Border cells and carbon in the rhizospheres of growth zones

This project goal is to quantify carbon associated with border cell release and exudation from the growth zone of the root. Border cells in rhizosphere soil were counted under a compound microscope after soil sonication and treatment with Tween surfactant. Carbon in the rhizosphere of the growth zone was measured with a Carlo Erba C/N analyzer on rhizosphere soil collected around the growth zone. For three different species, young seedlings (with roots shorter than 4 cm) release the most border cells. The apical 1 cm of Bromus carinatus has the potential to slough 800 border cells from young roots, but only 70 cells in older roots. Roots from Zea mays sloughs 17000 cells from young roots, and only 150 cells from older roots. The dicot Cucumis sativus sloughs 11000 and 300 border cells from younger and older roots, respectively. For a variety of native annual and perennial grasses and invasive annual grasses (Nassella pulchra, Bromus carinatus, B. diandrus, B. hordeaceus, Vulpia microstachys, Aegilops triuncialis, Lolium multiflorum, Zea mays) the rhizosphere of mature root systems contains between 34 and 48 μg carbon per g sand (compared to 16 μg for unplanted controls). The root tip provides carbon (22 μg per g sand compared to 13 μg for controls) to the rhizosphere in front of the growing tip and behind the root cap, and the largest deposition is to the rhizosphere surrounding the root cap (41 μg C per g sand for Zea mays and 130 μg C per g sand for Cucumis sativus.) These numbers from laboratory studies represent the maximum C contribution that might be released during flooding in soils. Scaling from organ to field scales requires a growth analysis to quantify root tip distributions in space and time.