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Article

Optimizing Indoor Hemp Cultivation Efficiency through Differential Day–Night Temperature Treatment

by
Gwonjeong Bok
1,
Seungyong Hahm
2,
Juhyung Shin
1 and
Jongseok Park
1,2,*
1
Department of Bio-AI Convergence, Chungnam National University, Daejeon 34134, Republic of Korea
2
Department of Horticultural Science, Chungnam National University, Daejeon 34134, Republic of Korea
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(10), 2636; https://doi.org/10.3390/agronomy13102636
Submission received: 14 September 2023 / Revised: 5 October 2023 / Accepted: 17 October 2023 / Published: 18 October 2023
Browse Figures
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Abstract

:
This study was conducted to determine the optimal temperature difference in day–night indoor cultivation conditions to enhance the flower yield and functional component contents of female hemp plants. Hemp clones were cultivated under five distinct day and night temperature differences (DIF) during the reproductive stage. The daytime and nighttime temperature settings were as follows: 18:30 °C (negative 12 DIF), 21:27 °C (negative 6 DIF), 24:24 °C (0 DIF), 27:21 °C (positive 6 DIF), and 30:18 °C (positive 12 DIF). Seven weeks after transplantation, the growth parameters, leaf gas exchange, total phenolic compounds, 2,2-diphenylpicrylhydrazyl scavenging activity, and cannabinoid contents were analyzed. The total shoot biomass based on dry weight was highest at 21:27, reaching 41.76 g, and lowest at 30:18, measuring 24.46 g. However, the flower biomass, which is the primary production site, was highest at 24:24 and lowest at 18:30, showing a 4.7-fold difference. The photosynthesis-related parameters were temperature-dependent and strongly correlated with biomass production. The cannabinoid content of the hemp leaves increased at 21:27, whereas that of the hemp flowers increased at 27:21. The findings of this study indicate that the optimal temperature condition for female hemp flower production in a limited space is positive 6 DIF treatment, which corresponds to 27:21 °C. These results can contribute to advancements in indoor crop cultivation technology.

1. Introduction

Cannabis sativa L. has been cultivated for thousands of years for multiple purposes, such as food, fiber, and medicine [1,2]. There are two representative types of cannabis, namely, marijuana and hemp. While marijuana is mainly used for recreational purposes, sometimes leading to addictive behavior, hemp is considered a valuable industrial resource due to its fibers and seeds [3,4]. In recent years, hemp cultivation for medical purposes has attracted considerable attention from both the scientific community and the public because of its promising therapeutic properties and diverse medicinal applications [5]. In contrast to marijuana, hemp contains cannabidiol (CBD), a non-addictive medicinal compound. The potential medicinal benefits of hemp have generated interest in its use for various health-related issues, including pain management and neurological disorders [6].
The Agricultural Improvement Act of the United States has legalized the industrial use of hemp, allowing its cultivation and trade; furthermore, many European countries are in the process of legalizing hemp [7,8]. However, hemp remains a regulated crop in many countries, including Republic of Korea. Traditional outdoor hemp cultivation methods have been complemented or replaced by innovative indoor farming techniques to ensure the security and consistent production of high-quality medical hemps [9,10]. Among these indoor farming techniques, plant factories have emerged as advanced methods for cultivating crops in precisely controlled environments [11]. Controlled conditions within plant factories allow researchers to manipulate various growth parameters, facilitating optimization and enhancement of productivity [12,13,14].
Enhancing harvest yields in confined spaces is crucial for addressing global food demand and promoting sustainable agricultural development [15]. As indoor farming systems evolve, research is underway to explore various cultivation techniques, encompassing the development of novel varieties [16], as well as chemical [17] and physical [18] treatment methods, all with the aim of boosting crop production within limited spatial constraints. Photosynthesis and biomass production in plants depend on environmental factors such as light and temperature. Typically, optimal temperature and light levels can enhance photosynthetic efficiency. However, excessively high temperature and light levels can have a negative impact on photosynthesis [19]. Therefore, it is crucial to determine the appropriate environmental conditions for indoor crop cultivation.
Temperature control is a critical environmental factor that plays a pivotal role in plant growth and development, including that of medical hemp [20,21]. The optimal temperature for hemp cultivation is approximately 24 °C [22], with temperatures above 30 °C or below 20 °C resulting in decreased photosynthesis and nighttime respiration rates [19]. In terms of cannabinoid contents, tetrahydrocannabinol (THC) levels have been reported to increase under low temperatures of approximately 23 °C compared to high temperatures of approximately 32 °C [23]. Plants exhibit various physiological responses to both daily average temperatures and the difference between daytime and nighttime temperatures (DIF). In general, negative DIF conditions are reported to suppress shoot growth in various plant species [24,25]. Considering these effects, DIF treatment can be used in crop cultivation to control plant height, as an alternative to pesticides, and to regulate flowering and fruit quality, among other applications [26,27].
However, the effect of DIF on medical hemp cultivated in indoor farming systems remains unexplored. The objective of this study was to determine the optimal DIF for enhancing the growth, yield, and chemical composition of medical hemp cultivated in an indoor farming system.

