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Seed Dormancy and Veronicastrum sibiricum: Comparison
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Please note this is a comparison between Version 2 by Vivi Li and Version 1 by SEUNG YOUN LEE.

Veronicastrum sibiricum is a perennial species distributed in Korea, Japan, Manchuria, China, and Siberia. This study aimed to determine the requirements for germination and dormancy break of V. sibiricum seeds and to classify the kind of seed dormancy. Additionally, its class of dormancy was compared with other Veronicastrum and Veronica species. V. sibiricum seeds were permeable to water and had a mature embryo during seed dispersal. In field conditions, germination was prevented by physiological dormancy, which was, however, relieved by March of the next year, allowing the start of germination when suitable environmental conditions occurred. In laboratory experiments, the seeds treated with 0, 2, 4, 8, and 12 weeks of cold stratification (4 °C) germinated to 0, 79, 75, 72, and 66%, respectively. After the GA3 treatment (2.887 mM), ≥90% of the seeds germinated during the four incubation weeks at 20/10 °C. Thus, 2.887 mM GA3 and at least two weeks at 4 °C were effective in breaking physiological dormancy and initiating germination. Therefore, the V. sibiricum seeds showed non-deep physiological dormancy (PD).

  • cold stratification
  • physiological dormancy
  • seed dormancy
  • seed germination
  • trait stasis
  • Veronica
  • Veronicastrum

1. Introduction

The most important role of seeds is to keep a species in existence. Accordingly, plants have evolved various strategies to ensure successful germination of seeds [1]. Germination is the first step in plant life history. The success of seed germination and seedling establishment can affect features for the propagation of plant species, which are of both economic and ecologic importance [2].
Veronicastrum sibiricum, which is endemic to Northeast Asia, is distributed in the central and northern parts of the Korean Peninsula, Far East Russia, Northeast China, Mongolia, and Northern Japan [3]. V. sibiricum is a perennial herb with a height of 50–90 cm, long, oval leaves, and light purple flowers in racemes blooming from July to August [3]. The species belongs to the Scrophulariaceae Juss., which includes approximately 220 genera and 4000 species, with 68 species in 25 genera distributed in South Korea. There are two forms of the same species of Veronicastrum, V. sibiricum and V. sibiricum f. albiflora T. Yamaz in Korea [4]. The species of Veronicastrum and its closely related genus Veronica are widely distributed in the Northern Hemisphere, and in several regions in the Southern Hemisphere [5]. Both Veronicastrum and Veronica are morphologically closely related. Veronicastrum flowers have short calyx lobes and long corolla tube, whereas Veronica flowers have long calyx lobes and short corolla tube [6]. The phylogenetic analysis based on DNA sequences, seed microstructure [7], and pollen [8] indicate similarity between the two genera.
Traditionally, in South Korea, V. sibiricum roots have been used to treat neuralgia, arthritis, and inflammation and the young shoots are used as edible herbs [4]. Pharmacological studies have indicated the presence of compounds, such as isoferulic acid and 3,4-dimethoxy cinnamic acid, which have anti-inflammatory and analgesic properties [9], diterpene, which has antioxidant and anticancer properties [10,11][10][11], and iridoids for treating common cold, leucorrhea, cystitis, and liver [12].
Seed dormancy is a survival strategy, wherein germination is blocked under favorable environmental conditions [13,14][13][14]. Global data on seed dormancy and germination of 5250 species in major vegetation zones indicated that seeds of 69.6% of the species are dormant when freshly matured [15]. Strategies to break seed dormancy differ between species [16]. The seeds of many native plants in temperate regions have relatively small immature embryos [17]. If embryo elongation and germination stages in such immature embryos are achieved within 30 d under environmental conditions favorable for germination, the dormancy type is classified as morphological dormancy (MD) [13]. If there is an additional dormancy mechanism that inhibits the germination of immature embryos, the dormancy type is classified as morphophysiological dormancy (MPD). On the other hand, if a germination-inhibiting mechanism is added to seeds with mature embryos, the dormancy type is classified as physiological dormancy (PD) [13,15][13][15].
Choi [18] and Martinez-Ortega and Rico [19] determined that the genera Veronica and Veronicastrum are closely related based on the phylogeny results. Furthermore, the seeds of Veronica parnkalliana, a species native to South Australia, have immature embryos and show MPD, but 80–90% of the seeds germinated after gibberellic acid (GA) treatment [20]. Fifteen species of Veronica were classified as having PD, and V. biloba and V. wormskjoldii were found to have non-dormant seeds [13]. Song et al. [21] reported that eight species of Veronica native to the Korean Peninsula had MD or MPD. Some closely related species show the same dormancy type (trait stasis; [22]), whereas some exhibit significant differences in the type of depth of dormancy (trait divergence; [23]). This implies that during adaptation to different environments, dormancy patterns are either preserved or changed. Breaking seed dormancy and different germination traits are ecological characteristics and physiological control mechanisms of plants [22].

