Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/12/1998
Publication Date: N/A
Citation: Interpretive Summary: Depletion of ozone in the upper atmosphere is of concern because that ozone screens out most of the biologically harmful ultraviolet (UV-B) wavelengths of sunlight. Doubling of UV-B at Earth's surface is possible with recent rates of ozone depletion. Intensified UV-B is known to harm humans and photosynthesis in plants. We evaluated the potential harm of five ecologically relevant intensities of UV-B for floral resources used by bees. The two plant species grown under intensified UV-B responded differently. The likelihood of flowering by plants of one species, a newly commercialized oilseed crop, diminished with doubling and tripling of ambient UV-B. The other, a useful wildflower for farming new agricultural pollinators, responded with a delay in bloom, smaller flowers bearing less pollen, and an abbreviated flowering season. The blue orchard bee, Osmia lignaria, accommodated these modest floral responses, foraging and provisioning nest cells without interruption despite the population being near carrying capacity for pollen. In the field, these UV-B effects could indirectly diminish a plant's abilities to attract pollinators as well as curtailing the reproductive success of its pollinators, but any direct effects were subtle enough to be frequently overshadowed by differences in soil, site fertility, weather and herbivory.
Technical Abstract: Intensified ultraviolet-B radiation or UV-B can delay flowering and diminish lifetime flower production in a few plants. Here we studied the effects of enhanced UV-B on floral traits crucial to pollination and pollinator reproduction. We observed simultaneous flowering responses of a new crop plant, Limnanthes alba, and a wildflower, Phacelia campanularia, to five lifetime UV-B dosages ranging between 2.74 and 15.93 kJ m-2d-1. Floral traits known to link plant pollination with bee host preference, host fidelity and larval development were measured. Intensified UV-B had no overall effect on nectar and pollen production of L. alba and P. campanularia flowers. A quadratic relationship between UV-B and nectar sugar production occurred in P. campanularia and showed that even subambient UV-B dosages can be deleterious for a floral trait. As UV-B dosage increased, L. alba plants were less likely to flower, but suffered no delays in flowering or reductions to lifetime flower production for those that did flower. Conversely, an equal proportion of P. campanularia plants flowered under all UV-B treatments, but these same plants experienced delayed onset to bloom and produced fewer flowers at greater UV-B intensities. Therefore, intensified UV-B elicits idiosyncratic responses in flowering phenology and flower production from these two annual plants. Overall, our results indicated that intensified UV-B can alter some flowering traits that impinge upon plant competition for pollinator services, as well as plant and pollinator reproductive success.