Introduction
As the interest and demand for Korean native plants grow, there is a need for methods to control flowering for the year-round cultivation of new ornamental crops. Among the Korean native plants, varieties of Lythrum salicaria, ‘Dwarf Pink’ was selected from a dwarf mutant of L. salicaria, characterized by its racemose inflorescence and a height ranging from 30–60 cm compared with L. salicaria ranging over 100 cm (Kim et al., 2011; Mattingly et al., 2023; Stevens et al., 1997; Stuckey, 1980; Thompson et al., 1987). Due to the dwarf phenotype, ‘Dwarf Pink’ has been considered a potential ornamental garden plant. The flowering time of many herbaceous perennials in green-houses and fields is affected by environmental factors such as light and temperature. By manipulating these factors, L. salicaria ‘Dwarf Pink’ can be cultivated year-round, introducing it as a new ornamental crop in the floricultural market.
Manipulation of flowering enables year-round production of herbaceous plants by controlling environmental factors such as photoperiod (Kim et al., 2011). Photoperiod, the length of daily light, regulates the initiation of flowering in many herbaceous perennial species (Foley et al., 2009; Heins et al., 1997; Im et al., 2021; Kim et al., 2011; Runkle et al., 1998, 1999; Whitman et al., 1996). Korean native plants show diverse photoperiodic responses; for example, Elsholtzia angustifolia flowered under specific short-day conditions, with delayed transfer from long-day or night interruption conditions enhancing both floral and vegetative growth (Im et al., 2021). However, Veronica nakaiana, a day-neutral plant, requires prolonged cold treatment to induce flowering, with its response being dependent on the duration of cold exposure and the age of the plant (Im and Lee, 2023).
Like photoperiod, the age of the plant, known as juvenility, can be crucial for sensing environmental conditions that induce flowering. The correlation between plant size and flowering timing suggests that only plants of a certain size and developmental stage can bloom, with larger plants flowering earlier and for a longer duration (Ollerton and Lack, 1998; Miranda-Jácome and Sosa, 2023). Lysimachia mauritiana requires a combination of sufficient cold treatment and a long-day photoperiod for flowering induction, with effectiveness also dependent on the plant growth stage (Im et al., 2020). Plant size had a positive effect on cumulative flower number over a five-year period in cactus Pilosocereus leucocephalus (Miranda-Jácome and Sosa, 2023). In Lotus corniculatus (Fabaceae), the first flowering was correlated with plant size, which means that larger plants flowered earlier and for a longer period (Ollerton and Lack, 1998). However, there was no effect of plant size on flowering at long branches than at short branches in Fouquieria splendens (ocotillo) (Bowers, 2005). Thus, it is important to identify the timing of an effective photoperiodic treatment according to the size of the plant.
Although research on L. salicaria has been performed for a long time, the studies specifically targeting the ‘Dwarf Pink’ cultivar are relatively rare in spite of its ornamental values. Previous research has shown significant growth restrictions under short-day conditions and identified a critical day length of approximately 13 hours for stem elongation in L. salicaria (Shamsi and Whitehead, 1974; Shamsi, 1976). Lythrum salicaria under the long-day conditions undergoes developmental shifts such as leaf expansion, stem elongation, dry matter production, and flower initiation (Shamsi, 1974). Specifically, a light-break treatment led to more dry matter being allocated to stems rather than leaves (Shamsi, 1976). A subsequent experiment on the ‘Dwarf Pink’ revealed that at least 14 hours of daylight is necessary to initiate flowering, with long-day conditions affecting various growth aspects such as plant height, plant weight, and the number of lateral shoots, but not altering the number of nodes significantly (Kim et al., 2011). Based on previous studies, this study was conducted to determine a photoperiodic requirement for floral induction and the effects of plant size on flowering characteristics in in Lythrum salicaria ‘Dwarf Pink’ seedlings.
Research Methods
Plant materials and seedling condition
Lythrum salicaria ‘Dwarf Pink’ seeds were received from the Korea National Arboretum (Yangpyeong, Korea). The seeds were sown in 105-cell plug trays filled with commercial soilless substrate (Sunshine Mix #4, Sun Gro Horticulture, Agawam, MA, USA). Germination started one week after sowing, and the seedlings were transplanted after 12 weeks of sowing. The seedlings were grown in the walk-in growth chamber which was maintained at 22°C and 60% relative humidity. The light sources were mixtures of T5 LEDs (LEDT5-9015-DHE, FOCUS Lighting Co., Ltd., Bucheon, Korea) and fluorescent lamps (TL-D 32 W RS 865, Philips Lighting Co., Ltd., Eindhoven, Netherlands). The average light intensity and photoperiod were 150 ± 10 μmol·m−2 · s−1 PPFD and 9/15 h (day/night hours), respectively. The seedlings were sub-irrigated using water-filled dishes and they were fertigated once a week with a water-soluble fertilizer (EC 0.8 dS·m−1; HYPONeX professional 20N-20P-20K; HYPONeX Japan Co., Ltd., Osaka, Japan).
