2016 Volume 85 Issue 3 Pages 248-253
The aims of this study were to examine the effects of different temperatures on seed germination and initial long-day (LD) or short-day (SD) duration on growth, floral initiation, and development of Hosta yingeri S.B.Jones. The germination percentage of H. yingeri seeds was > 90% at 5°C and between 15°C–30°C. The lowest time to the first germination (TFG) and the mean germination time (MGT) were observed at around 30°C. The optimum temperature for germination can be around 30°C considering the final germination percentage, TFG, and MGT together. Plant height, number of leaves, and leaf length increased as the initial SD duration decreased. The percentages of plant bearing buds were 50%, 50%, 20%, 10%, and 0% when the initial SD treatments were 0, 3, 6, 9, and 12 weeks with 29, 26, 23, 20, and 17 weeks of LDs, respectively. No plants flowered when they were exposed to the initial LD conditions. Flower spike length, number of visible buds, and number of open flowers increased as the duration of the initial SD decreased. Four-week-old H. yingeri required ≥ 26 weeks of LD duration to achieve > 50% flowering.
Hosta species are distributed in East Asia and Russia, but most species have been found in China, Japan, and Korea (Lee and Maki, 2015). Hosta yingeri S.B.Jones was discovered in 1985 in the Heuksan Archipelago, South Korea by Barry R. Yinger (Jones, 1989). Hosta yingeri is a Korean endemic plant, and is distinguished from other species of Hosta by its relatively thick, adaxially dark green leaves. A delicate raceme of flowers is spread evenly around the central axis of the inflorescence with 3-long and 3-short pairing stamens. Because its unique characteristics include short inflorescences and small flowers, this hosta has the potential to be developed as a commercial potting plant. Hosta yingeri has been generally available in the hosta trade, and has become a popular garden plant species internationally. However, its remote habitat and limited natural environment has slowed progress of research to develop H. yingeri as a commercial plant.
Hosta species are generally propagated by lateral shoot division or tissue culture (Feng et al., 2009; Wilson and Rajapakse, 2000). Because only a few shoots can be obtained from one taxon, satisfying the increased demand has been difficult. The rapid availability of high potential Hosta cultivars is in part due to the use of seeds as a means of commercial propagation. Optimum germination temperature differs by species, and is required for commercial propagation. Seed germination of H. sieboldiana occurs over a wide temperature range, from 10°C to 30°C, but is most rapid at 25°C and 30°C (Kanazawa et al., 2015). The optimum germination temperature of H. plantaginea is 25°C, but the germination percentage is very low (< 25%; Oh et al., 2003). Hosta yingeri seeds collected from several locations have resulted in plants that are quite uniform. Seed propagation offers potential for rapid mass production of H. yingeri. However, only limited information on the germination response of H. yingeri is available (Kim et al., 2005).
The photoperiod is a major factor that affects flower development of hosta, which is a long-day (LD) plant (Jeong, 2005). An understanding of when plants are sensitive to photoperiod is essential for the timing optimization of photoperiod treatments. The lengths of the photoperiod sensitivity phases can be quantified using reciprocal transfer experiments. In these experiments, plants are transferred at regular intervals between environments that are inductive and non-inductive for flowering (Wang et al., 1997). Short-day (SD) will not delay flowering in an LD plant if exposure is restricted to the photoperiod-insensitive juvenile phase, or photoperiod-insensitive phase of flower development (Adams et al., 2001). However, exposure to SD during the photoperiod-sensitive phase will delay flowering. Similarly, an LD will only hasten flowering in an LD plant if exposure occurs when the plant is photoperiod-sensitive. In commercial production, the ability to schedule potted plants to flower during periods of high demand is desirable because it allows precise crop scheduling, and improves production efficiency.
The objectives of this research were to examine the effects of different temperatures on germination of H. yingeri seeds. We also determined the photoperiod sensitivity duration for greenhouse-reared H. yingeri, which will support the development of this species as a commercial potted plant.
Seeds of wild H. yingeri were harvested from Geumwonsan Ecological Arboretum, Guchang, Korea (latitude 35°43'19.9''N, longitude 127°47'00.6''E) in October, 2013. The mean seed weight was 0.29 g (n = five replicates, 100 seeds per replicate). Mean seed size was 3.04 × 1.9 mm (n = 100).
