Open AccessArticle

Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting

Maitree Munyanont

^1,2

Na Lu

^3,*

Dannisa Fathiya Rachma

¹,

Thanit Ruangsangaram

¹ and

Michiko Takagaki

Graduate School of Horticulture, Chiba University, 648 Matsudo, Matsudo 271-8510, Chiba, Japan

Lamtakhong Research Station, Expert Center of Innovative Agriculture, Thailand Institute of Scientific and Technological Research, Pathum Thani 12120, Thailand

Center for Environment, Health and Field Sciences, Chiba University, 6-2-1 Kashiwanoha, Kashiwa 277-0882, Chiba, Japan

Author to whom correspondence should be addressed.

Agriculture 2024, 14(12), 2208; https://doi.org/10.3390/agriculture14122208

Submission received: 14 November 2024 / Revised: 1 December 2024 / Accepted: 2 December 2024 / Published: 3 December 2024

(This article belongs to the Special Issue The Influence of Light, Temperature and Irrigation on Crop Production and Quality)

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Abstract

Calendula is an edible flower with highly beneficial bioactive compounds for human health. Environmental factors such as light influence flower yield and quality. Calendula is cultivated under controlled environments in plant factories with artificial lighting (PFALs), which enhance its productivity. However, electricity is the main operating cost, with fees based on the time of use in some countries. This study aimed to investigate the effects of lighting patterns on calendula growth and yield. Two varieties of calendula seedlings were cultivated in a PFAL and subjected to six different lighting patterns, i.e., 6 h/6 h, 12 h/12 h, 6 h/2 h, and 18 h/6 h (light/dark) and two continuous lighting patterns with varying light intensities (24 h-200 and 24 h-400). The results indicated that plants cultivated under the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 conditions showed a more rapid appearance of the first flower bud than those cultivated under the 6 h/6 h and 12 h/12 h conditions. The number of flowers and the fresh and dried weights tended to increase with a longer photoperiod. Interestingly, the total carotenoid content (TCC) of “Citrus Orange” increased under 6 h/6 h and 12 h/12 h conditions compared with the others. For “Orange Gem”, continuous lighting (24 h) increased the total phenolic content (TPC) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity in flowers compared with the 6 h/6 h 12 h/12 h, and 6 h/2 h treatments. The energy use efficiency (EUE) under the 24 h-200 condition was the highest in terms of flower yield and secondary metabolite production. These results suggest that lighting patterns can be used to modulate the growth and flowering of calendula and to maximize EUE.

Keywords:

pot marigold; edible flower; LEDs; light cycle; artificial lighting

1. Introduction

Calendula (Calendula officinalis L.) is an annual ornamental plant belonging to the Calendula genus (family Asteraceae). It is also known as pot marigold, common marigold, Scottish marigold, or Chin Chan Hua in the Chinese language. It is native to central Europe and the Mediterranean region and, for decades, its edible flowers have been used in salads and soups as well as for stew seasoning. Additionally, this plant is used as a traditional herbal medicine and a natural dye for clothes [1]. Calendula has high concentrations of phytochemicals that provide important nutritional and medicinal benefits. For instance, the whole plant contains a variety of biologically active compounds, such as carotenoids, flavonoids, saponins, sterols, phenolic acids, and lipids [2]; nevertheless, the flower is the most compound-rich part. Dried calendula flowers, which are known for their high carotenoid concentration, find a use in cuisines as a spice and orange dye known as “fake saffron”. The Food and Drug Administration generally acknowledges that their use is safe. The flowers are also a rich source of natural antioxidants; specifically, phenolic compounds and other substances that target free radicals [3]. Furthermore, calendula can be widely applied as an antiseptic, anti-inflammatory, light antibacterial, and antiviral agent. Its essential oil contains mono- and sesquiterpenes, which give it a specific flavor and which are key compounds for use in traditional medicine [4].

