Agriculture – The light

What color is a plant? Green? Red? Our eyes received the image of an object through the reflection of light. If the object we see is white color, it indicates that the whole spectrum of light is reflected by the object and received by the receptor of our eyes. Before that, we need to know what is found in light, sunlight to be specific. A light can be diffused through a prism and projected seven colors: red, orange, yellow, green, blue, indigo, and purple or violet in decreasing order of their wavelength. Red has the highest wavelength which can be projected as far as possible, while purple has the lowest wavelength which can not be seen at a farther distance as compared to other colors. That is why red is used in the traffic light as the drivers can receive it at a distance that is far away from the traffic light before they stop.

How could the understanding of the application of light at different wavelengths help us in agriculture and horticulture? Light (physics) is projected on a plant (biology) that triggered a series of biochemistry. In most plants, their leaves and young stems are green in colors at different intensities. Most leaves possess an organelle, chlorophyll with photosynthetic pigments that gave different intensities of green with a density of the organelles present in the leaf tissues. Apart from that, red color is represented by the presence of lycopene and anthocyanin in the leaves, orange to yellow that shows the presence of carotene and beta-carotene, and blue to purple leaves that contain lycopene and anthocyanins which could be the signs of medicinal values apart from their attractive colors. In addition to photosynthesis, light determines the growth and development of the plants from seeds to flowering at different aspects. Photomorphogenesis that greatly influenced by the light can be used to regulate seed germination (photodormancy and photoblasty), synthesis of chlorophyll (photosynthesis), stem and leaf growth towards visible light (etiolation and phototropism), flowering time based on the length of the day, and light (photoperiodism), and the reaction to various light colors.

Light is electromagnetic radiation within the electromagnetic spectrum that can be perceived by the human eye. Visible light is defined as in the range of the wavelength between 400 to 700 nanometers, nm. For green plants, the light absorption by the chlorophyll, which can be split mainly into chlorophyll a and b, have different light absorption spectra but both do not absorb the spectrum of green color (495 – 570 nm). Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis that absorbs most energy from red to orange (at about 662 nm) and blue to violet (approximately 430 nm) light. Chlorophyll b is a form of chlorophyll that primarily absorbs blue light at approximately 380 nm and 500 nm. Since the green is not absorbed, it is reflected and that is why we see green color from most plants. Furthermore, chlorophyll is a carotenoid that adsorbs light in blue-green and violet (approximately 550 and 460 nm) regions that appear red, orange, or yellow to us.

How would the understanding of light help us in horticulture and agriculture? Hydroponic and indoor farming is increasingly demanded nowadays. As we can see, most of the systems involved artificial lighting systems mainly made of light-emitting diodes (LEDs) chips that often show magenta color. It could be the combination of red and blue LED chips at different percentages or ratios depending on the genotype of the plants based on previously reported research. It was found that by removing the green color from the spectrum, it might be great for the plants in terms of the efficiency of the light received by the plants through monochromatic LEDs as green is not absorbed by most plants especially green leafy vegetables. The advantages of using a specific wavelength of LEDs are the spectrum can be adjusted according to the stages of growth and development based on the plant genotype, the light is monochromatic (much focus instead of diverse light), and it is cost-effective for the long term since it produces less heat as compared to fluorescent light tubes.

LEDs have become increasingly popular due to the demand in the market for indoor farming. Commercial LEDs can be customized for the crops so that different wavelengths of the LEDs chips can be manufactured. Violet and blue color LEDs can be obtained by specifying the wavelength of the LEDs to 400-500 and 450-500 nm. Furthermore, the wavelength range less than 400 nm; it is ultraviolet (UV) light. Green would fall within the range of wavelengths that is 500-570 nm. Wavelengths from 570 to 590 nm would be yellow, while orange or amber would have the wavelength in between 590 to 610 nm. Red color LEDs will have a wavelength between 610 and 760 nm. More than 760 nm is known as infrared light that has several impacts on plant growth and development. Far-red LEDs fall just between red and infrared light, which is 700 – 850 nm.

LEDs are favored as an artificial light source for several reasons. One of the advantages is that LEDs are known as cool light that required low power. This means it produces less heat but high efficiency. Therefore, the cost of the air conditioner is greatly reduced due to the reduction of emission of heat by LEDs. Apart from that, LEDs have a relatively long life (25k to 100k hours) compared to typical fluorescent tubes (10k to 15k hours). The colors of LEDs can be manipulated for different stages of the growth and development of a crop. Besides, another benefit of having LEDs is because their size can be very small which is about 2 to 5 cm that made them ideal light sources for use in indoor farming and plant tissue culture. However, the drawbacks of LEDs are it is still costly as the cost of manufacture is high but its long life and electricity-friendly made it worthwhile to use. Moreover, if one of the LED chips goes out, the entire unit must be replaced.