2. Materials and Methods

2.1. Plant Materials and Experimental Design

This study was conducted using “V4(IT342821)”, a low-THC hemp strain developed by the Korean Rural Development Administration. To ensure consistency among experimental samples, 10 cm cuttings from the apical meristem were treated with a rooting promoter (Rootone, ISK, Japan) that was primarily composed of NAA, and then planted in rockwool cubes (width × depth × height = 2 × 2 × 4 cm) to induce root development. Vegetative and rooting stage plants were cultivated in a plant factory using artificial lighting, and the cultivation conditions are listed in Table 1. Irrigation was performed by dissolving equal amounts of Perticare 20-20-20 + 2MgO + micro and Perticare 13-0-1 + 16(CaO) + 5(MgO) (Yara International ASA, Oslo, Norway) in water and adjusted to the appropriate electrical conductivity for each growth stage.
After 4 weeks, which included the rooting and vegetative stages, the average shoot height of the clones was 12.58 cm, the leaf number and leaf area were 13.56 and 86.66 cm2, respectively, and the shoot biomass was 2.09 g. The clones were transplanted into 1 L pots filled with a substrate consisting of horticultural soil, perlite, and vermiculite in a ratio of 5:3:2. The plants were then transferred to growth chambers (DS-310S; Daewon Science Inc., Bucheon, Republic of Korea) for DIF treatment during the reproductive period. The internal dimensions of the chamber were 70 cm (width) × 60 cm (depth) × 120 cm (height), with a planting distance of 35 cm (initial planting density: 9.52 m−2). Four replicates were performed for each treatment. For the DIF treatments, DIFs of −12, −6, 0, +6, and +12 were established with daytime/nighttime temperatures of 18:30, 21:27, 24:24, 27:21, and 30:18 °C, respectively, based on an average daily temperature of 24 °C. At the beginning of the reproductive stage, the light intensity was set at 400 μmol·m−2·s−1 based on the apical meristem. Due to differences in plant growth according to DIF treatment, the distance from the apical meristem to the light source was adjusted uniformly for all plants once a week. The light intensity at the apical meristem of the plants increased to 700 μmol·m−2·s−1 just before harvesting.

2.2. Measurement of Plant Growth Parameters

The plant growth parameters were measured immediately after harvesting, which occurred after 7 weeks of the reproductive growth stage. To measure the height and width of the aboveground parts, each plant was rotated by 90° at the same position, and four shoot images were captured. Subsequently, the average height and width of the four images of each plant were calculated using ImageJ software (version 1.53t, National Institute of Health, Bethesda, Montgomery, MD, USA) (Figure 1). The chlorophyll content was measured using a nondestructive chlorophyll meter (SPAD-502; Minolta Camera Co., Ltd., Osaka, Japan). The leaf area was measured using a leaf area meter (Li-3100; LICOR, Lincoln, Lancaster, NE, USA), and the number of leaves was manually counted. The fresh and dried biomasses of the plants were measured using an electronic scale (MW-2N; CAS Co., Ltd., Seoul, Republic of Korea). After measuring the fresh weights, the samples were freeze-dried at −80 °C for 7 days using a freeze dryer (TFD550, IlshinBioBase, Dongducheon, Republic of Korea). Subsequently, the dry weights of the samples were measured and used for the analysis of the total phenolic compound content, 2,2-diphenylpicrylhydrazyl (DPPH) scavenging activity, and cannabinoid quantification.

2.3. Measurement of Leaf Gas Exchange

The net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular CO2 concentration were measured in well-developed leaves from the third to seventh leaves of the apical meristem of the main stem using a portable photosynthesis system (LI-6800; Li-COR). All parameters were measured 3 h after turning on the lights, with the CO2 level maintained at 400 μmol·mol−1, the light intensity set to 400 μmol·m−2·s−1, the humidity maintained at ambient levels, and the temperature adjusted according to the specific daytime treatment setting value for each treatment condition.

