The role of UV-B radiation in plant sexual reproduction

• Most floral parts are effectively UV-B protected by means of UV-absorbing compounds.
• Pollen is the most susceptible stage to suffer UV-B stress.
• UV-B can increase the development of flower and fruit colouration.
• UV-B can affect pollinators directly or through UV-B mediated changes in plants.
• As UV-B doses increase, annuals tend to delay flowering and decrease plant fitness.

UV-B radiation affects plant sexual reproduction, but, at present, general patterns about the nature of these effects and their underlying mechanisms remain elusive. In recent years, plant UV-B research has experienced a substantial conceptual change, from a stress-dominated view towards a more regulatory perspective. With this in mind, we have surveyed the published literature on the effects of UV-B radiation on plant reproductive traits and on plant–pollinator interactions.Most floral parts are effectively UV-B protected thanks to the accumulation of UV-B absorbing compounds. However, the least protected parts, such as pollen, are sensitive to high UV-B doses. Among UV-B absorbing compounds, flavonoids have a complex role in plant reproduction, since they are essential for UV-B protection and normal pollen function, while they also modulate flower and fruit colouration, which, in turn, affects visits by pollinators and frugivores. Effects of UV-B radiation on pollination can be direct, due to UV-B effects on pollinators, or indirect, due to pollinators responding to UV-B-mediated changes in plants. In the case of annual species, our literature survey revealed that, as UV-B doses increase, there is a tendency to delay the onset of flowering and to decrease fruit and/or seed production. Yet, the present review highlights the existence of complex dose–response curves that emphasize the need to use multiple UV-B doses in future studies of UV-B-mediated flowering responses. Moreover, species, populations or even cultivars originating from geographical areas with high impact of solar radiation (e.g. Mediterranean Basin) exhibit, in general, high protection against UV-B radiation, often showing positive responses to moderate UV-B increases.In conclusion, our survey of the literature reveals complex UV-B effects on plant reproduction. To fully comprehend these effects, integrated approaches, beyond those currently used, are required to analyze the complex mixture of direct and indirect, stimulatory and inhibitory UV-B responses.