Original paper: P. M. Pattison, J. Y. Tsao , G. C. Brainard, and B. Bugbee 2018. LEDs for photons, physiology and food. Nature. 563:493-500. https://doi.org/10.1038/s41586-018-0706-x
Compared to traditional lighting, LED lighting offers greater light control, improved performance, and decreased energy consumption. Due to these facts, LED lighting is beginning to be used for an array of new applications to improve human health and localize food production in controlled environments. For the first time in history, the use of LED lighting enables humans to engineer lighting of environments to elicit specific responses.
Four main features separate LED lighting from traditional lighting – light spectral control, light intensity control, control of light distribution in space, and ready integration with other technologies. LEDs for photons, physiology, and food outlines some of the applications and research avenues that LED lighting will enable in both humans and plants.
Lighting impacts both humans and plants greatly. In humans, light affects daily rhythms of sleep and wakefulness, body temperature, alertness, psychomotor performance, neurocognitive responses, and the secretion of hormones. Among the open questions posed regarding lighting for human health and productivity are the nature of the detailed pathways within the melanopsin-based photoreceptor system, interactions between the retinohypothalamic and primary optical tracts, the relationship between the dose of light and physiological regulation in everyday environments, and how to frame our understanding of the positive and negative effects of light. Light-emitting diodes will enable more precise and effective lighting research to be conducted relating to the aforementioned questions, which will enable LED lighting to be increasingly tailored to enhance human health and productivity.
Plants not only require light as fuel for photosynthesis but also use light as a signal to direct plant morphology and metabolite profile. Light sources and color filters have long been used to investigate plant responses to light. However, prior to LED lighting, many of these studies have been limited, mainly because they were conducted at low light levels on single leaves. LED lighting now enables research to be conducted at higher light intensities at the plant canopy level. Additionally, LED lighting allows light intensity, spectrum, and timing of light application to be precisely controlled, taking plant-light response research to new levels.
LED lighting has not only enhanced our understanding of plant-light responses but has also made it cost-effective to grow certain plants indoors for food. To demonstrate the efficacy of indoor agriculture, the authors calculate the grams of dry mass produced per mole of photons for various crops. In doing so, the authors conclude that the photon cost (% of dry market price) is 1% for microgreens, 5% for lettuce, 18% for tomatoes, 103% for general vegetables (i.e. broccoli), and 10,000% for staple crops (i.e. rice).
The main parameters driving the increased photon cost for the above mention crops are:
- Fraction of photons absorbed by the plant: Microgreens can be grown at a very high density, thus the fraction of photons absorbed by the plant is very high. However, as plant size increases, plant spacing must also increase. Increased spacing between plants leads to reduced radiation captured and thus reduces the fraction of photons absorbed by plants, as some of these photons will inevitability be lost in space between plants.
- Quantum yield (moles of carbon fixed per mole of photons absorbed): The more a particular crop benefits from increased light levels will dictate its quantum yield. Lettuce benefits from higher light levels than microgreens and tomato benefits from higher light than lettuce, thus the quantum efficiency of lettuce is lower than that of microgreens and the that of tomato is lower than lettuce.
- Harvest index (moles of carbon in edible product per mole of carbon in plant biomass): Microgreens and lettuce have a very high harvest index as the entire aboveground portion of the plants are edible. Alternatively, tomato stems and leaves are not edible, reducing harvest index. Other general vegetables and staple crops harvest index is further reduced, as these crops generally posses even less edible plant biomass. Thus, for crops with low harvest index, photons are being captured by non-edible plant biomass, leading to increased photon cost per dry mass.
Based on these parameters the authors concluded that “electric light input is a small cost for microgreens, a high cost for general vegetables, and an unacceptable cost for staple crops”. Currently, most indoor farms are focused on growing leafy greens. However, as LED lighting efficiency and technology continues to increase, more general vegetables will be attempted to be grown indoors. Nevertheless, according to this report, even if LEDs were 100% efficient, growing staple crops indoors would not be cost-effective.
It is clear that LED lighting will continually replace traditional lighting and become the standard light source for humans and plants. By 2035, it is estimated that 86% of electrical lighting installs in the U.S. will be LED, which will save roughly US$52 billion per year in direct energy costs. Research into physiological responses to light will allow lighting systems to be optimized and the full potential of LED lighting to be reached, which include improving human health and productivity, increasing the feasibility of local food production in controlled environments, and decreased energy consumption.
