Quantifying the Photosynthetic Quantum Yield of Ultraviolet‐A1 Radiation

Abstract

ABSTRACTAlthough 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.