereas certain PGPR-rootstock combinations improved tolerance mainly under moderate stress. These results highlight the importance of combining salt-tolerant rootstocks (P. atlantica, P. mutica, and Siirt) with appropriate microbial biostimulants to enhance pistachio establishment under saline conditions. The observed benefits mediated by PGPR should be interpreted in the context of the specific commercial microbial formulation used in this study, and broader generalizations to other PGPR products or strains require further confirmation.