Non-deep physiological dormancy and germination characteristics of Primula florindae (Primulaceae), a rare alpine plant in the Hengduan Mountains of southwest China

Abstract

Timing of seed germination is directly related to the survival probability of seedlings. For alpine plants, autumn-dispersal seeds should not germinate immediately because the cold temperature is not conducive to the survival of seedlings. Seed dormancy is a characteristic of the seed that prevents it from germinating after dispersal. Primula florindae is an alpine perennial forb endemic to eastern Tibet, SW China. We hypothesized that primary dormancy and environmental factors prevent seeds of P. florindae to germinate in autumn and allow them to germinate at the first opportunity in spring. We determined how GA₃, light, temperature, dry after-ripening (DAR) and cold-wet stratification (CS) treatments affect seed germination by conducting a series of laboratory experiments. Firstly, the effects of gibberellic acid (GA₃; 0, 20, and 200 mg L^-1) on germination of freshly shed seeds at alternating temperatures (15/5 and 25/15 °C) were immediately investigated to characterize seed with a physiological dormancy component. Then, the fresh seeds treated with 0, 3, and 6 months of after-ripening (DAR) and cold-wet stratification (CS) were incubated at seven constant (1, 5, 10, 15, 20, 25, and 30 °C) and two alternating temperatures (5/1, 15/5, and 25/15 °C) at light and dark conditions. Fresh seeds were dormant, which only germinated well (>60%) at 20, 25, and 25/15 °C in light but not at ≤15 °C and to higher percentages in light than in dark. GA₃ increased germination percentage of fresh seeds, and DAR or CS treatments increased final germination percentage, germination rate (speed), and widened the temperature range for germination from high to low. Moreover, CS treatments reduced the light requirement for germination. Thus, after dormancy release, seeds germinated over a wide range of constant and alternating temperatures, regardless of light conditions. Our results demonstrated that P. florindae seeds have type 2 non-deep physiological dormancy. Timing of germination should be restricted to early spring, ensuring a sufficient length of the growing season for seedling recruitment. These dormancy/germination characteristics prevent seeds from germinating in autumn when temperatures are low but allow them to germinate after snowmelt in spring.

Keywords: Alternating temperature; Cold stratification; Dry after-ripening; GA3; Light requirement; Mean germination time.

Figure 1. Effect of GA₃ and light condition on germination of fresh seeds.

Germination percentages of fresh seeds of Primula florindae at two alternating temperatures (15/5 and 25/15 °C) in light and in dark in various concentrations of GA₃. 0 means the treatment has no germination. Different uppercase letters indicate significant differences (p < 0.05) across all concentrations of GA₃ for the same light condition and different lowercase letters significant differences between light conditions at the same concentration.

Figure 2. Effect of temperatures on cumulative germination of fresh seeds.

Cumulative germination percentages of fresh seeds in Lulang and Sejila seed populations at seven constant and three alternating temperatures under light conditions. Scatters represent standard error of the mean.

Figure 3. Effect of DAR treatments and light condition on seed germination at constant and alternating temperatures.

Final germination percentage (GP) and mean germination time (MGT) at each constant and alternating temperature with respect to dry after-ripened (DAR) period (Fresh 0, control; D3 and D6, DAR at room temperature for 3 and 6 months, respectively) in Lulang and Sejila seed populations under light and dark conditions. 0, no seeds germinated in this treatment. Bars and scatters represent standard error of the mean. Bars with different uppercase letters are differ significantly (p < 0.05) for GP at the same temperatures of the three DAR periods. Scatters with different lowercase letters are differ significantly (p < 0.05) for MGT at the same temperatures of the three DAR periods under light condition. The unmarked bars and scatters showed no significant difference.

Figure 4. Effect of CS treatments and light condition on seed germination at constant and alternating temperatures.

Final germination percentage (GP) and mean germination time (MGT) at each constant and alternating temperature with respect to cold stratification (CS) period (Fresh 0, control; C3 and C6, CS at room temperature for 3 and 6 months, respectively) in Lulang and Sejila seed populations under light and dark conditions. 0, no seeds germinated in this treatment. Bars and scatters represent standard error of the mean. Bars with different uppercase letters are differ significantly (p < 0.05) for GP at the same temperatures of the three CS periods. Scatters with different lowercase letters are differ significantly (p < 0.05) for MGT at the same temperatures of the three CS periods under light condition. The unmarked bars and scatters showed no significant difference.