2.4. Analysis of the Total Phenolic Compound Content and DPPH Scavenging Activity

To analyze the total phenolic compound (TPC) content and DPPH scavenging activity, 20 mg of freeze-dried sample was mixed with 2 mL of 90% methanol and sonicated for 30 min. The mixture was centrifuged at 21,055× g for 10 min and the supernatant was isolated. The TPC content was determined according to the method described by Singleton and Rossi [28]. Folin–Ciocalteu reagent (10 μL), distilled water (150 μL), and supernatant (20 μL) were mixed and allowed to react for 5 min. Subsequently, 7.5% Na2CO3 30 (μL) was added to obtain a final volume of 200 μL, and the reaction was continued for an additional 30 min. The DPPH scavenging activity was determined according to the method described by Braca et al. [29]. Twenty milligrams of DPPH powder (Sigma Aldrich Chemical Co., St. Louis, MO, USA) was dissolved in 50 mL of 90% methanol to prepare a stock solution. Subsequently, the stock solution (10 μL), supernatant (20 μL), and 90% methanol (170 μL) were mixed to obtain a final volume of 200 μL, and the reaction was continued for 60 min under dark conditions. The absorbance was measured using a microplate reader (Epoch, BioTek, Winooski, VT, USA) at 765 and 517 nm for the TPC content and DPPH scavenging activity, respectively. A calibration curve was generated using gallic acid (Sigma Aldrich Chemical Co., Burlington, MA, USA) at concentrations ranging from 10 to 1000 ppm for accurate TPC quantification. The DPPH inhibition rate was calculated using the following formula:
[blank absorbance − sample absorbance)/blank absorbance] × 100.

2.5. Analysis of the Cannabinoid Content

The cannabinoid content was analyzed using high-performance liquid chromatography (Agilent1260, Agilent, Santa Clara, CA, USA) according to the method described by Hahm et al. [30] with modifications. The freeze-dried hemp sample was ground to powder using a blender and filtered through a 50 μm sieve. The sample powder (100 mg) was then mixed with 2 mL methanol/hexane (9:1) and sonicated for 30 min. Subsequently, the supernatant obtained by centrifuging at 21,055× g for 10 min was filtered through a 0.45 µm syringe filter and added to vials for quantitative cannabinoid analysis. In a gradient elution condition consisting of 0.1% formic acid in water and 0.1% formic acid in acetonitrile as the mobile phases, 10 μL of the filtered samples were passed through a Poroshell 120 EC-C18 column at a flow rate of 1 mL·min−1 and a temperature of 25 °C. The detection wavelength was 210 nm.

2.6. Calculation of the Total Biomass Yield and the Total Cannabinoid Yield

To calculate the total biomass production per square meter of hemp, the individual shoot areas were calculated using the average width values for each DIF treatment ((width/2)2 × π). Subsequently, this value was divided by square meters to calculate the per square meter density of the hemp for each DIF treatment. Finally, to calculate the total biomass yield per square meter (TBY) of hemp, the density values were multiplied by the dry biomass using the following formula:
TBY (g·m−2) = biomass (g) × [10,000(cm2)/{(plant width (cm)/2)2 × π}]
Finally, the total cannabinoid yield per square meter (TCY) was determined by summing the quantitatively analyzed total content of cannabinoids in each treatment group.
TCY (mg·m−2) = TBY (g·m−2) × total cannabinoid content (mg·g−1)

2.7. Statistical Analysis

Analysis of variance was performed followed by Tukey’s multiple comparison test for significance (p ≤ 0.05). All statistical analyses were performed using SPSS (Version 22.0.0.1, SPSS Inc., Chicago, IL, USA). Graphs were created using SigmaPlot 15 (Inpixon, Palo Alto, CA, USA) and OriginPro 2023 (OriginLab Co., Northampton, MA, USA).

