Arbuscular Mycorrhiza and Plant Growth-promoting Bacteria Alleviate Drought Stress in Walnut

Drought stress is one of the main constraints limiting worldwide crop production. Arbuscular mycorrhizae (AM) and plant growth-promoting bacteria (PGPB) such as Azotobacter chroococcum and Azospirillium lipofrum have been shown to alleviate drought stress effects. Therefore, the interaction effect of AM fungi [Glomus mosseae, G. etunicatum, and a mix of these (G. mix), and PGPB bacteria (Azotobacter chroococcum + Azospirillium lipofrum)] was investigated in 1-year-old walnut seedlings (cv. Chandler) under normal and drought stress conditions. Drought stress reduced growth (plant height, root length, number of leaves, and fresh weight) and leaf nutrient content (N, P, and Zn) significantly of walnut plants. In contrast, proline, total soluble sugar, starch peroxidase enzyme activity, and total phenolic content of walnut leaves increased under this stress. Application of fungi or bacteria, and especially their simultaneous use, alleviated the negative effects of drought stress on walnut seedlings. AM fungi and PGPB increased significantly the content of some metabolites, including total phenolic content, proline level, peroxidase activity, total soluble sugar, and starch content as well as peroxidase enzyme activity. This led to an increase in walnut plant growth under the drought stress condition. Among AM fungi, G. etunicatum was more effective in reducing drought stress symptoms than either G. mosseae or the G. mix of fungi. In conclusion, use of G. etunicatum, along with PGPB, can reduce negative effects of drought stress on walnut seedlings.