Radial Hydraulic Conductivity of Individual Root Tissues ofOpuntia ficus-indica(L.) Miller as Soil Moisture Varies

Abstract

The constraints on water uptake imposed by individual root tissues were examined for Opuntia ficus-indica under wet, drying, and rewetted soil conditions. Root hydraulic conductivity ( LP ) and axial conductance ( Kh ) were measured for intact root segments from the distal region with an endodermis and from midroot with a periderm; LP was then measured for each segment with successive tissues removed by dissection. Radial conductivity ( LR ) was calculated from LP and Kh for the intact segment and for the individual tissues by considering the tissue conductivities in series. Under wet conditions, LR for intact distal root segments was lowest for the cortex; at midroot, where cortical cells are dead, LR for the cortex was higher and no single tissue was the predominant limiter of LR . LR for the endodermis and the periderm were similar under wet conditions. During 30d of soil drying, LR for the distal cortex increased almost three-fold due to the death of cortical cells, whereas LR for the midroot cortex was unaffected; LR for the endodermis and the periderm decreased by 40 and 90%, respectively, during drying. For both root regions under wet conditions, the vascular cylinder had the highest LR , which decreased by 50–70% during 30d of soil drying. After 3d of rewetting, new lateral roots emerged, increasing LR for the tissues outside the vascular cylinder as well as increasing uptake of an apoplastic tracer into the xylem of both the roots and the shoot. The average LR for intact root segments was similar under wet and rewetted conditions, but the conductivity of the tissues outside the vascular cylinder increased after rewetting, as did the contribution of the apoplastic pathway to water uptake.