Introduction

Paprika (Capsicum annuum L.) is a vegetable crop of the Solanaceae family and is representative of crops grown through controlled horticulture in South Korea, alongside tomatoes and strawberries. With high water content, vitamin C, and antioxidants, paprika is recognized as a healthy food. It is primarily consumed fresh and in vegetable salads, and consumers perceive it as a high-quality food item (Jang et al., 2022). Among the fruits and vegetables produced in South Korea, paprika stands out due to its high exportation to Japan, leading to a significant increase in the cultivation area of paprika as an income crop that allows year-round production (KATI, 2023).

Paprika undergoes both vegetative and generative growth alternately. To maximize paprika fruit yield, it is essential not to overly favor one growth mode. When one type of growth dominates, the growth stage should be adjusted by controlling fruit conditions and numbers to restore balance. Excessive fruiting, aimed at increasing the crop load, can disrupt the carbon supply balance, resulting in reduced vegetative growth and overall yield (Hansen, 1979). Furthermore, paprika exhibits indeterminate growth. Increased consumption of photosynthetic assimilation products, such as through respiration, diminishes plant vigor, while flower and fruit thinning enhance (Aloni et al., 1999). With heightened vigor, leaves proliferate, diminishing light transmittance. Under these circumstances, trapped air reduces CO₂ exchange, leading to an overall decline in photosynthetic capacity (Ahmadi and Joudi, 2007). Defoliation improves photosynthetic efficiency by promoting airflow and light transmittance. Removing aged leaves with reduced photosynthetic rates and high respiration can enhance overall plant performance (Iqbal et al., 2012). Additionally, this approach offers the potential to reduce labor requirements (Lee et al., 2022).

Photosynthesis is a process of carbon assimilation whereby glucose is produced through a complex reaction involving light energy, CO₂, water, and temperature (Sharkey et al., 2007). It constitutes the most important reaction to create the energy and organic compounds that are needed to support the life of a plant (Jung et al., 2021; Nomura et al., 2022). Photosynthetic rate is strongly affected by the states of leaves and changes in microclimate, but it increases according to increasing light intensity or CO₂ concentration and can be used to estimate the point of saturation for these two drivers (Farquhar et al., 1980). The photosynthetic rate should also be interpreted in combination with environmental factors (e.g., level of transpiration) and stomatal conductance, indicating the level of CO₂ exchange (Kim and Lee, 2001b; Kim and Lieth, 2003). In a poor environment or under water stress, the closing of stomata reduces the rate of CO₂ absorption or H₂O transpiration and lowers the photosynthetic rate, leading to reduced crop growth and yield.

During winter, the combination of low temperatures and light intensity in greenhouses results in a decreased photosynthetic rate, leading to heightened inhibition of paprika growth, as well as issues like leaf and fruit dropping and malformation (Jeong et al., 2009). The crop load and management costs for winter paprika cultivation rise with the duration of the cultivation period and the number of days from flowering to harvest due to the limited day length hours, low light intensity, and cold temperatures (Nelson and Bugbee, 2014). To address these challenges, various methods aim to increase light intensity in winter greenhouses. These approaches include selecting an aggregate with a low frame ratio, using a film with high light transmittance, washing films, employing reflected light or light-scattering materials, and incorporating artificial lighting. Among these methods, supplemental light technology stands out as an active mechanism to consistently enhance light intensity within the greenhouse (Son et al., 2021). Kim’s study demonstrated that supplemental lighting during the night increases the growth and production of tomatoes regardless of the season and, this is attributed to the extended exposure to light due to supplemental lighting, which enhances chlorophyll content and photosynthesis in crops, promoting overall plant growth (Tewolde et al., 2016). Supplemental lighting, widely applied to fruits and vegetables like tomatoes, paprika, and cucumbers, has been the subject of both national and international studies. These studies consistently report enhancements in quality and production under supplemental lighting conditions (Demers et al., 1998; Hao and Papadopoulos, 1999; Lee et al., 2014a, 2014b).

