ULTRAVIOLET-B RADIATION AND OZONE EFFECTS ON GROWTH, YIELD, AND PHOTOSYNTHESIS OF SOYBEAN

The projected increase in solar ultraviolet-B (UV-B) radiation due to depletion of stratospheric ozone (O-3) has caused concern regarding possible UV-B damage to crops. At the same time, tropospheric O-3 is projected to remain at concentrations that are known to damage crops. Since these two stressors may co-occur, experiments were performed to determine their separate and joint effects on crop growth, yield, and photosynthesis. Open-top chambers, equipped with filtered UV-B lamp systems, were used in 3 yr of field studies to treat soybean [Glycine mau (L.) Merr.; 'Coker 6955','Essex', and 'S 53-34'] with supplemental UV-B radiation and/or O-3 from emergence through physiological maturity. Treatment levels of biologically effective UV-B radiation (UV-B-BE) simulated the increase in ground level W-B for stratospheric O-3 depletion up to 37% (approximately a doubling of ambient UV-B-BE). Ozone treatment concentrations ranged from 14 to 83 nL L(-1) (seasonal mean 12 h d(-1) concentrations). Ultraviolet-B radiation did not affect soybean seed yield in any of the 3 yr of the study. In 1 yr, UV-B affected pod and seed number and pod weight, but the treatment means were not consistently related to the UV-B dose. No O-3 x UV-B interactions were found for any yield component at final harvest. Biweekly harvests of Essex during the growing season did not reveal any persistent effects of increased UV-B radiation on growth. Net carbon exchange rate (NCER), stomatal conductance, and transpiration of Essex soybean leaves were not suppressed by supplemental UV-B radiation. On the other hand, O-3 treatment consistently induced visible injury, suppressed NCER and water use efficiency, accelerated reproductive development, and suppressed growth and yield. It is concluded that tropospheric O-3 poses a greater threat to soybean production than projected levels of UV-B radiation.