Tolerance to salinity and drought stresses in pistachio (Pistacia vera L.) seedlings inoculated with indigenous stress-tolerant PGPR isolates

Abstract

Salinity and drought are two major abiotic stresses that limit plant growth and productivity through their adverse effects on diverse physiological processes in plants, especially their photosynthetic capacity. Application of indigenous stress-tolerant plant growth-promoting rhizobacteria (PGPR) is among the eco-friendly strategies employed to ameliorate the harmful effects of salinity/drought stresses on plants. The present study was carried out to investigate the effects of stress-tolerant PGPR isolates on vegetative growth, nutritional, and photosynthetic parameters of pistachio (Pistacia vera L.) seedlings (cv. Badami-Riz–e-Zarand) under salinity and drought conditions. A factorial greenhouse experiment was conducted as a completely randomized design with three replications for 28 weeks in 2019. The treatments consisted of PGPR inoculations (control: non-inoculated, and inoculation with Staphylococcus sciuri, Zobellella denitrificans, and Arthrobacter endophyticus), three salinity levels (0, 1000 and 2000 mg NaCl kg−1 soil), and three irrigation intervals (C: irrigated every two days; D1: four days and D2: six days). The results indicated that, under high levels of salinity and drought stresses (2000 mg NaCl kg−1 soil + six-day irrigation intervals), separate inoculation of pistachio seedlings with Arthrobacter endophyticus, Zobellella denitrificans and Staphylococcus sciuri significantly enhanced the shoot and root dry weight, leaf area, leaf number, shoot and root K+ concentration and relative water content (RWC), which were accompanied by decreases in shoot and root Na+ concentrations. Moreover, the combined salinity and drought stresses had a greater effect on reducing total chlorophyll (TChl), carotenoids, chlorophyll fluorescence index (Fv/Fm), net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and water use efficiency (WUE), whereas application of the stress-tolerant PGPR isolates significantly increased these photosynthetic parameters compared to the non-inoculated seedlings at the corresponding stress levels. According to the results, the indigenous stress-tolerant PGPR isolates effectively improved the growth, physiological and photosynthetic parameters in pistachio seedlings subjected to salinity and drought stresses, which could be attributed to the local adaptation of the PGPR isolates as well as their plant growth-promoting properties that were unraveled in our in vitro studies, e.g. production of auxin, ACC-deaminase, siderophore, exopolysaccharides along with P/Zn solubilization activities.