Effects of water deficit stress and recovery on the root water relations of trembling aspen (Populus tremuloides) seedlings

Aspen (Populus tremuloides Michx.) seedlings were grown in sand culture and subjected to mild and severe water deficit stress by withholding watering. Severely-stressed seedlings were also rewatered for 24 h to determine the effects of water deficit stress and stress recovery on root water flow properties. Both stress levels and stress recovery treatment reduced leaf stomatal conductance and shoot water potentials. However, root volume flux density and hydraulic conductivity were inhibited only by the severe water deficit stress and did not recover within 24 h following rewatering. The inhibition of root hydraulic conductivity in severely-stressed plants was accompanied by an increase in the proportion of apoplastic root water flow, as determined by the trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate fluorescent tracer dye. In all treatments and control, the mean activation energy values for root water flow were lower than 6 kcal mol(-1), pointing to the presence of AQP-mediated transport. However, compared with earlier studies using solution culture-grown aspen seedlings, mercuric chloride had relatively little effect on root volume flux density. We interpreted this result as likely due to limited penetration of the root cortex by mercuric chloride through the exodermal layer. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.