Effects of Inoculation with Mycorrhizal Fungi and Azotobacter on Growth and Oxidative Responses of Wheat to Salinity and Cadmium Stresses

In recent decades, the increasing trend of concurrent soil salinity and heavy metal stresses in arid and semi-arid regions all over the world has become a serious threat for agricultural production and human food security. Cadmium (Cd), as a heavy metal, can be readily absorbed by plant roots, leading to disruptions in plant physiological and biochemical activities. A factorial pot experiment was carried out based on a completely randomized design with the aim of investigating the beneficial effects of root symbiosis with mycorrhizal fungi and azotobacter on the responses of two bread wheat (Roshan and Bahar) and two durum wheat (Yavarus and Behrang) cultivars grown in a naturally Cd-contaminated soil to salinity stress. Experimental factors were consisted of (1) irrigation water salinity in three levels including 0, 75 and 150 mM NaCl and (2) inoculation treatments in four levels including no-inoculation as control, plus separate inoculation by Rhizophagus intraradices and Funneliformis mosseae mycorrhizal fungi and Azotobacter sp. bacteria. Salinity stress was observed to cause an increase in shoot Cd concentration, antioxidant enzymes activity and the levels of proline, hydrogen peroxide and malondialdehyde, while it decreased shoot dry weight. Under saline condition, the salt tolerant cultivars (Roshan and Behrang) showed a lower increase rate of shoot Cd concentration as compared to salt sensitive cultivars (Bahar and Yavarus), while they showed a higher increase rate of antioxidant enzymes and proline content. Wheat plants inoculated by F. mosseae, Rh. intraradices and Azotobacter sp. revealed a higher shoot dry weight (20, 12 and 7%, respectively) as well as higher activities of catalase (22, 18 and 12%), peroxidase (39, 32 and 20%) and ascorbate peroxidase (64, 56 and 47%) antioxidant enzymes, while a lower shoot Cd concentration (24, 11 and 5%) and lipid peroxidation activity (14, 10 an 5%) as compared to non-inoculated plants. The results from this experiment showed that soil salinization increased cadmium concentration in wheat plants but the inoculation by mycorrhiza and rizobacter stimulated antioxidant enzyme defense system and reduced lipid peroxidation and cadmium absorption from soil.