Introduction
In floricultural crop production, managing flowering time is crucial to preserving the aesthetic value of crops, as their limited durability complicates cultivation scheduling (Proietti et al. 2022). Especially in Lilium, bulb enlargement stage and subsequent flowering stage estimation are crucial for bulb production. However, scheduling decisions based solely on calendar dates can be unreliable due to the variability in planting dates and the resulting climate conditions (Miller et al. 2001). Accumulated temperatures, particularly quantified through growing degree days (GDD), are key factors of plant phenological changes, as evidenced by their successful application in estimating the growth of crops like soybean and maize (Kim et al. 2017;Lee et al. 2019;Schwartz 1999). Consequently, GDD has been utilized as an agro-climatic indicator at the regional scale in mid-latitude climates (Grigorieva 2020). Accurate estimation of phenological changes enhances understanding of plant growth and enables more effective cropping plans (Dan et al. 2020). The objective of this research is to determine the GDD requirements of Lilium hybrids and utilize this information to estimate flowering dates.
Materials and Methods
Lilium FA hybrids were acquired from the National Institute of Horticultural and Herbal Science, while Lilium LA and Oriental hybrids were purchased from a commercial nursery farm (Wooriflowers Seeds Seedling CO., Ltd., Gwacheon, Korea). The lilies were cultivated in both open field and greenhouse conditions at Seoul National University Farm (Suwon, Korea; 37°27’N, 126°99’E). For greenhouse cultivation, ten Lilium hybrid ‘Serrada’ bulbs were planted in each bulb crate (40×20×60 cm) filled with horticultural substrates (Sunshine® Mix #4; Sun Gro Horticulture Canada Ltd., Agawam, USA). Plants were automatically irrigated twice a week using a sprinkler system. At planting, each crate was fertilized with 30 g of controlled-release fertilizer (Osmocote Plus 15N-4.8P-10.8K + 2Mg + TE; Everris International B.V., Heerlen, Netherlands). Phenological observations were conducted on an average of 20 replicates per lily cultivar. Air temperature was recorded at 30-minute intervals using a data logger (Watch Dog Model 1000 series, Spectrum Technologies Inc., Aurora, USA). GDD requirements were calculated using the following equations:
Where Tn denotes air temperature at each 30-minute interval, Tb is the base temperature (3°C) of Asiatic lily (Steininger and Pasian 2003), R (Tn) represents the forcing rate at each 30-minute interval, F(T) stands for the daily forcing rate, t0 represents the start date of forcing accumulation, tx is the date of phenological change, F* is the GDD requirement for phenological change. Estimated dates for phenological change were calculated using air temperature data recorded in the same greenhouse in 2022.
Results and Discussion
GDD requirements (F*) for phenological changes were calculated for Lilium hybrids (Table 1). Except ‘Golden Harmony’, FA and LA hybrids generally required more than 400°C GDD for floral bud visibility (Fig. 1A). ‘Dreamline’ required less than 500°C GDD, while the other Oriental hybrids required more than 600°C GDD for floral bud visibility. The beginning of flowering, defined as when the first flower opens, (Fig. 1B) required 900–1,000°C GDD in FA and LA hybrids, while Oriental hybrids needed more than 1,100°C GDD. Full flowering stage, defined as more than 50% of flowers being opened, (Fig. 1C) required 1,000–1,100°C GDD in LA and FA hybrid cultivars, which was the more than 1,200°C GDD requirement for Oriental hybrids. GDD requirements for plant phenological changes vary depending on the plant’s origin. Plants originating from mountainous areas typically exhibit lower GDD requirements for flowering, while those from tropical regions generally demonstrate higher GDD requirements (Lambert et al. 2010;Singh et al. 2014). Oriental hybrids, bred from Lilium auratum, Lilium speciosum, and Lilium henryi, which originate from more temperate habitats compared to LA hybrids, exhibit higher GDD requirements than LA hybrids (RDA 2020).
Flowering dates were estimated based on GDD requirements for phenological changes of Lilium LA hybrid ‘Serrada’ in greenhouse cultivation (Table 2). Estimated flowering dates were calculated as the date when the GDD requirement was exceeded, and then compared with observed dates. The maximum discrepancy between estimated and observed dates of phenological change was three days for full flowering stage, with a root mean square error (RMSE) of 91.79. Since the number of flowers is influenced by bulb size (Worley et al. 2000), an earlier date for reaching the stage would be attributed to the bulb size in this study, where smaller-sized bulbs were planted in greenhouse cultivation.
In this study, lily flowering dates were accurately estimated based on thermal time, regardless of planting site. In ornamental plants, accurate estimation of flowering dates is crucial for cut flower production and landscape use, especially in the context of global warming (Inouye 2008). Furthermore, thermal time accumulation has been used to calculate development rates in crop phenology models included in broader crop growth models (Ceglar et al. 2011). The results of this study would be used to estimate phenological stages for developing crop growth models for lilies.
