The role of light and spectrum on plant growth and metabolite production in CEA
We know light exerts a powerful influence on plant growth. These effects can range anywhere from seed germination to leaf expansion and from flowering to fruiting. But, did you know it’s not only plants that benefit from changes caused by light? Human health can also be boosted by light induced changes in the fruits and vegetables we eat. These systems are interconnected. Read on to find out how CEA farmers could hold the key to both higher crop yields and better human health through the smart use of spectral low intensity LEDs.
Color Survival
As humans we cannot survive without food, water, air, or shelter. Some maybe curious to go further in asking what’s the point in just surviving if you don’t live a healthy, and colorful life. So, what do we really mean by a colorful life? Perhaps the ‘joie de vivre’ could be loosely defined by variety, intensity, and vibrancy in our lives. The similarity to the definition of color, is patently obvious, correlating with chroma, value and hue. We want to show you how interconnected and highly dependent we are on light and color in growing fruitful crops with health promoting factors (aka; Natural products/Specialized Metabolites).
As farmers in CEA we are at the forefront of lighting technology, pushing the boundaries of understanding in the requirement to produce the best quality crops with the greatest impact on our health. Often we are told to ‘eat the rainbow’ in order to provide a range of nutrients for health. With this in mind it is even more important than ever for growers to use their knowledge and appropriate technology to increase their value proposition with efficient growing and marketing of their products.
What are the elements that make up color?
We see the world in a multi coloured spectrum of reflected light wavelengths. Of course ‘visible’ light is only a small part of the wide spectrum which as a whole also includes ultraviolet, and far red wavelengths. Structures called cones (rods are used for night vision and low photon light) in the back of our eyes refract visible light like a prism below to send a signal to our brains which helps us distinguish colors. Most often in life we see visible light split into its constituents when a rainbow forms and electrons are diffracted through raindrops. How much color is relevant and used by a given plant, we aim to find out.
Light penetrates air, water, and through our shelters if we don’t block it out so we must gather data on how much radiation traverses the greenhouse and other structures which give shelter to crops. Integral to this is the daylight integral or DLI, the optimal amount of light a plant needs over a day. This helps us establish when to add further efficiencies with supplemental light and tailored spectral recipes.
How do plants perceive light?
Plants are dependent on their ability to sense and interact to their surroundings to optimize their chances of survival. What happens in the plant world is very interesting and light has several known actions on plant growth and development.
Photosynthetic pathways are driven by light or more appropriately by energy exchange. When light is absorbed by leaves, photon light particles are transformed to a higher energy state in chloroplasts to synthesize glucose as an energy source from CO2 and this produces O2 as a by-product, along with this major function photons also send a signal to either increase or decrease plant growth regulators called auxins and cytokinins which control the direction of plant growth.
Figure showing chloroplast (light harvesting compartment) and the process of photosynthesis (light harvesting process) in a plant cell.
Deeper in the chloroplast within the thylakoids lie the photosystems that serve as the site for absorption of sunlight. Special structures called photoreceptors detect an array of wavelengths, allowing them to ‘perceive’ light and send a signal in the direction of growth. Similar to human eyes a wide range of photoreceptors exist, including phytochromes, cryptochromes, phototropins and ultraviolet-B receptors help plants discriminate light signals from ultraviolet to visible to far red wavelengths. Of course it’s much more complicated than we can talk about in this short article but essentially the plant has a control mechanism that distinguishes wavelengths through these photoreceptors and a metabolic switch to biological reactions.
In summary the leaf interface acts as a mini processor, where energy from excitatory photons hitting the thylakoids catalyze the photosynthetic pathway between carbon and water to produce glucose and oxygen. This directly impacts cell signaling, including metabolic, morphological and physiological changes in plants.
It is important to take into account not only spectrum but also efficacy of LED lamps as this determines the number of photons hitting the leaf surface. This means lights should be balanced for growth and less likely to be separated as defined by the image below. Instead they are low intensity LEDs incorporating blue wavelengths and appearing white, or balanced red and blue wavelengths, the latter appearing pink. It is important to take advice from a quality LED vendor and compare the market as not all LEDs are equivalent quality. Reach out if you need advice.
As more academic research into LED lighting becomes available, increased awareness of specific wavelength induced changes will help efficiency in new crops for higher biomass and increased stable levels of health promoting specialized metabolites for human health.
Can varying spectrum LED lighting increase crop traits and efficiency?
Light exerts a powerful influence on most vegetable tissues, and there can be no doubt that it generally tends to check their growth” – Charles Darwin, 1880 (The Power of Movement in Plants)
In CEA we have the advantage of an agricultural phenomenon that can harness data on each of the nine environmental variables that impact yield including diffracted wavelengths. This ultimately helps refine and optimize processes for farmers.
