Mechanisms Controlling Changes in Water Movement Through the Roots ofHelianthus annuusL. During Continuous Exposure to Oxygen Deficiency

The effects of oxygen deficiency on suction-induced water flux (Jv) through detached roots of hydroponically-grown sunflower were investigated over a period of up to 6 d. Jv was reduced by the time the oxygen partial pressure in the solution, sparged with oxygen-free N2, had fallen below 2 kPa (air − 20.6 kPa). This reduction resulted from a decline in the hydraulic conductivity of the radial pathway for water movement (Lp), although up to 46% of the decline was attributable to changes in the osmotic component of the driving force (σΔφ). Lp (mm s−1 MPa−1), for aerobic roots was 2.23 × 10−5 and for anaerobic roots during the initial 16 h, 1.21 × 10−5. At 22 h after the onset of anaerobic treatments, Jv and Lp increased, with Lp values becoming 3 times greater than those measured between 4 h and 16 h of treatment and 1.4 times greater than in aerated roots. However, Lp increased a further 15-fold when roots were killed by immersion in boiling water for 2 min. During and up to 6 d of anaerobic treatment, some roots retained Lp values similar to those at 22 h, while others displayed characteristics typical of dead roots. At no time was there any indication of reduced axial conductivity due to xylem vessel blockage. The results are discussed in terms of possible energy sources for the maintenance of root integrity and their importance to plant survival during long periods of severe oxygen shortage.