3. Results

3.1. Morphogenesis and Growth Parameters of Hemp

Notable differences were observed in the photographs of hemp plants subjected to DIF treatments. Specifically, the aboveground volume exhibited the most pronounced growth under the 21:27 treatment (Figure 1). Conversely, under the positive DIF treatment, a distinct enlargement in flower size was observed, which was easily confirmed through visual assessment (Figure 2). Images of apical meristem flowers showed that minimal development occurred in the negative DIF treatment, whereas normal flower development occurred in the 24:24 and positive DIF treatments.
The images show that the shoot height and width increased in the negative DIF treatments compared to the 0 DIF and positive DIF treatments (Table 2). The shoot height and shoot width increased by approximately 65% under the 18:30 treatment compared to the 30:18 treatment. Moreover, the leaf number, leaf area, and flower number were also significantly higher under the negative DIF treatments. Additionally, the SPAD values were higher at both 24:24 and 27:21, but did not exhibit significant differences.
The total hemp shoot biomass was highest in the 21:27 treatment, followed by the 18:30 and 24:24 treatments (Table 3). The stems exhibited the highest biomass in the 21:27 treatment, whereas the leaves had the highest biomass in the 18:30 treatment, and the flowers reached their peak biomass in the 24:24 treatment. The proportions of stems, leaves, and flowers within the total shoots were the highest in the 21:27, 18:30, and 27:21 treatments, respectively.

3.2. Photosynthesis-Related Parameters

Measurement of photosynthesis-related factors through leaf gas exchange in hemp revealed that net photosynthesis increased significantly under the 24:24 treatment (Figure 3). Furthermore, it showed an increasing trend as the positive DIF level, which corresponds to an increasing daytime temperature, increased. The stomatal conductance and transpiration rate did not show significant differences, but exhibited a trend similar to that of the net assimilation rate. The intracellular CO2 concentration increased significantly in the negative DIF treatments.

3.3. TPC Content and DPPH Scavenging Activity

The TPC content of the hemp leaves did not show significant differences, but were increased in the negative DIF treatments and decreased in the positive DIF treatments (Figure 4). In contrast, the TPC content of the hemp flowers showed significantly higher values in the 27:21 treatment. The DPPH scavenging activity showed a trend similar to that of the TPC content, with high antioxidant activity observed in the 18:30 treatment in the leaves and in the 0 DIF and positive DIF treatments in the flowers.

3.4. Quantitative Analysis of Cannabinoids

Table 4 shows the results of the cannabinoid content analyses. Ten types of cannabinoids were quantitatively analyzed in the hemp leaves and flowers. In the hemp leaves, the contents of the major cannabinoids—cannabidiolic acid (CBDA), tetrahydrocannabinolic acid (THCA), and their precursor cannabigerolic acid (CBGA)—were highest at 18:30. The lowest content was observed in the 24:24 treatment. In the flowers (Table 5), the CBDA and THCA contents were the highest and lowest at the 27:21 and 18:30 treatments, respectively.

3.5. Total Biomass Yield and Total Cannabinoid Yield

The dry biomass and cannabinoid yield of hemp per unit area were calculated separately for the leaves and flowers (Figure 5). These values were then summed, and a statistical comparative analysis was conducted. The results differed from those of the individual plant biomass (Table 3). The highest TBY was obtained in the 27:21 treatment and the lowest yield was obtained in the 21:27 treatment. TCY showed a similar trend, with a significantly higher yield observed in the 27:21 treatment.

4. Discussion

4.1. DIF Treatment Significantly Affects the Growth and Flowering of Hemp

Several studies have discussed the suppressive effect of negative DIF treatments on shoot elongation in crops; however, the internal plant mechanisms underlying this phenomenon remain unclear [31]. The current hypothesis suggests that changes in plant growth due to DIF treatment can be attributed to alterations in nighttime respiration [32] and variations in the expression of plant hormones, such as gibberellins and auxins, in response to temperature changes [33].
However, in this study, the aboveground morphology of hemp grown under negative DIF conditions exhibited results contrary to those of typical DIF treatments. Specifically, the results of this study revealed significant increases in shoot length, width, and biomass under negative DIF conditions (Table 2). This observation is consistent with previous findings in lemon balm, where negative DIF treatments led to increased shoot growth compared with 0 DIF and positive DIF treatments [34].
In particular, a notable difference in flower formation was observed under negative DIF treatments, with the flowers formed under these treatments being markedly poorer than those formed under 0 and positive DIF treatments (Table 3). This appeared to be similar to how the negative DIF treatments inhibited the impact of short-day cycles on short-day plants. According to Myster and Moe [35], in contrast to long-day plants, flowering in short-day plants such as poinsettia is affected by negative DIF conditions, which can be attributed to changes in the concentration of endogenous gibberellins resulting from high nighttime temperatures. In the case of lilies, the flowers exhibited increased stem height and leaf count along with reduced flower count under negative DIF conditions at a low daily average temperature of 15 °C. However, at a daily average temperature of 20 °C, the stem height, leaf count, and flower count increased compared to 0 DIF conditions [36]. In Arabidopsis thaliana, it has been reported that flower development is more dependent on nighttime temperatures than daytime temperatures under negative DIF conditions [37]. These responses are attributed to the regulation of flowering-related genes by temperature-sensitive phytochrome control, considered to operate independently within an autonomous pathway rather than being temperature-dependent [38]. These findings suggest that DIF may not universally affect shoot growth or flower development in all plant species.