Seeds were sterilized by briefly exposing them to 70% ethanol. They were incubated at constant temperatures of 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, or 35°C in a multi-room incubator (DS-13MCLP; Hanbaek, Hwaseong, Korea), using three replicates of 50 seeds per replicate. The seeds were placed on 9-cm diameter petri dishes with two layers of filter paper (Whatman No. 2; Whatman, GE Healthcare Life Sciences, Little Chalfont, UK). They were moistened with distilled water in the same incubator to control for the effect of water temperature, and were exposed to a 12-h photoperiod (20 μmol·m−2·s−1). Germination was defined as protrusion of the radicle through the seed coat (≥ 1 mm). Germinated seeds were counted daily, and then removed. Final germination percentage (FGP) and the time to the first germination (TFG) were recorded. The mean germination time (MGT) was calculated as:
MGT = Σ(ti × ni)/Σni
where ti was time (days) counted from the beginning of the test; ni was the number of germinated seeds at the time they were counted (Ellis and Roberts, 1981).
Four-week-old H. yingeri seedlings of 2.25 cm height and 3.5 true leaves were obtained from seed propagation. The plants were transplanted into 10-cm diameter plastic pots filled with commercial potting medium (Sunshine Mix #1; Sun-Gro Horticulture, Bellevue, WA, USA), and then grown in a growth room. Day and night temperatures inside the room were maintained at a constant 25°C. The plants were irrigated twice a week with tap water delivered via overhead irrigation. They were fertilized once a week with a nutrient solution at EC of 1.0 dS·m−1 (15N-15P-18K, Eco special series; HYPONeX Japan Co., Osaka, Japan).
The plants were exposed to a 10-h photoperiod (SD), or a 10-h photoperiod with a 4-h night-interruption (NI) (LD) for the initial photoperiod treatments of 0, 3, 6, 9, or 12 weeks. The plants were then moved to the opposite day length treatments for the later photoperiod treatments of 29, 26, 23, 20, or 17 weeks. The treatments were identified as LD0-SD29, LD3-SD26, LD6-SD23, LD9-SD20, LD12-SD17, SD0-LD29, SD3-LD26, SD6-LD23, SD9-LD20, and SD12-LD17 based on the photoperiod exposures (Table 1). The plants were exposed to NI from 22:00 to 02:00 using white fluorescent lamps (AL-2220D; A-lim Industrial Co., Ltd., Incheon, Korea) at 4–7 μmol·m−2·s−1. In the growth room experiments, the plants were grown under a mean photosynthetic photon flux of 56–60 μmol·m−2·s−1 provided by white fluorescent lamps (FLR40EX-W/A; Osram Korea, Ansan, South Korea) for a 10-h photoperiod from 07:00 to 17:00. The light intensities were measured using line quantum sensors (Apologee Instruments, Inc., Logan, UT, USA).
Photoperiod treatment exposures.
Plant height was measured from the soil level to the uppermost shoot. Leaf length was obtained by measuring the longest leaf from the base at the stem to the leaf tip. The numbers of leaves and plants bearing buds were also counted. At flowering, flower spike length, number of visible buds (VBs), and number of open flowers were recorded. Days to VBs and days to flower from the start of the treatments were measured. The data on growth and flowering were collected weekly.
A completely randomized design was used for the germination and photoperiod experiments. Ten plants from each treatment were randomly arranged on benches for each photoperiod treatment. Statistical analyses were performed using SAS for Windows software (version 9.2; SAS Institute, Inc., Cary, NC, USA). Differences among treatments were assessed using Tukey’s studentized range test. A P-value < 0.05 was considered significant. Regression and graph module analyses were performed using Sigma Plot software (version 10.0; Systat Software, Inc., Chicago, IL, USA).
The FGP of H. yingeri seeds was > 90% at 5°C and between 15°C–30°C (Fig. 1). It was 72.0% and 51% at 10°C and 35°C, respectively. The close relationships between temperature and TFG and MGT are shown in Figure 2, with R2 ranging from 0.89 to 0.97 (P < 0.001). Both TFG and MGT values tended to shorten as the temperature for seed germination increased up to 30°C (Fig. 2).
