What was done
The authors grew well watered and fertilized canola (Brassica napus L.) plants from the 30-day-old stage until 25 days after anthesis in 1-L pots within controlled environment chambers exposed to either 4.2 kJ m^-2 d^-1 of UVB radiation or no such radiation in air of either 370 or 740 ppm CO2, in order to determine the effects of these two parameters on the photosynthetic rates and water use efficiency of the maturing husks or siliquas that surround the plants' seeds.

What was learned
For the plants exposed to 4.2 kJ m^-2 d^-1 of UVB radiation, the experimental doubling of the air's CO2 concentration led to a 29% increase in siliqua net photosynthesis, an 18% decrease in siliqua transpiration, and a 58% increase in siliqua water use efficiency; while for the plants exposed to no UVB radiation, siliqua net photosynthesis was increased by a larger 38%, transpiration was decreased by a larger 22% and water use efficiency was increased by a larger 87% in the CO2-enriched air.

What it means
As noted by Zhao et al. (2004), "as a result of stratospheric ozone depletion, UV-B radiation levels are still high at the Earth's surface and are projected to increase in the near future (Madronich et al., 1998; McKenzie et al., 2003)." In the slightly more distant future, however, when stratospheric ozone levels are projected to decline as a result of the worldwide ban on stratospheric-ozone-depleting substances, the biologically-beneficial effects of the ongoing rise in the air's CO2 content will likely become ever greater as UVB levels at the earth's surface begin their anticipated long-term decline. As a result, the long-term agricultural and biospheric benefits of atmospheric CO2 enrichment may well be significantly greater than what has been suggested by past and current open-top chamber and FACE experiments.

References
Madronich, S., McKenzie, R.L., Bjorn, L.O. and Caldwell, M.M. 1998. Changes in biologically active ultraviolet radiation reaching the Earth's surface. Journal of Photochemistry and Photobiology B 46: 5-19.

McKenzie, R.L., Bjorn, L.O., Bais, A. and Ilyasd, M. 2003. Changes in biologically active ultraviolet radiation reaching the earth's surface. Photochemical and Photobiological Sciences 2: 5-15.

Zhao, D., Reddy, K.R., Kakani, V.G., Mohammed, A.R., Read, J.J. and Gao, W. 2004. Leaf and canopy photosynthetic characteristics of cotton (Gossypiuym hirsutum) under elevated CO2 concentration and UV-B radiation. Journal of Plant Physiology 161: 581-590.