We assessed the contribution of UV-induced violet–blue–green leaf fluorescence to photosynthesis in Poa annua, Sorghum halepense and Nerium oleander by measuring UV-induced fluorescence spectra (280–380 nm excitation, 400–550 nm emission) from leaf surfaces and determining the monochromatic UV action spectra for leaf photosynthetic O₂-evolution. Peak fluorescence emission wavelengths from leaf surfaces ranged from violet (408 nm) to blue (448 nm), while excitation peaks for these maxima ranged from 333 to 344 nm. Action spectra were developed by supplementing monochromatic radiation from 280 to 440 nm, in 20 nm increments, to a visible nonsaturating background of 500 μmol m⁻² s⁻¹ photosynthetically active radiation and measuring photosynthetic O₂-evolution rates. Photosynthetic rates tended to be higher with the 340 nm supplement than with higher or lower wavelength UV supplements. Comparing photosynthetic rates with the 340 nm supplement to those with the 400 nm supplement, the percentage enhancement in photosynthetic rates at 340 nm ranged from 7.8 to 9.8%. We suspect that 340 nm UV improves photosynthetic rates via fluorescence that provides violet–blue–green photons for photosynthetic energy conversion because (1) the peak excitation wavelength (340 nm) for violet–blue–green fluorescence from leaves was also the most effective UV wavelength at enhancing photosynthetic rates, and (2) the magnitude of photosynthetic enhancements attributable to supplemental 340 nm UV was well correlated (R² = 0.90) with the apparent intensity of 340 nm UV-induced violet–blue–green fluorescence emission from leaves.