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 the abaxial side, was dominated by the blue-green fluorescence of the epidermis. A detailed investigation of the adaxial epidermis and of possible candidates for its blue-green fluorescence was carried out. The possible 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-associated excitation and emission spectra were used to investigate further the blue-green fluorescence of the epidermis. In sugar beet epidermis, four kinetic components were resolved: the very fast (20-40 ps), the medium (0.8-0.9 ns), the slow (2.6-2.8 ns) and the very slow (7.1-7.6 ns) 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 characteristics of ferulic acid. Second, the fluorescence lifetime of this 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 (measured as the percentage of the blue-green fluorescence change per C°) matched that of ferulic add. The results of this work lead us to conclude that ferulic acid is the fluorophore responsible for most of the blue-green fluorescence emitted from sugar beet epidermis.
Morales, F., Cerovic, Z. G., & Moya, I. (1996). 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. Biochimica et Biophysica Acta - Bioenergetics, 1273(3), 251–262. https://doi.org/10.1016/0005-2728(95)00153-0