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The importance of aquaporins for root hydraulic conductance (L-P) was investigated along roots of the desert succulent Agave deserti in wet, dry and rewetted soil. Water channel activity was inferred from HgCl2-induced reductions of L-P that were reversible by 2-mercaptoethanol. Under wet conditions, HgCl2 reduced L-P for the distal root region by 50% and for the root region near the shoot base by 36% but did not affect L-P for the mid-root region. For all root regions, L-P decreased by 30-60% during 10 d in drying soil and was not further reduced by HgCl2. After soil rewetting, L-P increased to pre-drying values and was again reduced by HgCl2 for the distal and the basal root regions but not the mid-root region. For the distal region, water channels in the epidermis/exodermis made a disproportionately large contribution to radial hydraulic conductance of the intact segment; for the basal region, water channel activity was highest in the cortex and endodermis. The role of water channels was greatest in tissues in which cells were metabolically active both in the distal root region, where new apical growth occurs in wet soil, and in the basal region, which is the most likely root region to intercept light rainfall.