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Article

Physiological Dormancy and Germination Characteristics of Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. (Asparagaceae)

by
Un-Seop Shin
,
Da-Hyun Lee
,
Young-Ho Jung
,
Jun-Hyeok Kim
and
Chae-Sun Na
*
Division of Wild Plant Seed Research, Baekdudaegan National Arboretum, Bonghwa 36209, Korea
*
Author to whom correspondence should be addressed.
Horticulturae 2022, 8(11), 1057; https://doi.org/10.3390/horticulturae8111057
Submission received: 17 October 2022 / Revised: 2 November 2022 / Accepted: 7 November 2022 / Published: 10 November 2022
(This article belongs to the Collection Seed Dormancy and Germination of Horticultural Plants)
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Abstract

:
This study was conducted to elucidate the germination characteristics and dormancy types in Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. M. dilatatum seeds were collected from the Nari Basin on Ulleungdo Island, South Korea and used in a water imbibition test; the seed weight increased by approximately 20% within 48 h. The embryo-to-seed ratio at the time of seed dispersal was 0.57 ± 0.03. When the seeds were incubated under light and dark conditions, they germinated under dark conditions, and the germination rate was higher at 25 °C than at 20 °C. However, the final germination rates under dark conditions were 27.4 ± 3.6 and 47.1 ± 7.6%, respectively. Under 4, 8, 12, and 16 weeks of cold stratification treatment, the highest germination rate of 83.6 ± 3.6% was observed at 25 °C at 12 weeks of cold stratification treatment, the germination start date was decreased by more than 10 d, and the mean germination time (MGT) was shortened from 50 d to 39 d. However, the germination rate, germination start date, and MGT did not differ significantly among the gibberellic acid hormone treatments. Therefore, it was evident that M. dilatatum demonstrated physiological dormancy (PD) that can broken by cold stratification treatment and dark conditions.

1. Introduction

Biodiversity issues are becoming more important over time, and for this, not only in situ conservation, but also ex situ conservation and the development of propagation technologies are required. Seeds have a critical role in ex situ conservation of plant genetic resources [1,2,3]. The Kew Royal Botanical Garden in the UK has been compiling a seed information database (SID) by researching the storage characteristics and vitality of seeds. The International Seed Testing Association (ISTA) is active in various fields, ranging from quality management of seeds to quality checks for distribution, designation of quality certification laboratories by country, and education [4]. In Korea, the ISTA has approved the Korea Seed & Variety Service (KSVS) to issue a seed analysis certificate [5]. However, quality management standards for wild plant seeds are insufficient because only selected species of grasses, food, legumes, other crops, vegetables, and flowers are targeted. Therefore, the seed bank of the National Baekdudaegan Arboretum is developing a test method by studying dormancy types and breaking methods for each species to test the vitality of the collected wild plant seeds. Continuous research on seed germination and growth is required in order to safely store seeds in seed banks and effectively use the stored seeds [6].
Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. (Asparagaceae) are perennial herbs that grow in the lower parts of deciduous forests in alpine areas. In Korea, it is native to Ulleungdo and is distributed in Japan, China, Russia, and the United States. The white flowers of the raceme are beautiful, and the red fruits are also of ornamental value (nature; KPNI) [7]. According to the “Act on the Conservation and Utilization of Biological Diversity”, it is an important species that requires approval when exported abroad for biodiversity conservation [8]. Furthermore, it is a rare, least-concern (LC) perennial species in Korea [7]. These important species require in situ and ex situ conservation measures through the continuous monitoring and proliferation of their habitats.
As a survival strategy, seeds are dormant and inhibit germination in an environment unsuitable for growth [9,10]. Several mechanisms, including temperature, moisture, humidity, and light, are involved in seed germination, which is critical for seed dormancy [11]. Seed dormancy is broadly classified into five categories: physical dormancy (PY) due to impermeability of the seed coat, morphological dormancy (MD) due to immature embryos, physiological dormancy (PD) due to physiological factors of the seed, morpho–physiological dormancy (MPD), and combinational dormancy (PY + PD) in which two or more dormancy types occur simultaneously [12]. Dormancy type in M. racemosa was classified as deep simple double morphophysiological [13,14], M. bifolium was classified as MPD, and dormancy was broken through cold stratification treatment [15]. MPD is a common dormancy type, especially in East Asia and western North America [16,17,18,19,20,21,22,23]. Dormancy is overcome through warm or cold stratification treatment or a combination of both treatments [24,25,26,27]. Therefore, this study was conducted to establish genetic resource conservation and propagation technology by confirming that the seed dormancy type of M. dilatatum is MPD, and the development of additional dormancy breaking and germination promotion methods. In brief, we investigated the response of M. dilatatum seeds to light and temperature conditions with the aim of confirming seed germination characteristics. Moreover, we examined the effects of cold stratification and gibberellic acid (GA3) on seed germination improvement.

