Introduction

The lighting industry has experienced three revolutions, namely, the hot filament incandescent, gas discharging florescent and HID, and recently solid state LED. For plant growth and hydroponic applications, the second generation florescent and HID still dominate the market, and its lumen per wattage concept is prevalent in our general knowledge of horticulture lighting.

In the coming age of LED lighting, the concept of color rendering index (CRI) and lumen are about to be updated when considering the human eye’s cone cell and rod cell sensitivity [1, 2]. Likewise, different plant responses to light spectrum differently at various biologic stages such as rooting, vegetative growth and budding. This essay will cover the impact of Lumen, which is based on human vision, then Photo-synthetically active radiation (PAR) [3], and lastly more advanced Effective PAR concepts.

The Human Eye and Lumens

There are two ways to measure the "amount" of visible light – in Radiometric, using power of wattage for the light wave, while Photometric is in the term of perceived brightness by the typical human eye. A Lumen is a photometric unit, however Ultraviolet Light (UV) and Infrared Light (IR) are two bands of the light spectrum which the human eye can’t see, their lumen value is zero. Therefore, UV and IR can only be denoted by Radiometric units. The measurement of Lumen value is the weighted summation of radiometric spectrum power against the human photopic luminous efficiency, see Figure 1. Human photonics efficiency peaks at 550nm yellow wavelength and dwindles down above or below 555nm. Therefore, for light with a lot of yellow spectrum , such as High Pressure Sodium (HPS), will have a higher lumen value for a given wattage power of light, though the human eye will process that level of light with less efficiency.   

 

Figure 1: The typical Spectrums of a incandescent, HPS and phosphor converted white LED and human eye response curve.

Photosynthesis and PAR

Similar to the human eye response to light, plants absorb energy from light through the photosynthesis process. Chlorophyll is the main carrier for photosynthesis and this is what gives leaves their green color due to their spectrum preference.  In photosynthesis, the reaction is carried out at the atom and photon level. The speed of reaction is related to the amount of energy present and the number of photons. It is not directly related to a light’s lumen output. PAR value is thus introduced to measure of number of photons that have participated in the reaction. The PAR value measurement can either be through a weighted photon-count or energy weighted as shown in Figure 2. 

When measuring through a weighted photon count, PAR is quantified as µmol photons m⁻²s⁻¹, which is the unit for photosynthetic photon flux area density. PAR as described above does not distinguish between different wavelengths between 400 and 700 nm, and assumes that wavelengths outside this range have zero photosynthetic action.

By weighting energy factor to different wavelengths according to photosynthesis response, the photosynthetic photon flux density (PPFD) values in μmol/s can be modified to the yield photon flux (YPF) as shown in the red curve of figure 2.

Figure 2: The photon-weighted curve is for converting PPFD to YPF; the energy-weighted curve is for weighting PAR expressed in watts [3, 4]

Chlorophyll , Carotenoids and Effective PAR

PAR value is measure of photons in the range of 400nm to 700nm. The yield photon flux extend that range to 360nm to 760nm and weigh the measurement according to plant photosynthetic response [4]. Both PAR and YPF are much better parameters than lumen in quantifying actual photosynthesis. However, research has found that light interaction with plants is far more complicated than counting photons in the visible range. For instance, both IR and UV may play important roles as various stages for difference species. More importantly, different biological reactions need different spectrums and too much light may not be desirable [5].

PAR and YPF ‘s imperfection can be seen in Figure 3, where in three major components, namely Chlorophyll a, Chlorophyll b and carotenoids, are drastically different from the weighting factor for PAR as shown in Figure 2. Chlorophyll a is only one form of Chlorophyll that absorbs mostly violet-blue and orange-red light and reflects green- yellow light, which contributes to the observed green color of plants.  However, Chlorophyll b is another type of chlorophyll that primarily absorbs blue and red light. Besides Chlorephylls, carotenoids are also essential for horticulture. The key carotenoids include lycopene, vitamin A precursor β-carotene, and xanthophyll lutein, which is a vital nutrient in preventing age-related eye disease. Carotenoids are often masked by green tint from chlorophyll.  When chlorophyll is absent, such as seen in autumn leaves, carotenoids give the vibrate display of yellow, orange, brown and red. Carotenoid colors are also predominate in ripe fruits where chlorophyll is not present.

Comparing Figure 2 and 3, we will see that the weight factor of PAR in Figure 2 is the summation of the Chlorophyll a and b response curves. To distinguish the spectrum effects on individual process such as rooting, vegetative and flowering, the weight factor in Figure 3 needs to be further modified as well as the photon at the right wavelength is effective. Therefore, Effective PAR (EPAR) value is a better metric than that of PAR and YPF. The Effective PAR weighting factor covers a larger spectrum range and is not a fixed curve. EPAR will be different for each species and specific to stages such as rooting, vegetative growth and flowering, etc. 

DemeGrow Spectral Smart LED grow lamps have been designed to optimized the Effective PAR recipe for vegetative and flowering modes of a plant’s lifecycle. More importantly, the multi-diode, multi-channel design allows experienced user to define the recipe on wavelength to maximize Effective PAR depending on the plant subject including complete control over intensity on each wavelength channel as well as timing for photoperiodic control.

Figure 3: Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids [3]

References:

“Luminous efficacy of white LED in the mesopic vision state”,  Peng Jin, et al. Optoelectronics Letters,  July 2009, V5, P265-267;

“Spectral Effect of multi-chip LED on Color Contrast Sensitivity with SPD Tuning”，Peng Jin, Liting Jiang, Solid-State and Organic Lighting, Canberra Australia, December 2-5, 2014

“Photosynthetically active radiation”,  Wikipedia

“Accuracy of quantum sensors measuring yield photon flux and photosynthetic photon flux.” Barnes C, et al . HortScience. 1993 Dec; 28(12):1197-200.

