Since our understanding of mechanisms underlying fundamental
UV-B responses has substantially increased over the last two
decades, we can start to untangle the causes for the diverse UV-B
effects on fruit and/or seed production (Fig. 3). On one hand, stressful
conditions can lead to reduced photosynthetic yields and more
energy being directed to mitigate stress, which might explain the
often observed decreases in reproductive fitness under high UVB
doses. On the other hand, pollen viability can also be severely
decreased due to enhanced UV-B exposure (see section “Protection
of floral structures against UV-B radiation”). In addition, shifts in
the time (or duration) of flowering, which can result in insufficient
pollinators available at the time of flowering, or UV-B avoidance by
pollinators (see sections “UV-B effects on flowering phenology” and
“UV-B effects on pollination success”) may also contribute to the
trend of decreased reproductive success under high UV-B radiation.
In contrast, increases in reproductive success under more moderate
UV-B levels might reflect a range of stimulatory effects, including
the enhancement of flower colour (see section “The role of UV-B
in flower and fruit colour development”), increases in nectary size
and/or in the number of flowers (see section “UV-B effects on flower
size and number”), and increases in the length of flower visits by
pollinators (see section “UV-B effects on pollination success”).
Considering the important ecological, nutritional and economical
consequences of changes in seed production, it is essential
to accurately establish the underlying mechanisms that would
explain the impact of UV-B radiation on plant reproductive fitness.
Given the improved understanding of both stress and regulatory
UV-B responses in plants, future experiments need to be designed
in a way that distinguishes between these two different plant
responses to UV-B