Plant Physiol, May 2001, Vol. 126, pp. 352-362

Hydraulic Conductance and Mercury-Sensitive Water Transport for Roots of Opuntia acanthocarpa in Relation to Soil Drying and Rewetting¹

Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, California 90095-1606 (P.M., P.S.N.); and Department of Biology, Occidental College, Los Angeles, California 90041 (G.B.N.)

Drought-induced changes in root hydraulic conductance (L_P) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions of Opuntia acanthocarpa. During 45 d of soil drying, L_P decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil, L_P recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of L_P. Under wet conditions, HgCl₂ (50 µM), which is known to block membrane water-transport channels (aquaporins), decreased L_P and the radial hydraulic conductance for the stele (L_{R, S}) of the distal root region by 32% and 41%, respectively; both L_P and L_{R, S} recovered fully after transfer to 2-mercaptoethanol (10 mM). In contrast, HgCl₂ did not inhibit L_P of the mid-root region under wet conditions, although it reduced L_{R, S} by 41%. Under dry conditions, neither L_P nor L_{R, S} of the two root regions was inhibited by HgCl₂. After 8 d of rewetting, HgCl₂ decreased L_P and L_{R, S} of the distal region by 23% and 32%, respectively, but L_P and L_{R, S} of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in L_P in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L_{R, S} during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.