Magnesium deficiency in plants is a widespread problem, affecting productivity and quality in agriculture, yet at a physiological level it has been poorly studied in crop plants. Here, a physiological characterization of Mg deficiency in Beta vulgaris L., an important crop model, is presented. The impact of Mg deficiency on plant growth, mineral profile and photosynthetic activity was studied. The aerial biomass of plants decreased after 24 days of hydroponic culture in Mg-free nutrient solution, whereas the root biomass was unaffected. Analysis of mineral profiles revealed that Mg decreased more rapidly in roots than in shoots and that shoot Mg content could fall to 3 mg g−1 DW without chlorosis development and with no effect on photosynthetic parameters. Sucrose accumulated in most recently expanded leaves before any loss in photosynthetic activity. During the development of Mg deficiency, the two photosystems showed sharply contrasting responses. Data were consistent with a down-regulation of PSII through a loss of antenna, and of PSI primarily through a loss of reaction centres. In each case, the net result was a decrease in the overall rate of linear electron transport, preventing an excess of reductant being produced during conditions under which sucrose export away from mature leaf was restricted.