Biology of Dryland Plants
North, Gretchen .
Roots of Dryland Plants: Where and How Water Uptake Occurs.
For most plants, but particularly dryland perennials, roots become more suberized and lignified with age. Thus, older roots have been considered less permeable to water than are younger roots. Because rainfall in arid regions often wets only the top few centimeters of soil, roots near the surface are critical for water uptake. Investigations of desert succulents in rewetted soil following drought indicate that water uptake often precedes the development of new roots and thus must involve older, established roots. For cacti and monocotyledonous succulents, hydraulic conductance is often greater for older roots than for younger roots, due not only to greater axial conductance through mature xylem conduits but also to surprisingly high radial hydraulic conductance (through root tissues external to the xylem). Species in the Agavaceae, such as Agave deserti, Hesperoyucca whipplei, and Yucca schidigera, have highly absorptive regions in older roots near the base of the shoot where the endodermis has relatively thin cell walls and a high frequency of unsuberized passage cells. For A. deserti, water uptake in this basal root region is associated with aquaporins, which are more numerous or open than in the midroot region. These absorptive features occur in a root zone that contracts longitudinally (and expands radially) in all species of the Agavaceae that have been examined to date. Although contractile roots have not been observed in cacti, older roots near the shoot base often have fissured periderm that may help account for their relatively great hydraulic conductance. Older roots of one- to two-year old seedlings in the chaparral genera Ceanothus and Rhamnus also have greater hydraulic conductance than do younger roots, indicating again that the presence of periderm does not greatly impede water uptake. These findings suggest that the older roots of dryland species contribute more to water uptake than previously thought.
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1 - Occidental College, Biology, 1600 Campus Rd, Los Angeles, California, 90041, US
Presentation Type: Symposium or Colloquium Presentation
Location: Salon K - Austin Grand Ballroom/Hilton
Date: Wednesday, August 17th, 2005
Time: 8:15 AM