Different wavelengths help plants achieve various goals. In general plants exposed to blue light encourage vegetative leaf growth, stem elongation and rooting whereas red light, when combined with blue, switches on genes for plants to flower and fruit. This is not surprising when experiments show an increase in chlorophyll content in the PAR range of the spectrum. Green wavelengths reflect most light (hence why we see them as green) but this specific wavelength is known to be responsible for deeper canopy penetration and absorption balance of excess energy in some plants. The latter is an important physiological step, often overlooked as not all energy is used in photosynthesis (remember it’s rate limiting) and excess energy must be dissipated safely as heat.
Although we class the photosynthetically active region (PAR) between 425-695nm, a nice study by Paul Kusuma at Wageningen showed the power of far-red photons influencing leaf area and stem elongation. Essentially the higher ratio of far red light can help plants stretch at night. He also found lower energy of far-red photons makes them useful in reducing electrical power inputs.
UV light on the other hand can be used in pulses to disrupt bacterial DNA and prevent disease in plants. Short term UV treatment has been shown to improve performance for both seedlings and seeds that deliver long-term benefits, including improved crop consistency, increased yield and stronger disease resistance. This can increase the chance of producing healthy plants without viral invasion. As Darwin succinctly suggested, light provides nature’s way of balance.
Learn more from the experts in horticulture lighting spectrum here.
Different wavelengths in Turmeric
When plants are grown in tissue culture, light, humidity, and nutrients can be tightly controlled. Although TC is an artificial state with immature leaf structure, it could be useful in predicting a smart spectral recipe, taking into account the lack of stomatal development. Low light intensity LEDs are typically employed as a strategy to prevent heat damage in immature propagules but different wavelengths could be more advantageous for certain desirable traits. For instance red, blue, and green LEDs have been found to have specific effects on plant growth rate, developmental characteristics, and production of bioactive specialized metabolites.
We used turmeric as an example of how to control light for different growth and specialized metabolite requirements.
Under low intensity LEDs, we can encourage rooting in turmeric but also elongation of shoots. We can also combine factors we know control growth like levels of plant growth hormones, humidity, gas exchange, liquidity of substrate and additions like activated charcoal to help some species like turmeric root better and this can also increase plant biomass. Good rooting and biomass gives plants a head start during acclimation.
Turmeric shoots multiplied under Arize Lynk LEDs (red blue) as they continue simultaneous growth of both leaf area and roots in the multiplication phase. It’s not always desirable to let roots grow out in the multiplication phase as they tend to be more vulnerable to infection particularly if using high sucrose as a carbohydrate source. Reducing the ratio of blue can help reduce rooting during this stage. When in the multiplication phase, the level of cytokinin (shooting) to auxin (rooting) is increased but we also can utilize light to control growth as desired.
Acclimation
As turmeric acclimates and the plants develop mature leaves, Arize Lynk LEDs are better for leaf growth and an advantage to increase foliage biomass. We know from other studies that turmeric grown in the acclimation phase, under RB spectrum increases phytochemicals, such as polyphenols, flavonoids, sugars, and boosts curcumin biosynthesis.
Turmeric is a perennial spice that can reach a height of about 1m. To increase turmeric rhizome size requires higher light intensity light and increased oxygenation of roots during the growth stage taking up to a year to produce good harvestable yields in different systems. Prior to harvest, farmers should consider supplemental RB light and higher intensities, to increase anthocyanin content.
While we do not have the results from studies of isolated green light, we postulate that green light is efficiently absorbed deep into the canopy during rapid growth periods. If you time the crop season right, natural sunlight allows for a reduction in energy consumption while using the whole visible spectrum more efficiently (that is if wavelengths are not deflected from the structure you are growing within).
Growing plants like turmeric in CEA for the entire crop cycle is unsustainable and farmers should consider hybrid models to produce the best results and yields for the end user product and market they target. For instance if the product is for specialized metabolites then by all means grow and process in as close as possible to sterile environments but if the market is for color and curry, open fields are more realistic.
The power of hue in health.
Did you know sir Isaac Newton invented the first color wheel in 1666? I did not!
Artists have studied and designed other wheels based on Newton’s concept. Most color wheels have a total of 12 main divisions (as we see from the chart), but then subdivided again we have 24.The primary colors are red, yellow, and blue. The secondary colors are green, orange, and purple and the tertiary colors are yellow-orange, red-orange, red-purple, blue-purple, blue-green, and yellow-green. The problem is that color is not a quantifiable way to determine the anthocyanin content of a given fruit, leaf or rhizome.
