An overview of the recent developments and current status on the preharvest application of LED technology in controlled environment agriculture

ABSTRACT

The use of supplemental light has been proven to be effective in growing plants in controlled environments. These light sources have been proven effective in influencing both, developmental and phytochemical pathways. Among artificial/supplemental lights, light emitting diodes (LEDs) offer several advantages and unique properties. Consequently, the use of this novel technology has gained popularity in indoor plant cultivation. In this review, recent achievements and progress related to the preharvest application of LED technology are discussed, starting from the selection of appropriate LED wavelengths to varying photoperiod to enhance crop growth, yield, and phytochemical concentrations. The review further summarises the recent developments in the use of LED technology for cultivating crops hydroponically and the economic implications of LED usage is briefly discussed. Lastly, the future prospects and research directions are presented.

KEYWORDS:

• LED technology
• food security
• climate change
• sustainability
• hydroponics
• sustainable development goals

Introduction

The horticultural sector is one of the promising sectors that can assist in achieving the sustainable development goals (SDGs) to reduce or end hunger and improve the well-being of humans. This sector can sustainably improve food security and the global supply chain of food to meet the nutritional and food demands of the ever-increasing world population, which is expected to reach 8.6 billion and 9.7 billion in 2030 and 2050, respectively (UN, 2022). Horticultural production in a controlled environment is the key to helping meet the food demand because of its ability to modify the growing environments to improve food production efficiency in a sustainable manner (Nemali, 2022). As such, issues related to climate change and food production can be addressed through this innovative approach.

Light is one of the important requirements in controlled environment agriculture; plant growth and development depends on the light quality, intensity, and photoperiod. Sufficient light received by plants increases their photosynthetic activity, ultimately improving their quality and yield (Ngcobo & Bertling, 2023; Ngcobo et al., 2020b). In the last decades, conventional light sources, such as fluorescent, incandescent, and high-pressure sodium lights (HPS) (Figure 1) were commonly used in agriculture. However, due to their high energy conversion efficiency, high radiant heat emission, and low economic viability, they were substituted by light emitting diodes (LEDs) (Figure 1). These LEDs are a recent lighting technology that has gained popularity in controlled environment horticulture, due to several advantages and ability to emit light within the photosynthetic active radiation (PAR) range from 400 to 700 nm, the wavelengths spectrum needed by plants for photosynthetic activities (Zheng et al., 2019).

Figure 1. Conventional light sources previously used in agriculture (a- Fluorescent light; b-HPS (High pressure sodium) light and c-incandescent light) and full spectrum light emitting diode (d).

Blue and red LED light sources are commonly used to grow plants in greenhouses. These wavelengths or light sources affect physiological and morphological responses of plants, such as plant elongation, leaf expansion, closing and opening of the stomata, flowering, and resistance to abiotic stress, through manipulating photosynthesis and other physiological processes (Chen et al., 2004). In addition, the energy provided by the blue and red wavelengths is vital for plant growth and development. As such, various authors have tested the effectiveness of these wavelengths on growth and development in various crops (Gam et al., 2020; Izzo et al., 2021; Ngcobo et al., 2020b; Spaninks et al., 2020). Furthermore, there are other light wavelengths, such as green and purple, that can potentially affect plant growth and development positively, but they have not been given major attention yet (Zou et al., 2020). Most importantly, the choice of selecting and combining different wavelengths or light spectra is fundamental in enhancing growth, yield, and quality of horticultural commodities. This review, therefore, discusses the recent advancements in the application of LED technology in controlled environments, focusing on their effectiveness in amending crop growth and yield. The review will, further focus on both, commonly used light wavelengths and less popularly employed ones; additionally, the review briefly discusses the developments in the application of this novel technology in hydroponic systems, and lastly, the economic implications, limitations and future prospects in relation to preharvest application of this novel technology will be highlighted.

