Background
Loladze (2002) speculates that atmospheric CO2 enrichment will alter the plant tissue concentrations of a number of micronutrients, many of which are important to human health, worrying that if decreases predominate over increases, humanity could suffer severe adverse consequences.

What was done
In a study that is relevant to this hypothesis, the authors grew well-watered wheat (Triticum aestivum L. cv. HD-2285) plants from seed for 90 days in pots supplied with either high or low concentrations of soil nitrogen (150 or 75 kg N ha^-1, respectively) in sunlit open-top chambers maintained at atmospheric CO2 concentrations of either 350 or 600 ppm, after which their leaves, stems and roots were analyzed for concentrations of the micronutrients Mn, Zn, Cu and Fe.

What was learned
The extra 250 ppm of atmospheric CO2 decreased the leaf, stem and root concentrations of Fe at both soil nitrogen levels: -1.8% (leaves, high N), -17.8% (stems, high N), -15.7% (roots, high N), -15.9% (leaves, low N), -13.2% (stems, low N), -10.2% (roots, low N). However, it did just the opposite for the other three micronutrients. For Mn the observed increases were +22.4% (leaves, high N), +0.0% (stems, high N), +37.6% (roots, high N), +20.9% (leaves, low N), +3.3% (stems, low N), +41.2% (roots, low N), for Zn they were +12.9% (leaves, high N), +4.5% (stems, high N), +6.0% (roots, high N), +12.5% (leaves, low N), +9.3% (stems, low N), +9.1% (roots, low N), while for Cu they were +4.3% (leaves, high N), +42.4% (stems, high N), +13.9% (roots, high N), +7.9% (leaves, low N), +38.2% (stems, low N), +14.1% (roots, low N). Averaged across the three plant parts and both soil N levels, the CO2-induced changes in micronutrient concentrations within the wheat plants were: -12.4% (Fe), +20.9% (Mn), +9.0% (Zn) and +20.1% (Cu).

What it means
For three of the four micronutrients investigated in this study, atmospheric CO2 enrichment led to concentration increases, reminiscent of the findings of Lieffering et al. (2004), who observed CO2-induced increases in the concentrations of six out of six micronutrients (Zn, Mn, Cu, B, Mo, plus Fe) in a study of rice grains. Hence, there is reason to hope that the fears of Loladze will not be realized, and that perhaps just the opposite may prove to be true in the case of most food plants.

References
Lieffering, M., Kim, H.-Y., Kobayashi, K. and Okada, M. 2004. The impact of elevated CO2 on the elemental concentrations of field-grown rice grains. Field Crops Research 88: 279-286.

Loladze, I. 2002. Rising atmospheric CO2 and human nutrition: towards globally imbalanced plant stoichiometry. Trends in Ecology and Evolution 17: 457-461.