Despite the long-standing consumption of flowers by humans, only recently has there been a surge in research on their nutritional value. As evidenced by mentions in several cookbooks, magazine articles, fine-dining circles, and scientific studies, the demand for edible flowers is high, but the supply is unable to meet this demand due to issues associated with plant yield and quality. Since plant growth and development rely on a multitude of environmental factors, yield and quality are difficult to control through open-field farming. However, these problems can be solved by cultivating plants in plant factories with artificial lighting (PFALs). The use of PFALs is a form of controlled-environment agriculture, also known as vertical farming or indoor farming. Stacking cultivation units vertically within PFALs results in high plant productivity. A controlled environment can ensure not only year-round consistency in product quantity and quality but also the harvesting of pesticide-free and nutrient-rich plants [5]. Light conditions significantly influence the plant quality and visual and sensory characteristics, as well as the chemical components [6]. There are three parameters associated with light conditions: the photoperiod, the light intensity, and the light spectrum. In addition, the daily light integral (DLI) quantifies the amount of the photosynthetic photon flux (PPF) that a plant receives within a 24 h period. Hence, altering the photoperiod or light intensity can modify the DLI and create different lighting patterns while maintaining the same DLI.

Many studies have examined the impacts of the photoperiod, light intensity, and DLI on plant growth. For example, it has been reported that increased DLI levels boost plant growth, accelerate the flowering process, and increase the number of flowers [7]. Five ornamental plants, i.e., snapdragon (Antirrhinum majus L.), calendula (Calendula officinalis L.), impatiens (Impatiens wallerana Hook.f.), mimulus (Mimulus × hybridus Hort. ex Siebert & Voss) and torenia (Torenia fournieri Linden ex E. Fourn), showed increases in their shoot dry mass as DLI values increased from 10.5 to 17.5 mol·m⁻²·d⁻¹. In another study, different long-day herbaceous ornamentals, including tickseed (Coreopsis grandiflora), echinacea (Echinacea × hybrida), lavender (Lavandula angustifolia), lobelia (Lobelia × speciosa), and salvia (Salvia longispicata × farinacea), were grown in an environmentally controlled greenhouse and treated with different photoperiods (which results in different DLIs), and the results showed that, for all species, the time required to reach the flowering stage was decreased by longer photoperiods (higher DLIs) [8]. Furthermore, the majority of species exhibited an increased number of inflorescences and a more compact plant size in response to the higher DLI.

It appears that a higher DLI has a positive effect on crop production. However, increasing DLIs result in the extension of the photoperiod or an increase in light intensity, augmenting the costs of electricity for growing crops in PFALs. Electricity rates in several countries fluctuate based on the specific time of day. As a result, modifying the lighting pattern while maintaining the same DLI is an interesting approach to enhancing plant production efficiency in PFALs. As mentioned above, calendula has a high value due to its edible flowers and as a medicinal plant, but its cultivation in PFALs presents challenges in terms of cost and improvements in the flower yield and secondary metabolite productivity. The purpose of this study was to examine the response of calendula to different lighting patterns (different combinations of light intensity and photoperiod) to improve the EUE and to provide information with regard to the feasibility of cultivating this plant as a commercial crop in PFALs, with a specific emphasis on its potential as a source of edible flowers.

2. Materials and Methods

2.1. The Plant Materials and Growth Conditions

Two dwarf varieties of calendula, “Citrus Orange” (Sakata Seed Corporation, Kanagawa, Japan) and “Orange Gem” (Takii & Co., Ltd., Kyoto, Japan), were used as plant materials because of their dwarf characteristics, which allowed a more efficient space use for PFAL cultivation. Seeds were sown in a rock wool cube (5 × 5 × 5 cm) and placed in a dark chamber at 15 °C for 72 h. After germination, the seedlings were exposed to light. The light intensity was set at 240 ± 10 µmol m⁻² s⁻¹ with a photoperiod of 12 h. The temperature was maintained at 23 ± 2 °C for the entire day. Enshi nutrient solution with an electrical conductivity (EC) of 1.2 ± 0.1 dS m⁻¹ and a pH of 6.5 ± 0.1 was supplied daily for 15 min.

At 21 days after sowing (DAS), uniform seedlings were transplanted to a nutrient-film technique hydroponic system, applying the nutrient solution at an EC of 1.8 ± 0.1 dS m⁻¹ and a pH of 6.5 ± 0.1. The seedlings were subjected to a total of six lighting treatments, i.e., four lighting patterns (6 h/6 h, 12 h/12 h, 6 h/2 h, and 18 h/6 h light/dark cycles) and two continuous light treatments at different intensities (24 h-200 and 24 h-400). The DLI was 17.28 mol m⁻² d⁻¹ for all light treatments except for the 24 h-400 treatment, in which it was 34.56 mol m⁻² d⁻¹(Table 1). The temperature, humidity, and CO₂ concentration were 23 ± 2 °C, 50–70%, and 1000 ± 50 ppm, respectively.

