Effects of LED lighting technology on the productivity of leafy vegetables
Leafy salad green (LSG) vegetables are an essential component of a healthy diet, providing important nutrients and vitamins that are beneficial to health. Due to its well-recognised health benefits, the consumption of LSG has been greatly recommended by organization such as World Health Organisation and United State Department of Agriculture. As a result of efforts in promoting the consumption of vegetables, there has been an increase in the consumer demand of LSG due to consumers striving to make healthier eating habits. In addition, it was reported that a regular daily consumption of LSG reduces degenerative diseases including cardiovascular disease, cancer and ageing. Hence, it is foreseeable that the production and consumption of LSG will increase continuously in recent years.
According to (Horticulture Innovation Australia 2017), 52,356 tonnes of LSG was produced in the year end June 2017 and these refer to the leafy greens such as rocket, baby spinach, and lettuce. LSG is a high-value industry and it has a production value of $304.3 million and a fresh supply wholesale value of $343.2 million. Additionally, it was reported that the consumption per capita of LSG was 1.7kg in Australia, based on its supplied volume. This also represented that approximately 55% of Australian household purchases LSG, and buys an average of 177g of LSG per shopping trip. Furthermore, Australia is also a net exporter of LSG and it has an export value of $8.4 million. Base on the statistics in 2017, Australia has exported a total of 1,313 tonnes of LSG and they were sent to countries such as Singapore (471 tonnes), Hong Kong (279 tonnes), Malaysia (95 tonnes), Indonesia (90 tonnes) and Brunei (69 tonnes).
Despite its economic advantages, a greater demand increases the production volume and its production needs, thereby posing a challenge to its production sustainability and quality. This is due to the uncertainties in the availability of production land, invasive pest, along with other environmental concern like climate change. Subsequently, (Hazell & Wood 2008) has reported that the arable land per person is projected to decrease by 2050 to one-third of the amount available in 1970. Thus, the development of alternative technologies such as the Light-Emitting Diodes (LED) technology plays a critical role in balancing out the production needs and its uncertainties. LED is an artificial light source that is commonly used in indoor and urban farming, designed to stimulate the crop growth by providing an appropriate light spectrum required for photosynthesis.
The aim of this literature review is to present the effects of LED lighting technology on the productivity of LSG. The following review will consist of two components, the drivers for the use of LED in production and the effects on the productivity of LSG. The effects of the productivity will be discussed in relation to its growth and development, quality and phytochemical compounds.
Drivers for use of LED in LSG production
Firstly, LED is a promising technology for the LSG production industry as it supports adaptation, can be operated instantaneously using a control switched and it can be programmed to suit the requirements of various crop varieties. It can be used as an application when no light source is available or when supplemental lights are required. Additionally, the LED allows the adjustment of light intensity and duration, thus it can be used to enhance photosynthesis (Cocetta et al. 2017). For instance, during the winter season, when the amount of light received is insufficient for the desired LSG growth due to the shorter day length, LED can be used to extend the duration of light the crop received. Besides that, the production of LSG using LED in an enclosed system (indoor farming) provides a stable environment for production regardless of the weather conditions. Additionally, the enclosed system allows control of environmental factors thus it protects the cultivated LSG and minimizes its risk of weather-related crop failures due to drought or flooding (Benke & Tomkins 2017). Hence, LED enhances the production quality, increases yield and eliminates seasonality issues as the production occurs continuously all year-round.
With comparison to the operation lifespan, LED has a significantly longer operating lifespan of 100,000 hours as compared to the traditional lighting technology such as incandescent light and fluorescent light which is 1000 hours and 8000 hours respectively (Stutte et al. 2009). This is because electrodes are not used in LED therefore it does not burn out like the traditional lighting technology and only requires occasional replacement. Apart from that, LED has a low radiant heat output despite being programmed to produce high light intensity level, thus it does not ‘burn’ the LSG and it can be placed directly above the LSG unlike the traditional lightings which are not suitable for enclosed environments due to it generating high amounts of heat (Cocetta et al. 2017). By shortening the distance of installation, the LSG can receive more lights with less energy thereby enhancing efficiency. The lighting unit also comes in small size and volume which enables versatile design as well as space efficiency within facilities (Yano & Fujiwara 2012). Subsequently, LED is designed to be user and environment safe. It does not contain any hazardous material such as mercury and its made from semiconductor material materials instead of glass which is fragile and may be easily broken (Olle & Viršile 2013). Thus, its energy efficiency, longevity and crop safety have led to an increase utilization of LED in the production industry.
