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Life Sciences in Space Research 2 (2014) 43–53

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Life Sciences in Space Research

www.elsevier.com/locate/lssr

Signicant reduction in energy for plant-growth lighting in space using targeted LED lighting and spectral manipulation

L. Poulet a,∗, G.D. Massa b , R.C. Morrow c, C.M. Bourget c, R.M. Wheeler b , C.A. Mitchell a

a Purdue University, West Lafayette, IN 47907, USAb NASA Kennedy Space Center, FL, USAc ORBITEC, Madison, WI, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 January 2014Received in revised form 6 June 2014Accepted 9 June 2014

Keywords:CropESMFood productionLife-support systemsLight-emitting diodesLettuce

Bioregenerative life-support systems involving photoautotrophic organisms will be necessary to sustain long-duration crewed missions at distant space destinations. Since sufficient sunlight will not always be available for plant growth at many space destinations, efficient electric-lighting solutions are greatly needed. The present study demonstrated that targeted plant lighting with light-emitting diodes (LEDs) and optimizing spectral parameters for close-canopy overhead LED lighting allowed the model crop leaf lettuce (Lactuca sativa L. cv. ‘Waldmann’s Green’) to be grown using signicantly less electrical energy than using traditional electric-lighting sources. Lettuce stands were grown hydroponically in a growth chamber controlling temperature, relative humidity, and CO2 level. Several red:blue ratios were tested for growth rate during the lag phase of lettuce growth. In addition, start of the exponential growth phase was evaluated. Following establishment of a 95% red + 5% blue spectral balance giving the best growth response, the energy efficiency of a targeted lighting system was compared with that of two total coverage (untargeted) LED lighting systems throughout a crop-production cycle, one using the same proportion of red and blue LEDs and the other using white LEDs. At the end of each cropping cycle, whole-plant fresh and dry mass and leaf area were measured and correlated with the amount of electrical energy (kWh) consumed for crop lighting. Lettuce crops grown with targeted red + blue

LED lighting

used

50%

less

energy

per

unit

dry

biomass

accumulated,

and

the

total

coverage

white

LEDs

used 32% less energy per unit dry biomass accumulated than did the total coverage red + blue LEDs. An energy-conversion efficiency of less than 1 kWh/g dry biomass is possible using targeted close-canopy LED lighting with spectral optimization. This project was supported by NASA grant NNX09AL99G.

© 2014 The Committee on Space Research (COSPAR). Published by Elsevier Ltd. All rights reserved.

1. Introduction

Planetary exploration and expansion of humanity into the solar system to establish permanent settlements are grand challenges of the 21st century (NASA, 2010, 2012 ). The goal of human ex-ploration set by the Global Exploration Roadmap is a rst human mission to Mars by 2040 (ISECG, 2013 ). Standard mission-to-Mars scenarios envision a crew of 6 people and a total mission duration of approximately 1000 days (Drysdale et al., 2003 ), requiring a total mass of consumables (food, water, oxygen) that current heavy-lift vehicles are unable to launch at once. Resupplying consumables

* Corresponding author at: Eduard-Grunow Str. 24b, 28203 Bremen, Germany. Tel.: +49 17 664 738 184.

E-mail addresses: luciepoulet@yahoo.fr (L. Poulet), gioia.massa@nasa.gov(G.D. Massa), morrowr@orbitec.com (R.C. Morrow), bourgetm@orbitec.com(C.M. Bourget), raymond.m.wheeler@nasa.gov (R.M. Wheeler), cmitchel@purdue.edu(C.A. Mitchell).

with a cargo vessel like the Automated Transfer Vehicle (currently used to resupply the International Space Station) would not be cost effective, either, since the average cost to launch a kilogram of mass into Low-Earth Orbit has been estimated to be $10,000 (Futron Corporation, 2002 ). In-situ Resource Utilization might be used to recover water and oxygen from Lunar or Mars regolith,

but it does not directly enable generation of food (ISECG, 2013 ). Thus, bioregenerative life-support systems coupled to controlled-environment food-crop growth modules are needed for food pro-duction on the Moon or Mars (Massa et al., 2006; Wheeler et al., 2001, 2003 ). Such systems could sustain a crew at distant space destinations since seeds can be stored viably for a long time and/or (re)generated in space for very long missions.

High energy radiation on both the Moon and Mars also is a problem for both humans and higher plants that human explorers will have to address, most likely by using some form of radia-tion shielding. In addition, any pressurized structures at or near the planetary surface must withstand tremendous differences in

http://dx.doi.org/10.1016/j.lssr.2014.06.0022214-5524/ © 2014 The Committee on Space Research (COSPAR). Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.lssr.2014.06.002http://www.sciencedirect.com/http://www.elsevier.com/locate/lssrmailto:luciepoulet@yahoo.frmailto:gioia.massa@nasa.govmailto:gioia.massa@nasa.govmailto:morrowr@orbitec.commailto:morrowr@orbitec.commailto:bourgetm@orbitec.commailto:bourgetm@orbitec.commailto:raymond.m.wheeler@nasa.govmailto:raymond.m.wheeler@nasa.govmailto:cmitchel@purdue.eduhttp://dx.doi.org/10.1016/j.lssr.2014.06.002http://crossmark.crossref.org/dialog/?doi=10.1016/j.lssr.2014.06.002&domain=pdfhttp://dx.doi.org/10.1016/j.lssr.2014.06.002mailto:cmitchel@purdue.edumailto:raymond.m.wheeler@nasa.govmailto:bourgetm@orbitec.commailto:morrowr@orbitec.commailto:gioia.massa@nasa.govmailto:luciepoulet@yahoo.frhttp://www.elsevier.com/locate/lssrhttp://www.sciencedirect.com/http://dx.doi.org/10.1016/j.lssr.2014.06.002