2.2. Measurements

2.2.1. Growth and Flowering Parameters

Plant growth was evaluated based on dry weight. Shoot samples, including leaves, stem, and flower stalk, were collected after the last day of harvesting at 80 and 60 DAS for the “Orange Gem” and “Citrus Orange” varieties, respectively, and placed into a hot-air oven at 80 °C for 3 days. As previously stated, the flowers were harvested every 2 days until the final day of harvesting. Subsequently, the flower samples were placed in a hot-air oven at 35 °C for 10 days before conducting the dry-weight measurements and chemical analysis. The dry-weight values were combined with measurements of the shoot and flower parts. The flowering parameters, which included the days from sowing to the appearance of the first flower bud (which was defined as when its size reached 0.5 cm), fresh and dry weights of each flower, flower diameter, and flower color (L,a,b; color meter Konica Minolta CR-20, Tokyo, Japan), were measured. Then, the flower yield, which was calculated based on the total number of flowers and the total fresh, and dry weights of the flowers, was determined for each plant.

2.2.2. Secondary Metabolites and Antioxidative Activity

The TCC and TPC were analyzed to investigate secondary metabolite accumulation in calendula flowers, and antioxidative activity was determined via the DPPH radical scavenging activity assay. The TCC was measured from extracts obtained from dried calendula flowers. A total of 40 mg of dried flower powder was immersed in a glass vial containing 2 mL of N, N-dimethylformamide (DMF) and placed in the dark for 36 h. Subsequently, a spectrophotometer (SH−1300 Lab, Corona Electric Co., Ltd., Ibaraki, Japan) was used to measure the absorbance of the extracts at 480, 645, and 663 nm using DMF as a blank. Pigment concentrations were calculated using the following equations [9]:

C h l A = 11.65 A 664 - 2.69 A 647

(1)

C h l B = 20.81 A 664 - 4.53 A 647

(2)

T C C = \frac{(1000 A 480 - 0.89 C h l A - 52.02 C h l B)}{245}

(3)

The results were expressed as milligrams per gram of dry weight (mg g⁻¹ DW).

The extraction method for measuring the TPC and the DPPH radical scavenging activity was modified from that described in Qin et al. [10]. A total of 0.2 g of dried flower powder was weighed and added into a test tube containing 3 mL of 70% methanol (v/v) and mixed well before being incubated at 70 °C for 10 min in a water bath. The sample was centrifuged at 6000 rpm for 10 min, and then the supernatant was collected. The pellet was re-extracted using the same procedure, and then this supernatant was combined with the previously obtained supernatant. Then, 70% methanol was added to obtain a volume of 10 mL.

The sample’s TPC was measured using the Folin–Ciocalteu colorimetric assay [11]. The sample solution was subjected to a fourfold dilution with 70% methanol. Gallic acid solutions at concentrations of 0, 0.02, 0.04, 0.06, 0.08, and 0.10 mg mL⁻¹ were prepared to obtain the standard calibration curve. A total of 2.5 mL of 10% Folin–Ciocalteu reagent was combined with the sample and standard solutions. Then, 2 mL of 7.5% sodium carbonate solution was added, and the mixture was thoroughly mixed at room temperature. The absorbance at 765 nm was measured after 1 h using a spectrophotometer (ASV11D, As One, Corp., Osaka, Japan). The results were expressed as milligrams of gallic acid equivalents per gram of dry weight (mg GAE g⁻¹ DW).

The protocol for the spectrophotometric analysis described in Nguyen et al. [12], with some modifications, was used to determine the DPPH radical scavenging activity in calendula flowers. A sample solution was prepared via fivefold dilution with 70% methanol. Trolox solutions at concentrations of 0, 200, 400, 600, 800, and 1000 µM were also prepared for standardization. Then, 0.1 mL of sample solution or Trolox solution was added to 3.9 mL of DPPH solution (60 µM) in methanol. The mixture was thoroughly mixed at room temperature and then placed in the dark for 30 min. Then, the absorbance at 517 nm was measured using a spectrophotometer. The results were quantified as milligrams of Trolox equivalents per gram of fresh weight (mg TE g⁻¹ DW).