4.2. Physiological Changes in Hemp Based on Its Photosynthetic Capacity in Relation to Temperature

Plant photosynthesis exhibits temperature-dependent responses, and in the case of hemp, the optimal temperature range for photosynthesis varies widely depending on the variety [39]. In the results of Figure 3 in this study, it can be observed that photosynthesis increased sharply up to a daytime temperature of 24 °C and then the slope decreased. This phenomenon is believed to be influenced not only by the development of flowers and biomass accumulation during the reproductive growth stage, but also by the enhancement of antioxidant activity, including cannabinoids content.
The gas exchange necessary for plant photosynthesis occurs through the stomata of the leaves and involves the diffusion of water vapor along with the transpiration and absorption of carbon dioxide. Stomata are closely associated with photosynthesis and can be influenced by environmental factors such as light, carbon dioxide concentration, and temperature [39]. Furthermore, although photosynthetic assimilation and biomass production in plants vary among species, they are generally influenced by temperature. Plant transpiration rates depend on stomatal conductance [40], and our study showed similar results (Figure 3). Considering the results of Chandra et al. [41], an analysis of the changes in photosynthetic factors with respect to temperature was conducted for seven medical and fiber hemp varieties. The optimal temperature range for maximum photosynthesis varied depending on the plant variety. However, both photosynthesis and stomatal conductance showed similar patterns, with transpiration continuing to increase as the temperature increased within the range used in the experiments.
The intracellular CO2 concentration is associated with plant responses to environmental changes [42]. We observed that the intracellular CO2 values increased under conditions where photosynthesis did not occur normally, but decreased under conditions where photosynthesis occurred normally. Additionally, plant growth is closely related to both photosynthesis and respiration. Tomato growth has a strong correlation with nighttime and dark respiration, with a negative correlation with photorespiration [43]. In this study, it was suggested that the poor flower development and reduced physiological activity observed in hemp under the negative DIF treatments may have been influenced negatively by the higher nighttime temperatures.

4.3. Optimal DIF Conditions for Indoor Hemp Cultivation and the Practical Utilization Potential of Domestic Hemp

Plant phenolic acids are widely recognized as secondary metabolites and are considered as indicators of resistance to various stress conditions [44]. They are also known for their strong antioxidant activity and free radical scavenging ability [45]. DPPH scavenging is used as an indicator to measure the extent of antioxidant activity [46]. However, in this study, hemp exhibited a decrease in total phenolic content under the negative DIF treatments, where it did not show normal reproductive growth or development (Figure 4). This corresponds with research indicating that in lettuce, antioxidant and secondary metabolic activation may vary with the plant’s growth stages, and as plant growth progresses, normal growth temperature conditions are considered essential for the plant’s physiological activity [47]. These findings suggest the potential use of hemp’s total phenolic content and DPPH antioxidant activity as physiological active compounds for additional functional plant-based formulations when utilizing industrial hemp [48].
Cannabinoids are compounds with a C21 terpenophenolic backbone. Over 180 cannabinoids have been isolated from Cannabis sativa [49]. CBGA is generally synthesized in hemp via the alkylation of two precursors, olivetolic acid and geranyl pyrophosphate, and is further converted into THCA and CBDA in the hemp biosynthetic pathway [50,51].
In this study, ten cannabinoids were identified in the leaves and flowers. The compounds with the highest contents were CBDA, THCA, CBCA, and CBGA, and their combined total content accounted for 94.2% of the total cannabinoids detected. Regarding the pharmacological effects of each cannabinoid, CBD is known for its antioxidant and anti-inflammatory properties [52], and THC is reported to have psychoactive and anticancer effects [53]. However, research on the pharmacological activity of the remaining cannabinoids, including their acid forms, has been relatively limited, as indicated by the results of this study [54,55].
The total THC content, including that of THCA, was found to be in the range of 0.75 (18:30)~2.29 (27:21) mg·g−1, confirming that it falls within the acceptable THC levels for medical hemp standards in the United States and Europe [56].
The biomass yield of hemp in the 27:21 treatment was estimated to be approximately 2.3 tons·ha−1. The outdoor hemp harvest reported by the U.S. Department of Agriculture ranged from 1.2 to 3.4 tons·ha−1 depending on the moisture content of the soil [57], suggesting the potential feasibility of indoor hemp cultivation. Further research aimed at determining the optimal cultivation conditions for different growth stages through variation of DIF treatment levels may help increase indoor hemp cultivation yields.