Various methods of supplemental lighting have been proposed, placing increased emphasis on yield improvement under controlled greenhouse conditions during periods of low temperatures. However, no attempt has been made to manipulate the integral photosynthetic photon flux density (PPFD) absorbed by crops under supplemental lighting. Therefore, this study aimed to determine the treatment effects of supplemental LED lighting on the growth and yield of paprika. It compared the effectiveness of the general supplemental light (GSL) and daily integral supplemental light (DISL) methods and analyzed the growth and photosynthetic rate in each treatment group.

Materials and Methods

Plant variety and experimental design

The experiments in this study were conducted inside a plastic greenhouse (length 19.5 m, width 8.2 m, edge height 5 m) covered with a double-layer polyethylene (PE) film at Mokpo National University’s farm from October 19, 2020 to April 16, 2021 (Fig. 1). Red ‘Scirocco’ paprika (EnzaZaden, Enkhuizen, The Netherlands) plants were sown in rock-wool plugs on September 25, 2020, and the planting was conducted on October 19th, 2020, in a coco-peat slab (20 × 110 × 10 cm). The planting density was 3.9 plants per m², and whenever the solar energy reached 70 J·cm^-2, irrigation occurred. One drip watering session consisted of approximately from 100 cc to 150 cc and EC was set from 2.0 to 2.5 according to growth stages. The pH of nutrient solution was maintained at 5.0–6.0, irrespective of the growth stage. The irrigation system used MAGMA-1000 (GreenCS, Damyang, Korea). Greenhouse environmental data such as outside PPFD, temperature, and CO₂ concentration were measured and collected using a data logger (CR1000X, Campbell Scientific, Logan, UT, USA). The external PPFD data was collected after installing the sensor on November 19th.

Fig. 1.

(A) Experiment location (34°54'51.7"N 126°26'24.5"E), (B) cultivation greenhouse, and (C) LED light source.

The outside average daily temperatures during most days of the study period were below 15°C due to the influence of winter weather, an electronic heating device (SWE15F, Shinwoo Automatic, Daegu, Korea) whose calorific value was 12,900 kcal·h^-1 was used to maintain the temperature inside the greenhouse. While the lowest growth temperature of paprika was between 16–17°C, the initial temperature of the device was set at 18°C for the purpose of stable crop management, and the device was set to operate if the temperature fell below this threshold. On October 19th, the crops were planted. At that time, the operating temperature of the greenhouse heater was set to 15°C. However, on November 18th, when it was determined that it was not suitable for paprika cultivation, the operating temperature was adjusted to 18°C during the supplementary lighting treatment. The double-layer ceiling and side windows were closed an hour before sunset to reduce heat loss from night-time radiation and conduction, enhancing the thermal insulation effect. They were then opened gradually after sunrise. For the double-layer ceiling and side windows, 55% shield aluminum and multi-layer insulation screens were used. The fruiting position in paprika is determined based on the time of planting or growth state, which allows an efficient management of the distribution of photosynthetic products while reducing the crop load. For this reason, flower thinning was performed by leaving two flowers and removing either two or three, depending on the specific plant’s condition.

Method of supplemental light

A LED (LED bar, Allix, Korea) produced for use in plant growth was used as a light source, and it measured 100 μmol·m^-2·s^-1 at a distance of 1 m when the wavelength was between 400 nm to 700 nm. No supplemental lighting was used in the control group, and two treatment groups with possible supplemental lighting in the period from 06:00 to 22:00 hours were set up. The treatment groups consisted of general supplemental lighting (GSL) and daily integral supplemental lighting (DISL). Lighting in the GSL group was set to operate if external solar radiation was ≤100 W·m^-2 during the operation hours. Lighting in the DISL group was set to operate until a target daily amount of light was satisfied, measured as integral photosynthetic photon flux density (PPFD) using a PPFD sensor (SQ-212, Apogee Instruments, Logan, UT, USA) inside the greenhouse. PPFD means the wavelength of light used in photosynthesis of plants. The target amount of light was set at 12 mol·m^-2·d^-1 in reference to previous studies reporting favorable morphological and physiological responses and productivity at 9–12 mol·m^-2·d^-1 (Yan et al., 2021; Zhang et al., 2022). The supplementary lighting treatment commenced on November 18, 31 days after the planting.

