What was done
The authors grew twenty-day-old plants for an additional seven days within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 350 or 800 ppm in an iron (Fe)-sufficient medium with a soluble Fe source or under Fe-limited conditions in a medium containing the sparingly soluble hydrous Fe(III)-oxide, while measuring a number of pertinent plant parameters.

What was learned
Plant growth was increased by the elevated CO2 in both the Fe-sufficient and Fe-limited media, with shoot fresh weight increasing by 22% and 44%, respectively, and root fresh weight increasing by 43% and 97%, respectively. In addition, Jin et al. report that "the elevated CO2 under Fe-limited conditions enhance[d] root growth, root hair development, proton release, root FCR [ferric chelate reductase] activity, and expressions of LeFR01 and LeIRT1 genes [which respectively encode FCR and the Fe(II) transporter in tomato], all of which enable plants to access and accumulate more Fe." And they add, as would be expected, that "the associated increase in Fe concentrations in the shoots and roots alleviated Fe-deficiency-induced chlorosis."

What it means
Jin et al. write that the bioavailability of iron to terrestrial plants "is often limited (Guerinot and Yi, 1994), particularly in calcareous soils, which represent 30% of the earth's surface (Imsande, 1998)," and they thus conclude that "Fe nutrition in plants is likely to be affected by the continued elevation of atmospheric CO2, which, in turn, will affect crop production." And as their work strongly suggests, those important effects should be highly beneficial, while even wider biospheric benefits are suggested by the work of Sasaki et al. (1998), who have demonstrated that both the ferric reductase activity and Fe uptake capacity of the marine alga Chlorococcum littorale cultured in Fe-limited media have been significantly enhanced by elevated CO2 concentrations.

References
Guerinot, M.L. and Yi, Y. 1994. Iron: nutritious, noxious, and not readily available. Plant Physiology 104: 815-820.

Imsande, J. 1998. Iron, sulfur, and chlorophyll deficiencies: a need for an integrative approach in plant physiology. Physiologia Plantarum 103: 139-144.

Sasaki, T., Kurano, N. and Miyachi, S. 1998. Induction of ferric reductase activity and of iron uptake capacity in Chlorococcum littorale cells under extremely high-CO2 and iron-deficient conditions. Plant & Cell Physiology 39: 405-410.