2. Internal and External Seed Morphology

V. sibiricum seeds were light yellow and less than 1 mm in length (Table 1). The initial and prior to germination E:S ratios were 0.84 and 0.91, respectively (Figure 1), with no significant difference between the two E:S ratios (Figure 1).
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Figure 1. Embryo to seed length ratio (E:S ratio) at initial stage and seed coat split stage of V. sibiricum. Error bars are mean ± standard error (SE) of ten replicates. The NS represents no-significant differences as determined by paired t-test.
Table 1. Seed collection conditions and basic characteristics of V. sibiricum.
Scientific Name Collection Seed Length

(mm)		 Seed Width (mm) 100 Seeds Weight (mg) Collection Date
Veronicastrum sibiricum Collections of live specimens from genetic resources plot, Andong National University, Korea 0.75 ± 0.010 z 0.05 ± 0.010 z 7.74 ± 0.200 y 16 October 2019
-x - - 14 September 2020
z mean ± SE (n = 10). y mean ± SE (n = 3). x not measured.

3. Water Absorption Rate

Seed mass increased by more than 20% in 2 h compared to the initial weight, and increased by more than 30% after 24 h (Figure 2).
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Figure 2. Water uptake by V. sibiricum seeds incubated at 21–26 °C on filter paper moistened with distilled water for 0–48 h. Error bars indicate mean ± SE of four replicates.

4. Phenology of Germination

This experiment was conducted to describe the phenology of germination from seeds kept under field conditions. The first germination event occurred on 26 March 2021 at daily average maximum and minimum temperatures of 13.1 °C and 4.5 °C, respectively. In the next two weeks, until 9 April 2021, 95% of the seeds germinated. Thus, in the natural environment, the seeds germinated about five months after reaching maturity (Figure 3). The time when seeds germinate is affected by the soil moisture content in natural conditions. In this experiment, there was little rain at the end of March, but there was a sufficient amount of rain at the beginning of April. For this reason, germination could rapidly increase in early April.
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Figure 3. Phenology of germination of V. sibiricum seeds. The phenology experiment was conducted from 16 October 2020 to 16 April 2021. Vertical bars represent mean ± SE of four replicates.

5. Effect of Light and Temperature on Germination

The germination test showed that only 10% of the seeds germinated at 25/15 °C under light conditions after 4 weeks of incubation, but they did not germinate at 4, 15/6, and 20/10 °C in light or dark (Figure 4).
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Figure 4. Germination of V. sibiricum seeds as affected by light conditions in response to four temperature regimes. Germination (%) was calculated at four weeks after incubation. Error bars indicate mean ± SE of four replicates. Results of a two-way analysis of variance applied to germination percentages of V. sibiricum seeds affected by light and temperatures (*** significant at p < 0.0001).

6. Effect of Cold Stratification

A significant difference in the germination was found between the control group and the cold stratification treatment group. In the control group, germination was not observed, but the germination was more than 60% in all treatment groups after three weeks of culture (Figure 5).
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Figure 5. Germination of V. sibiricum seeds as affected by cold stratification periods (0, 2, 4, 8, or 12 weeks at 4 °C). Seeds were incubated at 20/10 °C after the stratification treatment. Error bars indicate mean ± SE of four replicates. The different letters represent statistically significant differences for germination percentages according to treatments in the fourth week as determined by the Tukey’s honestly significant difference (HSD) tests (p < 0.05).

7. Effect of GA3 Treatment

The germination significantly increased with an increase in the GA3 concentration (Figure 6). In the first week of culture, the germination was 0, 6, 24, and 76% at 0, 0.029, 0.289, and 2.887 mM GA3, respectively; subsequently, in the fourth week of culture, the germination was less than 20% at 0 and 0.029 mM GA3, 45% at 0.289 mM GA3, and 90% or more at 2.887 mM GA3 (Figure 6). Thus, the germination was the fastest at 2.887 mM.
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Figure 6. Effect of GA3 treatment on germination (%) in V. sibiricum seeds in light. Seeds were incubated at 20/10 °C (12/12 h). The seeds were soaked in 0, 0.029, 0.289, or 2.887 mM GA3 solution for 24 h. Error bars indicate mean ± SE of four replicates. The different letters represent statistically significant differences for germination percentages according to treatments in each week as determined by Tukey’s HSD tests (p < 0.05).

8. Effect of Light on Germination after Cold Stratification

The germination was more than 70% in the second week of culture under both light and dark conditions (Figure 7). Thus, the seeds could germinate even under dark conditions after cold stratification.
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Figure 7. Germination of V. sibiricum seeds as affected by light after cold stratification at 4 °C for 2 weeks. Seeds were incubated at 20/10 °C (12/12 h) after cold stratification. Error bars represent mean ± SE of four replicates.

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