A spectrofluorometric survey of UV-induced blue-green fluorescence in foliage of 35 species

Upon excitation by ultraviolet radiation (UV), plant foliage exhibits a blue-green fluorescence (BGF). In order to assess the prevalence and variability of this fluorescence we examined UV-induced blue-green fluorescence from foliage surfaces of 35 species comprising seven life forms (grasses/sedges, conifers, herbaceous dicotyledons, succulents, palms, woody deciduous dicotyledons, and woody evergreen dicotyledons) growing in Tempe, AZ, USA. Excitation (260-380 nm) and emission (400-600 nm) spectra of the adaxial and abaxial surfaces of foliage from five non-stressed plants of each species were measured with a spectrofluorometer. When excited with UV all species had Violet to blue emission peaks (range = 405-475 nm; adaxial mean = 443 +/- 1 nm (SE), abaxial mean = 442 +/- 1 nm), while about a third also had a well-defined green emission peak (range = 510-550 nm; adaxial mean = 523 +/- 1 nm, abaxial mean = 524 +/- 1 nm) and one species also had a yellow emission peak at 568 nm. Fluorescence excitation peaks ranged from 285-370 nm (mean adaxial excitation peak = 342 +/- 1 nm and mean abaxial excitation peak = 341 +/- 1 nm) among surveyed species. There was a significant positive correlation between adaxial and abaxial excitation peak wavelengths (r = 0.66), as well as between adaxial and abaxial emission peak wavelengths (r = 0.97). To quantify and compare emission peak intensities among species we determined a fluorescence yield index (FM) calculated as the emission peak energy divided by total incident excitation energy. The FM varied over an order of magnitude among species. On average, grasses/sedges and succulents had significantly greater FYIs than the other five life forms. The FYI of blue (<500 nm) adaxial emission was strongly correlated (r = 0.76) with abaxial emission. Our findings suggest BGF may be caused by several compounds in addition to cell-wall-bound ferulic acid which can vary among species but appear to be similar on adaxial and abaxial foliage surfaces within a species. The large range in FYI suggests some species may possess considerably greater amounts of BGF compounds than others.