ROOT-SOIL CONTACT FOR THE DESERT SUCCULENT AGAVE DESERTI IN WET AND DRYING SOIL

To investigate the extent and size of root-soil air gaps that develop during soil drying, resin casts of roots of the desert succulent Agave deserti Engelm. were made in situ for container-grown plants and in t he field. Plants that were droughted in containers for 7 and 14 d had 24 and 34% root shrinkage, respectively, leading to root-soil air gaps that would reduce the hydraulic conductivity at the root-soil interfa ce by a factor of about 5. When containers were vibrated during drough t, root-soil air gaps were greatly diminished, and the predicted condu ctivity at the interface was similar to that of the control (moist soi l). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was greater than for container-grown plants, but root-soil contact on the root periphery was greater, which led to a higher predi cted hydraulic conductivity at the root-soil interface. To test the hy pothesis that root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and sho ot of plants from vibrated containers (with gaps eliminated or reduced ) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting mo re rapid depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting greater water l oss by roots in the absence of root-soil air gaps. Thus, air gaps coul d benefit A. deserti by helping to maintain a higher root water potent ial in the early stages of drought and later by limiting root water lo ss at the root-soil interface when the water potential exceeds that of the soil.