2.2.3. EUE

The EUE of the LED lamps was defined by the following equation:

E U E = \frac{Y \times D}{K}

(4)

where Y is the total plant yield up until the last day of harvesting. This parameter can represent any yield; for instance, of the number of flowers (flower plant ⁻¹) or the fresh and dry weights of flowers (g plant ⁻¹). D is plant density (plant m⁻²), and K is the total electricity consumption of the LED lamps installed in the planting area until the last day of harvesting (kWh).

2.2.4. Statistical Analysis

The experiment was conducted using a randomized design. Data were analyzed by comparing the means using a one-way ANOVA and Tukey’s HSD post hoc test. All statistical analyses were conducted in SPSS (IBM SPSS Statistics, Version 23.0).

3. Results

3.1. Growth and Flowering

Various growth and flowering parameters were examined to understand the effect of different light patterns on calendula growth and flowering in PFAL cultivation. Figure 1 shows the morphological variations in calendula plants cultivated under different conditions at 55 DAS. In the cultivar “Orange Gem”, the plant dry weight increased with a longer photoperiod, and significant differences in this parameter were detected among the lighting patterns. However, no significant differences were detected between lighting patterns within the same photoperiod. The same results were obtained for the cultivar “Citrus Orange”, except in the 24 h-400 treatment, where the plant dry weight was lower than that observed in the 24 h-200 treatment (Figure 2A). In addition, the flower-to-shoot ratio was analyzed to examine the allocation of photosynthates by plants. In “Orange Gem”, no significant differences in the flower-to-shoot ratio were observed among the lighting patterns under the 18 and 24 h photoperiods, while those under the 12 h photoperiod resulted in a lower ratio. As observed for the plant dry weight, the different lighting patterns under the same photoperiod did not influence the flower-to-shoot ratio, except under the 18 h/6 h photoperiod for “Citrus Orange”, in which this parameter was significantly higher than under the 6 h/2 h photoperiod (Figure 2B).

Interestingly, the number of days from sowing to the appearance of the first flower bud decreased when the plants were subjected to a total photoperiod of longer than 12 h. As a result, the first flower bud appeared earlier in plants grown under the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 photoperiods than in those grown under 6 h/6 h and 12 h/12 h conditions. For “Orange Gem” and “Citrus Orange”, the growth period was shortened by approximately 5–9 days and 3–5 days, respectively (Figure 3A). When grown under 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 conditions, the fresh weight of “Orange Gem” flowers significantly decreased, while it increased under the 6 h/6 h and 12 h/12 h conditions. “Citrus Orange” plants grown under 12 h/12 h conditions yielded a higher flower fresh weight than those under the 6 h/2 h, 18 h/6 h, 24 h-200 and 24 h-400 conditions (Figure 3B). The flower dry weight of “Orange Gem” showed significantly differences. That for 12 h/12 h was higher than that under 18 h/6 h and 24 h-200 conditions. “Citrus Orange” did not vary significantly among all the lighting patterns. However, the flower dry weight tended to increase when the plants were cultivated under continuous lighting rather than divided lighting. For a lighting period of 24 h, the higher the light intensity, the higher the flower dry weight (Figure 3C).

As observed for the flower fresh weight, plants cultivated under the 6 h/6 h and 12 h/12 h conditions had a larger flower diameter than those cultivated under the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 conditions. The extension of the total photoperiod appeared to have an effect on the flower diameter, which was reduced in plants grown under a total photoperiod of 18 h (6 h/2 h and 18 h/6 h) and continuous lighting. No significant differences in flower diameter were detected among the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 treatments for “Orange Gem”. For “Citrus Orange”, the flower diameters under the 6 h/6 h and 12 h/12 h conditions were higher than those grown under the 6 h/2 h, 18 h/6 h and 24 h-400 conditions (Figure 4A). The lighting patterns also influenced flower color. In “Orange Gem” flowers, the redness (a-value) increased under the 6 h/6 h and 12 h/12 h conditions. For “Citrus Orange”, plants grown under the 12 h/12 h and 24 h-400 conditions exhibited increased flower redness. However, the 6 h/2 h and 18 h/6 h treatments resulted in a significant decrease in this parameter (Figure 4B). Unlike flower redness, the flower brightness (L-value) and greenness (b-value) remained constant under different lighting patterns (Figure S1).

