TIME-RESOLVED BLUE-GREEN FLUORESCENCE OF SUGAR-BEET (BETA-VULGARIS L)LEAVES - SPECTROSCOPIC EVIDENCE FOR THE PRESENCE OF FERULIC ACID AS THE MAIN FLUOROPHORE OF THE EPIDERMIS

Sugar beet (Beta vulgaris L.) leaves emit blue-green fluorescence when excited with ultraviolet light. Comparative spectral analysis showed that the blue-green fluorescence of leaves, on both the adaxial and th e abaxial side, was dominated by the blue-green fluorescence of the ep idermis. A detailed investigation of the adaxial epidermis and of poss ible candidates for its blue-green fluorescence was carried out. The p ossible candidates were two hydroxycinnamic acid derivatives (ferulic acid and p-coumaric acid) and two flavonoids (kaempferol and quercetin ). Each of these absorbed ultraviolet light, but ferulic acid was the only fluorophore that fluoresced similarly to epidermis. Decay-associa ted excitation and emission spectra were used to investigate further t he blue-green fluorescence of the epidermis. In sugar beet epidermis, four kinetic components were resolved: the very fast (20-40 ps), the m edium (0.8-0.9 ns), the slow (2.6-2.8 ns) and the very slow (7.1-7.6 n s) kinetic components. Several lines of evidence indicate the presence of ferulic acid as the main fluorophore of the epidermis. First, the very fast kinetic component, which contributes 70% to the overall blue -green fluorescence emitted from the epidermis, showed the spectral ch aracteristics of ferulic acid. Second, the fluorescence lifetime of th is kinetic component matched that of ferulic acid, which was 20-40 ps. Finally, the very fast kinetic component was also found in sugar beet leaves and in their phenolic extracts, and its thermal sensitivity (m easured as the percentage of the blue-green fluorescence change per C degrees) matched that of ferulic acid. The results of this work lead u s to conclude that ferulic acid is the fluorophore responsible for mos t of the blue-green fluorescence emitted from sugar beet epidermis.