Root Growth of Southern Magnolia Following Exposure to High Root-zone Temperatures

Abstract

Root growth of southern magnolia (Magnolia grandiflora Hort. `St. Mary') was studied for 16 weeks after an 8-week exposure to 30, 34, 38, or 42 ± 0.8C root-zone temperature (RZT) treatments applied for 6 hours daily. Immediately after RZT treatments, total root length of trees responded negatively to increased RZT in a quadratic pattern and the shoot and root dry weight of trees was similar. However, 8 and 16 weeks after RZT treatments, total root length responded linearly in a negative pattern to increased RZT, and shoot and root dry weight responded negatively to increased RZT in a linear and quadratic pattern, respectively. Root dry weight of trees exposed to 42C RZT treatment was 29% and 48% less than 38 and 34C RZT treatments, respectively, at week 8. By week 16, root dry weight as a function of RZT had changed such that the 42C RZT was 43% and 47% less than 38 and 34C RZT, respectively. Differences in root growth patterns between weeks 8 and 16 suggest that trees were able to overcome the detrimental effects of the 38C treatment, whereas growth suppression by the 42C treatment was still evident after 16 weeks.