2. Materials and Methods

2.1. Plant Material

The M. dilatatum seeds used in this experiment were collected from the Nari Basin, Ulleungdo, on 15 October 2020. After collection, the seeds were dried at room temperature for approximately two weeks and then used for experiments.

2.2. Basic Characteristics

To understand the internal and external morphology of the seeds, photographs and size measurements were recorded using a stereo microscope (DVM6; Leica Microsystems GmbH, Wetzlar, Germany). Whether the inside of the seed was full was determined using an X-ray device (EMT-F70; Softex, Tokyo, Japan), and the filled rate was calculated as (number of seeds full inside/total number of seeds inspected) × 100. To determine the moisture content of seeds, the change in seed weight was measured after drying for 17 h in a dryer at 103 °C [4]. The moisture content was calculated as follows: [(initial seed weight − dried seed weight)/initial seed weight] × 100.

2.3. Water Imbibition Test and Embryo Growth

Water imbibition tests were performed to determine whether the seed coat was impermeable. Two pieces of filter paper were laid on a 90 × 15 mm petri dish, distilled water was sufficiently injected, and then seeds were placed in four replicates of 25 seeds. The water absorption rate was calculated by measuring the weight of embedded seeds after 0, 3, 6, 9, 12, 48, and 72 h. The water absorption rate was calculated as Ws% = [(Wh − Wi)/Wi] × 100 [28], where Ws = percentage increase in seed weight, Wh = weight of water imbibed seed, and Wi = initial seed weight. To check whether the embryo was further elongated, embryo length and seed length were measured just before germination. From the measured length of the seed and embryo, the embryo-to-seed (E:S) ratio was calculated.

2.4. Effects of Light and Temperature

Seeds were exposed under a light/dark environment treatment of 12/12 h; for dark condition treatment, the seeds were wrapped in aluminum foil. The experiments were performed under two constant temperature conditions of 20 and 25 °C [29]. Four replicates, with 25 seeds per condition, were sown on the surface of 1% agar medium, and the germination rate was monitored for three months. Germination was defined as a protrusion of more than 2 mm. The germination rate was calculated as follows: (number of germinated seeds/total number of seeds sown) × 100. Rotten seeds were excluded from the germination rate analysis.

2.5. Effects of Cold Stratification Treatment

Cold stratification was carried out for 0, 4, 8, 12, and 16 weeks to investigate the dormancy-breaking and germination-promoting effects of this treatment. For the cold stratification treatment, seeds were wrapped in a paper towel with sufficient moisture and stored in a refrigerator at 4 °C. After cold stratification, the samples were treated under dark conditions at 20 and 25 °C. The germination rate was monitored for three months in the same manner as described above. In addition, the effects on germination start date and mean germination time (MGT) were compared. The average number of days required to germinate was calculated using MGT (days) = ∑(Ti × Ni)/N [30], where Ti is the number of days from the start of the experiment to the day of germination, Ni is the total number of days from the start of the experiment, and N is the total number of germinated seeds.

2.6. Effects of Gibberellic Acid Treatment

To investigate the dormancy-breaking and germination-promoting effects of GA3, hormone treatment was applied at concentrations of 0, 100, 250, and 500 mg·L−1. GA3 was directly injected into the 1% agar medium, and the temperature was maintained at 20 and 25 °C. This experiment was performed under dark conditions and the germination rate, germination start date, and MGT were determined for three months.

2.7. Statistical Analysis

For statistical processing and analysis, SAS 9.4 (SAS Inst. Inc., Cary, NC, USA) and SigmaPlot 10.0 (SPSS Inc., Chicago, IL, USA) were used. For comparison between the mean values of each germination test, treatment results were tested with Tukey’s HSD test (p < 0.05), and the t-test was used to compare the E:S ratio at the time of detachment and immediately before germination.

