9419841 Nobel The primary goals of the proposed research are to investigate water movement across the root-soil interface and from the root surface to the root xylem. The focus will be on how the constraints on radial flow in each component of the pathway change in concert with changes in soil moisture availability. Structural modifications in the root and at the root-soil interface during drought and subsequent rewetting will be analyzed with respect to their consequences for water uptake or loss by roots of the monocotyledon Agave deserti and the dicotyledon Opuntia ficus-indica. The pathway across the root is composed of several tissues, each with its own hydraulic conductivity that can change in response to moisture availability and as a consequence of root development. The limitations on radial water flow imposed by individual root tissues will be investigated by measuring hydraulic conductivity for intact and dissected root segments; the contribution of the apoplast to the radial pathway will also be assessed by fluorescent tracers. Anatomical changes that occur in roots during short-term drought (less than one month) will be assessed along with possible changes in tissue osmotic pressure; the importance of such changes in limiting water flow will be anlyzed with respect to two driving forces, hydrostatic versus osmotic pressures. The responses of roots to prolonged drought (four to six months) will also be examined, with the focus on anatomical and morphological changes that restrict water loss yet allow rapid water uptake upon rewetting. Roots of these and many other species shrink during drought, as perhaps does the soil, leading to the development of air gaps between roots and soil. Physical verification and measurement of air gaps around roots will be obtained from soil thin-sections surrounding roots, accompaied by experimental assesment of the effects of air gaps on water movement across the root- soil interface. In summary, the proposed research will investigate how the limitations on the movement of water in the root-soil system change with soil moisture availability, with particular emphasis on the capability of roots to function in drying soil.