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44 L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53

temperature between night and day (from − 233 ◦ C to + 123 ◦ C), large pressure differentials, and the likelihood of frequent micro-meteorite impacts. Thus, human habitats and crop-growth mod-ules very likely will be sheltered or located underground (ACCESS Mars, 2009 ). Moreover, sunlight will be reduced or not available at all times due to local conditions such as long dust storms on Mars, the periodically increased distance between Mars and the Sun, or extended dark periods on most locations of the Moon

(Cockell and Andrady, 1999; Cockell, 2001; Horneck et al., 2003;Rontó et al., 2003; Salisbury, 1992; Wheeler, 2004 ), making elec-tric lighting a more reliable option for growing food plants in space (Massa et al., 2006 ). However, it has been estimated that 40 to 50 m 2 of cropping area, in continuous use, would be necessary to fully sustain each crew member (Mitchell et al., 1997 ), which would require considerable energy for a traditional electric crop-lighting scenario (Drysdale, 2001 ). Ikeda et al. showed that 45% of the total power needed for growing lettuce in a controlled environ-ment under uorescent lights was consumed by lamps and ballasts and 35% by air-conditioning to reject waste heat (Ikeda, 1991 ).

Several ground-based bioregenerative Life-Support-System stud-ies have been conducted since the 1970s, all including elec-tric or hybrid (electric + solar) lighting (Gitelson et al., 1989;

Masuda et al., 2005; Nitta, 2005; Tako et al., 2010; Lasseur et al., 2010 ). Experiments using the Minitron II growth-chamber/cuvette system to determine maximum growth response for hydroponic lettuce using red-rich incandescent lighting had an associated power cost per unit growth area ranging from 1 to 10 kW/m 2(power density, where the area term refers to crop-growth area) and an energy consumption per unit dry biomass produced be-tween 953 and 1680 kWh/g (Knight and Mitchell, 1988 ). NASA’s Biomass Production Chamber (BPC), which also was not designed for optimal light delivery but as a closed plant-production sys-tem with HPS lamps (Wheeler, 1992 ), used 2.1 kW/m 2 of electrical power for lighting (Wheeler et al., 1996 ), which translated into 4.7 kWh/g for ‘Waldmann’s Green’ lettuce (Wheeler et al., 2008 ). An engineering concept for an inatable Mars surface greenhouse estimated that a greenhouse module of 90 m 2 using HPS lamps 12 h/day at 1000 µmol/m 2 /s would require 2.47 kW/m 2 (Hublitz et al., 2004 ). Using an intracanopy light-emitting diode (LED) system to grow cowpea crop stands in a controlled environment, Massa et al. (2005) reduced power density to 0.83 kW/m 2 , which corre-sponded to 1.02 kWh/g of dry plant biomass. More recently, Gomez et al. (2013) showed that intracanopy LED lighting during high-wire greenhouse tomato cultivation enabled the consumption of 4.3 times less energy for supplemental lighting than with over-head HPS lamps. In recent years, LED plant research has indeed become more active in the horticulture greenhouse industry be-cause of potential energy savings, as indicated by the study of Dueck et al. (2012) on hybrid supplemental lighting utilizing over-head HPS lamps and “interlighting” with LEDs.

LEDs also are promising candidates for space life support as their small size, mass, and ballast-free operation would contribute positively to reducing the Equivalent System Mass (ESM) of a space lighting system (Drysdale and Hanford, 1999 ) compared to tradi-tional high-intensity discharge (HID) lighting systems. In addition, their solid-state electronics ensure reliability and safety (Tibbitts et al., 1991 ). LED lifetime is more than twice as long as for any other type of light source for plant growth, which would spare mainte-nance time for astronauts on a given mission (Bourget, 2008 ) and reduce the launch mass of needed spares.

Efficiencies of the Philips Luxeon LEDs per se in 2012 reached 38% for red (630 nm) emitters and 50% for blue (455 nm) (Philips Lumileds Lighting Co., 2012 ). High-intensity discharge lamps such as HPS and Metal Halide (MH) have efficiencies comparable to red LEDs, but, because of their intensely hot lamp surfaces, must be placed much farther away from plants than LEDs (Tibbitts et al.,

1991 ), thereby resulting in much higher operating power required to get sufficient photosynthetic photon ux (PPF) at leaf level. An-other very important advantage of LEDs is that they emit pure colors, which can be selected to match the absorption peaks of plant pigments (Tibbitts et al., 1991 ) and thus improve spectral efficiency for optimal plant growth and development (Kim et al., 2007 ). Over the past decade, studies have shown that red and blue LEDs are an effective lighting source for plant growth (Yorio et al.,

2001 ). Even though blue light is photosynthetically less efficient than red light (Dougher and Bugbee, 2001; McCree, 1971/1972 ), it has important photomorphogenic effects on stem elongation, leaf expansion (Dougher and Bugbee, 2001; Hoenecke et al., 1992 ), and is important for water relations (Sharkey and Raschke, 1981 ).