On the last harvesting day, subjecting the plants to continuous lighting increased the total number of flowers. More flowers were produced under these conditions than under other lighting patterns, but this was not significantly different between 24 h-200 and 24 h-400 treatments for “Orange Gem”. The number of flowers was less abundant when plants were grown under 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h conditions compared with the 24 h-400 treatment for “Citrus Orange” (Figure 5A). The total flower fresh weight of “Orange Gem” showed this was higher under the 24 h-400 treatment than those from the 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h treatments. For “Citrus Orange”, the plants cultivated under the 24 h-200 and 24 h-400 conditions showed higher values than those from the 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h treatments (Figure 5B). Similar results were obtained for the total dry weight of “Orange Gem” flowers, which was higher under 24 h-400 of lighting than those under 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h conditions. In contrast, no significant differences in this parameter were observed in “Citrus Orange” grown under 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 conditions. However, in both varieties, the lower total dry weight of flowers was achieved in plants grown under the 6 h/6 h and 12 h/12 h conditions compared with the others (Figure 5C).

3.2. Secondary Metabolites and Antioxidative Activity

The different lighting patterns tested in this study resulted in significant differences in the TCC. Similar results were obtained for the “Orange Gem” and “Citrus Orange” plants. The TCC of the plants grown under the 6 h/6 h and 12 h/12 h lighting pattern were higher than those grown under the other conditions. When plants were subjected to a photoperiod of longer than 12 h, the TCC decreased. However, in the “Orange Gem” plants, no significant differences in this parameter were observed among the 6 h/2 h, 18 h/6 h, 24 h-200, and 24 h-400 treatments. For “Citrus Orange”, a lower TCC was achieved under the 6 h/2 h and 24 h-400 than under the 6 h/6 h and 12 h/12 h treatments (Figure 6A). A correlation was also detected between the TCC and redness (r = 0.806, p ≤ 0.01)

“Orange Gem” plants subjected to different treatments exhibited a significantly different TPC, which was not the case for “Citrus Orange” plants. The longer the photoperiod, the higher the TPC. High TPC was detected under the 18 h/6 h, 24 h-200 and 24 h-400 treatments, but significant differences were not found among these. In contrast, the 6 h/6 h, 12 h/12 h, and 6 h/2 h treatments resulted in a low TPC (Figure 6B). As observed for the TPC, increased DPPH scavenging activity was observed in “Orange Gem” plants subjected to the 24 h-200 and 24 h-400 treatments, although no significant differences were detected between them. Plants showed reduced DPPH scavenging activity when grown under 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h conditions. Significant differences in this parameter were observed in “Citrus Orange” plants under various lighting patterns. The values recorded under the 6 h/2 h treatment were higher than those for the 6 h/6 h, 24 h-200, and 24 h-400 treatments. (Figure 6C). Moreover, DPPH scavenging activity was more correlated with the TPC (r = 0.841; p ≤ 0.01) than the TCC (r = −0.321; p ≤ 0.01) (Figure 6D).

3.3. EUE

Calendula flowers are available for sale in several forms, such as fresh, dried, and counted flowers. Therefore, the plants’ EUE was calculated based on three types of flower yield, as defined by the number of flowers, fresh weight, and dry weight. The “Orange Gem” result showed a significant difference regarding the number of flowers. The 24 h-200 treatment showed the highest EUE. For “Citrus Orange”, a higher EUE was detected under the 24 h-200 treatment than under the 6 h/6 h and 12 h/12 h treatments (Figure 7A). When considering the total fresh weight of flowers, lower EUE values were obtained for plants cultivated under a shorter photoperiod. For “Orange Gem”, the 24 h-200 treatment resulted in a higher EUE than the 6 h/2 h treatment, whereas no significant differences were observed among 6 h/6 h, 12 h/12 h, 18 h/6 h, and 24 h-400 treatments. Similarly, for “Citrus Orange”, the 24 h-200 and 24 h-400 treatments resulted in higher EUEs than the 6 h/6 h, 12 h/12 h, 6 h/2 h and 18 h/6 h treatments (Figure 7B). When the EUE was computed using the total dry weight of flowers, significantly lower values were obtained for both varieties grown under the 6 h/6 h and 12 h/12 h treatments than under the others, although a significant difference was not found (Figure 7C).

