What was done
The authors grew well watered and fertilized tomato (Lycopersicon esculentum Mill cv. Cooperate 903) plants in hydroponic solution for 15 days within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 350 or 800 ppm, after which they harvested the plants and determined a number of physical and chemical characteristics of their roots.

What was learned
Total root biomass was increased by 189.3% by the extra 450 ppm of CO2, while root length was increased by 21.0%, root surface area by 45.4%, root diameter by 20.9%, root volume by 69.6% and the number of root tips by 113.2%. In addition, Wang et al. say that "root hairs of the control treatment were relatively sparse, short and thin, whereas in the elevated CO2 treatment, they were very intensive." Last of all, they report that indole acetic acid increased by 26.5% and 56.6% in the roots and leaves, respectively, and that increases in ethylene were nearly 100% in both roots and leaves.

What it means
The significance of the CO2-induced increases in various root characteristics is rather self-evident. As for the improvements in various root hair properties, the researchers write that "root hairs are crucial for phosphorus (P) uptake in tomato, in which about 50% of it is absorbed by the root hairs and up to 70% in soils with low P bioavailability," so that "the total root surface area increment from the development and elongation of root hairs induced by CO2 enrichment can have an important effect on nutrient uptake." Further to this point, they say that indole acetic acid "is involved in the regulation of lateral root development, root hair elongation, and its density under appropriate phosphorus conditions," and that it "can stimulate the biosynthesis of ethylene." With respect to the latter phenomenon, they state that "the effect of ethylene on cell wall formation and cell wall properties is to instigate the phase of root expansion and hair formation."

In light of these several observations, it can be appreciated that the ongoing rise in the air's CO2 content should greatly enhance the ability of tomato plants - and probably plants in general - to extract what they need from earth's soils in order to fully exploit the potential for greater growth and productivity that is provided by the aerial fertilization effect of atmospheric CO2 enrichment, which may well be one of the key reasons why the so-called progressive nitrogen limitation hypothesis has never been shown to operate in long-term CO2 enrichment studies.

For more on this latter topic, see Nitrogen (Progressive Limitation Hypothesis) in our Subject Index.