HYDRAULIC CONDUCTIVITY OF CONCENTRIC ROOT TISSUES OF AGAVE DESERTI ENGELM UNDER WET AND DRYING CONDITIONS

Water moves from the root surface to the root xylem through a series of tissues, each with properties that influence the root radial hydraulic conductivity (L(R)). To determine L(R) for intact roots, root hydraulic conductivity (L(R)) and axial (xylem) conductance (K-b) were measured for root segments of the desert succulent Agave deserti Engelm. As layers were successively removed, L(R) was calculated for each layer by considering the conductivities in series. For the distal root region, no single tissue limited L(R) under wet conditions, although the suberized exodermis and sclerenchyma/endodermis were more limiting per cross-sectional area of the root than were the cortex and stele. At midroot under wet conditions, L(R) for the cortex was higher than in the distal region, and cortical cells were dead. After 28 d in drying soil, L(R) for the suberized layers decreased by over 70 % and thereby became more limiting to L(R) of the intact segment, consistent with increases in extracted lipids in these layers. During drying, intercellular lacunae formed in the cortex due to cellular collapse, and cortical L(R) decreased. After 7 d of re-wetting following drying, L(R) increased for all tissues, but not to its value under wet conditions, reflecting persisting losses in conductivity, especially in the suberized layers. Thus, new apical growth and the development of new laterals are necessary for the complete restoration of root system conductivity. Within 20 mm of the root tip, substantial uptake of the apoplastic tracer 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS) occurred, although the exodermis restricted PTS uptake further than 20 mm back from the root tip under wet conditions and all along the root after drying. When the exodermis was removed, PTS moved freely through the cortex in both distal and midroot regions, further suggesting that the cortical apoplast is a significant pathway for radial water flow.