The EUE calculated based on the TCC, TPC, and DPPH scavenging activity showed significant differences among the different lighting patterns. For “Orange Gem”, the EUE of the TCC was higher when plants were subjected to the 24 h-200 rather than the 6 h/6 h, 12 h/12 h, 6 h/2 h and 24 h-400 treatments. A similar result was found for “Citrus Orange”. Additionally, the value under the 18 h/6 h treatment was also higher than those four treatments (Figure 7D). The EUE of the TPC showed similar results in both varieties. The value obtained from the 24 h-200 treatment was significantly increased compared with those under the 6 h/6 h, 12 h/12 h, 6 h/2 h, and 18 h/6 h treatments for “Orange Gem”. For “Citrus Orange”, the 6 h/2 h, 18 h/6 h, and 24 h-200 treatments showed a higher EUE than the 6 h/6 h and 12 h/12 h treatments (Figure 7E). Likewise, the EUE of the DPPH scavenging activity showed the same result as the EUE of the TPC (Figure 7F).

4. Discussion

4.1. A Long Photoperiod with Low Light Intensity Has a Greater Influence on the Growth and Flowering of Calendula Than a Shorter Photoperiod with High Light Intensity Under the Same DLI

Lighting strategies have been studied and developed for application in greenhouse cultivation over decades. The application of supplementary lighting improves plant growth and flowering [13,14]. However, these studies do not clarify whether such improvements are due to a longer photoperiod or a higher DLI. In PFALs, it is easier to provide plants with several combinations of photoperiod and light intensity under the same DLI than in greenhouses. Our study showed that shoot dry weight was higher for plants grown under a longer photoperiod with a lower light intensity than for those under a shorter photoperiod with a higher light intensity. In contrast, watercress (Nasturtium officinale L.) cultivated under a different combination of light intensity and photoperiod with the same DLI showed a decreased shoot dry weight under low light intensity with long photoperiod conditions [15]. Generally, some plants respond to shade (low light intensity) by leaf expansion, resulting in thinner leaves (increased specific leaf area) but with less chlorophyll content, leading to low light harvesting, lower net photosynthetic rates, and poor growth [16]. However, the higher plant dry weight observed in our study indicates that longer photoperiods with low light intensity are not an adaptation response to shade but, rather, a reflection of increased growth. Similar results have been reported in a study by Elkins and Iersel [17] on rudbeckia seedlings. As shown in Figure 1, according to the botanical characteristics of calendula, the first flower emerges at the apex of the shoot. The plant does not produce leaves along the stem; instead, it develops branches for flowering, accompanied by leaflets on the flower stalk, thereby enhancing the photosynthetic efficiency and growth. Moreover, generally, the quantum yield of photosystem II (Φ_PSII) increases while the electron transport rate decreases under a longer photoperiod with lower light intensity. This phenomenon leads to an improvement in the daily photochemical integral (DPI), which is the cumulative electron transport via photosystem II over a period of 24 h, indicating that plants are more efficient at photosynthesis [18].

According to their photoperiodic responses, flowering plants are classified into three groups: short-day plants, long-day plants, and day-neutral plants. Calendula is a facultative long-day plant that initiates flowering under any day length, but its flower buds can emerge earlier under long-day conditions [19]. In the present study, prolonging the photoperiod resulted in earlier flower-bud initiation. Nevertheless, some of the flower characteristics were altered in response to an increased duration of lighting, including a reduction in flower weight, flower size, and flower redness. Plants can adapt to environmental changes by changing the distribution of their biomass to different organs. In calendula, the flowers are considered a sink and the leaves a source. In this study, the 6 h/6 h and 12 h/12 h treatments resulted in lower flower-to-shoot ratios but produced flowers with a larger diameter and higher weight compared with the other treatments. This is in line with previous findings on the effect of the sink/source ratio on chrysanthemum flower size. In particular, when they kept only one flower on the stalk to reduce the sink activity, the sink/source ratio also decreased, resulting in a larger flower compared with the treatment without limiting flower numbers. Therefore, variations in the sink/source ratio affect flower size [20]. Flower size and number are functional characteristics associated with reproduction. According to the life-history theory, there is a trade-off between these two characteristics that potentially accounts for adaptation [21]. The experimental data obtained in the present study are consistent with this theory. The plants producing more flowers showed a smaller flower size and lower flower biomass. However, the effect of the photoperiod has been shown in the increased flower numbers. This also relates to the previously described growth increment. Doubling the DLI from 24 h-200 to 24 h-400 did not result in a significant change in growth and flower productivity. This result might be due to the photosynthetic pigments absorbing excess light energy, causing photoinhibition and limiting plant growth [22], which is different from the response of the nasturtium plant [23].

