CHANGES IN HYDRAULIC CONDUCTIVITY AND ANATOMY CAUSED BY DRYING AND REWETTING ROOTS OF AGAVE-DESERTI (AGAVACEAE)

Concurrent determinations of changes in hydraulic conductivity and tissue anatomy were made for roots of Agave deserti excised during drying and following rewetting in soil. At 30 d of drought, hydraulic conductivity had declined less than twofold for older nodal roots, tenfold for young nodal roots, and more than 20-fold for lateral roots ("rain roots" occurring as branches on the nodal roots). These decreases were consistent with increases in cortical lacunae caused by cell shrinkage and collapse. Similarly, reduction of lacunae in response to rewetting after 7 d of drought corresponded to levels of recovery in hydraulic conductivity, with young nodal roots showing full recovery, lateral roots returning to only 21% of initial conductivity, and older nodal roots changing only slightly. Increases in suberization in the exodermis, endodermis, and cortex adjacent to the endodermis in response to drying coincided with decreases in hydraulic conductivity. Measurements of axial hydraulic conductance per unit length before and after pressurization indicated that embolism caused reductions in axial conductance of 98% for lateral roots, 35% for young nodal roots, and 20% for older nodal roots at 7 d of drought. Embolism, cortical lacunae, and increasing suberization caused hydraulic conductivity to decline during drought in the three root types, thereby helping limit water loss to dry soil; the recovery in hydraulic conductivity for young nodal roots after rewetting would allow them to take up water readily once soil moisture is replenished.