Stimulation of border cell production in response to increased carbon dioxide levels

Field soil atmospheres have higher CO₂ and lower O₂ concentrations compared with ambient atmosphere, hut little is known about the impact of such conditions on root exudation patterns. We used altered levels of CO₂ and O₂ relative to ambient conditions to examine the influence of the atmosphere on the production of root border cells by pea (Pisum sativum) root tips. During germination, atmospheres with high CO₂ and low O₂ inhibited root development and border cell separation in pea seedlings. Later in development, the same atmospheric composition stimulated border cell separation without significantly influencing root growth, increased CO₂, not low O₂, was responsible for the observed stimulation of border cell number. High CO₂ apparently can override endogenous signals that regulate the number of border cells released from pea roots into the rhizosphere. The same conditions that stimulated border cell production in pea had no such effect in alfalfa (Medicago sativa).