Plant response to UV radiation in diffuse sunlight

Atmospheric conditions determine the properties of light that reaches the biosphere. Clouds and aerosols affect not only the amount of diffuse radiation but also its spectral composition. The consequences, for crop growth and yield, of climate change affecting spectral irradiance through clouds and aerosols have been modelled but long-term effects are difficult to validate experimentally. In this study, we tested how barley ( Hordeum vulgare L.) acclimates to long-term treatments creating diffuse radiation and changes to solar spectral composition. In a field experiment in southern Finland, we placed plastic filters that diffuse and attenuate spectral regions of solar radiation over a barley field throughout the growing season. We found that the response of specific plant traits to diffuse radiation was mediated by the UV (ultraviolet) region of solar radiation. Notably, even the attenuation of only shortwave solar UV radiation (< 350 nm) significantly affected traits related to carbon assimilation. When only shortwave solar UV radiation was attenuated, plants grown under diffuse light filters were taller and produced more leaves but exhibited impaired photosynthetic performance (lower maximum photosynthetic yield, and photosynthetic rate). Whereas, when all UV radiation (< 400 nm) was attenuated, plants under diffuse filters produced fewer leaves and no effects on photosynthesis were apparent. This study presents novel experimental evidence that the interaction between diffuse light and spectral composition changes leaf photosynthetic capacity, information that is crucial when predicting plant growth and crop yield under global climate change.