What was done
The authors grew cotton (Gossypium hirsutum L.) in sunlit controlled-environment chambers under favorable nutrient and water conditions at three levels of biologically effective UV-B radiation (0, 7.7 and 15.1 kJ m^-2 d^-1) - the latter of which levels was twice the current maximum observed in the United States mid-south cotton production area on sunny days between May and July - and two levels of atmospheric CO2 concentration (360 and 720 ppm) from emergence until three weeks after first flower stage.

What was learned
UV-B radiation of 7.7 kJ m^-2 d^-1, in the words of the authors, "did not affect cotton growth and development, but higher levels of UV-B radiation (15.1 kJ m^-2 d^-1), the amount expected to occur with a 30% depletion in the stratospheric ozone, significantly reduced stem elongation rate, leaf area and dry matter accumulation."  Elevated CO2, on the other hand, "significantly increased dry matter accumulation in leaves and stems at all sampling dates," and at final harvest the CO2-enriched plants had a 36% higher total biomass than plants grown at ambient CO2, averaged across the three UV-B treatments.  However, as the authors report, "elevated atmospheric CO2 could not alleviate the detrimental effects of high UV-B radiation on cotton net photosynthesis and growth."

What it means
Even under the current maximum level of UV-B radiation observed in the US mid-south cotton production area, atmospheric CO2 enrichment significantly enhanced cotton growth and development, although at twice that level the detrimental effects of UV-B radiation were dominant.  However, as recently documented by Newchurch et al. (2003), satellite-derived data "provide evidence of a slowdown in stratospheric ozone losses since 1997," which they describe as "the first stage of a recovery of the ozone layer" that they attribute to the implementation of the Montreal Protocol.  If correct, this observation suggests that the plant growth benefits of the ongoing rise in the air's CO2 concentration should continue into the foreseeable future.  Also suggesting the same is the fact that, contrary to the results of the current study, both Adamse and Britz (1992) and Rozema et al. (1997) did find an ameliorative effect of doubled CO2 in similar studies of high UV-B radiation effects on plants.

References
Adamse, P. and Britz, S.J.  1992.  Amelioration of UV-B damage under high irradiance.  I.  Role of photosynthesis.  Photochemistry and Photobiology 56: 645-650.

Newchurch, M.J., Yang, E.-S., Cunnold, D.M., Reinsel, G.C., Zawodny, J.M. and Russell III, J.M.  2003.  Evidence for slowdown in stratospheric ozone loss: First stage of ozone recovery.  Journal of Geophysical Research, in press.

Rozema, J., Lenssen, G.M., Staaij, J.W.M., Tosserams, M., Visser, A.J. and Brockman, R.A.  1997.  Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment.  Plant Ecology 128: 182-191.