Germination percentage of Hosta yingeri seeds at 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, or 35°C. The different letters indicate significantly different (P < 0.05) using Tukey’s studentized range test. Bars represent mean ± SE, n = three replicates, 50 seeds per replicate.
Relationship between germination temperatures and (A) time to first germination (TFG) and (B) mean germination time (MGT) of Hosta yingeri seeds (*** significance at P < 0.001).
The plant height did not increase after being transferred from the LD conditions to the SD conditions (Fig. 3A). Plant height at 18 weeks was < 5.0 cm for all the initial LD conditions. There was a greater increase in the height of the plants exposed to the SD0-LD29, SD3-LD26, or SD6-LD23 conditions compared with the plants exposed to the SD9-LD20 or SD12-LD17 conditions (Fig. 3B). The maximum plant height was 6.9 cm in the plants grown under the SD3-LD26 condition at 15–17 weeks after treatment. The minimum plant height was < 4.0 cm in the plants grown under the SD9-LD20 or SD12-LD17 conditions (Fig. 3B). After 6 weeks of treatment, more than 6 leaves were obtained in the plants grown under the LD9-SD20 or LD12-SD17 conditions, but there were no significant differences among the treatments after 14 weeks of exposure (Fig. 4A). From the beginning of treatments, the number of leaves in the plants grown under the SD0-LD29 condition was significantly greater (P < 0.001) compared with that of the plants grown under other the initial SD conditions (Fig. 4B). The number of leaves in the plants grown under the SD3-LD26 condition was greater than that of the plants grown under the SD6-LD23, SD9-LD20, or SD12-LD17 conditions after 15 weeks of treatment (Fig. 4B). Leaf length was continuously maintained in the plants exposed to the LD0-SD29 condition (Fig. 5A). Leaf length increased in the plants grown under both the SD0-LD29 and SD3-LD26 conditions until the end of the experiment (Fig. 5B).
Changes in plant height of Hosta yingeri as influenced by initial (A) long-day (LD) or (B) short-day (SD) photoperiod for 18 weeks after the beginning of treatment. Vertical lines represent the mean ± SE, n = five plants. Graph symbols are described in Table 1.
Changes in number of leaves of Hosta yingeri as influenced by initial (A) long-day (LD) or (B) short-day (SD) photoperiod for 18 weeks after the beginning of treatment. Vertical lines represent the mean ± SE, n = five plants. Graph symbols are described in Table 1.
Changes in leaf length of Hosta yingeri as influenced by initial (A) long-day (LD) or (B) short-day (SD) photoperiod for 18 weeks after the beginning of treatment. Vertical lines represent the mean ± SE, n = five plants. Graph symbols are described in Table 1.
The plants grown under the initial LD conditions did not exhibit any buds at any time throughout the experiment (Table 2; Fig. 6). Flowers developed in 50% of the plants grown under the SD0-LD29 or SD3-LD26 conditions, and 20% of the plants grown under the SD6-LD23 condition developed flowers (Table 2). The flower spike length in the plants grown under the SD0-LD29 and SD3-LD26 conditions was similar, and longer than other treatments. The plants developed more buds in the SD0-LD29 and SD3-LD26 conditions compared with the plants grown under the SD6-LD23 condition. The numbers of open flowers were greater for the plants grown under the SD0-LD29 and SD3-LD26 conditions compared with those grown under the SD9-LD20 condition (Table 2; Fig. 6). There were no significant differences in days to flower among plants grown under the SD0-LD29, SD3-LD26, SD6-LD23, and SD9-LD20 conditions (Table 2). None of the plants flowered in any of the initial LD conditions, or in the SD12-LD17 condition during the entire experimental period (Table 2; Fig. 6).
Effects of photoperiod treatment on reproductive growth in Hosta yingeri at 29 weeks after the beginning of treatment.
Effects of initial photoperiod of (A) long-day (LD) or (B) short-day (SD) on growth and flowering in Hosta yingeri at 29 weeks after the beginning of treatment. The definition of treatments is described in Table 1.