Lastly, LED has the capability to control the spectral composition and has the ability to emit a variety of narrow wavebands (red, blue and green) to mimic the outdoor environment. Although the traditional lighting can be used to enhance the photosynthesis of LSG, but it also provides wavelengths that are inefficiently in supporting the crop growth (Zhang et al. 2015). In comparison, LED provides a greater wavelength specificity and it enables elimination of the less important wavelength such as yellow and green, which in return enhances the energy efficiency (up to 49% for blue LED) and reduces the electrical cost (Yeh & Chung 2009; Olle & Viršile 2013). On top of that, LED is fully controllable by switch thus its light can be turned on and off periodically and such mechanism are not easily achievable through the traditional lightings (A Wardle et al. 2008). Therefore, LED is ideal for production as the wavelength can be adjusted to match the photoreceptor to deliver an optimal production and enhance the crop growth and development.
Productivity of LED lighting technology against traditional lighting
Growth and development
Several studies have demonstrated the possibility of achieving a higher productivity and quality through the red and blue LED. These colours red are the more popular ones used as they are more efficiently absorbed by chlorophyll molecules, with a maximum absorption range of 430-453m in blue wavelength and a range of 663-642nm in red wavelength (Son et al. 2012). Besides, blue LED drives photosynthetic reaction such as chlorophyll formation and chloroplast development. Additionally, it regulates the opening of stomata and inhibits stem and internode elongation (Folta & Childers 2008). (Son et al. 2012) has also documented that utilizing blue LED enables a higher accumulation of chlorophyll content. As shown in Figure 1, the presence of blue LED show significant differences in the chlorophyll content as compared the ones under solely red LED or the traditional lighting. In addition, the chlorophyll content correlates positively with the ratio of blue LED applied, as chlorophyll content increases up to 2 times with the presence of a higher blue LED ratio. Typically, the growth rate correlates with the chlorophyll content, thus having an abundance of chlorophyll also indicates a greater growth and higher biomass.
On the other hand, red LED is regarded as the basal lighting spectral for inducing the crop growth. As shown in Figure 2 and 3, the application of red LED over the traditional lights enhances several growth characteristics of lettuce by increasing up to 2.5 times for its fresh and dry shoot weight, up to 14% in its fresh and dry root weight, up to 3 times its leaf area and crop height. Such result was consistent with those from (Chen et al. 2016). This is because red LED activates phytochrome, which is a photoreceptor that plays a critical role in the crop growth. The activation of phytochromes promotes leaf expansion over stem elongation, initiates plastid development and controls the gene of chloroplast and nucleus (Folta & Childers 2008). Additionally, with an increased leaf area, it provides a greater surface area for light interception, thereby enabling biomass to increase significantly. Furthermore, the greater root weight also aids a higher biomass production as having a vigorous root system provides and allows a higher uptake of water and minerals for the shoot growth thus enabling a higher biomass.
However, (Stutte et al. 2009) has recorded that utilizing red LED solely records a declining photosynthetic rate and chlorophyll fluorescence. As shown in Figure 4, the chlorophyll fluorescence is approximately 0.78 which is 7% lower than a health crop value of 0.83, and this indicates that the crop is under stress. Nonetheless, the values increase with the input of blue LED, and under the combination of 59% of blue and 41% of red, the chlorophyll fluorescence manage to obtain the healthy value of 0.83 too. Thus, it indicates that although red LED does help in enhancing the growth but using it solely hinders the photosynthesis machinery from functioning to its full potential. In addition, (Cope et al. 2014) documented that a higher biomass of lettuce was achievable by using a combination of both red and blue LED, with about 15% of blue LED. Consistent to the results presented in Figure 2, although the biomass of lettuce under various red and blue LED might be slightly lower as compared to those exclusively under red LED, but overall the biomass recorded are still much higher as compared to those under the traditional lightings. Hence, the blue and red LED are commonly used as a combination to achieve a higher productively. But further research is required in studying the suitable ratio of the combination as its requirement varies according to crop.