Flowering response to different photoperiods (experiment 1)
After 12 weeks from sowing, seedlings with an average of 17.7 leaves were transplanted into 12 cm top-diameter plastic pots filled with the commercial soilless substrate and transferred to beds in a greenhouse at Seoul National University Farm (Suwon, Korea; 37°27’N, 126°99’E). The beds were controlled at 9/15 h of photoperiod using black screens, which opened at 9:00 and closed at 18:00. The light intensity for day extension was 3–4 μmol·m−2 ·s−1 PPFD at the canopy level, which was provided with LED strips (12 V SMD 5050 LED, CamFree Co., Ltd., Seoul, Korea). The low light intensity was used to minimize the increase in the daily light integral (DLI) caused by extending the photoperiod. The plants were irrigated three times a week with tap water using sprinklers. A controlled-release fertilizer (Osmocote Plus 15N-4.8P-10.8K+2Mg+TE; Everris International B.V., Heerlen, Netherlands) was applied at 3.0 g per pot.
Photoperiodic response to plant size (experiment 2)
Seedlings were transplanted into the same plastic pots filled with the same substrate as above. The seedlings were divided into two growth stages according to the number of leaves. The seedlings in the stage 1 group had an average of 8.5 leaves, while those in the stage 2 group had an average of 21.1 leaves. After transplanting the seedlings, the plants of each stage were moved to a greenhouse for photoperiod treatments (9/15, 12/12, 14/10, 16/8, and 24/0 h). The light conditions for day extension, irrigation, and fertilizer application were carried out the same as in experiment 1.
Data collection and statistical analysis
Experiment 1 was conducted from 19 May 2023 to 11 August 2023, and experiment 2 was conducted from 2 June 2023 to 25 August 2023. Ambient irradiance was measured every 30 min with a quantum sensor (LightScout, Spectrum Technologies Inc., Plainfield, IL, USA) connected to a data logger (Watch Dog 1650; Spectrum Technologies Inc., Plainfield, IL, USA). Air temperature was recorded every 30 min with the same data logger. During the experiments, the average, maximum, and minimum daily air temperatures in the greenhouse are shown in Fig. 1. Changes in average growing degree days (GDD) and daily light integral (DLI) in the greenhouse are shown in Fig. 2. The percent flowering, days to visible bud, days to first open flower, the number of nodes at first flowering, and number of inflorescences were measured. Days to visible bud were counted from the start of the photoperiod treatments, and days to first open flower were counted from the date of the visible bud. The time of flowering was based on the flowering of the floret, and the first flowering of a floret at the bottom of the raceme was regarded as the first open flower. Vegetative growth characteristics, such as the plant height and width, number of branches and leaves, and dry weights of shoot and root in both experiments were measured at 12 weeks of photoperiod treatment. A completely randomized design was used in this study. Under 14/10 h photoperiod conditions, there were only one or two plants that showed flowering in each replication. Therefore, they were excluded from the data analysis for flowering characteristics. Data were analyzed using ANOVA (analyses of variance) using the SAS system for Windows version 9.4 (SAS Institute, Inc., Cary, NC, USA). Differences among treatment groups were assessed by Duncan’s multiple range test at p < 0.05. Graph module analyses were performed using SigmaPlot software version 10.0 (Grafiti LLC, Palo Alto, CA, USA).
Results
Flowering response to photoperiod (experiment 1)
The flowering rate of ‘Dwarf Pink’ was 16.7% under 14/10 h photoperiod and 100% under 16/8 and 24/0 h photoperiod conditions (Fig. 3). Conversely, plants under short-day conditions (9/15 and 12/12 h photoperiods) did not show floral bud initiation and never flowered during this experiment. From the 5th week after photoperiod treatment, the first visible floral buds were observed under 16/8 and 24/0 h. Both the days to visible bud and days to first open flower did not show significant differences. The longer photoperiod seemed to show the shortened days to visible bud and first open flower, but there were no significant differences among them. The number of nodes at the first flowering of 16/8 h was significantly (p < .01) larger than that of 24/0 h. The number of inflorescences showed no significant differences among treatment groups (Fig. 4).
