WATER-UPTAKE AND STRUCTURAL PLASTICITY ALONG ROOTS OF A DESERT SUCCULENT DURING PROLONGED DROUGHT

Desert succulents resume substantial water uptake within 1-2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the mono cotyledonous leaf succulent Agave deserti Engelm, substantial root mor tality occurred only for lateral roots near the soil surface; nearly a ll main roots were alive at 180 d of drought. New main roots were init iated and grew up to 320 mm at soil water potentials lower than - 5.0 MPa, utilizing water from the shoot. The hydraulic conductivity of dis tal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored follo wing 45 d of drought but not after 90 and 180 d, The production of new lateral roots and the renewed apical elongation of main roots occurre d 7-11 d after rewetting following 180 d of drought. Hydraulic conduct ivity was higher in the distal region than at midroot and often increa sed again near the root base, where many endodermal cells lacked suber in lamellae, Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity alo ng a root allows d. deserti to capitalize on intermittent or heterogen eous supplies of water.