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

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