Plant photomorphogenesis

Indoor farms, also referred to as vertical farms, have increased in the U.S. since horticultural light emitting diodes (LEDs) became commercially available. LEDs are a promising technology to reduce the energy consumption of indoor growing systems. Before LEDs, fluorescent lights were used as the only source of light to grow plants indoors.

Fluorescent lights have a broad spectrum containing approximately 20% blue light (400 to 500 nm), 49% green light (500 to 600 nm), and 31% red light (600 to 700 nm), and less than 1% UV light (300 to 400 nm) and far-red light (700 to 800 nm)* (Figure 1a).

The light spectrum of fluorescent lamps is suitable to grow plants indoors; however, LEDs are now more efficient in converting electrical power into photons of light than fluorescent lights. The current efficiencies for high output fluorescent grow lights (T5-HO) is around 1.23** µmol of light per every second per every watt (W) of electrical power (1.23 µmol s^-1 W-1 or 1.23 µmol J^-1).

In contrast, current advertised horticultural LEDs have a range of 1.75 to 2.2 µmol s-1 W-1 efficiencies. This means that LED horticultural fixtures have 42 to 69% greater efficiency than common fluorescent grow lights, which directly translates into electrical power savings.

Maximizing plant growth

Current research efforts are underway to identify the LED spectrums that maximize horticultural plant growth. The idea is to provide the plants with only the necessary wavelengths to maximize growth rate while maintaining optimal plant morphology. The sun provides a full light spectrum and the specific photon flux of each color varies according to the season, geographical location, time of the day and greenhouse glazing.

For example, a solar spectrum measured at noon inside a greenhouse in Arizona with double-layer polycarbonate glazing had: 19% blue light (400 to 500 nm), 25% green light (500 to 600 nm), 28% red light (600 to 700 nm) and 27% far-red light (700 to 800 nm) (Figure 1b). With LEDs, it is possible to manipulate the light spectrum to increase growth rate or/and improve plant morphology (Figure 1c). More research is needed to identify versatile spectrums that can improve plant growth rate of horticultural crops suitable for indoor growing.

In addition to growth rate, another important component of the light spectrum is light’s effect on plant architecture (morphology) and development, often referred as plant photomorphogenesis.

Research has shown general effects of the different light colors on plant photomorphogenesis. For example, blue light can control stomatal opening, which can affect photosynthesis and water balances in the plant. Also, the stimulation of blue photoreceptors (cryptochrome) can decrease stem height and leaf size. The effect of green light is less understood; however, green light can trigger shade avoidance responses such as increase in stem extension, leaf area and petiole elongation.

Green light can also inhibit the responses triggered by blue light. Plant responses to red and far-red light are better understood. The phytochrome photoreceptor is able to perceive light in the red and far-red regions of the spectrum. Phytochrome-mediated plant responses are triggered by the relative amounts of red and far-red light in the light spectrum (phytochrome photostationary state). If far-red light is more abundant, then shade avoidance responses such as stem extension, increase leaf area, and petiole elongation are triggered. With the continuing development of LED lights, academic research is focusing on determining the optimal spectrum to promote desirable photomorphogenesis for horticultural crops when grown under indoor conditions.

For example, NASA research has shown that only a minimum amount of blue light (35 µmol·m^-2·s^-1) on an otherwise all-red light spectrum is need it to produce lettuce indoors. For ornamental plants such as impatiens, petunia and salvia, Michigan State University showed that under all-red light, plants were less compact than under the combination of red and 25%+ blue light.

In addition to ornamental and leafy greens, vegetable transplants are another suitable and economically feasible horticultural crop that can be produced indoors. At the University of Arizona, we have been working on research to find the optimal light spectrum to grow vegetable transplants indoors (Eguchi et al., 2016a, b; Hernández et al., 2016; Hernández and Kubota, 2016) (Figure 2). Tomato and cucumber seedlings were grown under different percentages of blue light (10%, 20%, 30%, 50%, 75% and 100%) on an otherwise red spectrum. For both tomato and cucumber seedlings, the greater percentage of blue light increased the compactness of the seedlings (reduced stem length and increased steam diameter) up to 75% blue treatment (Figure 3). The spectrum containing 10% blue light yielded the highest growth rate for cucumber seedlings, while 30 to 50% blue light yielded the highest growth rate for tomato seedlings.

We also compared plant growth between LEDs and the rarely available fluorescent lamps. Cucumber seedlings grown under 10%, 30% and 50% blue light (the rest being red) had the same growth rate as plants under fluorescent lamps. Tomato seedlings under 0%, 10%, 75% and 100% blue light had the same growth rate than in fluorescent lamps, and plants under 30% and 50% blue had higher growth rate than fluorescent lamps.

Research is underway to find diverse spectral recipes to grow plants indoors. For example, light recipes can be developed for the purpose of increased wet mass accumulation, increased dry mass accumulation (growth rate) or even increased nutritional content. Also, while developing optimal light recipes, it is important to address the interaction of light with other environmental factors such as CO₂ concentration, temperature, relative humidity, nutrition and air movement. Overall, the key metric under indoor production is to achieve the greatest production with the lowest energy possible (grams of production per kilowatt hour of energy) to increase the sustainability of indoor farming.

Ricardo is an assistant professor in Controlled Environment and Sustainable Energy, Department of Horticultural Sciences at North Carolina State University.

Eguchi, T., Hernández, R., and Kubota, C. (2016a). End-of-day far-red lighting combined with blue-rich light environment to mitigate intumescence injury of two interspecific tomato Acta Hort in press.

Eguchi, T., Hernández, R., and Kubota, C. (2016b). Far-red and blue light synergistically mitigate intumescence injury of tomato plants grown under UV-deficit light environment. HortScience in press.

Hernández, R., Eguchi, T., and Kubota, C. (2016). Growth and Morphology of Vegetable Seedlings under Different Blue and Red Photon Flux Ratios Using Light-emitting Diodes as Sole-Source Lighting. Acta Hort in press.

Hernández, R., and Kubota, C. (2016). Physiological responses of cucumber seedlings under different blue and red photon flux ratios using LEDs. Environmental and Experimental Botany 121, 66-74.

* Spectroradiometer scan of a T12 cool white fluorescent lamp 4500K

** Estimated from a conversion factor derived from side-by-side quantum and lux meter measurements of a T5-HO fluorescent growth light.