RADIAL HYDRAULIC CONDUCTIVITY OF INDIVIDUAL ROOT TISSUES OF OPUNTIA-FICUS-INDICA (L) MILLER AS SOIL-MOISTURE VARIES

The constraints on water uptake imposed by individual root tissues wer e examined for Opuntia ficus-indica under wet, drying, and rewetted so il conditions. Root hydraulic conductivity (L(p)) and axial conductanc e (K-h) were measured for intact root segments from the distal region with an endodermis and from midroot with a periderm; Lp( )was then mea sured for each segment with successive tissues removed by dissection. Radial conductivity (L(R)) was calculated from L(p) and K-h for the in tact segment and for the individual tissues by considering the tissue conductivities in series. Under wet conditions, L(R) for intact distal root segments was lowest for the cortex; at midroot, where cortical c ells are dead, L(R) for the cortex was higher and no single tissue was the predominant limiter of L(R). L(R) for the endodermis and the peri derm were similar under wet conditions. During 30 d of soil drying, L( R) for the distal cortex increased almost three-fold due to the death of cortical cells, whereas L(R) for the midroot cortex was unaffected; L(R) for the endodermis and the periderm decreased by 40 and 90%, res pectively, during drying. For both root regions under wet conditions, the vascular cylinder had the highest L(R), which decreased by 50-70% during 30 d of soil drying. After 3 d of rewetting. new lateral roots emerged, increasing L(R) for the tissues outside the vascular cylinder as well as increasing uptake of an apoplastic tracer into the xylem o f both the roots and the shoot. The average L(R) for intact root segme nts was similar under wet and rewetted conditions, but the conductivit y of the tissues outside the vascular cylinder increased after rewetti ng, as did the contribution of the apoplastic pathway to water uptake. (C) 1996 Annals of Botany Company