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Hybrid poplar (Populus × canadensis) saplings were subjected to acute ultraviolet-A (UV-A) irradiation (30 W m–2, ambient treatment, 60, 90 and 120 W m–2 of UV-A irradiance) to determine the effects on photosynthesis and biogenic volatile organic compound (BVOC) emissions in two different short-term experiments (i.e. sequential increase in UV-A irradiance and UV-A intensity–response relationships). Both intensity–response experiments showed that the UV-A ambient treatment did not affect photosynthesis and BVOC emissions. Whereas exposition at 60, 90 and 120 W m–2 of UV-A (first experiment), increasingly inhibited photosynthesis. This increasing inhibition was also detected by decreasing trends of both photochemical reflectance index (PRI) and fluorescence yield. Isoprene emission resulted to be very sensitive to increasing UV-A irradiances. Methanol was also very sensitive to high UV-A radiation, suggesting the occurrence of strong damages of cellular structures. The second experiment, which was performed both in the middle of July and repeated towards the end of the summer, showed a temporal variations in the UV-A intensity–response relationships. In fact, there were no longer significant differences in photosynthesis, PRI and isoprene emission in response to high UV-A radiation toward the end of the summer season. The adaxial flavonoid level increased significantly over the period monitored, resulting 85% higher toward the end of the summer than during the middle of the summer. This dramatic increase in the adaxial flavonoids may have played a protective role against UV-A radiation by shielding leaves. Our findings add to the understanding of physiological processes involved in plant response to UV radiation.