DEPLETION of stratospheric ozone will increase the intensity of solar mid-ultraviolet (280-320 nm) radiation reaching the biosphere1. Predictions of increases in biologically effective ultraviolet radiation require knowledge of both the solar spectral intensity and the wavelength-dependent sensitivity (action spectrum) for damaging the biological target2. A generalized action spectrum for plant damage encompassing wavelengths from 280 to 313 nm3-5 has been widely used to predict the consequences of ozone depletion. Calculations6 based on this spectrum and new satellite measurements of atmospheric ozone suggest that plants will be among those organisms most severely affected. Here we report an absolute action spectrum for cyclobutyl pyrimidine dimer induction in DNA in intact alfalfa seedlings, which reveals damage by wavelengths as long as 365 nm. Calculations based on this new action spectrum predict significantly smaller increases in biologically effective ultraviolet radiation resulting from ozone depletion, particularly at high latitudes, than calculations based on either the generalized plant action spectrum or the action spectrum for damaging unshielded DNA.
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