The authors mention many applications of LEDs for human and plant physiology. Can you think of any other LED applications that were not mentioned in this paper?
The authors discussed a broad range of possible LED applications, some more in depth than others. I am more interested in hearing specifically how the quality of light can affect plant morphology and growth. There are a wide range of targeted applications that can be achieved using LED lighting, like increased production of certain compounds, overall improved growth, or specific morphological goals. But, what are they? Where is that research currently?
Light quality is a large focus in the supplemental or sole source lighting industry, and much research is being done to target specific crops or achieve certain goals. A lot of this research is done by private companies, and this paper focuses mostly on academic research. Do the goals of industry R&D differ a large amount from academic research? I am curious to know more about this industry and specific goals within it.
Actually, much research is on going relative to light quality in academia. In next 5 years, I am hoping to see more established comprehensive understanding on the light quality effects on plant morphology and growth. Every 3-4 years, ISHS (international society for horticultural science) organizes a conference called ‘Light in Horticulture’ where you find many scientists in academia working in this research area.
If you were an LED researcher (plant or human), what type of experiments would you be most interested in conducting?
It seems like no matter how much development goes into increasing LED efficiency and making the spectra customizable (all good things) there will always be a loss of efficiency for crops unlike micro greens that have to be spaced apart. I would be interested in ways to reduce the loss of the energy not striking the plants directly from the LEDs. Could light beam size, angles, and intensities be optimized to strike a plant or a row of plants without so much loss to the open space? Could reflective surfaces (even just white paint?) be used to give the light a second chance to be intercepted? Where are the lights set – above the plants? Some LED columns within the canopy that release light 360 degrees? Can smaller plants that require less light be interspersed with plants that cause a less efficient use of the space? Treating the greenhouse more as an environment with many layers and pockets, in which differentially adapted plants could all coexist in, even thrive.
Cary Mitchell at Purdue University and his group work on ‘targeted lighting’ technology. In indoor production, that approach is the most effective in reducing the wasteful use of energy for lighting.
I would be very interested to see how LEDs affect work efficiency in closed environments. In my office, I prefer to work with the lights off and rely solely on natural light from my window; I tend to be more efficient. Additionally, there is one faculty member in our building that has blue light in his office when he is working.
This may be an area of interest in LED-human responses. As you know, some production facilities use red/blue lighting and workers express discomfort (and likely reduced work efficiency) in such an environment. Some workers would say not a problem as they get used to, and so it may be depending on workers.
In a sole source setting, I would be interested to see how sink-source flux and any intrinsic circadian rhythms affecting photosynthetic rate could be integrated into lighting strategy to increase energy efficiency. Intensity is adjustable in certain LED fixtures so could there be a real time analysis of photosynthetic rate (leaf temperature or chlorophyll fluorescence potentially) that could be fed back to the control system for lighting? Or if energy balance is favorable, could you increase light intensity to maximize photosynthesis when cooling is requiring high energy inputs?
Why do you think light quality likely affects sink-source?
I think that researching different types of lighting arrangements and installation strategies for LEDs would be interesting. For example, there’s a DOE grant looking at developing thin flexible lighting films that could alter the distribution/uniformity of lighting and I could see some potential applications for controlled environment growing.
If I were conducting human research with LEDs, I would be curious about light quality effects on the circadian rhythm and its relationship to sleep quality. Lighting in general is used indoors until people are about to sleep. Can specific LED wavelengths be targeted that can mirror natural human circadian rhythms to help people unwind/relax/prepare for sleep? Can people who take melatonin or other sleep aids replace these supplements with improved light quality or specific light patterns?
Human circadian rhythms are easily disturbed by varying light levels, and this would be an interest direction of research to see if human living conditions could be improved and reliance on supplements for sleep reduced.
On the other hand of melatonin, can proper light quality in the morning help reduce reliance on caffeine? There are interesting questions here to pose and answer related to sleep cycles, circadian rhythms, and human behavior.
LED technology has drastically impact Greenhouse production. The ability to significantly eliminate heat while producing specific light wavelength has reduced energy consumption while providing a more versatile light source. Where do you see LED moving next in Greenhouse production? do you think targeting or combining other wavelength will the next focus in LED?