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The Design and Production of a High-Intensity, Variable-Spectra LED Array for Plant Development in Challenging Environments

David Hawley1, Michael Stasiak1, Jamie Lawson1, Alan Scott2, Per Aage Lysaa3, Mike A. Dixon1. (1)Controlled Environment Systems Research Facility, University of Guelph, Guelph, Ontario, Canada; (2)COM DEV Ltd., Kanata Ontario, Canada; (3)Intravision Group AS, Snaroya

Norway.

INTRODUCTION EXPERIMENTS ARRAY DESIGN

Figure 2. (A) Seven clusters of seven different colours. (B) Optics applied to the LED stars. (C) Twelve stars arranged into one Snowflake array.

Figure 1. Emission spectra and relative PAR of seven Philips LEDs.

ABSTRACT

SELECTION OF LEDS

SUMMARIZED RESULTS

CONTACT

Plant development can be dramatically influenced by the quality of light present in their environment. Many factors, such as rates of photosynthesis and dry-matter production, synthesis of specific compounds, or time of flowering can be manipulated through the use of specific wavelengths of light. Researchers at the Controlled Environment Systems Research Facility (CESRF) have developed a high-intensity, narrow-bandwidth LED array with nine distinct wavelengths, ranging from short-wavelength UV LEDs to relatively long-wave far-red LEDs. Individually, each wavelength present is sufficient in intensity to drive associated pathways in plants. The intended use for the array is to develop light “recipes” that maintain a plant’s optimum light environment for whatever developmental characteristic is desired. The short term goal at the CESRF is to optimize dry matter accumulation in lettuce, tomato, and pepper plants for food production in challenging environments. Design considerations for deploying LEDs in this study are presented. Supported by Natural Sciences and Engineering Research Council (NSERC), Canadian Space Agency (CSA) and Industry partner COM DEV Ltd.

It was found that seven colours of light from the LEDs could be homogeneously distributed with the LEDs clustered in groups of seven, arranged in a hexagonal star. (figure 2a). Optics were fixed over the LEDs to focus light on the plants below (figure 2b). Twelve stars were then assembled into a “snowflake” design (figure 2c).

Germination Germination tests of several species were performed under seven monochromatic light qualities, an HPS lamp, and in darkness. Species tested included wheat, barley, radish, lettuce, tomato, soybean, pepper, and kale. Seeds were plated in moist germinating pods, maintained at 22 °C ± 0.5 °C. The intensity of each light treatment was set at 280 µmol/m2/s, with a photoperiod of 16 h. The number of seeds germinated was counted every 24 h, and germination rates under different light qualities were compared. Photosynthesis Lettuce, tomato, and pepper plants at the vegetative stage of development were exposed to 40 different light qualities of equal intensity, with a photoperiod of 16 h. Growth chambers were fixed at 22 °C. CO2 assimilation rates were measured, and photosynthesis under each light quality was compared on a leaf area basis. Adaptation Photosynthesis of vegetative lettuce was measured in the previously described conditions under seven monochromatic light qualities on day 1 (initial). The plants were then “adapted” to a light quality for three 16 h days. On day 5 (final), photosynthesis was again measured under the initial seven light qualities.

Seven colours of narrow-bandwidth LEDs were acquired from Philips. The emission peaks and relative intensity of each is summarized in figure 1 below.

Germination Some species showed a much greater sensitivity to light quality than others. Higher germination rates were achieved in some species using relatively long wavelengths, while in other species, this same treatment was completely inhibitory. Shorter wavelengths were generally inhibitory to germination, suggesting that there may be competition between photoreceptors in germination regulation. Photosynthesis In general, longer wavelengths were more effective in driving photosynthesis. However, other light qualities that included shorter wavelengths were shown to be equally effective in this regard. This may be useful in designing light qualities to elicit other responses while maintaining a high photosynthetic rate. Adaptation After adaptation, the photosynthetic capacity of plants under many light qualities was significantly impaired, indicating that there may be a necessary “recovery” period for plants in altering light qualities during their development.

