Elevated CO₂ alters deployment of roots in "small" growth containers

Previously we examined how limited rooting space and nutrient supply influenced plant growth under elevated atmospheric CO₂ concentrations (McConnaughay et al. 1993). We demonstrated that plant growth enhancement under elevated CO₂ was influenced more by the concentration of nutrients added to growth containers than to either the total nutrient content per pot or amount or the dimensions of available rooting space. To gain insight into how elevated CO₂ atmospheres affect how plants utilize available belowground space when rooting space and nutrient supply are limited we measured the deployment of roots within pots through time. Contrary to aboveground responses, patterns of belowground deployment were most strongly influenced by elevated CO₂ in pots of different volume and shape. Further, elevated CO₂ conditions interacted differently with limited belowground space for the two species we studied,Abutilon theophrasti, a C₃ dicot with a deep taproot, andSetaria faberii, a C₄ monocot with a shallow fibrous root system. ForSetaria, elevated CO₂ increased the size of the largest region of low root density at the pot surface in larger rooting volumes independent of nutrient content, thereby decreasing their efficiency of deployment. ForAbutilon, plants responded to elevated CO₂ concentrations by equalizing the pattern of deployment in all the pots. Nutrient concentration, and not pot size or shape, greatly influenced the density of root growth. Root densities forAbutilon andSetaria were similar to those observed in field conditions, for annual dicots and monocots respectively, suggesting that studies using pots may successfully mimic natural conditions.