THE COMBINED EFFECTS OF CO2 CONCENTRATION AND SOLAR UV-B RADIATION ONFABA BEAN GROWN IN OPEN-TOP CHAMBERS

The response of faba bean seedlings to the combined effects of increas ed atmospheric CO2 concentrations ([CO2]) and solar UV-B irradiance wa s studied using open-top chambers transparent to W-B radiation. The pu rpose of the study was to determine whether effects of increased [CO2] on growth and physiology are modified by the present solar UV-B fluen ce rate in the Netherlands. Seedlings were exposed to 350 or 700 mu mo l mol(-1) CO2. At both [CO2], solar UV-B irradiance was either present or reduced using polyester foil opaque to UV-B radiation. To obtain i nformation on the time dependence of increased [CO2] and UV-B radiatio n effects, three harvests were performed during the experiment, CO2 en richment resulted in increased biomass production at all harvests. At the final harvest, UV-B radiation did not affect biomass production bu t a significant decrease was observed after 14 d of treatment. A reduc tion of the UV-B fluence increased shoot length at both [CO2] througho ut the experiment, UV-B radiation slightly altered biomass allocation. Plants grown at reduced levels of UV-B radiation invested less biomas s in flowers and more in stem material compared to plants grown at amb ient UV-B levels. CO2 enrichment resulted in a stimulation of net phot osynthesis after 26 and 38 d of treatment. UV-B reduction did not alte r this response. After 26 d of treatment, photosynthetic acclimation t o CO2 enrichment was observed, which was probably the result of accumu lation of carbohydrates in the leaves. After 38 d, photosynthetic accl imation was no longer present. The UV absorbance of methanolic leaf ex tracts was increased by CO2 enrichment only. Both CO2 enrichment and s olar UV-B reduced the transmittance of radiation through intact attach ed leaves. Interaction between [CO2] and UV-B radiation was limited to UV-A transmittance of leaves. Under prevalent experimental conditions , UV-B radiation did not affect the measured physiological parameters. Most open-top chambers used for climate change research are construct ed of materials which do not transmit UV-B radiation. Our results indi cate that part of the 'chamber effects' on plant height often describe d in the literature might be explained by the absence of solar UV-B ra diation in these chambers.