Despite the energy-saving advantages of LEDs compared to tra-ditional crop-lighting sources, when using an overhead lighting system for rosette plants such as leaf lettuce, light in a xed-spacing growth system still is wasted falling on empty spaces between small plants before they grow. To avoid such losses, the concept of targeted lighting was investigated in the present study, switching on LEDs positioned only directly above individual plants. Changing the space between plants as they grow (variable spac-ing) is another option for solving this problem (Both et al., 2009;

Field, 1988; Davis, 1985; Prince and Bartok, 1978 ), as was done at Phytofarms of America using automatic spacing (Prince et al., 1981 ), but that solution was designed for large-scale terrestrial agriculture and required additional energy for daily plant-position adjustments. For space applications, simplicity of operation and ESM minimization are always preferred, and with effective targeted lighting, xed spacing between growing seedlings should not be critical.

The ultimate goal of the present study was to test the con-cept and demonstrate the energy efficiency of targeted lighting to grow ‘Waldmann’s Green’ leaf lettuce under optimizing red + blue LED lighting conditions, compared to total coverage red + blue or white LED lighting.

To achieve this objective, a three-part study was conducted for hydroponic lettuce in a growth chamber using red and blue LEDs for sole-source crop lighting. First, a temporal characterization was carried out to determine kinetics of lag and exponential phases of growth; secondly, a determination was made regarding optimiz-ing conditions of red:blue ratio during both growth phases; and nally, a comparison was conducted for efficiency of targeted vs. total coverage LED lighting. Previous studies found a power density of 2.47 kW/m 2 needed to grow plants using HID lighting (Hublitz et al., 2004 ), and 2.1 kW/m 2 was demonstrated for HPS lighting in the NASA Biomass Production Chamber (Wheeler et al., 1996 ). It was hypothesized that such gures may be lowered by at least an order of magnitude using targeted LED lighting technologies under optimizing spectral conditions, as tested herein.

2. Materials and methods

2.1. ORBITEC lighting system

Two identical custom LED lighting arrays were provided by the Orbital Technologies Corporation (ORBITEC, Madison, WI, USA). Both rectangular lighting arrays measuring 61 × 61 cm were ar-ranged in four two-by-two 27 .5 × 27 .5 cm panels, each containing 36 red LEDs (λ max 630 nm) and 9 blue LEDs (λ max 455 nm). The red peak is 19 nm Full Width at Half Maximum (FWHM), and the blue peak is 21 nm FWHM (Fig. 1). The red LEDs are 29% efficient and the blue 41% (manufacturer data). Fig. 2a shows a detailed image of the emission surface of the array. The irradi-ance of red and blue, as well as photoperiod, are adjustable from custom-control software, and light intensities were measured us-ing a spectroradiometer (Apogee) and a quantum sensor (Apogee).

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L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53 45

Fig. 1. Spectroradiometer scan of the ORBITEC red + blue LED array. (For interpre-tation of the references to color in this gure legend, the reader is referred to the web version of this article.)

Color uniformity was ensured at all times by keeping the LED panel at a threshold distance above the crop surface. Heat is removed from the interior of the array via air-cooled heat ex-changers and ductwork (Fig. 2b). Power and energy consumption were measured with current sensors, which were calibrated dur-ing the initial system checkout and monitored via custom software. Power use displayed on a control computer screen is that used by LEDs only, including LED driver circuits. The power supply is con-nected to an AC power source with input voltages in the range of

100–240 VAC, 50/60 Hz. 2.2. FlexFire LED lighting system

A third LED lighting array was built in-house using high-intensity UltraBright LED strip lights from the FlexFire LEDs Com-pany (Mountain House, CA). It was composed of four 77-LED, 54-cm-long strips, with two 7-LED, 5-cm-long-strips between rows. The LEDs are cool-white phosphor-based, with a Correlated Color Temperature (CCT) of 5914 K and a Color Rendering Index (CRI) of 69.0. There were two main peaks, one blue (448–452 nm) and one green (520–525 nm) (Fig. 3). A 100 to 0% dimmer was added so that light output could be controlled as needed. Electrical energy and power consumption were monitored with a Kill-A-Watt meter (P3 International Corporation, New York, NY, USA), which measures energy consumption with an accuracy of 0.2% (manufac-turer data).

2.3. Growth chamber

The growth chamber in which the experiments were conducted is an Environmental Growth Chambers (EGC, Chagrin Falls, OH) walk-in chamber of 9.29 m 2 oor area. Temperature, relative hu-midity, and CO2 concentration are controlled and monitored by EGC ControlNet software (version 4.5). When LED light was con-tinuous (from days 1 to 3 after sowing), temperature was main-tained at 25 ± 1 ◦ C and relative humidity at 80 ± 5%. When plants were subsequently maintained at a 16-h photoperiod on a 24-h light/dark cycle, temperature was 25 ± 1 ◦ C during the day and 20 ± 1 ◦ C during the night, and relative humidity was kept at

70 ± 5% during the day and 80 ± 5% during the night. Tempera-ture was measured continuously at a central point in the growth chamber, but additional temperature checks were performed peri-odically under the two lighting systems, which conrmed that the reported temperature of the growth chamber equated to the tem-perature of air under the LED arrays suspended above the plants.