The Munsell color scheme on the other hand could be the way to distinguish higher levels of anthocyanin. The color scheme comprises hue, value, and chroma. Anthocyanin pathways are complex and often unstable due to oxidation but if stabilized using supplemental LED lights it could be a quantitative roadmap. Using the Munsell system could help us understand color related health values in the same way that brix value quantifies sweetness in fruit sugars.
A change in the color of plant skin, leaf, fruit, and rhizome indicates when plants are ready for harvest. But do we ever consider we can control this process? It’s called the stress reaction in plants. Some fruits with purple skins will have higher Munsell values. We can correlate color intensity of blueberries, blackberries, strawberries and raspberries with higher levels of antioxidants. As a fluorescence scientist I know there are many spectrophotometer devices that could be used to quantify color values. An example of color versus phytonutrients can be seen in bilberries. Bilberries exhibit darker hues than farmed blueberries and have significantly higher anthocyanin content compared to the latter. Could we, in the future, have a hand held device for farmers to know the level of anthocyanin?
Other research articles reviewed the targeted use of LEDs, i.e. blue range (400-500 nm) of spectrum and found blue light is efficient in enhancing the accumulation of phytochemicals.
The flavonoid Curcumin in turmeric trapped in vesicles can range in diverse yellow-orange hues. Curcumin is a bright yellow chemical compound that gives turmeric its color. It is not readily soluble in water, but is in other carriers. Electrons in the curcumin molecule absorb energy from ultraviolet light and move to a more excited state. Try this interesting experiment if you are a teacher, it will get your students attention.
Stability of phytonutrients.
Curcumin in turmeric has been well proven in the lab to kill many types of cancer cells. Why does this not translate to the body? The biosynthetic pathways are highly unstable and curcumin has extremely low bioavailability. It is only when curcumin is combined with piperine that we see positive effects. Even then, the marketing of products containing turmeric has led us to believe they can cure ALL ills, when they cannot. We are not insinuating these functional plants don’t have potential, but we are concluding it is dependent on stability and bioavailability of active metabolites.
If you read our personal health journey’s you will discover like we did the best kept secret in medicine; that is if you are ill, a colorful plant based diet will give you a fighting chance.
Image of different zingiberaceae species courtesy of our friends at Spade and Clover, Johns Island, South Carolina.
Should we continue experimenting with environmentally friendly ways to produce the healthiest plants?
Absolutely, there is more to discover.
We know there is variability in the level of curcumin in commercial turmeric and native turmeric alone has low bioavailability. This means that, under normal circumstances, little is absorbed from the gut into the body. Increased stable levels of specialized metabolites could have potential to produce similar metabolic and physiological effects to what we see in the lab.
Light up your plants for Specialized Metabolites
Increasing the quantity and quality of curcumin using low intensity spectral LEDs at the correct time in the growing cycle can increase important specialized metabolites possessing various pharmacological properties providing increased carrier opportunity to cross membranes and produce stable physiological effects.
Is a turmeric based curry the healthiest meal you can eat?
In India, turmeric is commonly known as “haldi” (Sanskrit; haridra).
- Preceding Vedic culture, turmeric has been used for more than 4000 years in India, where it was used as an edible spice with ceremonial significance still practiced today.
- According to Ayurveda and Unani systems, turmeric has a long history of medicinal use in South Asia. In fact, in 1280 Marco Polo talked about turmeric as the new wonder spice having qualities similar to that of saffron.
- Susruta’s materia medica (250 BCE), mentioned a formulation of ointment containing turmeric as a major ingredient, having anti-inflammatory properties which helps in reducing the effects of food poisoning.
Indian recipes are a great way to boost the immune system, reduce inflammation, and improve cognitive functions. Turmeric and black pepper together have impressive health benefits, due to the metabolites curcumin and piperine. As piperine enhances curcumin absorption in the body by up to 2,000%, combining the spices magnifies the effects. You can read more about growing turmeric and the beneficial health effects from our previous article.
Now if this doesn’t inspire you to make a wonderful healthy grain inspired curry, and buy the freshest ingredients from your local farmer we haven’t done our job right.
All Images unless otherwise stated are the property of Urban Ag News, please ask for permission to reprint our articles. We are indebted to our friend Dr Shashank Saini for his diligent review of this article.
Janet Colston PhD is pharmacologist with an interest in growing ‘functional’ foods that have additional phytonutrients and display medicinal qualities that are beneficial to human health. She grows these using a range of techniques including plant tissue micropropagation and controlled environmental agriculture to ensure the highest quality control.
Unless otherwise stated all images are courtesy of The Functional Plant Company and property of Urban Ag News.
