Sugar beet ( Beta vulgaris L.) is an important and strategic agricultural product in Iran. About 23% of the sugar need in the country is supplied by sugar beet. To achieve the highest economic profit, in addition to high crop yield, root sugar content should also be high. Among the various agronomic and management factors, optimum nutrition management plays a significant role in increasing yield and sugar content of beet. Magnesium (Mg) is a forgotten element in crop production. Magnesium is one of the essential elements for the growth and development of plants, which despite the important role, Mg nutrition is often ignored. One of the important roles of magnesium is in relation to the activity of the key enzyme H-ATPase located on the root cell plasma membrane. This enzyme plays an important role in the regulation of solutes traorts across the root plasma membrane. The aim of this study was to investigate the effect of foliar application of Mg on plant growth, translocation of sugar from the leaves to the root, and activity of ATPase proton pump. Furthermore, the effect of foliar spray of Mg was investigated on proton secretion from the roots into the surrounding solution and as a result, solubility of insoluble Fe. A nutrient solution culture experiment was conducted with four fertilizer treatments in a completely randomized design with three replications in a greenhouse. Treatments were foliar spray of two sources of Mg fertilizer i.e., magnesium sulfate (MgSO 4 ) and magnesium complexed with lysine (Mg-lys). To determine the individual effects of lysine and Mg, a group of plants received only lysine free of Mg as foliar spray. A control treatment free of Mg and amino acid was also used. Uniform seedlings of sugar beet cultivar Jolgeh were transferred to a nutrient solution. One month after the plants were transferred to the nutrient solution, magnesium deficiency condition was imposed by replacing1 mM MgSO 4 with 1 mM Na 2 SO 4 in the nutrient solution. One week after the appearance of Mg deficiency symptoms, the foliar spray treatments were applied. Treatments were applied three times over a week. Leaf chlorophyll concentration, leaf and root sugar content, solubility of insoluble Fe(OH) 3 in the root environment, and root activity of the ATPase proton pump were measured. Foliar application of Mg from both sources of magnesium sulfate and Mg-Lys complex partially alliviated the magnesium deficiency symptoms on sugar beet leaves and resulted in higher concentration of chlorophyll and leaf and root sugar contents. In addition, foliar Mg spray significantly increased solubility of insoluble Fe(OH) 3 in the root environment at least in part due to higher passive traort of proton from the roots into the solution. The root activity of the ATPase proton pump was significantly lower in the plants receiving Mg in comparison with those not receiving Mg as foliar spray. In contrast, the passive traort of proton was higher in the Mg treatments as compared with the control. According to the results obtained from the present study, foliar spray of Mg is effective in improving plant growth and leaf-to-root translocation of sugar and can affect Fe solubility in the root media via passive release of proton. Keywords : sugar beet, sugar content, low magnesium, ATPase proton pump, solubility of iron