Document: WIL-3-32-20

The contribution of deep root functioning to water relations and xylem transport in Juniperus ashei.

POCKMAN, W.T.* ¹, J.MARTÍNEZ-VILALTA ² and R.B.JACKSON ³

University of New Mexico, Albuquerque, NM 87131-1091 ¹
Univ. Auṭnoma de Barcelona Bellaterra 08193, Barcelona Spain ²
Duke University, Durham, NC 27708 ³

Abstract:
Daily and seasonal measurements of whole-plant water relations, morphology, and xylem transport in Juniperus ashei were taken on the surface and 7 m underground in a cave system in central Texas, USA. Sap flux data indicated that over the growing season 24% of total daily transpiration on average came from below 7 m. During periods of high water availability sap flux density was similar in deep roots and the main stem, but during an extended drought sap flux density in the deep root was significantly less than in the main stem. Hygrometer measurements of deep root water potential, compared with sap flux density, indicated that hydraulic conductance of deep roots declined during the onset of the drought and increased again following precipitation more than two months later. Consistent with this decrease, deep roots were most vulnerable to xylem cavitation while branches were least vulnerable and shallow roots were intermediate. These differences in vulnerability to cavitation were accompanied by parallel differences in xylem tracheid diameter distribution. Mean tracheid diameter and hydraulically-weighted tracheid diameter were largest in deep roots, intermediate in shallow roots, and smallest in branches. These data demonstrate that deep roots can contribute significantly to total plant water use and suggest that an integrated understanding of whole root system function should incorporate differences in the physiological limits of its component parts.

Keywords: sap flux, deep roots, xylem cavitation, hydraulic conductance

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS