Characterization of blue-green fluorescence in the mesophyll of sugar beet (Beta vulgaris L.) leaves affected by iron deficiency

The mesophyll of sugar beet (Beta vulgaris L.) leaves emits red (chlorophyll a) fluorescence and blue-green fluorescence when excited with ultraviolet light. The intensity of blue-green fluorescence was increased in mesophylls affected by iron deficiency. This increase was large and progressive. It was concomitant with a decrease of photosynthetic pigments per unit of leaf area. Most of the increase in blue-green fluorescence can be explained by the decrease of the screening of ultraviolet light by chlorophylls and carotenoids. In addition, chlorophylls selectively reabsorb blue fluorescence, which leads to a change in the form of the fluorescence emission spectra. This effect induces an increase of the blue-to-green fluorescence ratio in control mesophylls that was concomitant with the decrease of chlorophyll per unit of leaf area. Iron deficiency induced a decrease of the blue-to-green fluorescence ratio that may be attributed to an accumulation of flavins fluorescing in the green. Time-resolved fluorescence measurements indicate that they are mostly riboflavin and/or flavin mononucleotide phosphate. Our data also indicate that the blue-green fluorescence emitted from the mesophyll contains fluorescence of nicotinamide nucleotides.