Prevalence and physiological significance of UV-induced blue fluorescence in foliage

Ultraviolet radiation induces blue fluorescence in plants. Among 35 species from seven growth forms, the peak wavelength for UV-induced fluorescence excitation averaged 342 nm (range = 285–370 nm) while the emission averaged 443 nm (range = 405–475 nm). Fluorescence peak wavelengths were correlated between abaxial and adaxial leaf surfaces. Peak wavelength variability suggests multiple primary fluorophores. Fluorescence intensity tended to be greater from the abaxial than adaxial surface. Grasses tended to have greater fluorescence intensity than other growth forms. A third of these species also had distinct UV-induced green fluorescence (average excitation = 342 nm and emission = 523 nm). Abaxial surface irradiance of Sorghum bicolor leaves with nonsaturating visible light and a polychromatic UV supplement that cutoff at 311 nm led to a 1% increase in photosynthetic rate by UV-induced fluorescence. This UV supplement provided more fluorescence excitation irradiance (320–350 nm) while cutting out more photosynthesis-inhibiting UV-B (280–315 nm) irradiance than other supplements (sharp-cutoff range = 280–382 nm). Increasing long-term water stress (grown under moderate or low water availability for 14 wks) either had no effect or caused surface fluorescence intensity to decrease in sorghum depending on the wavelength of excitation. Increasing water stress caused fluorescence intensity to decrease in cytoplasm extracts. The moderate water treatment caused fluorescence intensity to increase in wall-bound extracts, but low water treatment had no effect compared to the well-watered control. Short-term water stress involved a 15% decline in relative water content and caused peak blue fluorescence intensity to increase 7% (abaxial) and 18% (adaxial surface). The blue fluorescence emission signal has potential use as a sensitive signal to remotely sense short-term changes in leaf water status. Cellular localization of fluorophores in sorghum was assessed by sequentially extracting compounds from the cuticle, then cytoplasm and then those covalently bound to cell walls. A rise in the adaxial leaf surface fluorescence intensity after the cytoplasm extract was the result of photosynthetic pigment extraction. The only decrease in surface fluorescence intensity occurred after the wall-bound extract. This suggests wall-bound fluorophores are responsible for most surface UV-induced fluorescence. Wall-bound ferulic acid appears to be the primary source of surface UV-induced blue fluorescence in sorghum.