2.4. Hydroponic system

A deep-batch-recirculating hydroponics system was used con-sisting of a tub that served as a root compartment (35 L) with a 0.372 m 2 polystyrene foam lid that served as a growing sur-face, both mounted above a large reservoir (70 L) and pump that continuously recirculated nutrient solution between the reservoir and root compartment (Frantz et al., 2000 ). Active root-zone aera-tion was practiced within the root compartment using a bubbling wand and external aquarium pump. The nutrient solution used was Hoagland’s no. 1 (Hoagland and Arnon, 1950 ). During the rst 10 days of growth, quarter-strength nutrient solution was used, keep-ing solution conductivity at 0.6 dS/m. From day 11, concentration of the nutrient solution was incrementally increased on a daily ba-sis until it reached full strength with a conductivity of 2.4 dS/m

on day 18 after planting. The pH was maintained between 5.4 and 5.9. Conductivity and pH were monitored and adjusted as needed every other day during the rst 10 days of crop development, and then every day during the last 8 to 10 days of growth. This was done to avoid water status and mineral nutrition becoming vari-ables affecting plant growth. Conductivity was adjusted by adding more nutrient solution concentrate if the EC value was too low or by adding water if the EC value was too high; pH was adjusted by titrating with 0.1 N H2 SO4 if too high or 0.1 N KOH if too low.

2.5. Plant material and experimental design

The test species used was Lactuca sativa L. cv. ‘Waldmann’s Green’ leaf lettuce, provided by High Mowing Organic Seeds (Wol-cott, VT 05680), which was grown hydroponically in the man-ner of Frantz et al. (2000) . The growing surface was composed of 2-cm-thick extruded polystyrene insulation FOAMULAR (Owens Corning, Toledo, OH, USA) sheet in which holes were drilled to insert closed-cell polyethylene foam plugs (Log Home Center Inc., Noblesville, IN, USA). Pregerminated seedlings were placed and oriented root-down within a rolled polyester wick, which was in-serted into a slitted polyethylene foam plug, and then inserted into a pre-drilled hole in the polystyrene foam sheet.

Overhead LED lighting arrays were mounted above separate hy-droponics systems in the growth chamber by a series of ropes and pulleys so that height could be adjusted and the arrays lev-eled just above the growing surface on which lettuce plants were grown. CO2 level was uncontrolled at ambient levels during the rst 8 days of seedling development. From day 8, CO2 level was set at 700 ± 50 ppm and was increased by 100 ppm at the be-ginning of each photoperiod until it reached 1000 ± 50 ppm. Growth data collected immediately following harvest included, on a per plant basis, number of leaves, hypocotyl length, leaf area (Model LI-3000: LAMBDA Instruments Corporation, USA), shoot fresh and dry masses, and root dry mass. Lettuce was dried for 55 ± 7 hours in a forced-air oven (Model Blue M: Blue island, IL, USA) at 70 ± 5 ◦ C. Fresh and dry masses were obtained using an electronic balance (Model 1219 MP: Sartorius GmbH, Göttingen, Germany).

Preliminary experiments determined that yield was signicantly enhanced by having a reective, enclosed growth compartment (pink hydroponic growth surface together with reective white plastic walls) to prevent light from escaping the growth compart-ment vs. using a at-black growth surface and no wall lm.

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46 L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53

(a)

(b)

Fig. 2. ORBITEC LED array: (a) Bottom view of the combined four panels, each containing nine LED clusters with each cluster composed of four red LEDs and one blue LED, and zoom on one panel showing the position of red and blue LEDs. (b) Top view showing ductwork for fan-driven heat rejection from the back of the LED array.

Fig. 3. Spectroradiometer scan of the home-made FlexFire White LED array. Source: FlexFireLEDs.

2.6. Experiment 1: establishing lettuce-growth-optimizing red:blue ratio at low irradiance

Experiments were conducted to determine optimizing red:blue ratio for establishment of viable seedlings during the lag phase of growth (duration estimated from preliminary experiments). Sam-ple number per treatment was 8 plants grown for 10 to 14 days from seed to harvest. For germination, 30 seeds were sown on double-layer Whatmann number 1 lter paper in a Petri dish wet-ted with 2 mL of tap water, imbibed for 6 h under dim uorescent light (15 µmol/m 2 /s) at 22 ◦ C, followed by 24 h under dim 630-nm red LED light (about 2 µmol/m 2 /s) at 20 ± 0.5 ◦ C (8 h) and 25 ±0.5 ◦ C (16 h). After transplantation to hydroponics in the growth chamber, seedlings were kept for 6 h in darkness, followed by 2 days under continuous treatment light at 100 ± 5 µmol/m 2 /s. Then the photoperiod was set to 16 h. The lighting schedule is graphically described in Fig. 4. Since plants during lag phase did not response to high light, the goal was to keep seedlings healthy while saving energy, and thus low light intensities were used in

this experiment. PPF was increased by 25 µmol/m 2 /s daily until it reached 225 ± 10 µmol/m 2 /s. The LED panel was maintained 6 cm above the growing surface throughout light treatments.

