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Controlled-Environment Sunlit Plant Growth ChambersL. Liu a , G. Hoogenboom a & K. T. Ingram ba Department of Biological and Agricultural Engineering, The University of Georgia, Griffin,Georgia 30223-1797, USA; Fax: 770-228-7218; gerrit@griffin.peachnet.edu.b Department of Crop and Soil Sciences, the University of Georgia, Griffin, Georgia30223-1797, USAPublished online: 24 Jun 2010.

To cite this article: L. Liu , G. Hoogenboom & K. T. Ingram (2000) Controlled-Environment Sunlit Plant Growth Chambers ,Critical Reviews in Plant Sciences, 19:4, 347-375

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Critical Reviews in Plant Sciences, 19(4):347–375 (2000)

0735-2689/00/$.50© 2000 by CRC Press LLC

Controlled-Environment Sunlit Plant GrowthChambers

L. Liu,1 G. Hoogenboom,1* and K. T. Ingram2

1Department of Biological and Agricultural Engineering, The University of Georgia, Griffin, Georgia 30223–1797, USA; Fax: 770–228–7218; gerrit@griffin.peachnet.edu.; 2Department of Crop and Soil Sciences, theUniversity of Georgia, Griffin, Georgia 30223–1797, USA.

* Corresponding author.

TABLE OF CONTENTS

I. INTRODUCTION ................................................................................................................... 348

II. GENERAL DESIGN FEATURES ........................................................................................ 349A. Chamber Enclosures .......................................................................................................... 350

1. Transparent Materials ................................................................................................. 3502. Teflon Film .................................................................................................................... 3503. Enclosure Shape ........................................................................................................... 355

B. Air-Conditioning Methods ................................................................................................ 3551. Cooling Systems ............................................................................................................ 3552. Heating Systems ............................................................................................................ 3573. Humidification and Dehumidification Systems......................................................... 357

III. CONTROLLED ENVIRONMENTAL VARIABLES ........................................................ 358A. Fundamentals of Control Systems ................................................................................... 358B. Temperature ....................................................................................................................... 359C. Humidity ............................................................................................................................. 364D. Carbon Dioxide Concentration and Gas Exchange ....................................................... 366E. Air Movement..................................................................................................................... 367

IV. FURTHER NEEDS ................................................................................................................. 368A. Portability or Movability .................................................................................................. 368B. Guidelines ............................................................................................................................ 370

V. SUMMARY AND CONCLUSIONS ..................................................................................... 371REFERENCES ........................................................................................................................ 372

ABSTRACT: Controlled environment sunlit plant growth chambers have been built because of a great interestin plant responses to environmental variables under light intensities approaching those of natural sunlightconditions. Individual research projects have designed sunlit chambers that differ in size, structure, material, andenvironmental control systems dependent on the goals of the projects. Most literature describes plant organismresponses to environmental variables, whereas reports of system design and performance are few. The objectiveof this article is to present a review of the engineering aspects of the design, environmental control, andperformance of sunlit growth chambers that have been described in the literature. Most controlled environmentsunlit growth chambers have been constructed with experimental plants grown in either pots or soil bins. Although

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a few sunlit growth chambers were designed for field grown plants, precise environmental control was notavailable. Further progress in the development of precise controlled environment sunlit growth chambers shouldinclude portability (or movability) so these chambers can be used in multiple field sites for greater cost-effectiveness. Modifications for improvements in guidelines for the design and operation of controlled environ-ment growth chamber studies are also proposed.

KEY WORDS: environmental control, natural light, gas exchange, portability, soil-plant-atmosphere research,climate change, climate variability, environmental interactions, environmental control.

I. INTRODUCTION

Scientists and engineers have developed avariety of controlled environment systems forexperimental research on plant responses to theirenvironments in agricultural, biological, ecologi-cal, and environmental sciences. A common termfor controlled environment facilities is “plantgrowth chamber”, which is normally an artifi-cially lighted (indoor) enclosed space in whichenvironmental factors are controlled (Downs,1975). Air temperature and light (duration andintensity) are the variables normally controlled,while variables less often controlled include rela-tive humidity or dewpoint temperature, soil tem-perature and soil moisture, carbon dioxide, andother trace gasses of the aerial environment.

