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.