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 = 405475 nm; adaxial mean = 443 ± 1nm (SE), abaxial mean =442 ± 1nm), while about a third also had a welldefined green emission peak (range = 510–550nm; adaxial mean = 523 ± Inm, abaxial mean = 524 ± 1nm) 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 ± 1nm and mean abaxial excitation peak = 341 ± 1nm) 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 (FYI) calculated as the emission peak energy divided by total incident excitation energy. The FYI 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 ofBGF compounds than others.