Abstract

Seeds of Rhododendron catawbiense Michx. (Catawba rhododendron) and Rhododendron maximum L. (rosebay rhododendron) were germinated at 25C (77F) or an 8/16 hr thermoperiod of 25/15C (77/59F) with constant light after imbibed seeds were maintained in total darkness for 0, 9, 18, 27, 36, 45, 54 or 63 days at the same temperatures. Maintenance of imbibed seeds of R. catawbiense in darkness at 25C (77F) for up to 63 days caused no induction of secondary dormancy while induction occurred for seeds in darkness at 25/15C (77/59F). When imbibed seeds of R. catawbiense were subjected immediately to light following imbibition, 30-day germination at 25C (77F) was 98% compared to 95% for imbibed seeds maintained in darkness for 63 days and then exposed to light. If germinated at 25/15C (77/59F), immediate light exposure resulted in 99% germination which decreased significantly to 76% after 63 days of dark treatment. Seeds of R. maximum maintained in darkness developed secondary dormancy at both temperatures. Thirty day germination of seeds subjected immediately to light following imbibition at 25C (77F) was 82% which decreased to 29% after dark treatment for 9 days. Further reductions in germination continued as the length of dark treatment increased with < 10% germination after maintenance in darkness for 27 days. At 25/15C (77/59F) induction of secondary dormancy was not as dramatic as that at 25C (77F). Without dark treatment, 30-day germination at 25/15C (77/59F) was 99% which decreased significantly to 88% after dark treatment for 18 days. Reductions in germination continued up to 63 days with 67% germination. Partial removal of secondary dormancy in seeds of R. maximum was achieved by subjecting seeds to moist-chilling.