Because of a great interest in plant responsesto controlled environments under natural light,sunlit growth chambers have been built for plantsunder light conditions that are more characteristicof field situations. However, the number of sunlitgrowth chambers is much less than those of in-door growth chambers, which have been builtboth by individual researchers and by commercialcompanies. A comprehensive review of indoorgrowth chambers is beyond the scope of this ar-ticle. References for indoor growth chambers canbe retrieved from publications by Downs (1975,1980); Langhans (1978); Tibbitts and Kozlowski(1979); Kramer et al. (1980); Langhans andTibbitts (1996); and Tibbitts and Krizek (1997).The source of light is the principal differencebetween sunlit and indoor growth chambers. How-ever, sunlit chambers must deal with problems ofresponse time for an accurate control of tempera-ture and humidity to compensate the interferenceof dramatic disturbances of solar irradiance, whichcan change from full sunshine to completelycloudy within seconds. Furthermore, sunlit cham-

bers may have more problems with condensationeither inside or outside the chamber, dependingon the control of temperature and humidity withrespect to ambient weather conditions. Specialprotection is also required for the chambers andtheir control systems to tolerate all-weather rug-gedness, such as tornadoes, hail, and rain, ice, andsnow storms.

Greenhouses may be considered as sunlit plantgrowth chambers, although greenhouses are nor-mally larger in size and they do not have precisecontrol over environmental variables comparedwith most sunlit growth chambers. For the mostpart, literature on greenhouse design and perfor-mance focuses on economic production of highvalue vegetables and flowers. The design of green-houses as well as the operation has been standard-ized. For further information references such asAldrich and Bartok (1989) and Ortho Books (1991)can be consulted and are not reviewed here.

A special type of greenhouse is the tempera-ture gradient tunnel or chamber (TGC), which isdesigned to be portable and to provide a tempera-ture gradient along its longitudinal axis for re-search on field crop responses to different tem-peratures. Temperature gradient chambers mayalso be equipped with CO2 elevation to simulatethe effects of future climate changes on cropgrowth. The TGCs are normally cheap green-houses with a long tunnel shape. They enclose thecrops under normal soil conditions found in anexperimental field and with a transparent clad-ding material through which outside air is con-tinuously blown using fans mounted at one end.The TGCs provide a one-way airflow longitudi-nally to achieve near-ambient air temperature atthe inlet and gradually warming conditions alongits length by solar heating or supplementary heat-ing. The end-to-end temperature gradient can bemaintained constantly by controlling the speed of

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the fan that pushes or moves the air through thetunnel.

Temperature gradient chambers are open air-flow systems and normally have low power re-quirement due to the inexpensive and portablefeatures, thus no refrigeration is needed. How-ever, the special TGCs developed by Hadley et al.(1995) were equipped with heat pumps, whichenable temperature gradients to be maintainedunder low-radiation conditions. The heat pumpsalso allowed the chambers to be operated in aclosed mode so that the CO2 concentration couldbe increased without the necessity for large vol-umes of CO2 injection. More detailed informationon some of the TGCs can be found in the litera-ture, including Allen et al. (1992), Hadley et al.(1995), Horie et al. (1995), Okada et al. (1995),Rawson et al. (1995), and Sinclair et al. (1995).Due to the lack of control options of TGCs as atype of sunlit controlled environment chambers,TGCs are not reviewed in this article.

