A Study of Quantitative Lamp Spectra and Hydroponic Lettuce Growth
Key Participants
Tim Madden, President Biodynamicz, LLC 526 S. Main St. Akron, OH 44311
Roger Buelow, President Light for Life, [email protected]
Dr. Richard Hansler, Lighting Innovations Institute, John Carroll University; Cleveland, Ohio
Wil Hemker, The University of Akron Research Foundation (UARF); Akron, Ohio contact:
[email protected]
Ruth Zito, Horticulturalist NuLight ,LLC, [email protected]
Background:
Light has three principal characteristics: quantity, quality, and duration that affects plant growth.
Light quantity refers to the intensity or concentration of sunlight and varies with the season of
the year. The more light a plant receives, up to a saturation point, the higher the capacity that
plant has to produce plant mass through the process of photosynthesis. Light quality refers to the
color or wavelength reaching the plant surface and can be enhanced through the use of
supplemental lighting. A number of different light sources exist for that purpose. Incandescent
light is high in the red or orange range but generally produce greater heat output. Fluorescent or
cool-white light is high in the blue range of light quality and is used to encourage leafy growth.
These lights are excellent for starting seedlings. Fluorescent "grow" lights have a mixture of red
and blue colors that attempts to imitate sunlight as closely as possible. Solid state light emitting
diode or LED lights have a high efficacy (µmol per Joule), broad spectrum accuracy (in terms of
color), low thermal output and this lighting technology is well-aimed to produce an ideal
environment for growing plants anywhere. Light duration or photoperiod refers to the amount of
time that a plant is exposed to sunlight. Year round production of crops is possible by utilizing
supplemental lighting. To extend day length in crop growing the interest in studying artificial
lighting has grown. This study focused on the comparative characteristics of lamp spectra
qualities of commercial state of the art agriculture lamps of high pressure sodium (HPS),
inductive fluorescence and LED lamps.
Objective:
To compare a reference greenhouse food plant crop’s productivity when grown in like
hydroponic environments when the artificial light source is varied. The commercial light sources
are: Gavita Pro 1000 DE US (HPS lamp), IGrow ED-400W-GB (Inductive Fluorescent),
Heliospectra AB LX 60 Series LED, and Lumigrow Pro 325W (LED lamps). The outcome was
to characterize lamp spectra qualities and plant crop productivity for each commercial agriculture
lamp.
Materials & Methods:
The hydroponic lettuce growing was conducted in a controlled indoor environmental lab, using
Nutrient Film Technique (NFT) hydroponic channels to grow out the lettuce crop. Nutrient Film
Technique is a growing system that constantly re-circulates a continuous stream of water through
grade PVC thermoplastic channels which delivers nutrients to the crop (Figure 1). A baby gem
romaine lettuce variety (Lactuca sativa: v. Green Star), was used with a targeted maturation size
of 170 grams or six ounces. 6 heads of lettuce per channel were grown out for a forty five (45)
day duration for each lamp test condition. The geometrically balanced fertigation system was set
at a nutrient flow of 8-10 ounces per minute which is the common industry standard. Interference
from ambient light sources was minimized by using separate isolated room enclosure grow zones
for each of the four lamps tested. Accurate spectra quantifications were taken for each of the
lamp evaluated. Each light fixture was placed 5 feet above the surface of each NFT channel,
supporting each lettuce plant.
Seeds were germinated in pre-moistened Oasis® expanded foam soil-less media XL 1”X 1”X 1
½” cubes in dilute (600ppm) nutrient solution for four (4) weeks (Figure 2). Seedlings were then
transplanted into the NFT channels (figure 3) and grown out for the full forty-five (45) day
period. Each grow tent temperature was maintained between 68 and 70 degrees Fahrenheit with
40 percent humidity and CO2 at ambient levels. The nutrient solution used for ideal plant health
and growth was supplied by Smithers-Oasis Co. CWP indicators and probes (CWP Instruments
by Marco Industries) were installed in each solution tank to continuously measure PH, PPM, and
solution temperature. The pH was maintained at 6.0 by the addition of dilute phosphoric and
citric acid. PPM was maintained bet 800-825 by the addition of concentrated fertilizer solution
and source water. Solution tanks were manually mixed with R.O. (reverse osmosis) water and
fertilizer. Tanks were cleaned weekly and fresh nutrient solution was utilized.
