What was done
The authors grew well watered and fertilized three-month-old rootstock seedlings of Cleopatra mandarin (Citrus reticulata Blanco) and Carrizo citrange (Citrus sinensis (L.) Osb. x Poncirus trifoliata L.), with or without salt stress (an additional 50 mM NaCl), for eight additional weeks, one plant each in 1.5-liter containers located in controlled-environment greenhouses maintained at either 360 or 700 ppm CO2, during which time, and at the end of the experiment, they measured a number of plant properties and physiological processes that allowed them to test the hypothesis that "salinity tolerance of citrus rootstock seedlings would be increased when grown in elevated CO2."

What was learned
Garcia-Sanchez and Syvertsen report that "elevated CO2 increased plant growth, shoot/root ratio, leaf dry weight per area, net assimilation of CO2, chlorophyll, and water-use efficiency." The increase in the latter parameter was caused by both a decrease in transpiration and an increase in plant biomass, the latter of which plant properties was increased by 27% in the salt-stress treatment and 40% in the non-salt-stress treatment for Cleopatra mandarin, and by 49% in the salt-stress treatment and 43% in the non-salt-stress treatment for Carrizo citrange. In addition, they report that "elevated CO2 increased salinity tolerance in the relatively salt-sensitive Carrizo more than in the salt-tolerant Cloepatra."

What it means
As revealed in the comprehensive review of Idso and Idso (1994), atmospheric CO2 enrichment tends to help plants cope with various types of environmental stress. Such was also true in this study. In addition, the extra CO2 provided the greatest help to the genotype that needed it most.

Reference
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.