Light-emitting diodes (LEDs), when used in LED grow lights, have a variety of advantages over traditional forms of horticultural lighting. The advantages of LED grow lights is as a result of:

• Their small size
• Durability
• Long lifetime
• Cool emitting temperature, and
• The option to select specific wavelengths for a targeted plant response.

A revolution in horticultural lighting is underway due to new developments in wavelength availability, light output, and energy conversion efficiency.

LEDs and grow lights

An LED is composed of semiconductor materials that are sandwiched together. A semiconductor is not a metal, but not an insulator either. A semiconductor is somewhere between a metal and an insulator. Therefore, it doesn’t have free ranging electrons that can move around and it doesn’t have free electrons.

The two sandwiched semiconductors in an LED differ in the fact that one has extra electrons added to it and the piece has electrons removed from it. Subsequently, some of the electrons cross over and create what’s called a depletion region. The depletion region acts like a barrier. The depletion region shrinks when a voltage is applied.

The applied voltage changes the behaviour of the depletion region to act more like a check valve in the case of an LED. The high energy electrons that have been doped on the inside will jump over to the side that’s missing the electrons. The jump releases energy in the form of a photon, based on what is referred to as Planck’s law. Material scientists and chemists have been able to determine the different chemicals to dope into the semiconductor materials to make just about any colour.

LED grow light efficiency and spectrum

The efficiency of the LEDs depend on the materials that are in the chip, some are less efficient than others. Yet, they are similar in that they form a band. Therefore, they form a wavelength peak that has a point at the top, producing the different colours.

Early LED grow lights used LEDs with a high intensity in a very narrow band, such as red and blue. Different colours of these LEDs are placed together to provide the accumulated effect of a light spectrum. Additions to LEDs include primary or secondary optics to change the angle of the released photons. Phosphor and other chemicals can be used to narrow the band peak or drop it to lower energy wavelengths, thereby changing the colour of the light.

The solid state technology of LEDs provide efficient lighting, but the downside is an increase in cost. Fortunately,  the cost is decreasing, but it’s still a much more expensive option up front than HPS and other light sources.

Early testing of LED grow lights

The most likely the first recorded use of LED grow lights were by Bula et al. (1991) at the University of Wisconsin. The growth of lettuce plants under red LEDs supplemented with blue fluorescent (BF) lamps was equivalent to that under cool-white fluorescent (CWF) plus incandescent lamps. BF lamps were used as an alternative to blue LEDs, because blue LEDs were not widely available at the time.

Subsequent testing by that group showed that hypocotyls and cotyledons of lettuce seedlings became elongated under red (660 nm) LEDs and that the effect could be prevented by adding some blue light (Hoenecke et al., 1992). These findings inspired continued development of LED grow light systems for small plant growth chambers. LED grow light systems flew aboard  NASA’s Space Shuttle (Barta et al., 1992) and were used to grow:

• Wheat (Triticum aestivum L.) and Brassica rapa L. seedlings (Morrow et al., 1995),
• Potato (Solanum tuberosum L.) leaf cuttings (Croxdale et al., 1997),
• Arabidopsis thaliana (Stankovic et al., 2002), and
• Soybeans [Glycine max (L.) Merr] (Zhou, 2005).

The potential of LED grow lights

The potential of LEDs for terrestrial plant research continued:

•  A comparisons of red LED and xenonarc- illuminated kudzu [Pueraria lobate (Willd.) Ohwi] leaves showed slight differences in stomatal conductance (gS) but similar photosynthetic responses to photosynthetic photon flux (PPF) and CO2 (Tennessen et al., 1994).
• A comparison of photosynthetic rates of strawberry (Fragaria ·ananassa L.) leaves with red (660 nm) or blue (450 nm) LEDs showed higher quantum efficiencies under the reds (Yanagi et al., 1996a).
• Spectral measurements of red (660 nm) LEDs, red LEDs plus BF, red LEDs plus far-red (FR, 735 nm) LEDs, and metal halide (MH) lamps indicated similar phytochrome photostationary states but significantly higher levels of long-wave radiation from the MH lamps. Highlighting the thermal advantages of using LEDs in plant growth systems (Brown et al., 1995).
• Rice plants grown under a combination of red (660 nm) and blue (470 nm) LEDs sustained higher leaf photosynthetic rates than did leaves from plants grown under red LEDs only (Matsuda et al., 2004). The authors attributed this to higher nitrogen content of the blue light-supplemented plants.

The figure above illustrates why specific wavelengths is the main reason LED grow lights are able to outperform other light sources.

The importance of LED grow lights

Light quality plays a major role in the appearance and productivity of ornamental and food specialty crop species:

• Far-red light, for example, is important for stimulating flowering of long-day plants (Deitzer et al., 1979; Downs, 1956) as well as for promoting internode elongation (Morgan and Smith, 1979).
• Blue light is important for phototropism (Blaauw and Blaauw-Jansen, 1970), for stomatal opening (Schwartz and Zeiger, 1984), and for inhibiting seedling growth on emergence of seedlings from a growth medium (Thomas and Dickinson, 1979). The blue light photoreceptor class of cryptochromes has been found to work in conjunction with the red/FR phytochrome photoreceptor class to control factors such as circadian rhythms and de-etiolation in plants (Devlin et al., 2007).

The interactions are complex and continue to be unravelled at the molecular level (Devlin et al., 2007), but much of our understanding of these responses comes from studies with narrow-waveband lighting sources, in which LEDs provide obvious advantages. LED grow lights can be used to enhance desired characteristics for specific crops.

In addition to changes in appearance and productivity, plant responses to narrow bandwidth light sources or to supplemental LED lighting show increased suppression of pathogens in tomato and cucumber (Kim et al., 2005 and references therein).

References

This post was adapted from the article: Plant Productivity in Response to LED Lighting by Gioia D. Massa, Hyeon-Hye Kim, Raymond M. Wheeler and Cary A. Mitchell. Published in HORTSCIENCE VOL. 43(7) DECEMBER 2008. Additional information from KIS Organics Cannabis Cultivation and Science Podcast Episode 48.

In text references in order of appearance:

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• Hoenecke, M.E., R.J. Bula, and T.W. Tibbitts. 1992. Importance of blue photon levels for lettuce seedlings grown under red-lightemitting diodes. HortScience 27:427–430.
• Barta, D.J., T.W. Tibbitts, R.J. Bula, and R.C. Morrow. 1992. Evaluation of light emitting diode characteristics for space-based plant irradiation source. Adv. Space Res. 12:141–149.
• Morrow, R.C., N.A. Duffie, T.W. Tibbitts, R.J. Bula, D.J. Barta, D.W. Ming, R.M. Wheeler, and D.M. Porterfield. 1995. Plant response in the ASTROCULTURE flight experiment unit. SAE Technical Paper 951624.
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