Designed Outcome:
For each lamp tested the following response variables were measured; plant growth rate was
measured as fresh wet weight per head at harvest time, and dry weight. Plant quality was
observed throughout the growing process on a subjective basis, plant sugar quantity (brix) and
sensory qualities (bitterness, appearance, flavor, texture and overall acceptability) were
compared after harvest for each lamp evaluated. The sensory panel consisted of 5 members and
the sensory qualities were based on the Qualitative Descriptive Analysis method (QDA-see
Table 1) with ratings as follows: 0-3 rating (Poor /Unacceptable) 4-6 (Fair /color, crispness,
sweetness), 7-8 (Good/color, crispness, sweetness, liveliness) 9-10 (Excellent/color, crispness,
sweetness, very fresh, clean taste). The Refractometer Brix Scale for Lettuce is as follows: 4
Poor, 6 Average, 8 Good, 10 Excellent. (See below Table 2.).
Table 1
Attributes
Definitions
Color
Bright green color, characteristic of lettuce quality
Freshness
Liveliness and brightness before losing vigor
Brightness
Good condition and a feeling of freshness
Texture
Crisp, crunchy, soft-buttery ,soft tender
Taste
smooth, bitter, sweet, succulent, mild
Table 2. Lettuce (Lactuca sativa: v. Green Star) plants qualities following 45 days grow period
under four (4) different light sources
Light Source
Fresh Weight
Dry Weight
% Chg
Brix
Taste/Texture
Heliospectra
6.0 oz.
2.2 oz.
63.3%
6
9
IGrow
6.2 oz.
2.1 oz.
66.1%
3
7
Lumigrow
4.8 oz.
1.9 oz.
60.4%
4
6
Gavita
10.0 oz.
2.6 oz.
74.0%
3
7
Timeline
The lettuce growing was conducted over a 45 day period beginning on December 18, 2014 and
completion on January 30, 2015.
Results
After forty-five (45) days the observed growth rates for the lettuce varied between each light
source. The Heliospectra 600w, Lumigrow 325w and Gavita DE 1000w had measurable higher
biomass accumulation while the IGrow 400w Bloom had a lower biomass but had visible larger
leaf, was less compact and a broader surface. The plant’s morphology had two distinct
characteristics. Lumigrow, Gavita and Heliospectra had greater density and were more compact
with a thicker leaf as compared to the IGrow. The IGrow had greater leaf and stem stretching
with decreased leaf thickness. Increased lateral branching was more prevalent with the
Heliospectra, Lumigrow and Gavita lights. Brix testing results showed that Heliospectra had a
rating of 6 which is in the average range. Lumigrow, IGrow, and Gavita were below average
rating for the Brix test. The sensory qualities for Heliospectra ranked the highest with deep dark
green coloring of the leaf, sweet taste, crisp texture, and good liveliness. IGrow ranked average
with light green leaf coloring, soft tender leaf texture and mild taste, Gavita had good leaf
coloring but bitter taste and average liveliness and last was Lumigrow with dark green leaf color,
crisp texture but less than average bitter taste.
Conclusion:
This comparative leafy green lettuce crop growing study found that the Heliospectra AB LX 60
Series LED lamp grew the tastiest and most attractive lettuce for all four lamps having a similar
average sugar (brix value).
Pictures:
Figure 1. NFT Channel System
Figure 2. Lettuce seed germination flood table system
Figure 3. Growing channel showing
seedlings after transplant
Heliospectra
Heliospectra Week 1
Heliospectra Week 3
Heliospectra Week 6
Lumigrow
Lumigrow Week 1
Lumigrow Week 3
Lumigrow Week 6
IGrow
Igrow Week 1
Igrow Week 3
Igrow Week 6
Gavita
Gavita Week 1
Gavita Week 3
Gavita Week 6