Background
The authors state that "as a result of stratospheric ozone depletion, UV-B radiation (280-320 nm) 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 reference to this potential development, they also note that "increased levels of UV-B radiation are known to affect plant growth, development and physiological processes (Dai et al., 1992; Nouges et al., 1999)," stating that high UV-B levels often result in "inhibition of photosynthesis, degradation of protein and DNA, and increased oxidative stress (Jordan et al., 1992; Stapleton, 1992)."

What was done
The authors grew well-watered and well-fertilized cotton (Gossypium hirsutum L., cv. NuCOTN 33B) plants from seed in sunlit controlled environment chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm from emergence until three weeks past first flower stage under three levels of UV-B radiation (0, 8 and 16 kJm^-2d^-1).  On five dates between 21 and 62 days after emergence, they also measured a number of plant physiological processes and parameters.

What was learned
Over the course of the experiment, the mean net photosynthetic rate of upper-canopy leaves in the CO2-enriched chambers was boosted -- relative to that in the ambient-air chambers -- by 38.3% in the low UV-B treatment (from 30.3 to 41.9 m m^-2 s^-1), 41.1% in the medium UV-B treatment (from 28.7 to 40.5 m m^-2 s^-1), and 51.5% in the high UV-B treatment (from 17.1 to 25.9 m m^-2 s^-1).  In the medium UV-B treatment, the boost from the elevated CO2 was sufficient to raise net photosynthesis rates 33.7% above rates experienced in the ambient air and no UV-B treatment (from 30.3 to 40.5 m m^-2 s^-1); but in the high UV-B treatment the radiation damage was so great that even with the help of the 51.5% increase in net photosynthesis provided by the doubled-CO2 air, the mean net photosynthesis rate of the cotton leaves was 14.5% less than that experienced in the ambient air and no UV-B treatment (dropping from 30.3 to 25.9 m m^-2 s^-1).

It should be noted, however, that the medium UV-B treatment of this study was chosen to represent the intensity of UV-B radiation presently received on a clear summer day in the cotton production region of Mississippi, USA, under current stratospheric ozone conditions, while the high UV-B treatment was chosen to represent what might be expected there following a 30% depletion of the ozone layer.  Considered in this light, a doubling the air's current CO2 concentration and the environment's current UV-B radiation level would reduce the net photosynthetic rate of cotton leaves by just under 10% (from 28.7 to 25.9 m m^-2 s^-1), whereas in the absence of a doubling of the air's CO2 content, a doubling of the current UV-B radiation level would reduce their net photosynthetic rate by just over 40% (from 28.7 to 17.1 m m^-2 s^-1).  Hence, it can be appreciated that a doubling the air's current CO2 concentration compensates for over three-fourths of the loss of cotton photosynthetic capacity caused by a doubling of the environment's current UV-B radiation intensity.

What it means
As atmospheric CO2 concentrations continue to rise, they provide a powerful antidote for the detrimental effects on plant photosynthesis that could be caused by the increases in UV-B radiation that some scientists are predicting for the future.

References
Dai, Q., Coronal, V.P., Vergara, B.S., Barnes, P.W. and Quintos, A.T.  1992.  Ultraviolet-B radiation effects on growth and physiology of four rice cultivars.  Crop Science 32: 1269-1274.

Jordan, B.R., Chow, W.S. and Anderson, J.M.  1992.  Changes in mRNA levels and polypeptide subunits of ribulose 1,5-bisphosphate carboxylase in response to supplementary ultraviolet-B radiation.  Plant, Cell and Environment 15: 91-98.

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.

Nogues, S., Allen, D.J., Morison, J.I.L. and Baker, N.R.  1999.  Characterization of stomatal closure caused by ultraviolet-B radiation.  Plant Physiology 121: 489-496.

Stapleton, A.E.  1992.  Ultraviolet radiation and plants: Burning questions.  The Plant Cell 105: 881-889.