Root growth potential as an indicator of drought stress history

Container-grown quiescent Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) seedlings were air dried to plant water potentials of -0.2, -2.2 or -3.8 MPa (unstressed, moderate, and severe stress treatments, respectively). Trees from each treatment were either placed in root mist chambers held at 10, 20, or 28 degrees C for 28 days and root growth potential (RGP) and plant water potential (PWP) measured weekly, or potted in a 1/1 mix of peat and vermiculite, watered only once, and height growth and survival recorded after 10 weeks in an unheated greenhouse. Root growth potential of unstressed trees was greater than that of moderately stressed trees at all temperatures. Root growth potential of severely stressed trees was zero. Predawn plant water potentials of unstressed and moderately stressed trees were initially high, fell to -0.5 to -0.8 MPa, and then increased. Predawn plant water potential of severely stressed trees declined continuously over the 28-day experiment. Survival and height growth of the severely stressed trees were reduced compared to the unstressed and moderately stressed trees. Among the root growth potential measurements, RGP measured after 7 days at 10 degrees C was most sensitive to drought stress history and revealed differences in vigor that were not apparent from the survival and height growth data.