Pak. J. Bot., 52(4): 1441-1447, 2020. DOI: http://dx.doi.org/10.30848/PJB2020-4(7) EVALUATING EFFICACY OF PLANT GROWTH PROMOTING RHIZOBACTERIA AND POTASSIUM FERTILIZER ON SPINACH GROWTH UNDER SALT STRESS MUHAMMAD ZAFAR-UL-HYE * , FIZA MAHMOOD, SUBHAN DANISH, SHAHID HUSSAIN, MEHREEN GUL, RIZWAN YASEEN AND MUHAMMAD SHAABAN Department of Soil Science, Faculty of Agricultural Sciences and Technology, Bahauddin Zakariya University Multan, Pakistan *Corresponding author’s email: zafarulhyegondal@yahoo.com Abstract Soil salinity is a major constraint for crop production as it negatively impacts the supply of nutrients and produces ethylene that hampers crops productivity. Several studies have been conducted so far to devise robust strategies for alleviation of soil salinity stress on crops. Inoculation of plant growth promoting bacteria (PGPR) is one of the potential strategy to decrease salinity stress on the plants. The PGPR can produce 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase which decreases stress of ethylene generated by salinity. On the other hand, improving potassium (K) uptake is also an effective strategy to minimize the negative effects of salt stress on crops. Therefore, a pot study was conducted with co-application of two pre-isolated ACC deaminase producing PGPR (i.e. Bacillus amyloliquefaciens and Alcaligenes faecalis) with half (HRK) and full recommended dose of K (RK) fertilizer in spinach (Spinacia oleracea) under salt stress (control = 3.0 and 5.0 dS m –1 ). Co-application of B. amyloliquefaciens + RK significantly enhanced leaf and root fresh and dry weights of spinach. Maximum significant improvement in chlorophyll a (93%), chlorophyll b (50%), total chlorophyll (76%), K concentrations in leaf (70%) and root (60%), and reduction (26%) in leaf electrolyte leakage validated the effectiveness of B. amyloliquefaciens and A. faecalis with RK to mitigate salt stress. It is concluded that both B. amyloliquefaciens and A. faecalis with RK can alleviate salt stress in spinach. Key words: ACC deaminase; Chlorophyll pigments; Electrolyte leakage; Morphological attributes. Introduction Plant response to salt stress has been widely investigated by researchers (Lu et al., 2003; Qu et al., 2012; Shahzad et al., 2012; Yasmeen et al., 2013; Yasmeen et al., 2019). It is well documented that 20% of cultivable and half of irrigated area of world is saline (Abogadallah, 2010). Cultivation of crops in saline soils results in development of ionic toxicities, production of reactive oxygen species and osmotic stresses that significantly disturb plants growth (Locy et al., 1996; Flowers & Flowers, 2005; Munns et al., 2006; Zörb et al., 2019). In salt affected soils, Na + and Cl – restrict the development of root and optimum uptake of water by decreasing the water potential in rhizosphere (Rout & Shaw, 2001). Stress induced by salinity accelerates the synthesis of ethylene (Stearns & Glick, 2003; Roychoudhury et al., 2019) that imposes deleterious effects on plants (Van Loon & Fontaine, 1984; Stearns & Glick, 2003). The elevated level of ethylene in plants minimizes the supply of energy and restricts the water uptake due to poor development of root (Taiz & Zeiger, 2010; Ricardo, 2012). Higher rate of transpiration, minimum biological nitrogen fixation, stomatal closure and evoking of physiological responses are also major indicators of elevated ethylene biosynthesis and accumulation (Tamimi & Timko, 2003; Wang et al., 2003; Tanaka et al., 2005; Roychoudhury et al., 2019). Many scientists have observed that salinity stress stimulates ethylene precursor 1-aminocyclopropane-1- carboxylic acid (ACC) (Shah et al., 1998; Glick et al., 1999; Glick, 2004; Albacete et al., 2008; Roychoudhury et al., 2019). For mitigation of salinity stress, inoculation with ACC deaminase (ACCD) producing plant growth promoting rhizobacteria (PGPR) is frequently recommended (Penrose & Glick, 1997; Glick, 2004; Zahir et al., 2009; Zafar-ul-Hye et al., 2014; Ijaz et al., 2019). The enzyme deaminase breakdown ethylene into α- ketobutyrate and NH3 (Shah et al., 1998; Glick, 2004; Taiwo et al., 2018), and later one is mostly utilized by PGPR that inhibits the regeneration of ethylene (Shah et al., 1998; Roychoudhury et al., 2019; Ahmed et al., 2020). Besides the use of PGPR, most of the researchers also suggested to improve the uptake of potassium (K) to reverse the negative effects of salinity on plants (Yahya, 1998; Tzortzakis, 2010). Presence of K ions in large amount increases the selectivity of K over Na that significantly reduces the intake of Na in plants (Volkov et al., 2004). Better uptake of K ultimately increased the K/Na ratio in mesophyll cells which play an imperative role in regulation of proper enzymatic functioning in crops under salt stress (Volkov et al., 2004). However, despite all modern reclamation and mitigation techniques for salinity stress, the natural causes and agricultural practices are continuously contributing towards the development of saline soils (Munns & Tester, 2008). The need of the time is to explore the approaches that are associated with the solution of salinity stress for better production of crops (Chinnusamy et al., 2005; Ashraf & Akram, 2009). Spinach (Spinacia oleracea) is considered the richest source of beta carotene, calcium (Ca), vitamin C, iron (Fe), phosphorus (P) and potassium (K) (Dicoteau, 2000; Massa et al., 2018). It also plays an imperative role for the provision of supplement p-coumaric acid derivatives that show a significant antioxidant activity and glucuronic acid derivatives of flavonoids (Bergman et al., 2001; Edenharder et al., 2001; Pandjaitan et al., 2005; Caparrotta et al., 2019). A lot of work has been so far done on inoculation of ACCD producing PGPR and K