Light absorption and photosynthesis

As mentioned above, light plays a vital role in not only providing the necessary energy for plants to photosynthesise but also as a signal for various physiological responses. The three key components of light conditions are as follows: i) light intensity, ii) light quality, and iii) photoperiod (Singh et al., 2015). Light intensity refers to the quantity of light that a plant receives. This parameter mainly enhances photosynthesis, resulting in carbohydrates as an end product. The second key component, light quality, refers to the specific distribution of radiation emitted by all light sources. Plants respond strongly to blue and red LED lights, significantly affecting growth, flowering, and yield, while photoperiod controls the response of plants to light stress; the exposure time to light mainly affects flowering of plants (Lazzarin et al., 2021; Singh et al., 2015). Moreover, the most important region for photosynthesis is 400–700 nm, known as photosynthetically active radiation (PAR), and can be provided by LEDs. One more advantage of LEDs is that growers can select one wavelength based on what they want to achieve, unlike conventional lights. It is important to note that plants are very selective when absorbing wavelengths or light, as their absorption is based on the plant’s requirements. Photosynthetic pigments such as carotenoids and chlorophylls play a crucial role in the absorption of light, which ultimately increases the photosynthetic rate of plants (Figure 2). As such, the maximum absorption of light always occurs at the absorption peak wavelength; for example, ‘chlorophyll b’ has two peaks (640 nm in the red region and between 425 and 475 nm in the blue region) [Figure 2(b)]. This means that these are the points where chlorophylls are absorbing light significantly. On the other hand, other light wavelengths, excluding red and blue spectral lights, were also reported to play considerable roles in photosynthesis, but their role is neglected (Smith et al., 2017). Supplementation with far-red (FR) is a hot topic in controlled environmental horticulture and plant science and there is minimal information on this. This light source offers several benefits to plants including but not limited to plant growth and development, stem elongation, leaf expansion etc.

Figure 2. Absorption spectrum of photosynthetic organs (adopted from https://www.ledgrowlightshq.co.uk/chlorophyll-plant-pigments/).

Application of LEDs in greenhouses and closed growth chambers

The change in climate due to unfavourable stress conditions such as extreme temperatures and drought has forced food growers in most parts of the world to transition to controlled environment agriculture. This approach relies mostly on supplemental lighting for growth optimisation of various plant species. In the greenhouse industry, appropriate lighting is a very important for year-round production and optimal plant growth. Recently, extensive research has been conducted to evaluate the efficacy of different LED light sources on enhancing the physiological and morphological changes of various crops.

LED effects on fruit and vegetables

As mentioned earlier, blue and red-light spectra are commonly used LED light sources in controlled environments. Lettuce has been used as a model plant to evaluate the effect of LED light illumination (Hytönen et al., 2018; Loconsole et al., 2019; T. K. L. Nguyen et al., 2021; T. Zhang et al., 2018). It is interesting to see an increase in research on less popularly used wavelengths. For example, T. K. L. Nguyen et al. (2021) evaluated the effect of white LED lighting with specific shorter blue and/or green wavelengths (147, 50 ± 2.89 µmol m⁻² s⁻¹ maintained in all treatments) on the growth and quality of two lettuce cultivars in a vertical farming system. The authors discovered that the addition of green light had a positive effect on lettuce plant growth, in as much as green LED light is not as efficiently absorbed by chlorophyll as blue and red wavelengths, it still contributes towards photosynthesis. The authors further reported that some plants exposed to a combination of red and blue might change their appearance to humans, normally they appear purplish grey to human eye, and that discourages consumer purchase decisions. This problem can potentially be ameliorated by the addition of green LED light.

T. Zhang et al. (2018) further investigated the effects of eight different light treatments: white, purple, blue, red light, green light, yellow, red – blue light in a 9:1 ratio, and red – blue light in a 4:1 ratio (4 R/1B). These authors revealed that the response of lettuce differs based on the type of light it is exposed to. Specifically, the purple, blue, red, and the red-blue light combination increased the biomass of the aboveground part of lettuce, while the treatments with red or blue enhanced soluble protein, free amino acids, and vitamin C.

Other than lettuce, the effects of different LED light spectra have been extensively investigated on other plant species (Table 1). Very briefly, the light effect has been investigated on pepper (Capsicum annuum) (Claypool & Lieth, 2020; X. Liu et al., 2022), tomato (Solanum lycopersicum) (Kalaitzoglou et al., 2019; Ngcobo et al., 2020b, 2022; Yang et al., 2018). Interestingly, the preharvest LED light exposure affects the quality and storability of tomato fruit. Appolloni et al. (2023) demonstrate that supplementary LED underlighting, added to natural sunlight, during greenhouse tomato cultivation affects storability and nutritional quality of tomatoes post-harvest. In this study, red plus blue LED light and far-red light increased fruit firmness and maintained the concentration of lycopene and β-carotene which are the most important carotenoids in tomatoes. Additionally, other recent reports on various crops have proved that LEDs, in the spectral region that affects photosynthesis, positively affect plant growth and development (Hamedalla et al., 2022; Y. Li et al., 2019; Rahman et al., 2021; Tan et al., 2020; Wang et al., 2021). It must, however, be noted that it is not yet known how photosynthetic and physiological responses in plants are triggered by lights other than blue and red. T. Zhang et al. (2018) demonstrated that green and yellow LED lights inhibit lettuce growth, whereas Kaiser et al. (2019) reported that partial replacement of red and blue with green LED light positively enhance the biomass and yield of tomatoes. This implies that the response of crops to LED lights differs based on the LED spectra and duration of exposure.