Measuring photosynthesis rate

To measure the photosynthesis of a single paprika leaf, which was on the fifth position based on the growth point, estimated with a leaf age of 30 to 35 days, a portable photosynthesis system (Li-6800, Li-Cor, Lincoln, NE, USA) was used. The chamber conditions for photosynthesis measurements were set as follows: flow rate 600 μmol·s^-1, leaf temperature 25°C, relative humidity 55%, CO₂ concentration 400 μmol·mol^-1 and PPFD 1500 μmol·m^-2·s^-1. Levels of PPFD and CO₂ were adjusted depending on the type of measurement. When measuring the photosynthetic rate in response to light intensity, the level of PPFD was decreased from 2000 μmol·m^-2·s^-1 to 0 μmol·m^-2·s^-1, and CO₂ concentration maintained at 400 μmol·mol^-1. In measuring photosynthetic rate in response to CO₂ concentration, the initial level of CO₂ was set at 400 μmol·mol^-1 to simulate standard atmospheric conditions, then gradually decreased to CO₂ compensation point, after which measurements were taken at levels of 400 μmol·mol^-1 to 1,500 μmol·mol^-1 at the CO₂ saturation point, and PPFD maintained at 1,500 μmol·m^-2·s^-1.

To create a light photosynthetic response curve (Eq. 1), an equation suggested in a previous study was used, which described net CO₂ assimilation rate (A, µmol·m^-2·s^-1), initial light use efficiency when PPFD increase from 0 µmol·m^-2·s^-1 to 100 µmol·m^-2·s^-1 (𝜙), photosynthetic photon flux (I, µmol·m^-2·s^-1), light-saturated gross CO₂ assimilation rate (A_max, µmol·m^-2·s^-1) and the respiratory rate (R, µmol·m^-2·s^-1) (Kim and Lee, 2001a). To create a CO₂ photosynthetic response curve, Eq.1 was also used. A, A_max, and R were the same with light photosynthetic response curve, but was initial slope of the line when increasing from 50 µmol·mol^-1 to 100 µmol·mol^-1 and I was CO₂ concentration.

Plant growth measurement

The first growth measurement was performed on the day of planting when the supplemental light treatment was initialized. The second growth analysis was performed after 52 days, and subsequent analyses were performed at three to four-week intervals. The items tested in the growth measurement were plant height, number of leaves, fresh weight, dry weight, leaf area, number of fruits, fruit weight, yield, and sugar content. To determine plant height, the stem length between the soil surface and the meristem was measured. An electronic scale (IB-3100, InnoTem, Yangju, Korea) was used to measure the fresh weight and dry weight. The dry weight was measured after ≥72 h drying at 70°C in a heat chamber (JSOF-250T, JSR, Gongju, Korea). To measure leaf area, a leaf area meter (LI-3100C, Li-Cor) was used. Soluble solid content was measured using a portable Brix meter (Sugar-1 plus, CAS, Yangju, Korea). The fruiting rate was the ratio when excluding the numbers of flower thinning and fallen flower.

Growth measurements were conducted on five plants each time, while fruit measurements utilized a fixed set of five plants.

Plant growth analysis

For growth analysis of paprika plants during the experimental period, leaf dry weight (LW, g), leaf and stem dry weight (W, g), leaf area (LA, cm²), ground area occupied by the plant (GA, m²), and number of days after planting (t, day) were used to calculate the following equation (Koller et al., 1970; Wilson, 1981; Lambers and Poorter, 1992; Ni et al., 2000; González-Dugo et al., 2007; Lee and Cha, 2009; Nam et al., 2009; Xu et al., 2021; Kamara et al., 2022), and n is the date of the growth measurement, and n-1 is the date of the previous growth measurement.

Relative growth rate (RGR, mg·g^-1·d^-1) indicates the plant’s ability to produce dry matter (Eq. 2).

Leaf area ratio (LAR, cm²·g^-1) is the percentage of leaf area per unit weight of the plant (Eq. 3).

Specific leaf area (SLA, cm²·g^-1) is the leaf area per unit leaf weight as an indicator of the leaf compactness (Eq. 4).

Net assimilation rate (NAR, g·m^-2·d^-1) indicates the ability of dry matter production per unit leaf area and unit time (Eq. 5).

Leaf area index (LAI) is the percentage of leaf area in the cultivated area occupied by the plant (Eq. 6).

