What was done
White clover (Trifolium repens L. cv. Mervi) plants were grown for approximately three months in containers placed within two types of greenhouses that received atmospheric CO2 concentrations of 370 and 520 ppm.  The greenhouses were constructed of Plexiglas panels that were 3 and 5 mm thick, which effectively reduced incoming UV-B radiation fluxes to intensities that were only 88 and 82% of ambient, respectively.  Thus, the authors studied the interactive effects of elevated CO2 and variable UV-B radiation on the growth and development of white clover.

What was learned
Averaged across the two UV-B levels and monthly harvests, the relatively low 150-ppm increase in atmospheric CO2 concentration used in this study increased photosynthetic rates and total plant biomass by about 20 and 10%, respectively.  However, CO2-induced increases in growth were rarely observed at the lowest UV-B level.  At final harvest, for example, elevated CO2 increased total plant biomass by 16% at the 88% UV-B level, while it had no effect on biomass at the 82% UV-B level.  These observations suggest that CO2-induced growth responses may be severely underestimated or even completely unidentified in studies conducted in enclosures where UV-B levels are reduced due to the nature of the enclosure construction materials.

What it means
Scientists have been studying the effects of atmospheric CO2 enrichment on various plants for several decades.  Although some of this research has been conducted in open-top chambers, FACE arrays, and in the open air near natural CO2-emitting springs, much of it has been performed in research labs and within greenhouses or glasshouses where reduced UV-B radiation levels might have led to significant underestimations of CO2 effects.  Thus, the positive growth responses of many plants to atmospheric CO2 enrichment may be significantly larger than what has long been thought to be the case.  The results of this study therefore suggest that future indoor CO2 enrichment studies be performed within greenhouses and glasshouses constructed of materials that allow the greatest possible transmittance of UV-B radiation.