Conventional lighting systems for plant development do not allow for fine control of the spectrum emitted. This often results in light qualities that are not optimal for stimulating the desired developmental characteristics of the species being grown. Foremost of the many responses to light by plants is photosynthesis. In many species, previous studies have shown that a combination of red and blue light can effectively drive photosynthesis and fully develop a plant. More recent studies have begun to expand on this, supplementing red and blue with green light, or by adjusting the peak wavelengths of red and blue light. Other light responses: • Shorter wavelengths (UV / Blue): • Hypocotyl and petiole elongation • Cotyledon and leaf expansion • Stimulation of phenylpropanoid pathway • Plasma membrane depolarization • Circadian rhythm regulation

• Midrange wavelengths: • Increased electron transport rate • Effectively transmitted through canopies, lighting

lower leaves • Longer wavelengths (amber / red / far red): • Seed germination • Cell elongation • Nitrate reductase regulation • Flowering time This aim of this study was to develop an array capable of emitting variable spectra using multi-coloured, narrow-bandwidth LEDs. The array would then be used to design high-resolution light “recipes” for eliciting targeted responses.

David Hawley Controlled Environment Systems Research Facility; University of Guelph Email: dhawley@uoguelph.ca Phone: 519-824-4120 x54851 Website: www.ces.uoguelph.ca

655 nm

627 nm

591 nm

530 nm

505 nm

470 nm

440 nm

A B C

Each snowflake array had a total of 588 LEDs, and could emit a total PAR of approximately 7000 µmol/m2/s, measured 60 cm away from the array – an intensity approximately 3.5x brighter than the sun at ground level on a sunny day. The intensity of each colour could be independently controlled using an Arduino Due® microcontroller and custom software. This control allowed for the design of specific light “recipes” that could manipulate plant development in targeted ways.

INTENSITY CALIBRATION The intensity of each colour was mapped inside the single plant growth chambers as CESRF (table 1). The flux of photosynthetically active radiation (PAR) was measured in µmol/m2/s at 21 x and y coordinates on the plant support

surface within the chamber. This was repeated at three

distances from the array. From this data, equations were

derived to determine the intensity of each colour at any point within the growth chamber.

Colour 40 cm 60 cm 100 cm

Royal Blue

Blue

Cyan

Green

Amber

Red

Deep Red

1

2

3

4

5A B C D E

1300-14001200-13001100-12001000-1100900-1000800-900

1

2

3

4

5

A B C D E

1050-1150

950-1050

850-950

750-850

650-750

1

2

3

4

5A B C D E

700-800

600-700

500-600

1

2

3

4

5A B C D E

1800-2000

1600-1800

1400-1600

1200-1400

1000-1200

800-1000

600-800

1

2

3

4

5A B C D E

1200-1300

1100-1200

1000-1100

900-1000

800-900

1

2

3

4

5A B C D E

700-800

600-700

500-600

1

2

3

4

5A B C D E

1200-13001100-12001000-1100900-1000800-900700-800600-700

1

2

3

4

5A B C D E

700-800

600-700

500-600

400-500

1

2

3

4

5A B C D E

450-550

350-450

1

2

3

4

5A B C D E

500-600

400-500

300-400

1

2

3

4

5A B C D E

450-550

350-450

250-350

1

2

3

4

5A B C D E

250-300

200-250

1

2

3

4

5A B C D E

600-700

500-600

1

2

3

4

5

A B C D E

500-600

400-500

1

2

3

4

5A B C D E

350-400

300-350

1

2

3

4

5A B C D E

1450-1650

1250-1450

1050-1250

850-1050

650-850

1

2

3

4

5

A B C D E

1150-1250

1050-1150

950-1050

850-950

750-850

1

2

3

4

5A B C D E

700-800

600-700

500-600

1

2

3

4

5A B C D E

1050-1150

950-1050

850-950

750-850

1

2

3

4

5

A B C D E

800-900

700-800

600-700

1

2

3

4

5A B C D E

600-700

500-600

Table 1. Surface maps of light intensity (µmol/m2/s) of each colour. Maps were created at 40 cm, 60 cm, and 100 cm away from the light source.