CHANGES IN STRUCTURE AND HYDRAULIC CONDUCTIVITY FOR ROOT JUNCTIONS OFDESERT SUCCULENTS AS SOIL-WATER STATUS VARIES

Variations in hydraulic conductivity (Lp) and the underlying anatomica l and morphological changes were investigated for main root-lateral ro ot junctions of Agave deserti and Ferocactus acanthodes under wet, dry , and rewetted soil conditions. During 21 d of drying, L(p) and radial conductivity (L(R)) increased threefold to fivefold at junctions of b oth species. The increase in L(R) was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of th e junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. Du ring 7 d of rewetting, LR decreased for junctions of A. deserti, as ap oplastic water movement was not apparent, but L(R) was unchanged for F . acanthodes. Axial conductance (K(h)) decreased during drying for bot h species, largely because of embolism related to the degradation of u nlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admi t air bubbles at pressure differences of only 0.12 - 0.19 MPa. Rewetti ng restored K(h) for both species, but not completely, due to blockage of xylem elements by tyloses. About 40 % of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For t he dicotyledon F. acanthodes, which can form new conduits in its secon dary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral root s that persist during drought in the field compared with the case for the sympatric A. deserti.