Review articleUV-A radiation effects on higher plants: Exploring the known unknown

- The dependence of plant biomass production under UV-A radiation on genotype and/or further environmental conditions is analysed.
- Protection offered by the leaf structure and/or biochemical composition commonly prevents UV-A induced photosynthetic inhibition.
- UV-A regulates the accumulation of specific phenolic compounds rather than total phenolics.
- The overall lack of correlation between UV-A and UV-B effects does imply distinct molecular and physiological responses under the two wavelength bands.
- The review considers a role of photoreceptors (UVR8, phototropins, phytochromes and cryptochromes) in plant responses to UV-A exposure.

Ultraviolet-A radiation (UV-A: 315-400 nm) is a component of solar radiation that exerts a wide range of physiological responses in plants. Currently, field attenuation experiments are the most reliable source of information on the effects of UV-A. Common plant responses to UV-A include both inhibitory and stimulatory effects on biomass accumulation and morphology. UV-A effects on biomass accumulation can differ from those on root: shoot ratio, and distinct responses are described for different leaf tissues. Inhibitory and enhancing effects of UV-A on photosynthesis are also analysed, as well as activation of photoprotective responses, including UV-absorbing pigments. UV-A-induced leaf flavonoids are highly compound-specific and species-dependent. Many of the effects on growth and development exerted by UV-A are distinct to those triggered by UV-B and vary considerably in terms of the direction the response takes. Such differences may reflect diverse UV-perception mechanisms with multiple photoreceptors operating in the UV-A range and/or variations in the experimental approaches used. This review highlights a role that various photoreceptors (UVR8, phototropins, phytochromes and cryptochromes) may play in plant responses to UV-A when dose, wavelength and other conditions are taken into account.