Another type of open airflow sunlit chambersis the open top chamber (OTC), which normallyhas a cylindrical shape with a rigid frame coveredby transparent films. Blowers or fans are installedon the lower side of the OTCs to provide a con-tinuous high rate of vertical ventilation to keepthe inside temperature and humidity close to thoseof outside air. Open top chambers do not provideprecise environmental control and generally onlycontrol a single trace gas, although OTCs may beused to study elevated temperature effects (Allenet al., 1992). The OTCs can also be easily modi-fied to temporary closed or semiclosed systemsfor gas exchange measurement in the field. Theopen top chambers are the most widely used andthe most thoroughly studied experimental methodof exposing field grown plants to elevated CO2

and other atmospheric gases (Leadley and Drake,1993). Several reviews have been published, in-cluding those presented by Heagle et al. (1973,1979, 1989), Weinstock et al. (1982), Unsworthet al. (1984), Drake et al. (1985), Allen et al.(1992), and Leadley and Drake (1993). To avoidany duplication an in-depth review of OTCs arenot included in this article.

The controlled-environment sunlit growthchambers that are reviewed in this article arethose with “closed” air circulation systems (air

mixing within the systems), which provide a high-level environmental control for plants under nearlynatural solar radiation conditions. The term“closed” is relative, as there are always problemswith either leakage or infiltration with “closed”systems, and a perfect closed system does notexist for growth chambers. Moreover, the “closed”system sunlit chambers may be equipped withventilation system for necessary air exchange withambient. Thus, the “closed” systems of the sunlitchambers are best described as semiclosed aircirculation systems compared with open-air sys-tems in TGCs and OTCs. Environmental vari-ables that are controlled in sunlit chambers mayinclude air temperature, humidity or dewpointtemperature, the concentration of carbon dioxide,the concentration of other trace gases, soil tem-perature, soil moisture, or any combinations ofthe above. The simplest sunlit chambers controlonly air temperature.

The objective of this article is to present areview of the engineering design and performanceof some of the semiclosed sunlit plant growthchambers that have been documented in the lit-erature. We provide a general overview of differ-ent design features, environmental control meth-ods, and performance of these sunlit chambers.We also identify further needs in the design andapplication of the controlled environment sunlitgrowth chambers.

II. GENERAL DESIGN FEATURES

Sunlit growth chambers generally consist of achamber enclosure for housing plants and an air-handling system for environmental control withinthe enclosure. As mentioned in the previous sec-tion, sunlit chambers range from the very simpleones that control a single environmental variableto sophisticated ones that control multiple vari-ables. Chambers also differ from each other insize and structure, which are normally determinedby the nature of the individual research projects.When evaluating any chamber design, it is criticalto consider the original purpose for which thechamber was built. With this caveat, we comparethe two most important components, the enclo-sures and the air conditioning methods, for a se-

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lection of the sunlit growth chambers reported inthe literature (Table 1).

A. Chamber Enclosures

1. Transparent Materials

All sunlit chamber enclosures have a struc-ture with transparent material in order to use natu-ral sunlight. Most sunlit chambers are designed tomaintain light conditions within the enclosures asclose as possible to natural sunlight. The lightlevel that reaches the plant canopy inside thechamber enclosures depends on two factors: lighttransmissivity of the transparent material and shad-ing from the frame and other components of thechamber. Structural shading should be limited tothe minimum necessary to give adequate strengthto the enclosure. Generally, greenhouse-coveringmaterials can be used as transparent materials forsunlit growth chambers. There are three types ofgreenhouse-covering materials: glass, rigid plas-tic sheets, and flexible plastic films. Some charac-teristics of the covering materials are listed inTable 2. Readers are referred to Briassoulis et al.(1997a, 1997b) for more detailed mechanical prop-erties of greenhouse-covering materials.

Glass has been the traditional glazing mate-rial for greenhouses due to high solar radiationtransmittance, ranging from 71 to 92%, depend-ing on composition, thickness, and special treat-ments, for photosynthetically active radiation(PAR, 400 to 700 nm). Glass also has superiorresistance to heat and scratching. However, glassconsiderably attenuates ultraviolet (UV) light, andit is heavy and expensive. Among the list in Table1, only the Solardomes by Rafarel et al. (1995)were constructed with 3 to 4-mm-thick Sanaluxglass. Glass is rarely used for sunlit growth cham-bers.