During the rst two days after germination, light was contin-uous. The different light treatments during these 2 days had the following red:blue PPF ratios: 100:0, 95:5, or 90:10. After this pe-riod, each treatment had a red:blue ratio of 95:5 or 90:10, as summarized in Fig. 5.

2.7. Experiment 2: growth curve

For the growth curve experiment, 120 plants were split evenly between the two growing systems. The experiment lasted for 26 days from seed to nal harvest. During the rst 11 days in the growth chamber, plants in both systems were maintained un-der the same lighting treatment, with a red:blue ratio of 95:5. On day 12, one system was switched to 90:10, whereas the other system was kept under 95:5. From day 5, four plants per systemwere harvested every 2 days, and then harvested every day for

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Fig. 4. Detailed lighting schedule for experiment 1 (red:blue ratio experiment). The x-axis indicates days of treatment, while the y-axis displays PPF in µmol/m 2 /s.

Fig. 5. Six different red:blue ratio treatments tested during the lag phase of lettuce growth. The x-axis indicates days of treatment, while the y-axis displays treatment identication.

Fig. 6. Detailed lighting schedule for experiment 2 (growth curve experiment). The x-axis indicates days of treatment, while the y-axis displays PPF in µmol/m 2 /s.

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Fig.7. Detailed lighting schedule for experiment 3 (targeted vs total coverage experiment). The x-axis indicates days of treatment, while the y-axis displays PPF in µmol/m 2 /s.

the last 8 days. These regular harvests required plant-respacing adjustments, so that plants would always be equally spaced over the growing area. Plants were grown as described above with the exception that, since plants were grown longer than in experi-ment 1 (red:blue ratio), PPF was increased by 25 µmol/m 2 /s daily until it reached 350 ± 10 µmol/m 2 /s to accommodate their higher light-intensity needs. The lighting schedule is graphically described in Fig. 6.

During the rst 11 days in the growth chamber, plants in both systems were maintained under the same lighting treatment, with a red:blue ratio of 95:5. On day 12, one system was switched to 90:10, whereas the other system was kept under 95:5.

2.8. Experiment 3: targeted red + blue vs. total coverage red + blue vs. total coverage white

Under optimizing conditions determined from the growth-curve experiment and the red/blue-ratio experiments described above, the efficiency of targeted red + blue LED lighting was compared to that of a system in which all red and blue LEDs in the ar-ray were energized during the photoperiod of a crop-production cycle as well as a lighting array with all white LEDs energized (to-tal coverage). “Targeted LED lighting” refers to visual estimation of overhead LED position relative to plant position and plant size. At the beginning of each photoperiod, only LEDs that were located directly above individual plants were manually energized, and irra-diance was adjusted daily using the ORBITEC lighting-system con-troller.

This experiment was replicated three times for 21 days from seed to harvest each time. Plants were grown as described for experiments 1 and 2 with the following exceptions: after newly transplanted seedlings were kept for 6 h in the dark, they were then maintained for 1 day under continuous light of 75 ±5 µmol/m 2 /s and one day at 85 ± 5 µmol/m 2 /s; PPF was increased by 25 µmol/m 2 /s daily until it reached 350 ± 10 µmol/m 2 /s. The lighting schedule is graphically described in Fig. 7. This difference in light intensity compared to experiment 1 comes from the fact that plants were grown longer (lag plus exponential phases), thus requiring higher light intensities post-lag phase. The red:blue ratio used for both treatments was 95:5.

Sixteen plants were mounted on each 0.372 m 2 growth board, giving a xed plant density of 43 plants/m 2 . Plants were assembled

Fig. 8. Diagram to scale representing plant spacing on growing lid for experiment 3. Circles represent where a plant was placed. All dimensions are in cm.

in 4 clusters of 4 plants and spacing between them did not change during the course of an experiment. Each plant in a cluster was separated from the other by 10 cm, and clusters were separated from each other by 15 cm, due to the lighting-array conguration, which is separated in four identical panels (Fig. 8).

2.9. Statistical analysis

Statistical analysis was performed using SAS software (SAS 9.2). To perform analysis of variance (ANOVA) within treatments, the ANOVA procedure and Duncan’s multiple range test were used. For experiment 3, data were pooled and treated as one sample with a larger N number and the assumption was made for complete randomization of treatments.

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Table 1Growth responses of lettuce (per seedling) subjected to different red:blue ratios of LED lighting for the rst 9 days of lag-phase seedling development following transplanting. Treatments I and III had a constant red:blue ratio throughout the experiment. Treatments II, IV, V and VI were split into 2 parts: 2 days of continuous light at a given red:blue ratio and the remaining 8 days at another red:blue ratio and 16 h photoperiod. N = 8 plants per treatment. Comparisons made within rows. Different letters within rows indicate signicant treatment differences at P < 0.05. SLA is “specic leaf area”.

