We conducted three experiments to examine the influence of ultraviolet‐B radiation (UV‐B; 280–320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV‐B levels that simulated either an ambient stratospheric ozone level (control), 16% (“low enhanced”), or 32% (“high enhanced”) ozone depletion levels at Morgantown, WV, USA in mid‐March. In the first experiment, we examined whether UV‐B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by ≈50% under both enhanced UV‐B levels relative to ambient controls. While plants under high‐enhanced UV‐B produced over 40% more flowers than plants under the two lower UV‐B treatments, whole‐plant production of viable pollen was reduced under high‐enhanced UV‐B to 17% of that of ambient controls. Whole‐plant production of viable pollen was reduced under low‐enhanced UV‐B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV‐B levels and examined whether source‐plant UV‐B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced‐UV‐B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high‐enhanced UV‐B levels for 6 h, viability of the pollen from plants grown under ambient UV‐B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV‐B treatments was reduced to a much lesser extent: only from ≈43 to 22%. Thus, ambient source‐plant pollen was more sensitive to enhanced UV‐B exposure. In the third experiment, we used pollen collected from source plants under the three UV‐B levels to fertilize plants growing under ambient‐UV‐B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV‐B treatments, than from ambient UV‐B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV‐B exposure of their pollen source. Our findings demonstrate that enhanced UV‐B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa.

We assessed the effects of enhanced ultraviolet-B radiation (UV-B; 280-320 nm) on biomass allocation to roots, shoots, leaves and flowers in the annual Brassica rapa. In addition, we investigated how concentrations of chlorophyll and UV-B-absorbing compounds in leaves, ovaries and pollen changed in response to enhanced UV-B. Plants were grown for 38 d in a greenhouse under lampbanks providing daily biologically effective UV-B doses equivalent to those under ambient mid-March stratospheric ozone levels or 16% ('low-enhanced UV-B') or 32% ('high-enhanced UV-B') ozone depletion levels for Morgantown, WV, USA. Total and aboveground biomass of plants was less under low-enhanced UV-B, but similar to ambient controls under high-enhanced UV-B. Concentrations of UV-B-absorbing compounds in leaves (area basis) increased under high-enhanced UV-B by about 20%, but were similar to ambient controls under low-enhanced UV-B. More effective protection due to higher screening-compound concentrations in plants under high-enhanced UV-B may explain why biomass production was not reduced. Plants under high-enhanced UV-B also had more reproductive biomass and produced more flowers, and had less root mass, than plants under ambient or low-enhanced UV-B. Concentrations of leaf total chlorophyll were not affected by UV-B treatment. While UV-B treatment had no affect on concentrations of UV-B-absorbing compounds in ovaries, concentrations in pollen from plants under both enhanced-UV-B treatments were >40% greater than ambient controls.

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