Dormancy Breaking and Germination Requirements of Jeffersonia Dubia

Jeffersonia dubia (Maxim.) Benth. et Hook. has been recognized as a promising horticultural crop which has attractive light purple flowers and heart-shaped leaves. However, seeds of this Korean endemic have proven to be very difficult to germinate. It takes a long time (about 9 months) for the seeds to germinate due to seed dormancy. For a horticultural use, a rapid germination is needed, but little information has been reported. We examined their embryo elongation and germination in regard to the effect of the temperature and gibberellic acid (GA₃). In incubators, seeds did not germinate at 25/15 °C (light 12 h/dark 12 h alternating temperature), 20/10 °C, 15/6 °C, or constant 5 °C with a 12-h light photoperiod for 12 months. Seeds germinated to 88% when incubated for 9 months with a sequence of 25/15 °C (warm stratification) → 20/10 °C → 15/6 °C → 5 °C (cold stratification) → 15/6 °C. In several laboratory experiments, embryos grew at 20/10 °C (light 12 h/ dark 12 h) following warm stratification (25/15 °C or 25 °C). Radicle emerged from seeds with fully elongated embryos at field conditions after cold stratification at 5 °C for 10 weeks. Thus, warm stratification followed by cold stratification is required for germination. GA₃ increased the embryo growth in seeds kept at constant 20 °C, but only 12.6 ± 1.8% of them germinated. Thus, GA₃ substitutes for warm but not cold stratification. However, seeds incubated at 15/6 °C or 20/10 °C for 8 weeks germinated to 74.2 ± 2.9% or 30.1 ± 5.9%, respectively,  after addition of 1,000 mg·L^-1 GA₃. Although seeds require 8-9 months for germination after dispersal in nature, it took only 2 months with the combination of alternative low temperature 15/6 °C and 1,000 mg·L^-1 GA₃. This will contribute to the rapid propagation of Jeffersonia dubia plants for commercial production.