Quantifying the Photosynthetic Quantum Yield of Ultraviolet‐A1 Radiation

Although it powers photosynthesis, ultraviolet-A1 radiation (UV-A1) is usually not defined as photosynthetically active radiation (PAR). However, the quantum yield (QY) with which UV-A1 drives net photosynthesis rate (A) is unknown, as are the kinetics of A and chlorophyll fluorescence under constant UV-A1 exposure. We measured A in leaves of six genotypes at four spectra peaking at 365, 385, 410 and 450 nm, at intensities spanning 0-300 μmol m s^-1. All treatments powered near-linear increases in A in a wavelength-dependent manner. QY at 365 and 385 nm was linked to the apparent concentration of flavonoids, implicating the pigment in reductions of photosynthetic efficiency under UV-A1; in several genotypes, A under 365 and 385 nm was negative regardless of illumination intensity, suggesting very small contributions of UV-A1 radiation to CO₂ fixation. Exposure to treatment spectra for 30 min caused slow increases in nonphotochemical quenching, transient reductions in A and dark-adapted maximum quantum yield of photosystem II, that depended on wavelength and intensity, but were generally stronger the lower the peak wavelength was. We conclude that UV-A1 generally powers A, but its definition as PAR requires additional evidence of its capacity to significantly increase whole-canopy carbon uptake in nature.