Shoot growth parameters

95R:5B (Treatment I)

100R:0B then 95R:5B (Treatment II)

90R:10B then 95R:5B (Treatment III)

90R:10B (Treatment IV)

100R:0B then 90R:10B (Treatment V)

95R:5B then 90R:10B (Treatment VI)

Shoot f resh mass (mg) 174 a 109 c 139 b 133 b 114 c 160 aShoot dry mass (mg) 23 a 17 a 19 a 18 a 17 a 20 aHypocotyl Length (cm) 0.76 b, c 1.3 a 0.83 b 0.61 c 1.3 a 0.65 b, cLeaf area (cm 2 ) 7.0 a 5.4 b, c 5.4 b, c 5.2 c, d 4.5 d 6.2 bSLA (cm2 /g) 327 a 366 a 280 a 306 a 314 a 340 a% Dry Mass 13.0 a 15.6 a 14.2 a 14.9 a 15.2 a 12.7 a

3. Results

3.1. Experiment 1: establishing lettuce-growth-optimizing red:blue ratio at low irradiance

Preliminary experiments (results not presented), compared ef-fects of 5, 10, 15, or 20% blue light at low irradiance on lettuce-

seedling development

during

the

lag

phase

of

seedling

develop-ment. Those experiments indicated that seedlings grown under

low-irradiance red light (< 20 µmol/m 2 /s) during the lag phase led to stem breakage and seedling death. A consistent trend observed from preliminary experiments was that lag-phase plants developed shorter, stronger stems with increasing proportions of blue light, but above a certain level of blue, leaf expansion was inhibited.

Results for the lag-phase red:blue-ratio experiment 1 are sum-marized in Table 1 . The highest seedling fresh and dry masses, plus leaf area, were achieved with treatment I (red:blue ratio of 95:5). Treatment VI (red:blue ratio of 95:5 initially, then 90:10) gave sim-ilar results (not signicantly different) for fresh and dry masses. Minimum hypocotyl length occurred for treatment III (red:blue ra-tio of 90:10). Treatments II and IV (continuous red light for the rst two days) gave similar results: same hypocotyl length (signif-icantly different from other treatments) and dry mass, and similar fresh mass. Specic leaf area and percentage of shoot dry matter were not signicantly different between treatments. Similar results for fresh and dry mass, percentage of dry matter, and hypocotyl length were found for plants that grew with the same light treat-ment during the rst two days (red:blue ratios of 100:0, 95:5 or 90:10), independently of the treatment they were under for the rest of the experiment.

3.2. Experiment 2: growth curve

Regression analysis was used to determine transition time be-tween lag and exponential growth phases for four important met-rics of plant productivity, including leaf area, shoot fresh mass, shoot dry mass, and root dry mass (Fig. 9a, b, c, d). Given the sig-nicantly different results found for lag-phase testing of different sole-source LED red:blue ratios (experiment 1), a difference also was expected for exponential-phase growth metrics between the two treatments tested (95:5 vs. 90:10), but treatments were not statistically different for any metric. Thus, growth data were pooled and only one regression curve is presented per variable.

For each growth parameter measured, two growth phases were identied, including a lag phase, when growth was very slow, fol-lowed by an exponential phase when growth accelerated at an increasing rate. The beginning of exponential growth corresponds to intersection of the horizontal asymptote of the lag phase and the oblique asymptote of the exponential phase. On average for all variables, exponential growth began on day 15 for the envi-ronmental conditions of these experiments. Energy expended for

lighting per unit dry mass of edible lettuce parts during lag phase was 3.22 kWh/g, but was only 0.40 kWh/g during exponential growth (Table 2 ). Specic leaf area during lag phase averaged 331 cm 2 /g versus 201 cm 2 /g during exponential phase. Percentage dry matter in the shoot (ratio between dry mass and fresh mass) was 45% larger during exponential phase (6.84 %) than during lag phase (4.67 %).

3.3. Experiment 3: targeted R:B vs. total coverage R:B vs. total coverage white light for the entire production cycle

Three replicate experiments were performed and results are summarized in Tables 3a and 3b. Hypocotyl length was signi-cantly shorter for lettuces grown under total coverage white LEDs. Leaf and whole-plant dry masses and leaf area were statistically larger for the total coverage red + blue treatment than for the other two treatments. On average, total and edible biomass pro-duced by this treatment also was higher. However, percent dry matter in the shoot was highest for lettuces grown under the tar-geted red + blue treatment and statistically higher than that of lettuce grown under the total coverage white treatment.

The total coverage red + blue LEDs used 2.5 times more abso-lute energy and almost twice as much energy per unit dry biomass accumulated than did the targeted red + blue LEDs; the total cov-erage red + blue LEDs also used 2.2 times more absolute energy and 1.5 times more energy per unit dry biomass than did the white LEDs. Crop productivity per unit area was highest for the total cov-erage red + blue LEDs with 13 to 16 g/m 2 more than for the two other treatments, that is to say 32% greater than targeted red +blue and 50% greater than total coverage white.

4. Discussion

4.1. Red:blue ratio experiments

Light spectrum (i.e., red:blue ratio) was addressed for lag phase to obtain normal morphogenesis while minimizing light intensity, which otherwise causes fatal hypocotyl elongation and insufficient leaf expansion. The results revealed that lettuce grew best under a red:blue ratio of 95:5 giving the better fresh and dry mass and leaf area. Our results agree with the ndings of Hoenecke et al.(1992) and Dougher and Bugbee (2001) that stem length of lettuce decreases with increasing percentage of blue light. The trend found by Dougher and Bugbee (2001) that leaf area of lettuce decreases with increasing blue light did not appear clearly in our results. However, treatments IV and V (respectively 90R:10B and 100R:0B followed by 90R:10B), with the largest percentages of blue light, also had the smallest leaf area.

