Fraxinus chinensis subsp. rhynchophylla (Oleaceae), hereafter F. rhynchophylla, is an important timber species in northeast China; however, little is known about its seed dormancy and germination, which hinders regeneration of the species from seeds for reforestation and conservation. Our aim was to determine the class of seed dormancy and how to break it. Studies were conducted to determine the permeability of the seed coat to water, changes in embryo development during cold stratification and effects of cold stratification on germination over a range of temperatures. The seeds were water-permeable, and the embryo was fully developed and filled the embryonic cavity. Cold stratification at 5 °C for 8 weeks was effective in breaking dormancy; thus, we conclude that the seeds have nondeep physiological dormancy (PD). As cold stratification time was increased, the ability of seeds to germinate at low temperatures (e.g., 10 °C and 15 °C) increased, indicating the presence of Type 2 nondeep PD, in which the minimum temperature for germination decreases during dormancy-break. Nondormant seeds germinated to high percentages and rates at constant temperatures of 25 °C (germination percentage was 63%) and at alternating temperature regimes of 35 °C/25 °C, 30 °C/15 °C, 25 °C/20 °C and 20 °C/10 °C (germination percentage was 66%, 67%, 65% and 66%, respectively). To produce seedlings, we recommend 8 weeks of cold stratification at 5 °C before sowing the seeds at temperatures ranging from 15 °C to 30 °C.

To determine the optimal germination temperature for Pyrus ussuriensis seeds and whether they experienced the phenomenon of thermodormancy and its inciting factors, several germination tests were conducted using non-dormant P. ussuriensis seeds for comparison. The results showed that the highest germination rate of P. ussuriensis seeds was reached at a constant temperature of 5 °C and variable temperature (night/day) of 5 °C/10 °C. Constant temperatures of 25 °C for three days induced thermodormancy, triggering significant drops in seeding emergence. Thermodormancy was related to the inhibitory effect of endogenous substances in the seed coat and an elevated abscisic acid concentration. The embryo, by contrast, remained non-dormant. Thermodormant and non-dormant seed embryos showed higher germination rates than dormant seed embryos when applied exogenous abscisic acid and gibberellic acid. We found that P. ussuriensis seeds showed thermodormancy; thus, during early spring sowing, high temperatures should be avoided to prevent low seed germination capacity. Additionally, applying exogenous gibberellic acid, shading and increasing soil moisture can be helpful to enhance the species seed germination.