Introduction

With increasing interest in utilizing native plants for indoor and outdoor gardening and landscaping, research on their propagation, growth, and flowering responses under various environmental conditions has been conducted (Hong and Zimmerer 2022; Im et al. 2022; Lee et al. 2024;Yang et al. 2022). In particular, Korean native Aster species have long been used for medicinal and edible purposes, with numerous studies on their medicinal properties (Kim et al. 2010;Woo et al. 2008). Beyond their practical uses, Aster species also have aesthetic value in landscaping. Several Aster species and hybrids are commercially cultivated for various purposes, including cut flowers, potted plants, and garden use (Jung et al. 2022;Wallerstein et al. 1992b). The genus Aster generally inhibits flowering under long-day conditions and promotes it under short-day conditions. However, the flowering response to photoperiod can vary depending on the species or cultivar (Lopez et al. 2006;Wallerstein et al. 1992a, 1992b). This study examined the photoperiodic flowering responses of three Korean native Aster species: A. hayatae H.Lév. & Vaniot (Nungaetssukbujaengi), A. spathulifolius Maxim. (Haeguk), and A. koraiensis Nakai (Beolgaemichwi). These species are used for ornamental and horticultural purposes due to their light purple flowers, easy propagation, and tolerance to environmental stress. A. hayatae branches horizontally before flowering upright, A. spathulifolius has hairy, thick leaves and is semi-evergreen, and A. koraiensis is used as ground cover due to its spreading rhizomes (KNA 2025). Understanding their photoperiodic flowering requirements will provide valuable data for developing them as new horticultural crops.

Materials and Methods

Aster hayatae, sown March 2024, germinated after 1 week and was transplanted to 10 cm pots 6 weeks later. Overwintered suckers of A. spathulifolius and A. koraiensis, collected April 2024, were stored at 5°C for 4 days, treated with (0.4% 1-Naphthylacetamide; ISK Biosciences Korea Co., Ltd., Seoul, Korea), and propagated. Rooted suckers were transplanted to 10 cm pots after 5 weeks. All three Aster species were maintained under natural greenhouse conditions until photoperiod treatments. The photoperiod treatment was carried out in a greenhouse at the Seoul National University Farm (Suwon, Korea; 37°27'N, 126°99'E) from June 28 to November 5, 2024. To examine the effects of photoperiod on flowering, plants were exposed to four different photoperiods: 9, 12, 14, and 16 hours. A base photoperiod of 9 hours of light and 15 hours of darkness was established using black plastic films, which were opened at 09:00 _HR and closed at 18:00 _HR. Supplemental white LED lighting (12 V SMD 5050 LED, CamFree Co., Ltd., Seoul, Korea) was then used to extend the day length and achieve the target photoperiods. The supplemental light intensity was kept low (3 μmol･m^-2･s^-1 PPFD) to minimize the effect of the daily light integral. Three replications per treatment were used, with five plants per replication. Flowering was assessed by the percentage of flowering plants and the number of days from the start of the treatment to visible floral bud formation (Fig. 1A and E) and first open flower (Fig. 1D and H). Flowering was examined for 19 weeks after treatment, and photos of each treatment were taken at the 12th week, when flowering could be observed. Data were analyzed using ANOVA (analysis of variance) in SAS 9.4 (SAS Institute Inc., Cary, NC, USA), followed by Tukey's HSD test for post-hoc comparisons at α = 0.05. A t-test was used for comparisons between two groups. Graphs were generated using SigmaPlot 10.0 (Grafiti LLC, Palo Alto, CA, USA).

Results and Discussion

Aster hayatae and A. spathulifolius exhibited flowering under the tested conditions, while A. koraiensis did not flower under any photoperiod (Fig. 2). A. hayatae flowered under all photoperiods, with flowering rates of 92%, 85%, and 65% at 9, 12, and 14 hours, respectively. However, the flowering rate decreased to 27% under the 16 hours long-day condition (Fig. 3). Days to visible bud and flowering were significantly shorter under shorter photoperiods, indicating flowering inhibition under long-day conditions (Fig. 3 and 4). A. spathulifolius flowered only under photoperiods of 12 hours or less, with flowering rates of 27% and 13% at 9 and 12 hours, respectively. There was no significant difference in days to visible bud and flowering between these two treatments (Fig. 3 and 4).

Wallerstein et al. (1992b) reported that long-day conditions promote stem elongation but inhibit flowering in Aster 'Dark Pink Star' and 'White Butterfly' cultivars, requiring short-day conditions for flowering in elongated stems. Consistent with the general understanding of Aster as a short-day plant (Lopez et al. 2006;Wallerstein et al. 1992b), both A. hayatae and A. spathulifolius exhibited a tendency towards short-day flowering promotion. While A. hayatae flowered under all photoperiods, its enhanced flowering under short-day conditions classifies it as a facultative short-day plant. In contrast, A. spathulifolius, which flowered exclusively under short-day conditions of 12 hours or less, is classified as an obligate short-day plant.

According to Schwabe (1985), A. novi-belgii undergoes stem elongation and branching under long-day conditions in spring and summer after winter vernalization, with flowering initiated under short-day conditions in autumn. Similar to Chrysanthemum (De Jong, 1989), rosette shoots of Aster species are known to be juvenile and unresponsive to inductive short-day conditions (Wallerstein et al. 1992b). Therefore, the absence of flowering in A. koraiensis (Fig. 2) may be attributed to the lack of vernalization, but more likely to insufficient growth duration for maturity. The low flowering rate observed in A. spathulifolius is also presumed to be due to maturity. Further research on vernalization and juvenility is needed to accurately classify the flowering responses of these Aster species.