Background
The authors write that "salt stress has a threefold effect on plant health," in that it (1) reduces water availability, (2) causes ion imbalance, and (3) causes toxicity, all of which phenomena, as they describe it, "curtail growth, photosynthesis, protein synthesis, energy storage, and lipid metabolism," as described in detail by Munns (2005) and Parida and Das (2005).

What was done
In a study designed to explore the negative consequences of potentially greater salt stress in a CO2-enriched world of the future, Perez-Lopez et al. grew barley (Hordeum vulgare) plants in pots containing a 3:1 mix of perlite:vermiculite within controlled-environment chambers maintained at either ambient or elevated atmospheric CO2 concentrations (350 or 700 ppm) for the last 14 days of a 28-day post-planting period, where they also instituted four different salt-stress treatments on the 15th day by supplying the plants then and thereafter with water possessing one of four different degrees of saltiness (0, 80, 160 or 240 mM NaCl), while at the end of the full 28-day period they measured a number of plant physiological properties and processes related to the maximal rate of net photosynthesis (Amax) exhibited by the first fully-expanded attached leaf of each plant.

What was learned
In the words of the five Spanish scientists, "in the zero-saline treatment, elevated CO2 increased the Amax by 49% compared with the Amax measured at ambient CO2," while "under ambient CO2 conditions, saline treatments (80-, 160- and 240-mM NaCl) reduced the Amax by 18, 32 and 39%, respectively." But they add that "these reductions were lower at elevated CO2: 8, 22 and 28% for 80-, 160- and 240-mM NaCl." And based on the graphical representations of their results, we calculate that the CO2-induced enhancements of Amax in the four saline treatments (0-, 80-, 160- and 240-mM NaCl) were, respectively, 49%, 68%, 71% and 76%.

What it means
In a nutshell, the greater the salinity-induced percentage reduction in barley Amax becomes, the greater the CO2-induced percentage increase in barley Amax becomes.

References
Munns, R. 2005. Genes and salt tolerance: bringing them together. New Phytologist 167: 645-663.

Parida, A.K. and Das, A.B. 2005. Salt tolerance and salinity effects on plants: a review. Ecotoxicology and Environmental Safety 60: 324-349.