Crop growth rate (CGR, g·m^-2·d^-1) is an indicator of total dry matter production per unit area in a crop community (Eq. 7).

Statistical analysis

The experimental setup was arranged in a randomized block design and repeated three times. The statistical analysis of the indoor conditions and crop growth was performed using R (v. 4.2.1, R Foundation for Statistical Computing, Vienna, Austria). Statistical significance was determined using a one-way analysis of variance (ANOVA). Mean separation treatments were analyzed by least square difference (LSD) tests was performed to test for differences among groups. SigmaPlot (14.5, SYSTAT, Palo Alto, CA, USA) was used for regression analysis and figures.

Results and Discussion

Greenhouse environment

The internal and external conditions of the experimental greenhouse are shown in Fig. 2. The temperature and CO₂ inside the greenhouse were maintained without large deviation across the treatment groups, while the average outside PPFD increased over time. The temperature inside the greenhouse increased during the day due to solar radiation. The nighttime temperature inside the greenhouse was initially set at 15°C, but it was considered too low for pepper cultivation. Therefore, it was adjusted to 18°C along with supplementary lighting treatment. When it decreased to under 15°C and 18°C during the night, an electronic warm-air circulator was used to minimize day-to-night temperature difference and attendant crop stress. After sunset, an aluminum screen was used to prevent cold damage to the meristem due to radiative heat loss (Kwon et al., 2016). For those reasons, from November to March, the average temperature during the cultivation period was approximately 20.7°C. CO₂ concentration in greenhouse displayed a repeating pattern with increasing levels at night due to the closed windows and plant respiration and decreasing levels after sunrise due to carbon fixation through assimilation.

Fig. 2.

Changes in (A) inside temperature, (B) inside CO₂ concentration, and (C) outside PPFD during the experimental period from October 2020 to April 2021 (control: no supplemental lighting; GSL: general supplemental lighting; DISL: daily integral supplemental lighting).

The accumulated PPFD, which combined both solar and artificial light over the course of a day, was shown in Fig. 3. The final integral PPFD on the cloudy day of February 26th was 2.90 mol·m^-2·d^-1, 5.38 mol·m^-2·d^-1, and 8.14 mol·m^-2·d^-1 in control, GSL, and DISL, respectively. All treatments did not reach the target light quantity of 12 mol·m^-2·d^-1, but compared to the control group, GSL was 1.85 times higher, and DISL was 2.81 times higher. However, the clear day, March 5th was 5.58 mol·m^-2·d^-1 in control, 9.09 mol·m^-2·d^-1 in GSL, and 12.00 mol·m^-2·d^-1 in DISL. The DLI was higher in GSL than in control by 3.56 mol·m^-2·d^-1, presumably due to the additional supplemental lighting before sunset and after sunrise under preset operation at external solar radiation under 100 W·m^-2. For DISL, supplemental lighting was provided until the target cumulative amount of light inside the greenhouse was achieved, and it was then discontinued at 20:00 upon reaching the target level. Table 1 represents the monthly average daily accumulated light intensity from November 2020 to March 2021. The control exhibited a daily average of 8 mol·m^-2·d^-1 or less in all months. In contrast, GSL did not meet the targeted integral PPFD of 12 mol·m^-2·d^-1 from November to February, but in March, it exceeded the goal with a measured value of 14 mol·m^-2·d^-1. Finally, DISL, although falling short of the target light intensity until January, showed a cumulative light intensity 1.5 to 2 times higher than other treatment groups. For these reasons, both GSL and DISL are effective in overcoming the low solar radiation during winter compared to the control group. However, it can be observed that DISL is more efficient in utilizing the accumulative light intensity and energy needed for crop growth. In addition, the coefficient of variation appeared to be 0.6 or less in all treatments.

Fig. 3.

The accumulated PPFD in the greenhouse over 24 hours according to the supplemental lighting treatments on (A) cloudy day (February 26th) and (B) clear day (March 05th) (control: no supplemental lighting; GSL: general supplemental lighting; DISL: daily integral supplemental lighting).