Rigid plastic sheets such as single- or double-layer acrylic and polycarbonate also have a highPAR light transmissivity, ranging from 79 to 93%,are resistant to adverse weather conditions, anddo not affect transmission of UV. In addition,they are lighter in weight than glass. Sunlit cham-bers with rigid plastic sheets do not require aframe and less maintenance than chambers with

plastic films (Phene et al., 1978; Parsons et al.,1980; Jones et al., 1984; Tissure and Oechel,1987; Adaros and Daunicht, 1985; Buxton andWalker, 1991).

Three plastic film materials have been usedas greenhouse covers: polyethylene (PE), polyes-ter (PET, tradename “Mylar”), and polyvinyl fluo-ride (PVF, tradename “Tedlar”) films. Manygreenhouses use PE film because it is inexpensiveand easy to install. It has fair light transmittance,for example, less than 85% PAR, and high UVtransmission. Unfortunately, it can only survive afew months of exposure to solar radiation unlessstabilizers have been added to the film (Norris etal., 1996). No sunlit growth chambers have usedPE films as cover material. Tedlar film was usedin the sunlit climate chambers reported by Hoffmanand Rawlins (1970), because of its high light-transmitting properties (92% PAR), ability towithstand weathering, and durability. Tedlar filmhas high tear resistance, but tears easily whenpunctured. Mylar film is the most widely usedfilm in sunlit growth chambers. Mylar film hasgood light transmittance (85 to 88% PAR) andhigh resistance to puncture and tearing. Sunlitchambers that have used Mylar film were thosedeveloped by Musgrave and Moss (1961), Bakerand Musgrave (1964), Egli et al. (1970), andPickering et al. (1994). However, Mylar is UVdegradable unless it is specially treated.

2. Teflon Film

Most sunlit growth chambers have been de-signed for gas exchange measurements, or forplant interaction studies with air pollutant gases,such as CO2, O3, NOx, and SO2. Therefore, it isessential to use chemically inert and impermeablematerial to eliminate confounding sources of con-tamination from the interior chamber environ-ment (Knight, 1992). Teflon FEP (FluorinatedEthylene Propylene copolymer) film is now con-sidered to be the best material for growth cham-ber design. It has been used for chambers withpollutant studies because it is the most inert ofavailable plastic films (Musselman et al., 1986;Meyer et al., 1987; Stokes et al., 1993; Tingey etal., 1996). Teflon FEP film is chemically inert,

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TABLE 1General Design Features of Selected Sunlit Growth Chambers (In Chronological Order)

Reference Purpose Enclosure Air-conditioning method

Musgrave measure CO2 flux 2.74 × 1.52 m, 3.66 m high to Direct expansionand Moss, of field corn the center of the roof which is1961; curved to sides

Baker and 3-mil (0.08 mm) Mylar film 2-ton (7 kW) compressor withMusgrave, attached to 2 × 2 lumber or flexible refrigerant hoses1964 steel conduit frame connected to an evaporating unit

for each chamber

Egli et al., Measure CO2 1.52 × 2.30 × 1.02 m Direct expansion1970 assimilation of

field soybean 2-mil (0.05 mm) Mylar film 3-ton air conditionerstretched over aluminum tubing framework

Hoffman Determine 3.66 × 2.74 m, 6.10 m high Direct expansionand Rawlins, quantitative effects with 3.05 m for soil profile1970 of temperature and studies and 3.05 m for plant 20 hp (14.9 kW) compressor with a

humidity on shoot growth centrifugal fan to deliver airvarious crops through a cold coil, a hot water

Aluminum frame with gable heat exchanger, and a steamroof design, covered with humidifier, then to the plant2-mil (0.05 mm) Tedlar film chambers