Quality and phytochemicals
Not only the crop growth, light is also critical for the biosynthesis of nutraceutical components of the crop, for its sugars and phytochemical compounds. With specific light spectrum, the nutritional quality of LSG can be improve using LED. The most identifiable response will be the accumulation of sugars in leaves due to photosynthesis. Having a high sugar content may also benefit the taste of the LSG by enhancing its sweetness. According to (Lin et al. 2013), red and blue LED does influence the accumulation of sugar content of the crop. It was recorded that lettuce grown under the combination of red and blue LED obtains about 25% higher sugar content as compared to the rest of the traditional lights. Thus, it justifies that the composition of lights may indirectly affect the overall quality of a crop.
Additionally, utilization of various red and blue LED was also noticed to increase the phytochemicals concentration in the crop (Li & Kubota 2009). Its results as presented in Figure 5, the anthocyanins concentration has the greatest value using blue LED and it increases up to 31%. The carotenoid concentration also shows a positive increase of up to 6% with blue LED. Similarly, the phenolics concentration was observed to increase up to 6% under the red LED. These results were consistent with who reported that red and blue LED enhances the accumulation of phytochemicals in lettuce (Samuolienė et al. 2011; Chen et al. 2016). Hence, its concludable that the quality and phytochemical differs with light composition and red and blue LED are capable in improving the quality and enhancing the phytochemical accumulation.
In conclusion, production of LSG using LED enables a higher productivity as compared to the traditional lights. The increase use of LED are due its advantages such as its energy efficiency due to its wavelength specific which enables elimination of less important wavelength, its low emission of heat which allows installation of lights near the crop as well as reducing the energy required due to shorter distance, its small size which enables design versatile, longer lifespan as compared to traditional lights and lastly its support in adaptation which enables an all-year round production regardless of any environmental issues like flood, seasonality issues and drought. In comparison with the traditional lights, the various combination of red and blue LED induces the growth by increasing its fresh and dry weight, crop height and leaf area, drives photosynthetic reaction and increases its chlorophyll content which indirectly enhances the growth of the crop. Additionally, the LED combination also improve the crop quality by achieving a higher sugar content and a greater accumulation of phytochemical concentration within the crop. Although the start of cost to set up the LED might be slightly more expensive than traditional lights but a greater return is achievable through this system as the production continues all year-round in a safe environment and it produces high yielding and quality crop. Hence, it achieves a higher productivity as compared to traditional lights and is a promising tool for the LSG production industry.
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Figure 1: Chlorophyll content recorded in two lettuce cultivars, Summang and Grand Rapid TBR under different combination of red and blue LED, and its control under florescent light over 4 weeks (Son et al. 2012).
Figure 2: Biomass and morphological characteristic of two lettuce cultivar, Summang and Grand Rapid TBR under different combination of red and blue LED, and its control under florescent light over 4 weeks (Son et al. 2012).
(W= White LED, WY = Yellow LED, WR = Red LED, WFr = Far red LED, WG = Green LED, WB = Blue LED)
Figure 3: Morphological characteristic of lettuce “Green Oak Leaf” under different combination LED (Chen et al. 2016).
Figure 4: Chlorophyll fluorescence of lettuce cultivar under different combination of red and blue LED, and its control under florescent light over 4 weeks (Son et al. 2012).
(W1/W2= control white light, WUV = UV Red, WR = Red LED, WFR= Far red LED, WB = Blue LED, WG = Green LED)
Figure 5: Accumulated phytochemicals of baby lettuce cultivar under different various LED combination (Li & Kubota 2009).
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