4.2. Increased Light Intensity Affects the TCC, While an Increased Photoperiod Influences the TPC Under the Same DLI

Environmental factors, such as light, greatly affect the synthesis and accumulation of secondary metabolites in plants. As mentioned above, the main function of a flower is not photosynthesis but to attract pollinators. Instead of chlorophyll, calendula petals produce a yellow–orange pigment that is classified as a carotenoid [24]. The results of our study indicated that the TCC present in calendula petals, which is related to redness, decreased when the plants were exposed to a lower light intensity. Similarly, it has been reported that orchids exhibit a decrease in pigment production (specifically, anthocyanin) when subjected to a lower light intensity. The primary role of pigments in flowers is to attract potential pollinators, as flowers serve the purpose of reproduction. Pollinators can easily detect bright colors in a bright-light environment. The decrease in pigment concentration in calendula flowers under low-light conditions may be an adaptive response to enhance their floral visibility to pollinators under such conditions [25]. In contrast, light intensity was shown to regulate the TPC. Plants subjected to a photoperiod of longer than 12 h showed increased antioxidative activity. Generally, the exposure of plants to light conditions that fully saturate the process of CO₂ assimilation and that generate excess excitation energy leads to the accumulation of reactive oxygen species, consequently triggering photo-oxidative stress [26]. Therefore, plants produce antioxidants, such as phenolic compounds, to protect themselves against oxygen damage to the photosynthetic systems. Xu et al. [23] reported that nasturtium cultivated under 24 h of lighting adapted to the excessive light by increasing its antioxidative capacity. Calendula can also be considered a light-adaptive plant tolerant to continuous lighting.

4.3. Application of Lighting Patterns Could Optimize the EUE and Alleviate Production Costs in PFALs

The primary factor affecting the operational costs involved in PFAL cultivation is the electricity consumption attributed to lighting. Evaluating the EUE of plant production is crucial for assessing the feasibility of PFALs. Several studies have investigated the effect of light on plant growth and yield in such controlled environments. However, EUE has not been sufficiently investigated. Calendula flowers can be sold in different product forms. For example, fresh flowers can be sold by number, while fresh or dry flowers can be sold by weight. Our study demonstrated that the EUE calculated based on calendula yield, including the number of flowers and both their fresh and dry weights, varied among plants grown under different lighting patterns with the same DLI. Because the DLI was equal for all treatments, the electricity consumption of the lighting was also equal. Although the yields obtained under the 24 h-400 and 24 h-200 treatments did not differ significantly, the electricity consumption was higher (because of the higher DLI) under the former, which resulted in a lower EUE. Similar results were obtained for EUEs in the production of secondary metabolites. The highest EUEs calculated based on the TCC, TPC, and DPPH scavenging activity were observed under the 24 h-200 treatment. Tong et al. [27] suggested the following possible solutions to improve the EUE: (1) improving the conversion coefficient from electric to PAR energy by using high-efficiency lamps; (2) increasing the light absorption by plants; and (3) appropriately controlling environmental factors such as temperature, humidity, and wind velocity. According to the findings in our study, changing lighting patterns is another method for improving the EUE of PFALs. Moreover, in some countries, electricity rates are different at different times of the day. For example, there is a peak time during 9:00–22:00 and the off-peak time during 22:00-9:00 in Thailand [28]. By taking the example of the 18 h/6 h (light/dark) lighting pattern, the electricity consumption of the lighting is 0.164 kWh per day per m² (plantable area [29]). The electricity fee will be reduced from THB 12.4 to THB 11.4 (THB: Thai currency) when we maximize the use of off-peak time as a lighting period (the cost of the lighting based on the basic electricity plan), which means a 9% saving in electricity costs. Therefore, appropriate lighting patterns can improve the EUE and, when applied to match off-peak hours, can also reduce the energy cost of PFALs.