"The phenomenon of photoinhibition of photosynthesis and its importance in reforestation". Alves P, et al. The Botanical Review 68 , 2002.

 

Written by Dr. Peng Jin
Technical Director/Owner, DemeGrow, Inc.
Director, Green Lighting System Center at Peking University
Research Area:  Horticulture lighting, smart lighting and human color vision

 

 

University of Florida associate professor Kevin Folta and other scientists at the university's Institute for Plant Innovation are studying the impact specific light wavelengths can have on plant characteristics. Photo by Tyler Jones, UF/IFAS Photography.

With the ability to deliver specific light wavelengths with LED lights, growers, retailers and consumers could eventually manipulate the scent, color, flavor, post-harvest life and other characteristics of ornamental and edible crops.

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Both ornamental and edible plant growers are using supplemental lighting. Some use light to control photoperiod. Others use supplemental light to hasten plant development by increasing the rate of photosynthesis.

What if you could use light to increase the flavor, aroma, color intensity, insect and disease resistance and post-harvest life of edible crops? What if you could use light to increase the fragrance, color intensity, insect and disease resistance, flower timing and post-harvest life of ornamental flowering plants? Sound like science fiction? Read on.

Talking to plants

Kevin Folta, interim chair and associate professor of the Horticultural Sciences Department at the University of Florida, said the fundamental idea of using light to manipulate plants is an old one.

“We’ve known for a long time that light can affect photosynthesis, but we are now starting to understand how light can regulate specific plant responses,” Folta said. “It’s no big surprise that light could manipulate something like flavors or any other aspect of plant metabolism.”

Working with other scientists at the university’s Institute for Plant Innovation, Folta said initial research indicates red, far red and blue light are the three major wavelengths that affect volatile accumulation in plants. The researchers have studied the impact of light wavelengths on strawberries, blueberries, tomatoes and petunias.

“Volatiles are the chemicals that contribute to the aroma and flavor that are released,” Folta said. “Volatiles are the chemicals that are emitted that allow you to smell and taste a piece of fruit. These are the compounds that are really important in providing flavor to fruit and vegetables.”

Folta said similar changes could be made to flowering plants by manipulating the light wavelengths that the plants are exposed to.

“For ornamentals we could affect aromas, colors and flower timing by changing the light environment—the specific wavelengths,” he said. “It would be possible to synchronize an entire greenhouse of plants to flower at the same time just by flipping a switch. By understanding the light spectrum and how a plant sees it, it could allow us to manipulate how a plant grows.

“It’s almost like we can talk to the plants. It’s a language that is essentially a vocabulary of light wavelengths and that we can use to influence how a plant grows.”

Focused on LEDs

Folta said all of the research being done involves the use of LED lights.

“LEDs allow us to deliver very precise amounts of specific wavelengths,” he said. “LEDs allow us to mix the light conditions precisely. We can pick and choose the light we want to use.”

Folta said one of the ways different light wavelengths could be used is to customize what the final fruit, vegetable or flower would look, taste and smell like.

“For example, maybe we could put the plants under blue light for a few days and then switch to far red and then red. We know that such sequential treatments allow us to bump up the pigments, then the nutrients and then the flavors,” he said. “This treatment could change the way we grow, ship and sell crops, as well as how consumers store them at home.

“All plant traits are a combination of genetics and the environment. The genetics are already in place to make a quality fruit, vegetable or flower, so the LEDs allow us to manipulate what’s already there. We can tweak the environment with the LEDs to alter plant characteristics. Maybe an LED light would be placed in a box of roses. When a consumer opens the box there would be this incredible aroma released.”

Folta said the research has tremendous potential for both edible and ornamental crops.

“This research would probably have happened a longtime ago, but LED lights were prohibitively expensive,” he said. “Now that the cost of LEDs and narrow band width lighting is becoming more affordable, we realistically see LED arrays being used in greenhouses to manipulate the way plants grow.”

Endless potential

Although the initial research has focused on changing the taste of fruit and vegetables, Folta said the use of light could easily be expanded to manipulate other plant characteristics.

Kevin Folta said growers may eventually be able to synchronize an entire greenhouse of plants to flower at the same time just by flipping a switch for LED lights. Photo by Tyler Jones, UF/IFAS Photography.

“There is an increasing body of research literature that indicates some of the compounds emitted by plants and their fruit deter insects or deter fungal growth,” he said. “It may be possible that we could affect insect and disease resistance. For example, by using LED lights we could change the metabolic profile of the plant so that poinsettias would be more resistant to whitefly. This might be done by stopping production of plant compounds that attract whiteflies, or producing compounds that scare them away or even better than that may attract a predator of the whitefly."

“What we are doing is manipulating the plant metabolism or changing it in ways that we don’t necessarily understand 100 percent yet, but we know we can do it.”

An example of one of the results of the research he doesn’t completely understand has occurred with strawberry plants.

“In the lab we have exposed strawberry plants to LED lights and they don’t get spider mites,” he said. “We don’t know if there is something that the LEDs are doing to change the development of the spider mite. Or the light maybe doing something to the plant that causes it to produce a chemical the spider mites don’t like so they choose to go to a different plant. This is something that we still need to test.”

Folta said most of the previous research that involved the same type of plant process manipulation involved inserting a gene, spraying a chemical or other types of treatments that were labor intensive and required other inputs.

“Now we are looking at basically flipping a switch to turn on a low energy device,” he said. “Adding value at a low cost would be a great thing for the horticulture industry.”

This article was written by David Latchman for Hort Americas Corporate Blog.