3. Results and Discussion

3.1. Basic Characteristics

Seeds of M. dilatatum were round in shape with a length and width of 3.38 ± 0.08 and 3.54 ± 0.04 mm, respectively. The initial moisture content of the seeds was 9.22 ± 0.47%, and the seeds were 100% full (Table 1).

3.2. Water Imbibition Test and Embryo Growth

In the water imbibition experiment, seed weight increased by nearly 20% after 48 h (Figure 1). Dormancy caused by impermeability of the seed coat or pericarp is classified as PY [31], and the seed is judged to possess water permeability when the amount of water absorbed is more than 20% of the initial weight of the seed [32]. It was determined that there was no PY in the seeds of M. dilatatum, as seed weight increased by 20% compared to the initial weight of the seeds after water absorption. Also, microscopic observation revealed a fully developed linear embryo inside the seed (Figure 2), and the E:S ratio at the time of detachment was 0.57 ± 0.03 (data not shown). In general, when the E:S ratio is >0.5, the embryo is regarded as fully developed inside the seed [33]. No additional embryo elongation was observed when the E:S ratios at the time of detachment and immediately before germination were compared. Therefore, it was judged that there was no MD due to undeveloped embryos, as there was no additional elongation of the embryo inside the M. dilatatum seeds.

3.3. Effects of Light and Temperature

The seeds exhibited almost no germination under light conditions (data not shown); final germination rates under dark conditions were 27.4 ± 3.6% and 47.1 ± 7.6% at 20 and 25 °C, respectively (Figure 3). There was a significant difference in the effects of both light and temperature, and the correlation between light and temperature was not significant (Table 2). Therefore, light and temperature seem to affect M. dilatatum seed germination. M. dilatatum appeared to be particularly affected by the presence or absence of light, and a germination temperature of 25 °C was appropriate. The start time for germination of M. dilatatum seeds was greater than 30 days in the untreated group (Figure 4). Generally, if germination is delayed for more than 30 days, even though the embryo is fully grown at the time of detachment, it is classified as PD [9]. Therefore, M. dilatatum appears to be a PD species. According to Nikolaeva [26,34], although PD seeds are permeable, the appearance of roots in the embryo can be suppressed by a physiological mechanism. Baskin and Baskin [35] classified plants of the genus Maianthemum as MPD [36].

3.4. Effects of Cold Stratification Treatment

Cold stratification treatment was performed for 0, 4, 8, 12, and 16 weeks, and the germination rates were 27.4 ± 3.6%, 52.7 ± 6.7%, 68.0 ± 6.6%, 74.2 ± 6.9%, and 73.6 ± 6.1%, respectively, at 20 °C. The germination rates of the treated group were more than twice that of the untreated group and showed a tendency to increase (Figure 5) with an increase in treatment duration. Even at 25 °C, the germination rate increased as the duration of cold stratification treatment increased, and the germination rates observed were: 47.1 ± 7.6%, 70.1 ± 6.0%, 67.0 ± 2.4%, 83.6 ± 3.6%, and 78.4 ± 6.0%, at 0, 4, 8, 12, and 16 weeks, respectively. In the case of the cold stratification treatment, temperature and treatment period affected the germination rate, but there was no correlation (Table 2). In particular, the effect of the cold stratification treatment period was evident. In the absence of cold stratification treatment, germination started after more than a month and start days at 20 and 25 °C were 39.0 ± 2.6 d and 37.5 ± 2.8 d, respectively (Figure 4). However, in the case of cold stratification treatment, germination started rapidly and lasted more than 10 days when subjected to four weeks of treatment. The MGT also took more than 50 days when subjected to four weeks of cold stratification treatment at 25 °C, but was shortened to 39 days after eight weeks of treatment. In Japan, the germination rate was reported to be more than 80% when subjected to cold stratification treatment for more than four months and when cultured for more than six months [29]. In contrast, M. dilatatum native to Korea showed a germination rate of more than 80%, even after cold stratification treatment for only two months. At the same temperature, the germination rate was the highest at 25 °C, and the seeds germinated well under dark conditions. In the case of Penthorum chinense Pursh (Penthoraceae), the germination rate improved under the condition of alternate temperatures; in Korean and Japanese native plants, improved germination was observed when this difference was 16 and 10 °C, respectively [37]. Regional differences may even occur within the same species.