5. Conclusions

Calendula is a plant that produces edible flowers containing considerable quantities of beneficial phytochemicals and shows great potential as a profitable crop for cultivation in PFALs. Our research investigated the response of PFAL-cultivated calendula plants to different light patterns. The results showed that changing the light pattern affected growth and flowering. Plants grown under 24 h of lighting exhibited higher plant dry weights, flower yields, TPC, and antioxidative activity than those grown under the 12 and 18 h photoperiods. When the photoperiod was longer than 12 h, the number of days to the appearance of the first flower, the flower weight, flower diameter, and the TCC all decreased. The quantity and quality of flowers can be enhanced by modifying the lighting pattern while maintaining the same amount of electricity consumption. The 24 h-200 treatment resulted in the highest EUE value. We suggest that for calendula grown in PFALs, it is possible to achieve a higher yield or production of secondary metabolites at low electricity costs by exposing the plants to continuous lighting at 200 PPFD. The application of various types of lighting technology has been extensively researched in controlled-environment agriculture to enhance crop yield and quality. However, the interaction between light properties and other environmental factors, such as the light spectrum and fertilizer application, still needs to be examined in future research.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture14122208/s1, Figure S1: Flower brightness (A) and flower greenness (B) of calendula plants grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. There are no significant differences among treatments based on Tukey’s test (p < 0.05).

Author Contributions

Conceptualization, M.M., N.L. and M.T.; methodology, M.M. and N.L.; software, M.M.; validation, M.M., N.L. and D.F.R.; formal analysis, M.M.; investigation, M.M., D.F.R. and T.R.; data curation, M.M. and T.R.; writing—original draft preparation, M.M.; review, editing, and visualization, M.M. and N.L.; supervision, N.L. and M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article and supplementary material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Morphology of calendula plants grown in a PFAL under different lighting patterns at 55 DAS.

Figure 2. Plant dry weight (A) and flower-to-shoot ratio (B) of calendula grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Figure 3. Number of days from sowing to the appearance of the first flower bud (A), and the fresh and dry weights of calendula flowers (B,C) obtained from plants grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Figure 4. Flower diameter (A) and flower redness (B) of calendula plants grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Figure 5. Yield of calendula (total number of flowers (A), total fresh and dry weights of flowers (B,C)) grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Figure 6. Total carotenoid content (A), total phenolic content (B), DPPH scavenging activity (C), and the correlation between secondary metabolites and antioxidative activity (D) of calendula grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Figure 7. EUE of yield (total number of flowers; (A) total flower fresh weight; (B) and total flower dry weight; (C)), secondary metabolites (total carotenoid content; (D) and total phenolic compound content; (E)) and DPPH scavenging activity (F) of calendula grown in a PFAL under different lighting patterns. Data are shown as the mean ± SD; n = 6. The different letters show significant differences among treatments applied to the same variety based on Tukey’s test (p < 0.05).

Table 1. Summary of the lighting patterns applied in this study. The arrows indicate the lighting hours.

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MDPI and ACS Style

Munyanont, M.; Lu, N.; Rachma, D.F.; Ruangsangaram, T.; Takagaki, M. Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting. Agriculture 2024, 14, 2208. https://doi.org/10.3390/agriculture14122208

AMA Style

Munyanont M, Lu N, Rachma DF, Ruangsangaram T, Takagaki M. Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting. Agriculture. 2024; 14(12):2208. https://doi.org/10.3390/agriculture14122208

Chicago/Turabian Style

Munyanont, Maitree, Na Lu, Dannisa Fathiya Rachma, Thanit Ruangsangaram, and Michiko Takagaki. 2024. "Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting" Agriculture 14, no. 12: 2208. https://doi.org/10.3390/agriculture14122208

APA Style

Munyanont, M., Lu, N., Rachma, D. F., Ruangsangaram, T., & Takagaki, M. (2024). Lighting Patterns Regulate Flowering and Improve the Energy Use Efficiency of Calendula Cultivated in Plant Factories with Artificial Lighting. Agriculture, 14(12), 2208. https://doi.org/10.3390/agriculture14122208

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