Root water flow and growth of aspen (Populus tremuloides) at low root temperatures

Effects of root zone temperature on growth, shoot water relations, and root water flow were studied in 1-year-old aspen (Populus tremuloides Michx.) s eedlings. Seedlings were grown in solution culture and exposed to day/night air temperatures of 22/16 degrees C and solution culture temperatures of 5 , 10, or 20 degrees C for 28 days after bud flush. Compared with root growt h at 20 degrees C, root growth was completely inhibited at 5 degrees C and inhibited by 97% at 10 degrees C. The 5 and 10 degrees C treatments severel y reduced shoot growth, leaf size, and total leaf area. Root water flow was inhibited by the 5 and 10 degrees C treatments. However, when seedlings we re grown for 28 days at 5 degrees C and root water flow was measured at 20 degrees C, there was an increase in flow rate. This increase in root water flow was similar in magnitude to the decrease in root water flow observed w hen seedlings were grown for 28 days at 20 degrees C and root water flow wa s measured at 5 degrees C. Reduced root water flow of seedlings grown at 5 and 10 degrees C resulted in decreased stomatal conductance, net assimilati on, and shoot water potentials. Root water flow was positively correlated w ith leaf size, total leaf area, shoot length, and new root growth. Transfer ring seedlings from 5 to 20 degrees C for 24 h significantly increased root water flow, shoot water potential, and net photosynthesis, whereas transfe rring seedlings from 10 to 20 degrees C resulted in only a slightly increas ed shoot water potential. Transferring seedlings from 20 to 5 degrees C gre atly reduced root water flow, stomatal conductance, and net photosynthesis, whereas shoot water potential decreased only slightly.