Hydraulic conductivity and anatomy along lateral roots of cacti: changes with soil water status

To help understand root function for desert plants at different levels of water availability, cellular and water-conducting properties were examined for young lateral roots of Ferocactus acanthodes (Lem.) Britton & Rose and Opuntia ficus-indica (L.) Miller. Hydraulic conductivity (L_p ), radial conductivity (L_R ), and axial conductance (K_h ) were determined for root segments in wet soil, during drought, and after rewetting. All three parameters increased with distance from the root tip under wet conditions and during drought for both species, with larger increases occurring under wet conditions. L_p decreased with drought duration up to 35 d for both species, due to dehydration of cortical cells, suberization of periderm, and embolism of xylem vessels; L_p and L_B decreased more for O. ficus-indica, and K_h decreased more for F. acanthodes. The increase in K_h with distance from the root tip and the decrease with duration of drought were associated with the changes in the number of conducting xylem vessels. After 7 d of rewetting, new secondary lateral roots developed, enhancing water uptake, especially for O. ficus-indica. Rewetting also caused water uptake for primary lateral roots to return to approximately the original value under wet conditions, mainly because decreases in emboli led to recovery of axial flow.