Photoperiod treatments significantly correlated with vegetative growth parameters such as plant height, number of branches, and dry weight (Table 1). Plant height and the number of branches were significantly promoted with increasing photoperiods (p < .001). Shoot dry weight was heavier under 16/8 h and 24/0 h than short-day of 9/15 h (p < .05), whereas root dry weight showed an opposite trend. Root dry weight was significantly lighter under long-day conditions than 12/12 and 9/15 h (p < .01). There was no significant difference in plant width, the number of leaves, and the total dry weight of ‘Dwarf Pink’.
Flowering response to plant size (experiment 2)
The flowering rate of ‘Dwarf Pink’ was 8.3%, 36.4%, and 100% under 14/10, 16/8, and 24/0 h photoperiod conditions, respectively, when the plants were in the stage 1 group (Fig. 3). In the stage 2 group, the flowering rate was 16.7%, 90%, and 100% under 14/10, 16/8, and 24/0 h photoperiods, respectively. There was no floral bud initiation under short-day conditions (9/15 and 12/12 h photoperiods) during this experiment at both stage sizes.
From the 5th week after photoperiod treatment, the first visible floral bud was observed under 16/8 and 24/0 h in both stage groups (Fig. 5). The days to visible bud and first open flower did not show significant differences, but the longer photoperiod seemed to show slightly earlier days to visible bud and first open flower. The number of nodes at first flowering of 16/8 h was significantly (p < .05) larger than that of 24/0 h photoperiod. The number of inflorescences had no significant differences among photoperiod treatments. The smaller plant size seemed to show the earlier days to visible bud under 16/8 h photoperiod, but there were no significant differences among them. The plant size and photoperiod did not affect the days to first open flower, with no significant differences. The number of nodes at the first flowering also did not show significant differences between the plant size. The number of inflorescences in the stage 2 group was significantly (p < .05) lower than in the stage 1 group under 16/8 and 24/0 h photoperiods. The correlation between the plant size and photoperiod on flowering characteristics was not significant.
Photoperiod treatments showed significant differences with vegetative growth parameters such as plant height, plant width, number of branches, number of leaves, and dry weight (Table 2). With increasing photoperiods, plant height, plant width, number of branches, the number of leaves, shoot dry weight, and total dry weight were significantly promoted (p < .001). Shoot dry weight was heavier under long-day conditions above 16/8 h than short-day of 9/15 h (p < .001), whereas root dry weight showed an opposite trend. Root dry weight was significantly lighter under long-day conditions than 12/12 and 9/15 h (p < .001). Total dry weight was heavier under long-day conditions above 16/8 h than short-day of 9/15 h (p < .01).
Plant size showed significant correlations with vegetative growth parameters such as plant height, width, number of branches, and dry weight. Plant height was significantly promoted with increasing plant size (p < .05) under the above 12/12 h photoperiod. Plant width was significantly wider with increasing plant size (p < .01) under all photoperiod conditions. The number of branches was significantly (p < .05) lower in the stage 2 group than in the stage 1 group under 16/8 and 24/0 h photoperiods. Conversely, the number of branches was lower in the stage 1 group than in the stage 2 group under below 14/10 h photoperiod. The number of leaves did not show significant differences. Shoot and total dry weight were significantly heavier with increasing plant size (p < .001) under all photoperiod conditions, and root dry weight showed a similar trend (p < .01).
Meanwhile, there was no significant correlation between stage and photoperiod on plant height and width, number of leaves, and dry weight. The number of branches only had a significant difference (p < .01) in the correlation between stage and photoperiod. In the stage 1 group, the number of branches was more than doubled above 14/10 h photoperiod, and in the stage 2 group, the number of branches was gradually promoted with increasing photoperiod. In other words, the smaller the plant size, the greater the effect of the long-day treatment under the above 16/8 h photoperiod. The bigger the plant size, the greater the number of branches below 14/10 h photoperiod than the smaller plant.