Table 1. Effects of various LED light spectra on horticultural species.

Light supplemented	Commodity	Effects on plants	Reference
White, red and blue LED lights	Butterhead and romaine lettuce (L. sativa var. capitata)	White LEDs with shorter blue peak wavelength (437 nm) enhanced the growth and development of lettuce.	T. K. L. Nguyen et al. (2021)
Red (R), blue (B) and far-red (FR) LED lights and their combination (R + B, R + FR, B + FR, R + B + FR)	Brassica microgreens	The influence of LED light wavelengths on the major growth and nutritional quality as well as the glucosinolates were dependent on the species.	Demir et al. (2023)
Red and Blue (RB), Red and Blue + Far-Red (FR) with an intensity of 180 µmol m^−2 s^−1 for Red and Blue, plus 44 µmol m^−2 s^−1 for Far-Red	Tomatoes (Solanum lycopersicum ‘Siranzo’)	In this study, red plus blue LED light and the far-red light increased fruit firmness and maintained the concentration of lycopene and β-carotene which are the most important carotenoids in tomatoes.	Appolloni et al. (2023)
White, purple, blue, red light, green light, yellow, red – blue light in a 9:1 ratio, and red – blue light in a 4:1 ratio (4 R/1B).	Lettuce(Lactuca sativa)	Purple, blue, red, and the red-blue light combination increased the biomass of the aboveground part of lettuce, while the treatments with red or blue enhanced soluble protein, free amino acids, and vitamin C.	T. Zhang et al. (2018)
Red, blue, and red + blue LEDs (1:1)	Super-Hot chilli (Capsicum annuum)	Supplemental blue LED light at 100–200 μmol m^−2s^−1 showed higher growth and yield of chilli plants than control but did not differ with red or mix LED (red + blue).	Yap et al. (2021)
Three blue peak emission wavelengths with photosynthetic photon flux density (PPFD) maintained at 200 ± 10% μmol m^−2 s^−1	Kale (Brassica oleracea L. var. acephala)	Different wavelengths did not affect plant biomass nor leaf physical characteristics but enhanced the concentration of carotenoids and chlorophylls in kale leaves.	Ashenafi et al. (2023)
Red and blue (R:B) (30:70, 50:50, and 70:30) emitting 80, 100, and 150 µmol m^−2 s^−1 respectively	Cucumber (Cucumis sativus L)	Cucumber seedlings grown under the 70 R:30B showed a significant increase in growth as well as photosynthetic capacity.	Hamedalla et al. (2022)
Red LED was combined with a blue LED light at a ratio of (1:1) with a combined photosynthetic photon flux density (PPFD) of 138 ± 5 µmol m^−2 s^−1	Tomato (Solanum lycopersicum)	The treatment enhanced growth, yield, and antioxidant phytonutrients in tomato plant and fruit.	Ngcobo et al. (2022)
Green, yellow and purple LED lights	Lettuce (Lactuca sativa L. var. youmaicai)	Green, yellow and purple light significantly regulated the biomass, photosynthesis and soluble sugar content in lettuce	H. Liu et al. (2018)
Yellow light, green light, and blue LED lights	Potato plantlets (Solanum tuberosum L.)	The addition of yellow to the combined spectra of red and blue contributed positively to the growth, development, and morphogenesis	H. Liu et al. (2018)

Application of LEDs in hydroponics

Climate change and environmental degradation have caused a disruption in global food security which has posed enormous pressure to farmers as it is becoming harder to meet the fast-growing food demand. To address this crisis, some growers have considered shifting to hydroponic cultivation system. This system has a potential to mitigate the rising threat of global hunger as it can produce high-quality crops in a small space and short period of time regardless of the climatic conditions and soil quality (S. Khan et al., 2020). In addition, hydroponics is a more flexible technology as compared to soil cultivation method, this is because this system can efficiently grow crops in desert areas, mountainous regions, infertile land, etc. Even though lettuce is the commonly cultivated vegetable crop in hydroponics, several studies have demonstrated that hydroponic systems could play a crucial role in relieving rising threat of global hunger through enhancing food stability and sustainability (F. A. Khan, 2018; Martin & Molin, 2019).