Shoot fresh mass and hypocotyl length were similar and not statistically different between pairs of treatments that had in com-mon their rst two days of growth but different red:blue ra-tios from days 2 to 9. Light quality during the rst 2 days of

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50 L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53

T a

b l e 2

E n e r g y c o n s u m p t i o n a n d l e t t u c e d r y m a s s a c c u m u l a t i o n –

E x p e r i m e n t 2 ( G r o w t h C u r v e ) . D

a t a p o o l e d f o r b o t h l i g h t t r e a t m e n t s ( r e d : b l u e r a t i o o f 9 5 : 5

a n d 9 0 : 1 0 )

. N =

1 2 0 .

T o t a l e n e r g y

c o n s u m p t i o n

( k W h )

E d i b l e b i o m a s s

a c c u m u l a t i o n ( g )

E n e r g y s p e n t p e r

u n i t e d i b l e

b i o m a s s

( k W h / g e d i b l e

)

T o t a l b i o m a s s

a c c u m u l a t i o n ( g )

E n e r g y s p e n t p e r

u n i t o f b i o m a s s

p r o d u c e d

( k W h / g t o t a l

)

C o n v e r s i o n

e ffi c i e n c y

1

( g / k W h )

S p e c i c l e a f a r e a

( c m

2 / g )

% D r y m a t t e r i n

t h e s h o o t

L a g p h a s e

( d a y s 1 t o 1 4 )

1 7

5 . 2 8

3 . 2 2

5 . 9 6

2 . 8 5

0 . 3 5

3 3 1

4 . 6 7

E x p o n e n t i a l p h a s e

( d a y s 1 5 t o 2 5 )

2 6 . 8

6 6 . 2

5

0 . 4 0

7 4 . 4

4

0 . 3 6

2 . 7 8

2 0 1

6 . 8 4

1 C o l u m n 6 i s t h e i n v e r s e o f c o l u m n 5 .

growth thus seems critical for seedling establishment and future plant developmental characteristics. Continuous red light during the rst 2 days did not result in acceptable growth, especially because hypocotyls became too long, conrming that sole-source-LED-lighted lettuce needs a minimum amount of blue light early on for healthy seedling development.

4.2. Growth curve

This study allowed us to determine precisely when the lag phase of seedling growth ends and when the exponential phase starts, during which time lettuce seedlings are most responsive to optimizing environments. There actually are more phases, but the experiment was intentionally stopped before plateau and senes-cence phases because the aim was to characterize the lighting sys-tems during the most active phase of growth, and phases beyond exponential are not productive in terms of biomass accumulation. Determining the inection point between the two phases had two direct consequences on subsequent experiments:

• Nutrient-solution strength and light level could be increased incrementally beginning on the appropriate day for optimal use by lettuce during its exponential growth phase.

• Energy expenditure for each phase of growth could be deter-mined in terms of kWh per gram dry mass of edible biomass produced.

The exponential phase of crop growth started on day 15, and the crop was terminated before an inection point occurred curv-ing over to plateau phase. Total dry biomass accumulation during this phase was 12.5 times more than during lag phase, which also is reected in the percentage of dry matter in the shoot biomass (6.84% for exponential phase versus 4.67% for lag phase).

Absolute energy consumption was least during lag phase (17 kWh versus 26.8 kWh during exponential phase), which was predictable. However, energy-biomass conversion efficiency dur-ing exponential growth was 8-fold higher than that during the lag phase (2.78 g/kWh versus 0.35 g/kWh). This stresses the im-portance of lag-phase light-use optimization to further increase overall energy savings over an entire cropping cycle.

One objective of experiment 3 (targeted vs. total coverage light-ing) was to minimize energy consumption during the lag phase of growth, when plants are small, by focusing incident light on plant material and not wasting it on empty space.

4.3. Targeted red + blue vs. total coverage red + blue vs. total coverage white

The shortest hypocotyls occurred for the lighting treatment us-ing white LEDs (Table 3a ), which have a rich blue component of approximately 40% (Fig. 3). This conrms previous ndings of Dougher and Bugbee (2001) , who found a decrease of 72% in let-tuce stem length when increasing blue light from 0 to 2%, and a further 13% decrease when increasing blue-light percentage from 2 to 6%. The blue light used in treatments using red + blue LEDs accounted for 5% of total PPF. Similar trends were found for the specic leaf area (SLA) index. It was highest for the targeted red +blue treatment, though not signicantly different from other treat-ments, and lowest for the total coverage white LED treatment. This also reinforces the ndings of Dougher and Bugbee (2001) , which showed that specic leaf area of lettuce decreased with increasing blue-light fraction from 5 to 25%.

The targeted red + blue treatment was the most efficient of the three treatments: 20% more biomass produced per kWh than with total coverage white; energy consumption per unit dry mass

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L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53 51

Fig. 9. Leaf area (a), root dry mass (b), shoot fresh mass (c), and shoot dry mass (d) of lettuce plants over 25 days of growth, average per plant. Circles represent data for treatment 1 (90 R:10 B) and triangles represent data for treatment 2 (95 R:5 B). The line is the regression curve.

Table 3aGrowth responses of lettuces (per plant) subjected to three different lighting treatments (Targeted R + B, Total coverage R + B, and Total coverage White) – Experiment 3, cumulative growth after 21 days of treatment – N = 16 in each treatment – comparisons are within rows. Different letters within rows indicate signicant treatment differences at P < 0.05.

