Hydraulic Properties of Shallow vs. Deep Lateral Roots in a Semiarid Shrub, Gutierrezia sarothrae

-Hydraulic conductivity of the deep and shallow lateral roots in the xerophyte Gutierrezia sarothrae Shinners (broom snakeweed) was measured for plants grown under two soil-water regimes. Hydraulic conductivity was 3.4-fold greater in the white, nonsuberized deep roots than in the brown, suberized shallow roots in dry soil; but was only 67% higher under favorable soil moisture conditions. Deep roots had 40% more large xylem vessels (diameters -20 ,tm) than shallow roots in the plants growing in the field during the normal summer dry season. This may result in lower axial resistance to water flow in the xylem conduits in deep roots, but theoretical axial resistances calculated with the PoiseuilleHagen equation could only explain 23% of the difference in hydraulic conductivity between deep and shallow roots. Apparently, increased radial resistance in the suberized shallow roots was mainly responsible for the low hydraulic conductivity observed during the dry season. Suberized roots of G. sarothrae regained high hydraulic conductivity 1 day after soil rewetting. This indicates a rectifier-like behavior of root systems in this half-shrub, as has been documented for desert succulents.