Generally, light plays a pivotal role in enhancing photosynthetic activities, which in turn results in an increase in the growth and development of plant species. Similarly, in hydroponics, the use of LED light is gaining popularity. This technology is important for optimising the light spectrum to meet the light requirements of plant species, ultimately resulting in high quality and yield (Kalaitzoglou et al., 2019). The number of people growing fruit and vegetables hydroponically is increasing, due to the benefits associated with the use of this technology compared with conventional soil agriculture. Talukder et al. (2018) tested the combined effect of spraying amino acids and exposing strawberry plants, in recycled hydroponics, to red, blue, and white LED lights. The authors found that a combination of amino acids and Red: Blue (8:2) LED light of 567 μmol m⁻² s⁻¹ NaN Invalid Date NaN improve the growth, yield, and quality of strawberry plants. As mentioned above, blue and red LED lights are directly aligned with certain sensory pigments which affect photosynthetic and physiological responses in plants, on the other side amino acids used in this experiment potentially ameliorated the negative effects of autotoxicity which is common in strawberry plants. Similarly, J. Li et al. (2021) investigated the effect of different LED lights, namely, red-blue (RB), red-blue-green (RBG), red-blue-purple (RBP), and red-blue-far-red (RBF) on growth, quality, and nitrogen metabolism of lettuce under recycled hydroponics. The study revealed that adding purple, green, and far-red light had a negative effect on lettuce growth because it decreased the effective PPDF for chlorophyll absorption; however, unsurprisingly the R:B combination had a positive effect on growth.

Considering the significant impact that hydroponic cultivation might have in addressing food security, there is an urgent need to enhance knowledge and understanding of the use of hydroponics for food production. This will make it popular and eventually contribute to alleviating global food security issues. A summary of the LED effects on horticultural crops is presented in Table 2.

Table 2. Effects of various LED wavelengths on horticultural crops grown hydroponically.

Lighting supplemented	Commodity	Effects on plants	Reference
The hydroponic spinach plants were grown under four light spectra: LED lamps with ratio of red to blue light (R:B ratio) of 0.9, 1.2, and 2.2 and fluorescent lamps with R:B ratio of 1.8 as control	Spinach (Spinacia oleracea L.)	Th LED lights with different R:B ratios significantly enhanced growth, and nutritional quality and spinach yield.	Gao et al. (2020)
Continuous light (CL) for 48 h was combined with red and blue LEDs supplemented with or without green LED	Lettuce (Lactuca sativa L. cv. capitata (Butterhead)	In this study, green light supplementation significantly increased nitrate reductase (NR) and nitrite reductase (NiR), glutamate synthase (GOGAT) and glutamine synthetase (GS) activity. The same treatment improved the nutritional quality and photosynthetic rates.	Bian et al. (2018)
The following experimental LED treatments were exposed to Kale plants: 1) white LED for 37 d; 2) 5% Blue/95% Red for 37 d; 3) 20% Blue/80% Red for 30 d; 4) 20% Blue/80% Red for 25 d; 5) 20% Blue/80% Red for 20 d; and 6) 20% Blue/80% Red for 15 d prior to harvest	Kale (Brassica oleracea var. acephala)	The study found that shifting to higher blue light LED treatments closer to harvest result in decreased plant height and increased leaf width of the kale plants.	Metallo et al. (2018)
The effect of white – green light (W:G = 4:1, W4G1), red – green light (R:G = 4:1, R4G1), red – green – blue light (R:G:B = 4:1:1, R4G1B1), and four types of red – blue lights (R:B = 3:1, R3B1; R:B = 2:1, R2B1; R:B = 1:1, R1B1; R:B = 1:6, R1B6) was investigated in hydroponically grown wheat	Winter wheat (Triticum aestivum L., cv. zhongmai 1062)	Regardless of growth rate and production, winter wheat grown under various LED light treatments produced grains; more notably, R4G1 treatment enhanced growth, yield and starch content significantly	Guo et al. (2023)
A combination of red and blue LEDs (R660/B450 = 80/20) at different light intensities (90, 140, 190 and 240 µmol/m^2/s^−1) on the growth, photosynthesis, and leaf microstructure of hydroponic cultivated spinach	Spinach (Spinacia oleracea L.)	The authors found that 190 µmol/m^2/s^−1 light intensity as the appropriate intensity for growth of spinach	T. Nguyen et al. (2019)
The effects of high light irradiation (HLI, 500 μmol m^−2·s^−1) provided by red and blue LEDs (4 R:1B) on lettuce grown was evaluated	Purple lettuce (Lactuca sativa var. ‘Zishan’)	The study revealed that short-term HLI promotes biomass production and enhances secondary metabolite production in lettuce	Shao et al. (2020)

