Microbial Ecology Improved Tolerance of Acacia nilotica to Salt Stress by Arbuscular Mycorrhiza, Glomus fasciculatum may be Partly Related to Elevated K/Na Ratios in Root and Shoot Tissues Bhoopander Giri, Rupam Kapoor and K. G. Mukerji Department of Botany, University of Delhi, Delhi 110007, India Received: 17 February 2007 / Accepted: 26 February 2007 / Online publication: 20 March 2007 Abstract A pot experiment was conducted to examine the effect of arbuscular mycorrhizal fungus, Glomus fasciculatum, and salinity on the growth of Acacia nilotica. Plants were grown in soil under different salinity levels (1.2, 4.0, 6.5, and 9.5 dS m _ 1 ). In saline soil, mycorrhizal colonization was higher at 1.2, 4.0, and 6.5 dS m _ 1 salinity levels in AM-inoculated plants, which decreased as salinity levels further increased (9.5 dS m _ 1 ). Mycorrhizal plants maintained greater root and shoot biomass at all salinity levels compared to nonmycorrhizal plants. AM-inoculat- ed plants had higher P, Zn, and Cu concentrations than uninoculated plants. In mycorrhizal plants, nutrient concentrations decreased with the increasing levels of salinity, but were higher than those of the nonmycor- rhizal plants. Mycorrhizal plants had greater Na concen- tration at low salinity levels (1.2, 4.0 dS m _ 1 ), which lowered as salinity levels increased (6.5, 9.5 dS m _ 1 ), whereas Na concentration increased in control plants. Mycorrhizal plants accumulated a higher concentration of K at all salinity levels. Unlike Na, the uptake of K increased in shoot tissues of mycorrhizal plants with the increasing levels of salinity. Our results indicate that mycorrhizal fungus alleviates deleterious effects of saline soils on plant growth that could be primarily related to improved P nutrition. The improved K/Na ratios in root and shoot tissues of mycorrhizal plants may help in protecting disruption of K-mediated enzymatic processes under salt stress conditions. Introduction Salinization of soils is a serious land degradation problem in arid and semi-arid areas and is increasing steadily in many parts of the world including India [3]. Globally, almost 1000-million-ha land (7% of all land area) is affected by soil salinity [43]. Out of 1.5 billion ha cultivated land, about 77 million ha (5%) are affected by excess salt content [30]. In India alone, salinity affects 7 million ha of land, which is mainly attributed to irrigation with ground water of high salt content, sodic and alkaline parent material [1, 19]. Among the most common effects of soil salinity is growth inhibition by Na _ and Cl _ [44]. Elevated Na + in soil solution inhibits the uptake of other nutrients by disrupting the uptake of nutrients directly by interfering with various transporters in the root plasma membrane, such as K + -selective ion channels and inhibiting root growth by the osmotic effects of Na + on soil structure [46]. Thus, the uptake of water and essential mineral nutrients, such as P, K, Fe, Cu, and Zn, and the growth of soil microorganisms can be reduced. Moreover, high Na + /K + ratio disrupts various metabolic processes such as protein synthesis in the cytoplasm [44]. Arbuscular mycorrhiza (AM) occurs naturally in saline soils [39]. Salinity affects the formation and function of mycorrhizal symbiosis [19, 24, 28]. However, several studies have demonstrated that inoculation with mycorrhizal fungi improves growth and productivity of plants under a variety of salt stress conditions [6, 13, 14, 18]. Owing to the importance of AM fungi under salt stress conditions, they have been considered as bio- ameliorators of saline soils [13, 14]. It is well established that mycorrhizal fungi uptake immobile nutrients, particularly phosphorus (P). The improvement in the plant P status has been suggested as the most important strategy of salinity stress tolerance in AM colonized Present address of Bhoopander Giri: Biotechnology and Management of Bioresources Division, The Energy and Resources Institute (TERI), Darbari Seth Block, India Habitat Center, Lodhi Road, New Delhi 110003, India Correspondence to: Bhoopander Giri; E-mail: bhoopg@yahoo.com DOI: 10.1007/s00248-007-9239-9 & Volume 54, 753–760 (2007) & * Springer Science + Business Media, LLC 2007 753