What was done
The authors employed micropropagation techniques to grow hybrid poplar clones that were eventually potted and transferred to growth chambers where they were kept at day/night temperatures of 20/15°C on a 14/10 hours rotation schedule at either ambient (360 ppm) or elevated (700 ppm) concentrations of atmospheric CO2 for a period of three months.  Excised leaves were then subjected to two different stresses: chilling temperatures at high light intensity or submersion of the leaf's petiole in a solution of paraquat, after which a variety of measurements were made on the treated leaves and non-stressed control leaves.

What was learned
In the words of the authors, "photosynthesis was less diminished and electrolyte leakage was lower in stressed leaves from poplar trees grown under elevated CO2 as compared with those from ambient CO2."  Although severe stress did cause pigment and protein degradation in all stressed leaves, the damage was expressed to a lower extent in leaves from the elevated CO2 treatment.  The CO2-induced protection was determined to be accompanied by a rapid induction of superoxide dismutase activity, as well as slightly higher stabilities of other antioxidative enzymes.

What it means
"Although stress amelioration under elevated CO2 was limited," say the authors, "it will still be advantageous, especially for coping with fluctuating adverse environmental conditions."  Hence, as the air's CO2 content continues to rise, we can expect plants to become ever-better equipped to tolerate more challenging environments than they can currently.  This development will be a boon to both agriculture and the natural environment, enabling plants to become more robust and productive and to thereby extend their ranges into territories where they could not grow before.  And if by some chance the planet's overall environment were to become more stressful in the future, earth's vegetation should be more fit for the task of dealing with that situation.