SCIENCE TO THE GROWER: A Nobel idea for plant lighting

by Richard Evans

Whew! I barely got this article done in time for the newsletter because I have been busy writing my acceptance speech for the Nobel Prize. As it turns out, I’ve been passed over once again and won’t need that speech now, but I’m a glass-half-full type of guy. I’m optimistic about the chance that I’ll win next year. I’m also optimistic about the chance that the tuxedo I bought for the awards ceremony will still fit next year. 

One of this year’s Nobel Prizes caught my eye because it’s pertinent to greenhouse flower growers. The physics prize went to three Japanese scientists for their invention of the blue light-emitting diode (LED). Prior to that invention, we had to be content with red and green LEDs. Their energy-efficient light production probably saves money for big-box stores when they are all lit up for the Christmas season, but their incomplete light spectrum offered limited benefits to growers seeking efficient supplemental lighting for greenhouse crops. For example, Eskins (1992) showed that Arabidopsis, the guinea pig for plant scientists, takes nearly three times longer to flower when grown in red light than it does in blue light or a combination of blue and red. 

Blue LED technology has many commercial applications, but its impact becomes evident when it is combined with red and green LEDs to make white light. All of you have felt that impact when you have watched cat videos on your smartphones while sitting in the audience enduring my extension talks. But you may eventually feel the greatest impact when you use white LED lamps for lighting your crops. 

The obvious uses for white LEDs in ornamental horticulture are for increasing plant growth by supplementing natural light in winter months, and for controlling flowering by manipulating photoperiod with night interruptions. Growers have relied primarily on metal halide lamps for supplemental light and incandescent lamps for photoperiod control, but both light sources also emit large amounts of energy at wavelengths that plants can’t use. That wasted energy is costly. LEDs have a low heat output, adjustable light intensity and color, and high efficiency of energy conversion to light. They also can be pulsed at high frequencies. Tennesen and others (1995) found that when all of the energy from continuous light was packed into 1.5 microsecond pulses from LED lamps every 150 microseconds, plants produced the same amount of photosynthesis as in continuous light. The rapid pulses of light saturate the plant’s photosynthetic apparatus, and the pauses between pulses allow the plant to make use of the light energy they captured. That might not have a practical application right now, but it sure would annoy your neighbors. 

Jao and Fang (2004) compared efficiency of fluorescent lamps and arrays of red and blue LED lamps in a tissue culture system. The highest growth rate was achieved with LED lamps that flashed on and off every 0.7 milliseconds. Growth was only slightly slower, and energy use was 80% lower, when the LED lamps flashed on and off every 2.8 milliseconds. That’s a substantial energy savings, and I’m sure it still would annoy your neighbors. 

Recently, Chang and others (2014) devised a scheme for greenhouse lighting that takes advantage of all of the variables afforded by LED lamps. Their system adjusts the spectrum and intensity of supplemental light as the spectrum and intensity of natural light changes during the day, and also makes adjustments according to the developmental stage of the plants (e.g., changing lighting to affect both vegetative growth and flowering). 

Right now, LED bulbs are expensive, and their high cost doesn’t make them a viable option for most commercial horticulture applications. However, energy costs may rise again, and the price of LED bulbs is sure to come down. In the meantime, I’ll gladly buy LED bulbs for everyone after I win the Nobel Prize next year.

Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.

References

Chang CL, Hong GF, Li YL. 2014. A supplementary lighting and regulatory scheme using a multi-wavelength light emitting diode module for greenhouse application. Lighting Research and Technology 46: 548–566. 

Eskins K. 1992. Light-quality effects on Arabidopsis development. Red, blue and far-red regulation of flowering and morphology. Physiologia Plantarum 86: 439-444. 

Jao RC, Fang W. 2004. Effects of frequency and duty ratio on the growth of potato plantlets in vitro using light-emitting diodes. HortScience 39: 375-379. 

Tennessen DJ, Bula RJ, Sharkey TD. 1995. Efficiency of photosynthesis in continuous and pulsed light emitting diode irradiation. Photosynthesis Research 44: 261-269.