Hydraulic conductance in aspen (Populus tremuloides) seedlings exposed to low root temperatures

Low root temperatures significantly reduced root hydraulic conductivity and increased resistance to water flow through the roots of aspen (Populus tre muloides Michx.) seedlings. Increased resistance to water flow could not be fully explained by the corresponding increase in water viscosity at low te mperatures. The shapes of Arrhenius plots of root water flow and the activa tion energies were dependent on the direction, sequence and extent of tempe rature change. The Arrhenius plots suggested that the effect of low root te mperature on root water flow was mediated by an effect on root metabolism. The low root temperatures tested did not induce root electrolyte leakage no rmally associated with cell membrane injury. Although a decrease in root te mperatures to 7 or 4 degreesC induced a reduction in stomatal conductance, this reduction lagged the decline in root water flow by several hours. In c ontrast, when soil temperatures were raised from 4 or 7 degreesC to 25 degr eesC, root water flow presumably increased, and stomatal conductance respon ded rapidly and was temporarily higher than before the cold treatment was i mposed.