Imbibition and germination of seeds of Colubrina oppositifolia (Rhamnaceae), a federal-endangered tree species endemic to Hawaii

Considerable variation in seed dormancy has been reported for Colubrina species. Most freshly-matured seeds of Colubrina oppositifolia have water-impermeable seed coats (physical dormancy). At 28(12 h)/20(12 h), 25(12 h)/15(12 h) °C and ca. 24°C (room temperatures), 99-100% of the seeds imbibed, and 96-100% of the seeds that germinated did so in 27-34 days. Daily weighing of individual seeds showed that start of imbibition (= 5% increase in mass) at room temperature ranged from < 1 to 20 days. Thus, at high temperature, physical dormancy is broken so rapidly that technically most seeds would be classified as nondormant (i.e. germinate in ≤ 30 days). At 15(12 h)/6(12 h) and 20(12 h)/10(12 h) °C, 93 and 96%, respectively, of the seeds had imbibed after 30 days, but only 12 and 50%, respectively, of the seeds that germinated did so in 30 days. Eleven months of dry storage at room temperatures increased rates of imbibition and germination of seeds incubated at 28/20 and 20/10°C. Based on the relatively rapid rates at which seeds become water-permeable and germinate when incubated on a moist substrate, we predict that the soil seed bank of this endangered species may be only short-lived. Thus, seeds would imbibe and germinate (or die) during the first wet season following their dispersal.