Effects of LED lighting technology on the productivity of leafy vegetables

Introduction

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 conten