5. Conclusions

Owing to climate and weather-related food shortages, the importance of indoor farming continues to increase. Maximizing crop production within a limited space is a major challenge in indoor farming, and techniques aimed at maximizing crop production may facilitate more efficient management of energy input into cultivation environment control systems. Through this study, it was found that the positive 6 DIF condition significantly promoted the development of female hemp flowers, resulting in the highest biomass production per unit area and cannabinoid production efficiency. These results highlight the potential of DIF treatments as an environmental control technology for industrial applications. They can be used to optimize product harvesting timing and maintain quality control standards, enhancing overall indoor farming efficiency and sustainability. In future research, analyses of different varieties and the energy requirements for temperature control could aid in the effective utilization of DIF treatments as indoor farming cultivation techniques.

Author Contributions

Experimental design and execution, data collection, and manuscript writing by G.B.; experimental setup preparation and data collection by S.H.; data analysis and protocol development by J.S.; project management, supervision, and manuscript review by J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This work was carried out with the support of the “Cooperative Research Program for Agriculture Science and Technology Development (project no. PJ017063022023)”, Rural Development Administration, Republic of Korea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work was supported by Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No.RS-2022-00155857, Artificial Intelligence Convergence Innovation Human Resources Development (Chungnam National University)).

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 02636 g001
Figure 1. Frontal images of hemp, cultivated under five DIF treatments immediately before harvest. The length of the red bars is 10 cm.
Figure 1. Frontal images of hemp, cultivated under five DIF treatments immediately before harvest. The length of the red bars is 10 cm.
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Figure 2. Images of apical meristem hemp flowers cultivated under five DIF treatments 1 week before harvest.
Figure 2. Images of apical meristem hemp flowers cultivated under five DIF treatments 1 week before harvest.
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Agronomy 13 02636 g003
Figure 3. Photosynthesis-related parameters of hemp according to DIF treatment, including net photosynthesis (A), transpiration rate (B), intercellular CO2 concentration (C), and stomatal conductance (D). The scatter plots represent the means, and error bars indicate the standard errors (n = 3). Different letters denote significant differences among the treatment groups, determined through one-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test at a significance level of 0.05.
Figure 3. Photosynthesis-related parameters of hemp according to DIF treatment, including net photosynthesis (A), transpiration rate (B), intercellular CO2 concentration (C), and stomatal conductance (D). The scatter plots represent the means, and error bars indicate the standard errors (n = 3). Different letters denote significant differences among the treatment groups, determined through one-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test at a significance level of 0.05.
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Figure 4. Total phenolic compound (TPC) content and antioxidant capacity of hemp according to DIF treatment. TPC content in the leaves (A), TPC content in the flowers (B), DPPH scavenging in the leaves (C), and DPPH scavenging in the flowers (D). The bars represent the means, and error bars indicate the standard errors (n = 3). Different letters denote significant differences among treatment groups, determined through one-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test at a significance level of 0.05.