The effect of LEDs could be amazing as a different combination of LEDs (spectrum) would greatly influence the growth and production of the crops. Studies had found that certain spectra of LEDs are known to boost the production of secondary metabolites that have medicinal values. Alternatively, the physiology of the plant reacts accordingly to the application of LEDs at different spectra. Certain spectrum is proved to boost the number of shoots, expansion in the leaf area, production of roots, stimulation of flowering, and fruiting. Furthermore, it is found that LEDs at certain wavelength alter the taste of the vegetables (due to the changes in the concentration of the secondary metabolites), which may change the perspective from the consumer side towards the vegetables. Some studies had shown that LEDs could be used to control and reduce the postharvest diseases of some crops like strawberries.  However, monochromatic LEDs with narrow peak emission may cause an imbalance of the distribution of light energy between the photosystem I and II of the chlorophyll. Therefore, optimization on the spectrum and light intensity must be established for the crop before a mass scale production. As we can see most of the hydroponic system employed purple LEDs or the combination of blue and red LEDs at a different ratio, this is due to the reflection of green color in nature. Therefore, LEDs that are customized without the spectrum are manufactured for such a situation. However, recent studies had proved that full-spectrum with green spectrum gave higher yield and productivity in terms of biomass and biochemical contents of the plants compared to the combination of blue-red LEDs at a different ratio.

Besides, UV LEDs are essential in our life. The main usage of this light is disinfection, which is mainly used in food production especially food safety technology. Apart from that, it is used to induce mutation for crop improvement as the plants have photoresponse to UV light. UV can be further split into UVA, UVB, and UVC which are ranged in between 320-400 nm, 280 – 320 nm, and 100 – 280 nm respectively. UVA radiation has a strong influence on the morphological effects such as the reduction of leaf area and its impacts on biomass production and net photosynthesis is species-specific. It is allowed to reach much deeper tissues in the leaf and penetrate the canopy. Many studies had conducted mainly to study the effect on UVA and partly UVB. This is because UVA and UVB rays from the sunlight are transmitted through the atmosphere, while the ozone layer of the Earth absorbs all UVC and some UVB rays. Exposure to UV rays could be dangerous in the long term and causing abiotic stress to the plants. For a human, long term exposure to a high amount of UV can lead to skin cancer and aging. Therefore, it is always suggested to apply sunscreen with a sun protection factor of around 30 before going out for outdoor activity. For plants, UV triggers the breakdown of molecules into radicals, thus causing radical oxidative stress (ROS) in plants. Certain plant self-defense mechanisms can reduce ROS.

Electromagnetic radiation such as x-ray and gamma-ray are frequently used in crop improvement by inducing physical mutations apart from sterilization. Light is crucial for plants to conduct photosynthesis that produces food as a producer in the ecological chain. Plants react to light via phototropism, whereby it has the proteins called phototropin, which are the main photoreceptors responsible for light detection. Phototropism is found to interact with plant growth regulators like auxin that promotes cell elongation that caused the plant to grow more on the shady sides which bend in the direction of the light. The absence of light caused etiolation, which is a process in flowering plants characterized by long and weak stems and smaller leaves due to longer internodes. It is followed by a pale yellow color, known as chlorosis. However, most of the seeds required a dark condition to germinate into seedlings. The study of light is pivotal apart from optimizing the morphology and biochemistry of the plants; it can be used to trigger flowering by induction of infrared light and adjustment of photoperiod at the physiological level.

There is a critical dark period for photoperiodism. Some plants are known as short-day plants that require minimal light per day in which the light received by the plants is needed less than the critical dark period. However, if there is any flash of light after the critical dark period, the interrupting prevent flowering in the short-day plants. Rice is one of the examples of a short-day plant. Long-day plants like spinach and sugar beets required the minimal light received that longer than the critical dark period. The interruption of light in dark conditions will induce flowering in a long-day plant. Nowadays, there is increasing attention on the application of far-red LEDs as it greatly impacts the canopy of the plants and the induction of flowers.

Plants have specialized proteins, which are known as phytochromes, that play an essential role in controlling flowering apart from the regulation of seed germination, chlorophyll synthesis, seedling elongation, and the leaves movement. Photochrome (P) exists as a dimer formed by two monomers in which it has two forms that are photo-reversible, namely Pr and Pfr. Pr can be changed into Pfr when the plant is exposed to red light, whereas, Pfr will transform into Pr when exposed to far-red light. Pfr is responsible for inducing flowering in a long-day plant. Therefore, the induction of red light that caused the conversion of Pr to Pfr which induced flowerings. On the other hand, Pris used to induce flowering in a short-day plant and it can be improved by subjecting the plants with far-red light that converts the Pfr into Pr.

With the advanced agrotechnology, LEDs are widely monitored by using algorithms with data science that customized for different crop species in-home applications, greenhouse, research, as well as indoor farming. The programmed light system for indoor farming eventually boosted the yield and productivity according to the seasonal demand to secure the food supply chain.

Further readings:

Sharrock, R. A. (2008). The phytochrome red/far-red photoreceptor superfamily. Genome biology9(8), 230.

Gupta, S. D., & Jatothu, B. (2013). Fundamentals and applications of light-emitting diodes (LEDs) in in vitro plant growth and morphogenesis. Plant Biotechnology Reports7(3), 211-220.

Koutchma, T. (2019). Ultraviolet light in food technology: principles and applications (Vol. 2). CRC press.