Dormancy-release and germination improvement of Korean bellflower (Campanula takesimana Nakai), a rare and endemic plant native to the Korean peninsula

Abstract

Korean bellflower (Campanula takesimana Nakai) is a rare and perennial herb with medicinal and ornamental values, is endemic to the Ulleung Island of Korea. In this study, we investigated the dormancy-release and germination characteristics of C. takesimana (Campanulaceae) seeds by subjecting them to varying temperatures (5, 10, 15, 20, and 25°C and diurnal/nocturnal temperatures of 15/6, 20/10, and 25/15°C), cold stratification periods (0, 4, 8, or 12 weeks at 5°C), and gibberellic acid (GA3) concentrations (0, 10, 100, or 1,000 mg·L-1 at 15/6°C and 25/15°C) to identify the ideal seed propagation conditions. The seeds were stimulated to germinate (at 25°C, 12-h photoperiod with fluorescent lamps at 40 ± 10 μmol∙m-2∙s-1) after cold stratification. To examine the germination characteristics, the seeds were tested for water imbibition and found to readily absorb water. The seeds exhibited underdeveloped embryos during dispersal, showed final germination of 37.00% ± 4.43 at 25°C and were not influenced by temperature. The seeds subjected to 0, 4, 8, or 12 weeks of cold stratification germinated at a success rate of 22.00% ± 4.76, 87.00% ± 6.80, 79.00% ± 2.52, and 77.00% ± 1.91, respectively. Additionally, the germination characteristics, which were based on final germination, mean germination time, and germination velocity (Timson index), were significantly greater in the seeds pretreated with 1,000 mg·L-1 GA3 at 25/15°C than in seeds pretreated with 0 mg·L-1 GA3. Overall, the seeds broke dormancy with GA3 and short-term cold stratification. Therefore, we concluded that C. takesimana seeds have non-deep, simple, morphophysiological dormancy, and pretreatment with cold stratification and GA3 is required for effective seed propagation.

Fig 1. Percentage increase in mass of Campanula takesimana seeds during water incubation.

The seeds were incubated at ambient temperature (approximately 23 ± 2°C) on filter paper moistened with distilled water for 96 h. The vertical bars represent the standard deviation from the mean (n = 4). Different letters in the same column indicate significant differences based on Tukey’s HSD test (p ≤ 0.05). *** means significant at p ≤ 0.001.

Fig 2. Morphology and anatomy of Campanula takesimana seed and embryo.

(A) Seed surface, (B) longitudinal section of the underdeveloped embryo at the dispersal stage, (C) longitudinal section of the fully developed embryo at the germination stage, (D-E) seed photographed using a scanning electron microscope (SEM), and (F) Embryo/seed (E:S) ratio in the seeds of Campanula takesimana at seed dispersal and just before germination stages. The vertical bars represent the standard deviation from the mean (n = 10). Each E:S ratio at dispersal and just before germination stages was compared using a paired t-test. *** Significant at p ≤ 0.001.

Fig 3. Effects of constant temperature (5, 10, 15, 20, and 25°C) and fluctuating temperature regimes (15/6, 20/10, and 25/15°C) on the cumulative germination of Campanula takesimana seeds after 7 weeks of incubation.

The vertical bars represent the standard deviation from the mean (n = 4). Different letters in the same column indicate significant differences based on Tukey’s HSD test (p ≤ 0.05).

Fig 4. Effects of cold stratification pretreatment (0, 4, 8, or 12 weeks at 5°C) on the cumulative germination of Campanula takesimana seeds under 4 weeks of incubation.

Seeds were incubated at 25°C after the stratification treatment. The vertical bars represent the standard deviation from the mean (n = 4). Different letters in the same column indicate significant differences based on Tukey’s HSD test (p ≤ 0.05).

Fig 5

Effects of GA₃ pretreatment (0, 10, 100, or 1000 mg·L^-1) and two temperature regimes (15/6 and 25/15°C) on germination (A, B); mean germination time (C, D); and Timson index (E, F) of Campanula takesimana seeds under seven weeks of incubation. Vertical bars represent the standard deviation from the mean (n = 4). Different letters in the same column indicate significant differences based on Tukey’s HSD test (p ≤ 0.05). NS, *, **, and *** indicate no statistically significant difference and significant difference at p < 0.05, 0.01, and 0.001, respectively.