Figure 4. Total phenolic compound (TPC) content and antioxidant capacity of hemp according to DIF treatment. TPC content in the leaves (A), TPC content in the flowers (B), DPPH scavenging in the leaves (C), and DPPH scavenging in the flowers (D). The bars represent the means, and error bars indicate the standard errors (n = 3). Different letters denote significant differences among treatment groups, determined through one-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test at a significance level of 0.05.
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Agronomy 13 02636 g005
Figure 5. Bar graphs indicating the biomass yield per square meter (A) and the cannabinoid yield per square meter (B) of hemp according to DIF treatment. The bars represent the means, and the error bars indicate the standard errors (n = 3). Different letters indicate significant differences among treatments for the combined values of the leaves and flowers, based on one-way analysis of variance with Tukey’s post-hoc test at a significance level of 0.05.
Figure 5. Bar graphs indicating the biomass yield per square meter (A) and the cannabinoid yield per square meter (B) of hemp according to DIF treatment. The bars represent the means, and the error bars indicate the standard errors (n = 3). Different letters indicate significant differences among treatments for the combined values of the leaves and flowers, based on one-way analysis of variance with Tukey’s post-hoc test at a significance level of 0.05.
Agronomy 13 02636 g005
Table 1. Specific environmental conditions for each cultivation stage.
Table 1. Specific environmental conditions for each cultivation stage.
Growth StageTemperature
(DT:NT z,
°C)		Humidity
(%)		CO2 Concentration
(μmol·mol−1)		PPFD y
(μmol·m−2·s−1)		Photoperiod
(DT:NT, h)		Cultivation
Period
(week)		Electrical Conductivity (dS·m−1), pH
Rooting248040010020:431.0, 5.8
Vegetative 246040020020:411.5, 5.8
Reproductive18:30
21:27
24:24
27:21
30:18		40400400
~
700		12:1272.0, 5.8
~
2.5, 5.8
z DT, daytime; NT, nighttime. y Photosynthetic photon flux density.
Table 2. Growth parameters of hemp according to DIF treatment.
Table 2. Growth parameters of hemp according to DIF treatment.
DIF
Treatment		Shoot
Width
(cm)		Shoot
Height
(cm)		Leaf
Number		Leaf
Area
(cm2)		Flower
Number		SPAD
Value
18:30 z46.55 b y55.98 bc715.9 a4440 a100.7 b56.73
21:2756.85 a77.85 a548.0 ab3519 a209.3 a56.70
24:2445.00 bc58.49 b202.0 b1568 b88.3 b65.23
27:2136.95 cd53.69 bc161.3 b1039 b70.7 b64.70
30:1834.31 d47.17 c150.0 b822 b56.0 b52.53
Significance x***************n.s
z Daytime temperature/nighttime temperature. y The data represent the means and standard errors (n = 3). Different letters indicate significant differences among treatments at the 5% significance level, as determined by Tukey’s test. x Significant differences were determined using one-way analysis of variance (ANOVA) and are denoted as follows: *** p < 0.001. n.s. indicates no significance.
Table 3. Biomass of hemp by different parts according to DIF treatment.
Table 3. Biomass of hemp by different parts according to DIF treatment.
DIF
Treatment		Fresh Weight (g)Dry Weight (g)
StemLeafFlowerTotal
Shoot		StemLeafFlowerTotal
Shoot
18:30 z56.49 a y
(38.2) x		87.39 a
(59.2)		3.85 c
(2.6)		147.73 ab
(100)		11.50 ab
(32.7)		22.76 a
(64.7)		0.93 d
(2.6)		35.18 ab
(100)
21:2770.41 a
(41.6)		66.17 a
(39.1)		32.65 b
(19.3)		169.22 a
(100)		16.07 a
(38.5)		18.69 a
(44.8)		6.99 c
(16.7)		41.76 a
(100)
24:2430.46 b
(21.0)		35.02 b
(24.2)		79.50 a
(54.8)		144.98 ab
(100)		8.36 bc
(22.6)		10.38 b
(28.1)		18.23 a
(49.3)		36.97 a
(100)
27:2123.53 b
(19.5)		24.57 b
(20.4)		72.29 a
(60.0)		120.39 ab
(100)		6.45 c
(21.0)		7.08 b
(23.1)		17.17 ab
(55.9)		30.69 ab
(100)
30:1819.88 b
(19.4)		24.71 b
(24.1)		58.03 a
(56.5)		102.62 b
(100)		4.91 c
(20.1)		6.60 b
