A SUPPLEMENTAL ULTRAVIOLET-B RADIATION SYSTEM FOR OPEN-TOP FIELD CHAMBERS

Studies suggest that increased ultraviolet-B (UV-B) radiation at the earth's surface due to stratospheric ozone (O3) depletion may affect crop production. Current levels of tropospheric O3 are known to decrease crop yields. To assess the combined effects on plants of increased UV-B radiation and chronic exposure to O3, a commonly used constant-addition supplemental UV-B radiation system was modified for use in open-top field chambers. Lamp banks containing 14 filtered UV-B-313 fluorescent lamps were suspended in 33 chambers to which either charcoal-filtered, nonfiltered, or nonfiltered air plus O3 was added. Lamp banks provided ample levels and distribution of biologically effective UV-B (UV-B(BE)) radiation for stimulating up to a 29% loss of column O3, according to a radiative transfer model. However, ground-based measurements of solar UV-B(BE) radiation provided a more realistic base than the model for simulations of column O3 loss using supplemental UV-B radiation. Shading by the lamp bank and chamber assembly reduced the daily solar UV-B(BE), UV-A, and visible irradiances inside the chamber on average by 24% +/- 5% compared with ambient levels. Biweekly adjustments of the supplemental UV-B(BE) irradiance level and exposure period provided treatments that were proportional to the seasonal trend in solar UV-B(BE) radiation. Control of supplemental UV-B(BE) irradiance in proportion to solar UV-B(BE) irradiance during the day would better simulate the effects of stratospheric O3 depletion. However, the constant-addition system was a simpler and less expensive method than the proportional-addition system for examining the relative sensitivity of plants to the combined effects of increased UV-B radiation and O3 under field conditions.