The optimal temperature for seed germination is the temperature that results in the highest germination percentage within the shortest time (Mayer and Poljakoff-Mayber, 1989). In H. yingeri seeds, the highest FGP was obtained between 15°C–30°C, and the shortest TFG and MGT were observed at around 30°C (Figs. 1 and 2). The optimal temperature for H. yingeri seed germination is around 30°C considering FGP, TFG, and MGT together. High germination temperatures may be preferred by H. yingeri species because the maximum temperature in July and August has been between 26.3°C–27.9°C for the last 10 years (Korea Meteorological Administration, 2015; http://www.kma.go.kr/weather/climate/past_table.jsp?stn=169&yy=2015&obs=08&x=20&y=11; 17 October 2015), and plants are exposed to a short day for most of the year in their native habitats (about 34° latitude) in Korea. Some species have a specific temperature range for seed germination because temperature regulates germination under natural conditions, and prevents germination at thermal conditions disadvantageous for seedling growth (Bewley and Black, 1994). In the present study, H. yingeri seeds germinated at 5°C, and an increase in temperature above 15°C hastened TFG and MGT. These results indicate that the spring and early summer seasons are appropriate times for H. yingeri germination.
Hosta grown in the SD condition produced an initial flush of leaves, and then entered a vegetatively dormant state. Growth was not observed in hosta after transfer from LD to SD photoperiods; growth was likely inhibited by physiological factors within the crown. Fausey (1999) reported that H. plantaginea and H. ‘Royal Standard’ resumed growth when they were transferred to the LD condition after 15 weeks of SD exposure. In the present study, LD with NI promoted vegetative growth, which was indicated by an increase in leaf number in the plants grown under the SD0-LD29 condition (Fig. 4B). Plants grown under the initial LD conditions produced an initial flush of leaves, then entered a dormant state. Vegetative growth ceased, and no flowering occurred within a few weeks in the plants shifted from LD to SD conditions. These results indicate that H. yingeri eventually becomes dormant under SD conditions. Similarly, when LD-grown spinach plants are transferred from LD to SD conditions, petiole growth almost stops by the next day, and then continues at a rate comparable to that of plants kept continuously under SD conditions (Zeevaart, 1971). Exposure to SD conditions at an early stage induced growth retardation in H. yingeri.
To determine the photoperiod sensitive stage and required inductive days for flowering can be important to predict time to flower. When plants are transferred from inductive to non-inductive conditions, the first time at which transferred plants results in a decrease in leaf number coincides with the time at which the meristem becomes committed to flowering (Adams et al., 1998). The activation of the floral meristem in H. yingeri can be estimated at 14–17 weeks after treatment because the reduction in the number of leaves commonly occurred in plants grown under the initial LD conditions during this time (Fig. 4A). Photoperiod duration control allows plants to continue to form leaf primordia, or induce flowering. A quantitative or continuous response to a photoperiod is a prerequisite for a plant that is permanently sensing a changing photoperiod environment, and has a continuous capacity to respond to changes. The reproductive stage was reached by the plants grown under the SD0-LD29 and SD3-LD26 conditions, and most with more than 6 leaves were capable of flowering. The photoperiod sensitivities of different developmental stages seem to be relatively independent, and vary between cultivars (Slafer and Rawson, 1994). Our results indicated that days to visible buds and flowers tended to decrease as the duration of the initial SD decreased (Table 2). All plants failed to flower without exposure to LDs for the later photoperiods. Flowering only occurred when plants were exposed to over 20 weeks of the LD condition. The later photoperiod of LD might be replaced with the initial LD photoperiod if the exposed duration of LD is over 20 weeks during the entire growth period of the plants. In conclusion, a germination temperature of around 30°C enhanced seed germination of H. yingeri. For greenhouse production of H. yingeri, long-day lighting (NI) should be provided from the early stages of H. yingeri, when the natural day length is < 14 h in late winter, or early spring. Four-week-old H. yingeri required more than 26 weeks of LD duration to achieve > 50% flowering.
We appreciate the support of Sun Woo Chung (Seoul National University) for statistical analysis and some helpful discussions.