Potential of LEDs to induce tolerance of plants to biotic and abiotic disorders

So far, the effects of various wavelengths of light on growth and development of several crops have been reported. To fine-tune these responses, however, other technologies might become instrumental. Lazzarin et al. (2021) reviewed the current knowledge on the effect of wavelengths on plant growth in comparison with what the plant directs towards defence. Reviewing the ‘growth-defence trade-off’, they argued that the stress brought about by excess light results in the production of reactive oxygen species (ROS) and that this synthesis of ROS has various effects. While energy is redirected from photosynthesis to the production of compounds protecting plants from ROS attacks, compounds produced to ‘detoxify’ ROS, support plant metabolism in such a way that they make plants more resilient against environmental stress factors. This investment into the defence system clearly reduces plant growth, but the plant becomes resistant to other stress types, particularly herbivory (Courbier & Pierik, 2019; Sharath Kumar et al., 2020). The impact of this from a plant-growing perspective is large, as this reaction is almost similar to that of the vertebrate immune system, where stimulating the production of certain defence compounds shields the organism against a wider range of stress factors. In plants, through triggering ROS production of plants can respond to stress exposure (LED exposure) by producing antioxidant compounds. These compounds can ‘de-activate’ (capture) ROS. The more ROS are produced, the more the plant becomes resilient to other stressors; however, this system works at the expense of plant growth and development, meaning – in a plant production system – reduced growth and, therefore, reduced yields.

Other means to harness the positive impact of ROS production have, therefore, been attempted. The timing of light exposure and the provision of short pulses of supplemental light are such possibilities (Sharath Kumar et al., 2020). While the effects of light pulses on plant growth have already been investigated (e.g. effects of end-of-day far-red treatments on flowering and overall quality of ornamental crops) (M. Zhang & Runkle, 2019), further investigation of the consequences for host-plant resistance and pest management is needed (Lazzarin et al., 2021).

Economic implication of LED usage in horticulture/LED effects on health-promoting compounds in fruit

While LED technology can initiate profound effects of biomass accumulation and the production of secondary plant products (Al Murad et al., 2021), vegetative growth is adversely affected. This growth-defence trade-off (Lazzarin et al., 2021) has obvious economic implications, as smaller plants will be produced, resulting in lower yields. The quality of the produce, however, also needs to be considered, increasing the value of the produce and allowing its sale at a premium price. The increase in health-promoting compounds produced due to LED usage (Ngcobo & Bertling, 2021; Ngcobo et al., 2020a) could increase consumer demand for such products, thereby potentially enhancing the price of these products. On the other side, the increase in health-promoting compounds in the produce due to LED usage has significant implications on human well-being. Zushi et al. (2020) demonstrated that illuminating tomato fruit with blue, but not red, LED light enhanced the fruit’s ascorbic acid concentration. Further, artificial daylength enhancement (pre-sunrise and post-sunset) with blue and red LED lights positively impacted tomato plant development, yield, and fruit nutritional quality, such as β-carotene, total soluble solids (TSS), phenolics as well as vitamin C concentrations of cherry tomato (Ngcobo et al., 2022).

Most importantly, when considering investments into LED technology in protected cultivation, the producer needs to carefully weigh up the price for the additional installation of LEDs (including the need for power supply to these lights, although they have a very low energy consumption). Compared to other light sources, LED usage is economically favourable, this is because of their low maintenance costs and long-life expectancy (Nelson et al., 2014).

Conclusion

Although the benefits of light application to increase horticultural productivity have long been realised, the recent use of LED light technology has resulted in further possible applications. Considering that LEDs have proven to be effective in enhancing growth and yield of various crops, the increase in bioactive compounds, particularly in health-promoting compounds, seems to be an avenue that requires further investigations. This will help achieve sustainable goal 3 (good health and well-being) thereby ensuring nutrition security. The advantages of LEDs, particularly in protected cultivation, to enhance fruit growth and development might become a new technology in horticulture, not only to enhance certain compounds present in these commodities but also for a more sustainable, less-energy requiring production of greenhouse crops. Another avenue that needs further investigation is to understand the physiological responses induced by other LED light sources, besides red and blue wavelengths. This will enhance the knowledge on the regulation of plant morphogenesis using different portions of the light spectrum independently or cooperatively.

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No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing is not applicable to this article as no new data were created or analysed in this study.

Additional information

Funding

This work was not supported by any organisation or institution.