Response of southern magnolia to supraoptimal root-zone temperatures

1. 1.|Rooted cuttings of southern magnolia (Magnolia grandiflora Hort. 'St. Mary') were exposed for 6 h daily to root-zone temperatures (RZT) of 28, 35 or 42°C for 8 weeks during the spring or fall of 1988. 2. 2.|Length of survival for 8-month old rooted cuttings was shortened when roots were exposed during the spring to 42 or 35°C compared to 28°C. Survival of 13-month old trees was unaffected by RZT treatments applied during the fall. 3. 3.|Electrolyte leakage from excised root tissue exposed for 30 min to temperatures ranging from 25 to 70°C, was used to assess cellular injury of 13-month old rooted cuttings after RZT treatments. 4. 4.|The critical killing temperatures (CT₅₀) of root tissue pretreated at 28, 35 or 42°C RZT were 52.5 ± 0.9°C, 54.0 ± 0.4°C, respectively, and indicated differences in root membrane thermostability. 5. 5.|In another experiment, histological and scanning electron microscopy studies showed the proliferation of cells that was probably a callus response to temperature-mediated root injury induced by 38 or 42°C RZT after 20 or 8 days, respectively.