Comparison of paprika photosynthesis

A comparison of photosynthetic rates of paprika leaves by treatment in response to changes in light intensity and CO₂ concentration is shown in Fig. 4. Rate changes in response to light intensity revealed that DISL yielded the highest photosynthetic rate, followed by GSL and control. Photosynthetic rate in response to changes in CO₂ concentration did not vary significantly at low concentrations but showed a positive effect of the DISL treatment compared to control at high concentrations. This indicated that the supplemental LED method positively enhanced the photosynthetic rate in the supplemental light treatment groups with increased light exposure time, compared to the control group grown under short day length hours and low light intensity in winter. This was presumably due to the difference in crop adaptation to the light and agreed with reported variations in photosynthetic rate measured in a paprika greenhouse under varying levels of day length hours and light distribution (Kim et al., 2021). The results show that the photosynthetic rate was influenced by crop growth conditions. Table 2 displays the regression equations, R², and RMSE of the curves depicted in Fig. 2. The regression of Photosynthetic rate was used. In addition, R² represents the coefficient of determination, which is a measure of the goodness of fit of a regression equation, and RMSE stands for Root Mean Square Error, which signifies the discrepancy of observed values as the average squared difference. The regression equation was analyzed assuming that the dark respiration rate is consistent across the same experimental treatment. With a high determination coefficient (R2) exceeding 9.70 and a low RMSE, it can be inferred that the estimated equation showed minimal deviation from the actual values.

Fig. 4.

Photosynthesis rate of ‘Scirocco’ under various supplemental lighting conditions on March 23rd. (A) Response to light intensity, (B) response to CO₂ concentration (control: no supplemental lighting; GSL: general supplemental lighting; DISL: daily integral supplemental lighting). Values are the means ± standard error (n = 5).^*The measurement of the paprika leaf was conducted on the fifth leaf, estimated with a leaf age of 30 to 35 days from the top, based on the growth point.

Comparison of paprika growth

There were no significant treatment-dependent differences in the growth of the paprika plants except for plant height (Table 2). Plant height was highest in the control group and similar in the two treatment groups. The leaf count remained consistent across all groups due to continuous defoliation up to the joint of the fruit at the time of fruit coloration. Previous studies have shown that defoliation can prevent pests and diseases through the management of humidity with increased aeration at the lower part of the plant while at the same time facilitating fruit coloration by increasing the number of day length hours reaching the fruits (Jang et al., 2021; Lee et al., 2022). As weight tended to vary across different organs in each treatment group, dry matter weights were compared. Respective to the control, GSL and DISL, the dry matter weight of the leaf was 12.2%, 13.6%, and 13.5%, and the dry matter weight of the stem was 16.7%, 18.9%, and 18.0%. This may imply a trend for a positive effect of the supplemental light treatment on biomass increase. The conversion to the leaf area of a single leaf resulted in areas of 104 cm², 87.6 cm^2, and 97.9 cm² for the control, GSL, and DISL, respectively. This difference in the control group was likely due to the inadequate flower bud development (indicative of generative growth) and the dominant growth of leaves and stems (indicating vegetative growth) in this group. The reason for these results was particularly in the control group, insufficient floral differentiation occurred due to low integral light, which resulted in higher values in plant height than other treatment groups.

Paprika growth analysis

Dry matter productivity was analyzed using leaves and stems to examine the growth of the paprika plants (Fig. 5). Paprika in this study exhibited a pattern of initial high RGR and subsequent gradual decrease, presumably due to the increased proportion of generative growth and a natural decrease in division speed (Lambers and Poorter, 1992). After 50 days of planting, LAR gradually decreased from the initial high level, as a result of the plant’s growth progression and characteristic of indeterminate growth, leading to a lower structural proportion of photosynthetic organs within the entire plant. Initial SLA level was high, but a gradual decrease was observed after the onset of generative growth (Nam et al., 2009). The reason for the changes in LAR and SLA is that these formulas are composed of leaf area and either plant weight or leaf weight. As sunlight decreases during the winter season, adjustments in leaf number are necessary, leading to a decrease in these values. NAR fluctuated according to the growth stage, presumably based on the paprika fruiting node order (Kamara et al., 2022). NAR has the most significant impact on the efficiency of leaf assimilate production, but it is also influenced by factors such as the amount of assimilates in the sink organ or respiratory processes. The substantial variability in the numerical values observed in Fig. 5D, particularly during the cultivation of paprika that exhibit indeterminate growth, seems to be attributed to flower thinning after a certain number of fruit sets, aimed at preventing biased vegetative growth. In previous studies, flower thinning was used to efficiently manage the distribution of photosynthetic products by adjusting crop load. In this experiment as well, flower thinning is performed by adjusting the position of fruit clusters based on planting time or growth state (Lee and Cha, 2009). LAI exhibited an initial rapid increase, which leveled off after defoliation. This was because the ground area remained constant while leaf area increased, maintaining a consistent number of leaves through the removal of lower and older leaves (González-Dugo et al., 2007). CGR was calculated by multiplying NAR and LAI, and it exhibited a similar pattern to NAR in terms of growth (Koller et al., 1970; Wilson, 1981; Ni et al., 2000). Table 3 shows the effects of various supplemental lighting treatments on the growth of 'Scirocco' paprika as of April 16th. The control group had the tallest plants, but there were no significant differences in other parameters among the treatments. However, the GSL and DISL treatment groups, which received supplemental lighting, showed increased dry weight and dry matter percentages.