Discussion
Photoperiod can enhance flowering in herbaceous ornamental plants (Foley et al., 2009; Im et al., 2020; Kim et al., 2011; Runkle et al., 1998, 1999; Whitman et al., 1996). In this study, the critical photoperiod of ‘Dwarf Pink’ was verified as 14/10 h, and the flowering rate was 16.7% under the condition that the previous study discovered (Kim et al., 2011). However, under 16/8 and 24/0 h photoperiods, the percentage of flowering rate was much higher, and the days to visible bud were lower. Therefore, for the manipulation of uniform cultivation, the optimal photoperiod treatments are 16/8 and 24/0 h. The results of experiment 1 and 2 show that the flowering rate increases as the photoperiod increases. There was a small difference in percentage of flowering between experiment 1 and 2. In stage 2, under 16/8 h photoperiod, the percentage of flowering in experiment 1 was lower than in experiment 2. Although the average leaves of stage 2 was larger than experiment 1, only 90% of flowering rate was shown. It would be attributed to the difference in average and minimum temperatures throughout the duration of each experiment (Fig. 1). Plants from higher latitudes with cold temperatures began flowering earlier and at a smaller size compared to those from lower latitudes, even under uniform conditions in a greenhouse (Montague et al., 2008). Furthermore, as indicated by the average of growing degree days (GDD), experiment 1 showed lower GDD than experiment 2 (Fig. 2A). This suggests the possibility that L. salicaria ‘Dwarf Pink’ shows a higher flowering rate when exposed to slightly cold temperatures. There is a need to confirm the interaction between GDD and photoperiodism on the flowering response. Thus, additional research about other environmental condition such as GDD and DLI is necessary to estimate the flowering reseponse according to various conditions.
However, for the vegetative growth, L. salicaria grew less in plant height, shoot and root dry weight, stem diameter and dry weight with shorter photoperiod under the high latitude with cold temperature (Bastlová D et al., 2006). The other possibility is that it could also be due to the higher average number of leaves at the start of stage 2 in experiment 2 compared to experiment 1. Moreover, while flowering was relatively delayed under a 16/8 h photoperiod condition in experiment 1, the flowering rate reached 100%, but the daily light integral (DLI) increased faster than in experiment 2, and there were fewer flower stems in experiment 1 (Fig. 2B, Fig. 4, and Fig. 5). Thus, the percentage of flowering under a 16/8 h photoperiod seems to be influenced by a combination of various environmental factors such as temperature and DLI. If plants are not sufficiently mature, they may not flower even when subjected to long-day conditions, as they might not be able to respond to the environmental cues (Im et al., 2020). So it is suggesting that additional experiments on the temperature and DLI are necessary for uniform flowering quality.
The number of nodes at the first flowering decreased as the photoperiod lengthened in both experiment 1 and 2 regardless of plant growth stage (Figs. 4 and 5). These findings could be associated with the increase in plant height with longer photoperiod (Tables 1 and 2). It can be interpreted as the plant absorbing more light during the vegetative growth stage and growing taller when given a longer light length than the critical photoperiod. In this experiment, we were able to eliminate the difference in DLI by providing natural light up to 9 hours and then very small light intensities for extended periods of time. However, the longer photoperiods put the plants in a very low light intensity, which could have caused a shade avoidance response, which manifested as a stem elongation response. It is difficult to interpret these results as a quantitative response of GA of long-day condtion, and I suggest further discussion. In addition, since the DLI during this experiment was much lower than in a usual environment, there seems to be more to consider.
The number of inflorescences was lower in larger plants than in smaller plants in both experiments (Figs. 4 and 5). It is because under long-day conditions, the vegetative and reproductive growth stages of smaller plants lasted longer, leading to an increase in the number of branches and flowers compared to the larger plants (Table 2). The number of branches of larger plants increased gradually than the smaller plants when the day length longer; however, the ratio of the number of branches to the number of inflorescences was similar in both sizes of plants. Thus, it is assumed that all the branches did not change to the reproductive phase from the vegetative phase. In Fouquieria splendens (ocotillo), long branches produced three times as many flowers as short branches with no effect on plant size (Bowers, 2005). Every branch does not bloom, because the short branches need time to transition from vegetative to reproductive phase. Although larger plants under long-day conditions may have bigger size and taller height, they do not necessarily have more inflorescences. The results could be explained by the fact that campanula cultivars ‘Champion Blue’ and ‘Champion Pink’ have short juvenile phases which indicate the capability of perceiving photoperiod induction for flowering at early stages, but flowering rate is higher when photoperiod treatment is applied at mature age (Ha, 2014).
Conclusion
This study demonstrates the impact of temperature and photoperiod conditions on the flowering percentage, the number of nodes at the first flowering, and the number of inflorescences in plants. Although the flowering percent was lower than that over 16/8 h, the flowering occurred under 14/10 h photoperiod conditions. These results indicated that L. salicaria ‘Dwarf Pink’ could be categorized as an obligate long-day plant and the critical photoperiod was verified as 14/10 h. With bigger plants, the number of branches gradually increased as the photoperiod became longer. Under the long-day conditions, the plant size did not affect the days to flowering. However, when the size of plant was significantly large under long-day conditions, the number of inflorescences and the number of branches decreased, affecting the quality of flowering.
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