Effect of enhanced UV-B radiation on pollen quantity, quality, and seed yield in Brassica rapa (Brassicaceae)

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 W-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by approximate to 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 W-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 W-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 W-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 approximate to 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.