Research Journal of Agriculture and Biological Sciences, 4(5): 413-415, 2008 © 2008, INSInet Publication Corresponding Author: Mahtab Salehi, Department of Agronomy and Plant Breeding, College of Agriculture, University of Tehran, Karaj, Iran Tel: +98-0918-363-4906 E-mail address: mhtb.salehi@gmail.com 413 The Effect of Salinity on the Nitrogen Fixation in 4 Cultivars of Medicago sativa L. in the Seedling Emergence Stage Mahtab Salehi, Foroozan Salehi, Kazem Poustini and Hossein Heidari-Sharifabad 1 2 1 3 Department of Agronomy and Plant Breeding, 1 College of Agriculture, University of Tehran, Karaj, Iran Department of Horticulture, College of Agriculture, University of Tehran, Karaj, Iran 2 Institute of Seed and Plant Breeding, Karaj, Iran 3 Abstract: In order to study the effect of salinity on the nitrogen fixation in Medicago sativa L. seedling emergence stage, this experiment was conducted in research greenhouse of Karaj Agriculture college, in spring 2006 using a factorial as complete block design with four replications. Treatments included a combination of salt stress in 4 levels (control, 40 mM, 80mM, 120 mM, 160 mM) on alfalfa cultivars (Bami, Yazdi, Shoorkat, Yugoslavi) supplemented with MS26 Rhizobium and planted in pots that were filled by perlit. Hoagland solution, without nitrogen, was used to supply nutrition. At first, the pots were irrigated with tap water. After the emergence of main leaflet, irrigation with Hoagland solution was started.After 21 days, salinity treatment was followed with Hoagland solution and it was continued for 46 days. The results showed plant dry weight, the number of active nodules and nitrogen content were affected under salt stress in all tested cultivars. Plant dry weight decreased by about 82% with the highest salt dosage (160 mM NaCl). The number of active nodules and Nitrogen content decreased significantly in the highest level of salinity. Because of the growth stage in which the salinity was employed, all the cultivars have a similar response to salinity treatments and none of them could show its ability and difference. Key word: Nitrogen Fixation, Nodule, Salinity Stress, Hoagland Solution. INTRODUCTION Salinity is a serious threat to agriculture in arid and semiarid regions . Nearly 40% of the world’s land [12] surface can be categorized as having potential salinity problems . Increasing salt concentration may have a [4] detrimental effect on soil microbials as a result of direct toxity as well as through osmotic stress . [15] As with most cultivated crops, the salinity response of legumes varies greatly and depends on such factors as climatic conditions, soil properties and the stage of growth . [5] Alfalfa is the most important forage crop for the arid and semi-arid areas where increased salinity of irrigated fields is one of the major constraints that limit crop productivity . [8] Alfalfa may be affected by salinity throughout its growth. Three developmental stages have been studied, although these stages have often not been clearly defined. These three stages are germination seedling [1] [2] emergence and growth mature plant . [3] [13] Salinity may affect symbiotic N2 fixation in alfalfa in various ways. Included among these are effects on both the Rhizobium bacteria, which infects alfalfa (Rhizobium meliloti L.) andcarries out N2 fixation, as well as the alfalfa host plant itself, which provides photosynthates to fuel fixation . Salinity [2] retard initiation or growth of new nodules reduced the efficiencyof fully formed nodules which had developed earlier under nonsaline conditions and decrease the [2] proportion of those nodules that are initiated in saline conditions, that are able todifferentiate fully in to active N2-fixing nodules . [17] The purpose of this study was to investigate the effect of salinity on tow important traits including biological nitrogen fixation and the number of active nodules in a special stage in Medicago sativa life. Since finding the resistant cultivar was not the aim, the seeds were chosen from saline regions. MATERIAL AND METHODS In a pot experiment, 4 cultivars of Alfalfa (Medicago sativa L.) (Bami, Yazdi, Shoorkat,