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Gretchen B. North and Evan A. Baker

Desert succulents are renowned for their rapid recovery of shoot function after periods of drought as a result of shoot succulence, root–shoot interactions, and key root properties. Near the base of the shoot, the proximal (generally older) roots appear to play a major role in the rapid uptake and delivery of water, especially after a period of soil drying when the rest of the root system has a lower hydraulic conductance. In all of the cacti, agaves, and yuccas examined to date, hydraulic conductance for the proximal root zone is unexpectedly high. Substantial water uptake by older roots of cacti is associated with much higher axial (xylem) hydraulic conductance in proximal than in distal root regions. For agaves and yuccas, proximal roots have other anatomical and physiological traits that enhance water uptake near the shoot base. Despite suberization (waxiness) of external root tissues in the proximal region, internal cell layers appear more characteristic of young root regions with living cortical cells and a high proportion of unsuberized passage cells in the endodermis. Both features may be related to the existence of a contractile root zone possessed by all members of the Agavaceae surveyed to date. Passage cells in the endodermis and aquaporins (protein water channels) in the metabolically active cortex expedite radial water transport. The radial swelling that accompanies longitudinal root contraction may also help maintain root-to-soil contact that would otherwise be diminished in dry soil. Modifications that increase axial and radial hydraulic conductance in the proximal root region suggest that water uptake by older roots is critical to the success of desert succulents.