Changes in root water flow properties of solution culture-grown trembling aspen (Populus tremuloides) seedlings under different intensities of water-deficit stress

To study the effects of water-deficit stress on root water flow properties in trembling aspen (Populus tremuloides Michx.), seedlings were grown in solution culture and subjected to water-deficit stress by placing their roots in sealed high humidity chambers. After 17 h of stress treatment, seedlings showed mild stress (MS) symptoms with a decline in shoot water potentials. Within 20 h, shoot water potentials rapidly declined, and severe stress (SS) symptoms were present. Root hydraulic conductivity (L-pr) increased more than two-fold and the relative concentration of apoplastic tracer dye trisodium 3-hydroxy-5, 8, 10-pyrenetrisulphonate (PTS3) in xylem exudate decreased by 73.6% in MS seedlings. Conversely, L-pr decreased (55.3%) and PTS3 increased (28.6%) in SS seedlings. Treatment of roots with 0.1 mM mercuric chloride decreased root volume flow density (J(v)) by about 29.0% in control and MS plants with no decrease measured in SS seedlings. Mercuric chloride also increased PTS3 concentration in xylem exudate of control (59%) and MS (86%) seedlings with no change observed in SS plants. The results suggest that aquaporin-mediated transport is important in the regulation of root water flow under drought stress and that root water flow properties are strongly affected by the stress level. Regulation of root water flow may represent an important drought-stress resistance mechanism.