3.5. Effects of Gibberellic Acid Treatment

In the GA3 treatment, germination rate, germination start date, and germination energy did not show any significant differences between treatments. When treated with 0, 100, 250, and 500 mg·L−1 concentrations, the germination rates were 27.4 ± 3.6%, 31.2 ± 6.5%, 29.9 ± 3.9%, and 17.4 ± 1.5% at 20 °C, respectively, and 47.1 ± 7.6%, 41.3 ± 9.5%, 41.3 ± 9.4%, and 38.6 ± 10.8% at 25 °C, respectively (Figure 6). The germination start dates were 39.0 ± 2.6 d, 38.5 ± 3.0 d, 41.0 ± 1.4 d, and 45.0 ± 2.7 d at 20 °C, respectively, and 37.5 ± 2.8 d, 35.5 ± 3.1 d, 37.3 ± 2.6 d, and 36.3 ± 2.8 d at 25 °C, respectively (Figure 7). The MGT was statistically significant between the temperatures, but the relationship with hormone concentrations was not consistent; therefore, it was judged that there was no effect of hormone treatment.

3.6. Effect of Combined Cold Stratification and Gibberellic Acid Treatment

When the cold stratification treatment and GA3 application were combined, the germination rates were 73.6 ± 6.1%, 85.0 ± 1.1%, 73.4 ± 6.4%, and 81.9 ± 2.0% at 20 °C (even at 16 weeks), and at 25 °C, the germination rates were 78.4 ± 6.0%, 85.7 ± 4.1%, 91.4 ± 3.8%, and 80.1 ± 1.2% (Figure 8). The longer the cold stratification treatment duration, the higher the overall germination rate; however, there was no difference in the effects of hormone concentration. The correlation between the cold lamination treatment and hormone levels was not significant (Table 2). The germination start date and MGT also showed similar results (data not shown).

4. Conclusions

Physiological dormancy is the most common dormancy type in temperate regions and can be broken by light conditions, cold stratification treatment, and hormone treatment [38]. It is further classified into deep, intermediate, and non-deep PD, according to the strength of the PD [26]. Deep PD-type dormancy cannot be overcome with GA3 treatment, but it can be overcome with 3–4 months of cold or warm temperature stratification treatment. Intermediate PD is limited to some species to break dormancy through GA3 treatment, and it is mainly possible with cold stratification treatment for 2–3 months [12]. Non-deep PD is the most common type of dormancy, and it is possible to break the dormancy of these seeds using all treatments, such as 1–2 months of cold stratification, post-ripening treatment, and GA3 treatments [12,38]. In this study, the dormancy type was classified as intermediate PD because the dormancy of M. dilatatum seeds was broken under dark conditions and cold stratification treatments lasted more than eight weeks. In conclusion, there was no PY because the seeds of M. dilatatum absorbed nearly 20% of water within 48 h, and there was no MD because the seeds had a mature embryo at the time of detachment. Our findings suggest that dormancy in M. dilatatum can be classified as PD, especially intermediate PD. Although some species of the same genus have been classified as MPD [35], the dormancy type of M. dilatatum was concluded to be PD. These results will serve as basic data for seeds of native plants; such data are still insufficient, and will be important for future use.

