Manipulating Plant Growth Responses with LEDs

by Nicholas Claypool and Heiner Lieth

A 2014 issue of UCNFA News included an article about LED lighting technology (Evans 2014) describing some of the exciting effects that light emitting diodes (LEDs) have on plant growth that could be used in horticultural production. Most discussions around lighting systems focus on photosynthetically active radiation (editor’s note, see previous article for definition), with particular emphasis given to the economics of light production and initial investment costs. In this article, we focus on the effects of LED lighting in manipulating plant growth responses that are controlled by phytochrome, rather than overall plant growth as a result of photosynthesis. Phytochrome is a plant light receptor, a pigment that plants use to detect light. It is sensitive to light in the red (660 nm) and far-red (730 nm) regions of the visible spectrum. Phytochrome has two different chemical structures named after the color of light that they absorb: Pr (physiologically inactive form) absorbs red light and Pfr (physiologically active form) absorbs far-red light. The two forms of phytochrome are interconvertible and the change from one form to the other acts as a control mechanism to regulate various stages of plant growth.

Photoperiodic manipulation of flowering has long served as an integral management practice for floricultural production.  Typically night interruption has been achieved with incandescent lighting because the incandescent light spectrum is suitable for converting the active form of phytochrome to the inactive form.  This allows for the photoperiodic flowering responses that growers have come to rely on. 

Outside of the photoperiodic response, growers typically do not control plant growth by manipulating the light spectrum.  This is largely because the spectrum of a given light type is fixed.  High pressure sodium (HPS) lamps, for example, all have more or less the same spectrum — a high portion of green and yellow-orange light with relatively little red or blue light.  While these spectra can be modified to some extent with various coatings and engineering changes, the final spectra will be fixed. Thus HPS and other types of HID lamps in commercial greenhouses are primarily used to enhance photosynthesis and increase plant biomass, not for responses regulated by phytochrome.

LEDs provide a narrow bandwidth of light and come in a variety of wavelengths.  This allows for the creation of custom light spectra, where the combination of diodes results in the overall light spectrum delivered.  The spectrum can be further modified by adjusting the light output of diodes emitting a certain wavelength within a fixture, allowing for different spectral output for different stages of plant growth.

This customization enables more precise activation and control of the phytochrome response.  Using Arabidopsis as a model plant, researchers have identified many other plant growth responses that can be controlled besides flowering, ranging from stem elongation and leaf area control to root development (Franklin and Quail 2010).  By changing root development, one could conceivably also control nutrient uptake and growth rate.

Thus by adjusting ratios of certain wavelengths, compact plants with smaller, more numerous leaves could potentially be produced.  This hypothesis is supported by research conducted by Hogewoning and colleagues (2012) who grew cucumber plants under a variety of light spectra and demonstrated clear differences in leaf size and plant compactness.  Plants grown under red and blue LEDs (with no far red light) were the most compact while those grown under artificial sunlight (with a light spectrum that promoted inactivation of phytochrome) were the least compact; plants grown under artificial sunlight were similar in appearance to plants grown using red/blue/far-red LEDs in a ratio that induced the same phytochrome-balanced state as natural sunlight. This research demonstrates the importance of LED spectral customization. By adding far-red diodes the plants were encouraged to elongate, resulting in less compact plants than plants grown under red and blue LEDs without far red light. 

Furthermore, phytochrome signaling based on the red to far-red light ratio represents just one light spectral quality response in plants.  Plants also possess other light receptors that respond to ratios of blue to UV light and ratios of blue to green light.  Each of these receptors controls different functions in the plant, with the strength of influence affected by the plant developmental stage.  So, the plant may be relatively insensitive to certain signals during vegetative growth, but may increase in sensitivity during flowering and fruit development.

The information gleaned from Arabidopsis studies offers many potential horticultural applications.  Plant appearance can be altered, since red to far-red ratio responses exist for petiole elongation and leaf color (Frank and Quail 2010).  Likewise, both blue and red light can impact stem length and leaf area; also blue and red light signaling can influence flowering time, flower number and flower diameter (Frank and Quail 2010, Huche-Thelier et al. 2016).  In addition to modification of appearance, the plant’s growth rate can also be influenced, since these light signals also influence photosynthesis, nutrient uptake and plant defense (Frank and Quail 2010, Huche-Thelier et al. 2016). 

While illumination is still needed in this area to determine the specifics — ideal spectral compositions for desired effects, range of possible effects and species limitations — the prospects for LED application in horticultural production is bright.

Nicholas Claypool is a graduate student in Horticulture and Agronomy and Heiner Lieth is Professor Crop Ecologist and UC Cooperative Extension Crop Ecologist, Department of Plant Sciences, UC Davis. 

References

Evans R. Science to the grower: A nobel idea for plant lighting. 2014. UCNFA News 18(3):7. http://ucnfanews.ucanr.edu/Articles/Science_to_the_Grower/A_Nobel_idea_for_plant_lighting/. 

Franklin KA, Quail PH. 2010. Phytochrome functions in Arabidopsis development. J. of Exp. Bot. 61:11-24.

Hogewoning SW, Trouwborst G, Meinen E, van Ieperen W. 2012. Finding the optimal growth-light spectrum for greenhouse crops. Acta Hort. 956:357-364.

Huche-Thelier L, Crespel L, Le Gourrierec J, Morel P, Sakr S, Leduc N. 2016. Light signaling and plant responses to blue and UV radiations — Perspectives for applications in horticulture. Env. and Exp. Bot. 121:22-38.