Fig. 5.

Growth analysis of 'Scirocco' paprika with supplemental lighting treatments from October 19, 2020, to April 16, 2021. (A) Relative growth rate (RGR), (B) leaf area ratio (LAR), (C) specific leaf area (SLA), (D) net assimilation rate (NAR), (E) leaf area index (LAI), and (F) crop growth rate (CGR) (control: no supplemental lighting; GSL: general supplemental lighting; DISL: daily integral supplemental lighting). Values are the means ± standard error (n = 5).

Soluble solid content and yield of paprika fruit

The soluble solid content of paprika fruits did not vary significantly by treatment (control 3.0%, GSL 3.5%, and DISL 3.7%) (Fig. 6). In the treatment groups, the net photosynthesis of the crop exceeded that of the control group, allowing a higher level of translocation and a consequent increase in Brix; however, no significant difference was found between the two treatments (Kwon et al., 2023). In the two supplemental light treatment groups, the paprika yield for DISL (3.1 kg/plant) and GSL (2.8 kg/plant) had 29% and 19% increase in yields, respectively, compared to the control (2.4 kg/plant). This difference is attributable to inadequate flower bud differentiation in control due to insufficient light intensity in the greenhouse and the higher number of fruits in the supplemental light treatment groups (Lee et al., 2014b; Park et al., 2018). The fruit weight was higher in the control group, but the number of harvested fruits and the fruiting rate of harvest were the highest in DISL (Table 4).

Fig. 6.

Comparison of (A) accumulated yield and (B) brix contents of ‘Scirocco’ paprika under supplemental lighting treatments until April 16th (control: no supplemental lighting; GSL: general supplemental lighting; DISL: daily integral supplemental lighting). Values are the means ± standard error (n = 5). Within-graph means followed by the same letter are not significantly different by Duncan’s multiple range test at p < 0.05.

Conclusion

Paprika cultivation demands high light intensity, and during winter, the growth and yield of paprika decline due to a reduced duration of sunshine and solar radiation. To address this light deficiency inside the greenhouse, supplemental lighting emerges as a direct solution. In this study, the outcomes of supplemental lighting treatments under two regimes (GSL and DISL), regulated differently in response to external solar radiation, were compared. The photosynthetic rate was higher than in the control group for both treatments, emphasizing the effectiveness of supplemental lighting. The growth analysis clearly illustrated the indeterminate growth characteristics of paprika. While the sugar content in paprika fruits did not significantly vary with treatment, there was a notable effect on increasing fruit yield. Collectively, these results suggest that the DISL treatment positively influenced paprika growth and fruit production, emphasizing the impact of increased light availability on crop photosynthetic rates. Supplemental lighting proves beneficial in satisfying the light requirements of paprika for efficient cultivation in winter. Further studies should explore diverse supplemental lighting methods tailored to varying light intensity conditions suitable for different crop types. However, it’s crucial to note that DISL requires additional sensors compared to traditional lighting methods. Additionally, as the experiments utilized LED lights instead of the high-pressure sodium lamps commonly found in large-scale farms, applying the results directly to actual farms may pose challenges. Thus, supplementary experiments considering the characteristics of the light source appear necessary for practical implementation.