Author Contributions

Conceptualization, U.-S.S. and C.-S.N.; methodology, U.-S.S. and C.-S.N.; formal analysis, U.-S.S.; investigation, U.-S.S.; data curation, U.-S.S.; writing—original draft preparation, U.-S.S.; writing—review and editing, U.-S.S. and C.-S.N.; resources, U.-S.S., D.-H.L., Y.-H.J. and J.-H.K.; project administration, C.-S.N.; funding acquisition, C.-S.N.; supervision, C.-S.N. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the R&D Program for Forest Science Technology (Project No. 2021399B10-2225-CA02 and No. 2021400B10-2225-CA02) of the Korea Forest Service (Korea Forestry Promotion Institute).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Changes in the mass of the Maianthemum dilatatum seeds during water imbibition test. Error bars indicate mean ± S.E. (n = 4).
Figure 1. Changes in the mass of the Maianthemum dilatatum seeds during water imbibition test. Error bars indicate mean ± S.E. (n = 4).
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Figure 2. Microscopic observation of seeds: (A) outside and (B) inside at dispersal stage; (C) outside and (D) inside at germination stage of Maianthemum dilatatum. (Scale bar = 1 mm).
Figure 2. Microscopic observation of seeds: (A) outside and (B) inside at dispersal stage; (C) outside and (D) inside at germination stage of Maianthemum dilatatum. (Scale bar = 1 mm).
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Figure 3. Effect of light and temperature on germination of Maianthemum dilatatum. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05). *** is significant at p ≤ 0.001.
Figure 3. Effect of light and temperature on germination of Maianthemum dilatatum. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05). *** is significant at p ≤ 0.001.
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Figure 4. Effect of cold stratification treatment on mean germination time and start germination time in Maianthemum dilatatum. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05).
Figure 4. Effect of cold stratification treatment on mean germination time and start germination time in Maianthemum dilatatum. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05).
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Figure 5. Effect of cold stratification treatments (0, 4, 8, 12, or 16 weeks at 5 °C) on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05). *** is significant at p ≤ 0.001.
Figure 5. Effect of cold stratification treatments (0, 4, 8, 12, or 16 weeks at 5 °C) on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05). *** is significant at p ≤ 0.001.
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Figure 6. Effect of GA3 treatments (0, 100, 250, or 500 mg·L−1) on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant.
Figure 6. Effect of GA3 treatments (0, 100, 250, or 500 mg·L−1) on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant.
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Figure 7. Effect of GA3 treatments on mean germination time and start germination time in Maianthemum dilatatum. Seeds were incubated at 20 °C; (A) and 25 °C; (B) for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant.
Figure 7. Effect of GA3 treatments on mean germination time and start germination time in Maianthemum dilatatum. Seeds were incubated at 20 °C; (A) and 25 °C; (B) for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant.
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Figure 8. Effect of cold stratification treatments and GA3 treatments on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant. Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05).
Figure 8. Effect of cold stratification treatments and GA3 treatments on Maianthemum dilatatum seed germination. Seeds were incubated at (A) 20 °C and (B) 25 °C for three months. Error bars indicate mean ± S.E. from four replicates (n = 25). NS is not significant. Different letters in the same column indicate significant differences based on Tukey’s multiple range test (p < 0.05).
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Table
Table 1. Maianthemum dilatatum seeds’ basic information.
Table 1. Maianthemum dilatatum seeds’ basic information.
NameLength
(mm)		Width
(mm)		Moisture Contents (%)Filled Rate
(%)
Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr.3.38 ± 0.083.54 ± 0.049.22 ± 0.47100.0 ± 0.0
Table
Table 2. Results of a two-way ANOVA applied to germination percentages of Maianthemum dilatatum seeds affected by light, temperature, cold stratification, or GA3 treatments. (GA3 was applied only under 25 °C conditions).
Table 2. Results of a two-way ANOVA applied to germination percentages of Maianthemum dilatatum seeds affected by light, temperature, cold stratification, or GA3 treatments. (GA3 was applied only under 25 °C conditions).
TreatmentsSignificance
Control
Light***
Temperature*
Light-TemperatureNS
Cold stratification
Duration***
Temperature*
Duration-TemperatureNS
GA3
ConcentrationNS
Cold stratification***
Concentration-Cold stratificationNS
NS, not significant; * and *** are significant at p < 0.05 and 0.001, respectively.
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Shin, U.-S.; Lee, D.-H.; Jung, Y.-H.; Kim, J.-H.; Na, C.-S. Physiological Dormancy and Germination Characteristics of Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. (Asparagaceae). Horticulturae 2022, 8, 1057. https://doi.org/10.3390/horticulturae8111057

AMA Style

Shin U-S, Lee D-H, Jung Y-H, Kim J-H, Na C-S. Physiological Dormancy and Germination Characteristics of Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. (Asparagaceae). Horticulturae. 2022; 8(11):1057. https://doi.org/10.3390/horticulturae8111057

Chicago/Turabian Style

Shin, Un-Seop, Da-Hyun Lee, Young-Ho Jung, Jun-Hyeok Kim, and Chae-Sun Na. 2022. "Physiological Dormancy and Germination Characteristics of Maianthemum dilatatum (A. W. Wood) A. Nelson and J. F. Macbr. (Asparagaceae)" Horticulturae 8, no. 11: 1057. https://doi.org/10.3390/horticulturae8111057

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