What was done
The authors performed a meta-analysis of published experimental data to determine the growth responses of plants to atmospheric CO2 enrichment when they are exposed to unfavorable growing conditions, including those resulting from inadequate levels of water, light and soil nutrition, as well as those resulting from stressful air temperatures, soil salinity and exposure to ozone and other aerial pollutants.

What was learned
In general agreement with the conclusions of an earlier review of the subject (Idso and Idso, 1994), the present study demonstrates that reduced water supplies significantly increase the percentage plant growth response to atmospheric CO2 enrichment, as does the presence of aerial pollutants like ozone.  In addition, the data show a non-significant increase in the percentage growth increase resulting from atmospheric CO2 enrichment when light levels are less than optimal.  Also in agreement with Idso and Idso (1994), the current review reveals no change in the effect of elevated CO2 on the growth responses of plants over a wide range of soil salinities.  However, the new review indicates that exposure to inadequate levels of soil fertility or low air temperatures may in fact decrease the percentage plant growth response to elevated CO2, which was also observed by Idso and Idso (1994), who found a small reduction in the percentage growth response of nutrient-stressed plants exposed to a 300 ppm increase in atmospheric CO2 concentration (but not to a 600 ppm or greater CO2 increase), and no CO2-induced growth response of plants exposed to air temperatures of 8°C or lower.

What it means
As the CO2 content of the air rises, it is likely that most plants will exhibit increased rates of photosynthesis and biomass production, even when subjected to stressful growing conditions.  In many such situations, in fact, the relative CO2-induced growth increase may actually be greater than it is under optimal plant growth conditions.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.