To detect the stress state of sugar beet suffering magnesium restriction, three stages of development were studied. Leaf reflectance and solvent extraction were used to analyse the pigments. Chlorophyll fluorescence monitoring has been carried out with direct and modulated techniques. Although Mg is the atom in the centre of the chlorophyll molecule, its deficiency becomes apparent in the plant at several levels (such as electron transport and non-light dependent rate constants) before the chlorophyll content. Distinct depressions in chlorophyll concentration parallels interveinal chlorosis toward the distal end of the recently mature leaves. Therefore, the fluorescence transient is a potential biosensor that monitors photosynthetic activities for basic, agricultural and ecological projects.