Intensity of ultraviolet-B radiation (UV-B) differs between northern and southern hemispheres. Therefore, exotic plants species that originate from the northern hemisphere provide an opportunity to study the effects of UV-B on plant physiology and growth, and their implications for the role of adaptation and phenotypic plasticity during plant invasion on the southern hemisphere.

We conducted a growth-chamber experiment with and without UV-B on Hieracium pilosella plants from Germany, where the species occurs natively and New Zealand (NZ), where it is invasive. We tested the hypothesis that: (i) H. pilosella plants respond to UV-B with high phenotypic plasticity, demonstrating the ability to react to changes in UV-B, and (ii) NZ plants are better adapted to UV-B than German plants as a result of directional selection of well adapted phenotypes.

Consistent with our first hypothesis H. pilosella plants reacted plastically to UV-B by producing longer foliar hairs and a higher leaf dry matter content (LDMC) when they were treated with UV-B regardless of their origin. Additionally, irrespective of the treatment, plants from NZ displayed a higher LDMC and grew less but longer leaves than German plants. Plants showed typical stress responses and a reduction in growth caused by the UV-B treatment: below-ground biomass and production of ramets were reduced when plants were treated with UV-B. H. pilosella proved to be particularly well predisposed to grow in areas of high UV-B radiation.

Our findings hint at the necessity to consider UV-B radiation in future research on mechanisms of invasions in regions with high UV-B irradiation. Species that provide the ability to respond directly to UV-B might have an advantage to invade these areas. As UV-B intensity is likely to change in the future due to ongoing ozone depletion, research addressing the effects of UV-B during plant invasions is of increasing importance.