NITROGEN NUTRITION OF HYBRID POPLARS By

By Shan Zhao, M.S. Washington State University August 2006 Chair: Jon D. Johnson Hybrid poplars are the result of interspecific crosses in the Populus genus and are genetically predisposed to grow faster and have wider adaptability than either parent species. They have been widely adopted for use in short rotation plantations in the Pacific Northwest for chips to make paper and for solid wood products. Nitrogen (N) can be one of the most limiting factors for growth and therefore precise N management is needed. In the present study, the physiological control of N use and a field-portable method of estimating N level of hybrid poplars were investigated in two greenhouse studies. Two hybrid poplar clones GR-4284 (Populus. deltoides × P. trichocarpa) and 57-276 (P. trichocarpa × P. deltoides) were selected based on a previous field study as plant materials in two experiments. In experiment 1, the clonal and physiological responses of 57-276 and GR-4284 to N were examined. The steady-state experimental technique with a customized semihydroponic system was used to provide nutrients. Two relative addition rates (RAR) 1.5% and 10%, were used to create low and high N treatments. Results showed clonal response to N. In comparison with GR-4284, clone 57-276 was more sensitive to low N, but more responsive when N increased. This response can be attributed to different