Growth parameter Targeted R + B Total coverage R + B Total coverage White

Hypocotyl length (cm) 0.50 a 0.53 a 0.14 bLeaf area (cm 2 ) 410.56 b 510.53 a 345.47 bLeaf dry mass (g) 0.82 b 1.06 a 0.70 bRoot dry mass (g) 0.12 a 0.15 a 0.12 aShoot/root ratio 7.17 a, b 7.98 a 6.35 bWhole plant dry mass (g) 0.97 b 1.28 a 0.85 bSLA (cm2 /g) 514.09 a 508.49 a 502.09 a% Dry matter in the shoot 6.9 a 6.2 a, b 6.1 b

Table 3bRelationship between total biomass of plants subjected to three different lighting treatments (Targeted R + B, Total coverage R + B, and Total coverage White) and their respective energy and power consumptions – N = 16 – Experiment 3, cumulative growth after 21 days of treatment – comparisons are within rows.

Growth parameter Targeted R + B Total coverage R + B Total coverage White

Total edible dry biomass (g) 13.59 18.06 11.74Total dry biomass (g) 15.48 20.41 13.64Total energy consumption (kWh) 9.6 23.6 10.8Energy per unit edible biomass 1 (kWh/g) 0.71 1.31 0.92Energy per unit total biomass 2 (kWh/g) 0.62 1.16 0.79Conversion efficiency (g/kWh) 1.61 0.86 1.27Power per unit area 3 (W/m 2 ) 82.2 202 92.5Dry biomass per unit area (g/m 2 ) 41.6 54.9 36.7

1 The edible biomass is the biomass of the leaves.2 The total biomass is the biomass of the shoot and the root together.3 Power averaged on lighted periods.

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52 L. Poulet et al. / Life Sciences in Space Research 2 (2014) 43–53

(a)

(b)

Fig. 10. ORBITEC LED lighting system providing close-canopy overhead lighting of a young hydroponic lettuce crop (a) with photons falling on empty spaces between plants in a total coverage lighting scenario testing a 95:5 red:blue ratio, (b) with photons falling only on plants in a targeted lighting scenario testing a 90:10 red:blue ratio. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)

of lettuce was reduced by almost half compared to the total cover-age red + blue. Light falling between plants on a growing surface is wasted because it is not used efficiently by plants. This is partic-ularly true when small plants are in the lag phase and thus widely spaced (6 cm) apart from each other (Figs. 10a and 10b ). The ex-cess light of the total coverage R + B treatment, however, had a positive effect on crop productivity, which was 21% higher than for the other two treatments. The most likely explanation is re-ectance of PAR off of the pink polystyrene growth surface onto the leaves of nearby plants, thereby enhancing their photosynthe-sis.

The energy expenditure per gram of dry biomass produced with the total coverage white LEDs (0.79 kWh/g) was not far from that of the targeted red + blue (0.62 kWh/g), and it was 32% lower than for the total coverage red + blue LEDs. This efficiency likely is be-cause the white LED technology was 3 years newer than that of the red and blue LEDs used in the present study. Another possibility is that broad-band (white) light contains wavelengths promoting cer-tain photomorphological characteristics affecting productivity in a positive way that neither red nor blue LEDs alone can provide. An interesting follow-on study would be to include a targeted white-LED treatment of this same technology.

For the three LED lighting treatments tested, energy expendi-ture per unit dry biomass formed was about 80% lower than that reported by Wheeler et al. (1996, 2008) for ‘Waldmann’s Green’ lettuce in the BPC, which consumed 4.7 kWh/g dry biomass. The targeted red & blue LEDs spent 39% less energy per unit dry biomass accumulated and the white LEDs 23% less than the sole-

source intracanopy (vertical) red + blue LEDs reported by Massa et al. (2005) , which consumed 1.02 kWh/g dry cowpea biomass. The total coverage red + blue LED treatment, however, used 14% more energy per unit dry biomass accumulated than what Massa et al.(2005) reported. This leads to two conclusions:

• Targeted lighting enables signicant savings of energy expen-diture.

• Ongoing progress in LED technology should further reduce the cost of lighting in coming years.

The three LED lighting treatments compared here (targeted red + blue, vs. total coverage red + blue or white) used at least 90% less power per unit growing-surface area than did the HPS lamps of the BPC, which consumed 2.1 kW/m 2 (Wheeler et al., 1996 ).

5. Conclusions

Regardless of light treatment, the types of LEDs used in the present study performed better in terms of energy savings than in the previous lighting experiments cited, with power-use per unit growing-surface area reduced by at least an order of magnitude. It should be noted, however, that previous lighting studies were not designed for energy-use optimization but rather to explore maxi-mum limits of crop productivity.

The ndings of the present investigation reinforce that tar-geted lighting coupled to efficient LED technology plus optimiza-tion of red/blue ratio are key to reducing the overall energy cost of growing food crops in controlled environments, including those intended for life support in space.

Acknowledgements

The authors would like to thank Judith Santini for help with statistical analysis of the data, as well as Cannon Cheng, Michael Dzakovich, Celina Gomez, David Lotz, and Miranda Smith for their help in experiment management.

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