(27.0)		12.95 b
(52.9)		24.46 b
(100)
Significance w************************
z Daytime temperature/nighttime temperature. y The data represent the means and standard errors (n = 3). Different letters indicate significant differences among treatments at the 5% significance level, as determined by Tukey’s test. x Percentage of each part within the total shoot. w Significant differences were determined using a one-way analysis of variance (ANOVA) and are denoted as follows: *** p < 0.001.
Table 4. Cannabinoid content of hemp leaves according to DIF treatment.
Table 4. Cannabinoid content of hemp leaves according to DIF treatment.
DIF
Treatment		CBDA y
(mg·g−1)		CBCA
(mg·g−1)		THCA
(mg·g−1)		CBGA
(mg·g−1)		CBL
(mg·g−1)		CBT
(mg·g−1)		CBC
(mg·g−1)		CBG
(mg·g−1)		CBD
(mg·g−1)		THC
(mg·g−1)
18:30 z9.7 c x0.960.643 b0.206 bc0.215 a0.224 b0.198 b0.0526 b0.092 c0.0828
21:2712.9 a0.860.699 a0.247 a0.212 a0.344 a0.195 b0.0691 a0.079 c0.0899
24:249.9 c0.820.584 c0.175 d0.195 b0.348 a0.208 a0.0562 b0.083 c0.0869
27:2111.7 b1.030.652 b0.218 b0.215 a0.368 a0.199 b0.0660 a0.116 b0.0900
30:189.4 c0.910.598 c0.192 cd0.216 a0.393 a0.200 b0.0670 a0.151 a0.0893
Significance w***n.s.********************n.s.
z Daytime temperature/nighttime temperature. y CBDA, cannabidiolic acid; CBCA, cannabichromenic acid; THCA, tetrahydrocannabinolic acid; CBGA, cannabigerolic acid; CBL, cannabicyclol; CBT, cannabitriol; CBC, cannabichromene; CBG, cannabigerol; CBD, cannabidiol; THC, tetrahydrocannabinol. x The data represent the means and standard errors (n = 3). Different letters indicate significant differences among treatments at the 5% significance level, as determined by Tukey’s test. w Significant differences were determined using a one-way analysis of variance (ANOVA) and are denoted as follows: ** p < 0.01, *** p < 0.001. n.s. indicates no significance.
Table 5. Cannabinoid content of hemp flowers according to DIF treatment.
Table 5. Cannabinoid content of hemp flowers according to DIF treatment.
DIF
Treatment		CBDA y
(mg·g−1)		CBCA
(mg·g−1)		THCA
(mg·g−1)		CBGA
(mg·g−1)		CBL
(mg·g−1)		CBT
(mg·g−1)		CBC
(mg·g−1)		CBG
(mg·g−1)		CBD
(mg·g−1)		THC
(mg·g−1)
18:30 z11.6 e x0.89 d0.76 d0.32 e0.230 e0.341 ab0.224 e0.040 d0.075 e0.077 d
21:2729.2 d1.89 c1.33 c1.26 a0.370 d0.326 b0.240 d0.137 c0.096 d0.080 d
24:2431.3 b1.81 c1.50 b1.05 b0.398 c0.332 b0.252 c0.185 b0.141 c0.087 c
27:2140.3 a2.60 a2.12 a0.99 c0.519 a0.390 a0.302 a0.293 a0.231 a0.110 a
30:1830.3 c2.13 b1.52 b0.91 d0.429 b0.360 ab0.258 b0.196 b0.214 b0.095 b
Significance w*****************************
z Daytime temperature/nighttime temperature. y CBDA, cannabidiolic acid; CBCA, cannabichromenic acid; THCA, tetrahydrocannabinolic acid; CBGA, cannabigerolic acid; CBL, cannabicyclol; CBT, cannabitriol; CBC, cannabichromene; CBG, cannabigerol; CBD, cannabidiol; THC, tetrahydrocannabinol. x The data represent the means and standard errors (n = 3). Different letters indicate significant differences among treatments at the 5% significance level, as determined by Tukey’s test. w Significant differences were determined using a one-way analysis of variance (ANOVA) and are denoted as follows: ** p < 0.01, *** p < 0.001.
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Bok, G.; Hahm, S.; Shin, J.; Park, J. Optimizing Indoor Hemp Cultivation Efficiency through Differential Day–Night Temperature Treatment. Agronomy 2023, 13, 2636. https://doi.org/10.3390/agronomy13102636

AMA Style

Bok G, Hahm S, Shin J, Park J. Optimizing Indoor Hemp Cultivation Efficiency through Differential Day–Night Temperature Treatment. Agronomy. 2023; 13(10):2636. https://doi.org/10.3390/agronomy13102636

Chicago/Turabian Style

Bok, Gwonjeong, Seungyong Hahm, Juhyung Shin, and Jongseok Park. 2023. "Optimizing Indoor Hemp Cultivation Efficiency through Differential Day–Night Temperature Treatment" Agronomy 13, no. 10: 2636. https://doi.org/10.3390/agronomy13102636

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