Louwerse Measure gas 1.5 × 1.33 × 0.5 m for grass Secondaryand exchange of field 1.5 × 1.33 × 1.2 m for potatoesEikhoudt, crops 0.8 × 0.8 × 2.5 m for maize Two flexible tubes were connected1974 and wheat to the chamber: one as by-pass,

the other equipped with a coolingPerspex rim covered with body filled with running water of 1°C,PVC film, sealed on a metal which was cooled by a refrigeratorframe, the latter being (2.32 kW) and circulated by ahammered into the soil to a water pumpdepth of 15 cm

Heating with two heating elements(1.5 kW each)

Phene et al., Control aerial Canopy: 2.0 × 0.5 × 1.5 m, Direct expansion1978; (temperature, constructed from 3.2 mm thick

Parsons et al., humidity, CO2) clear acrylic sheets bolted to Air temperature controlled by a1980 and soil an aluminum angle frame and 5.6 kW air conditioner and a 5.8 kW

(temperature) sealed with sealant electric heater with flexible tubingenvironment for connected to the aerial chamberwhole plant Welded steel soil bin:research 2.0 × 0.5 × 1 m Soil temperature was controlled by

a brine flowing through coppertubing encircling each soil bin

Jones et al., Provide control of Canopy: 2.0 × 1.0 × 1.5 m, Secondary cooling system1984 air temperature, constructed from clear acrylic

dewpoint sheets; all joints were bonded Chilled water provided by a 27 kWtemperature, and with metal screws and sealed chiller for six chambersCO2 concentration with silicon caulk (each having peak cooling

capacity of 4.5 kW)Welded steel lysimeter:2.0 × 1.0 × 1.0 m A heater of 3.8 kW used to reheat

the air to the setpoint

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TABLE 1 (continued)General Design Features of Selected Sunlit Growth Chambers

Reference Purpose Enclosure Air-conditioning method

Prudhomme Control temperature Surface area: 1.65 m2 Direct expansionet al., 1984 and CO2 for Volume: 1.35 m3

measuring CO2 A 0.25 kw compressor attachedflux in tussock Greenhouse had 12.7 mm to a remote heat exchanger insidetundra PVC tubing frame covered the greenhouse for cooling fan in

with 0.8 mm clear plastic greenhouse operated continuouslysheeting (flexible) galvanized to ensure adequate air mixingsheet metal was attachedaround the base of the frameand was sunk 10 to 15 cminto the soil

Tissue and Study A modification of the system Direct expansionOechel, 1987 photosynthetic and described by Prudhomme

growth responses et al. (1984) Same refrigeration unit asof tussock tundra Prudhomme et al. (1984),to elevated CO2 Surface area: 1.2 × 1.2 m modified with a hot gas bypassand temperature Volume: 1.02 m3 to the evaporator so that it could

heat as well as coolEnclosure material:rigid clear acrylic Humidity controlled by misting

Adaros and Study ventilation Canopy: 1.6 × 1.0 × 1.2 m SecondaryDaunicht, effects on small1985 plant stands Chamber cover of clear Air-conditioning equipment

double-layered acrylic; mounted at lower section ofwith wheels for mobility to chamber, including a blower,change place and direction a heat exchanger (connected to

a remote refrigeration unit of12.5 kW cooling capacity), andan electric heating coil (6 kW)

Buxton and Study interactions 1.22 × 1.53 × 1.07 m EvaporativeWalker, 1991 of environmental

factors in Chambers constructed of Air continuously ventilatedproduction acrylic sheets, angle aluminum through an evaporative-coolinggreenhouses strengthened all corners, system, a heating duct and a

and all edges sealed flexible tubing to the chamberwith sealant

Chambers installed inside agreenhouse

Pickering Study the effects of Canopy: 2.0 × 1.0 × 1.5 m Secondaryet al., 1994 CO2 and Soil: 2.0 × 1.0 × 0.6 m

temperature on Air circulated within an air-handlersoybean and rice Canopy enclosure constructed that contained cold and hot water

from aluminum frame covered heat exchangers and an electricwith Mylar film heater (5 kW), two 70 kW chillers

with a 3800 L storage tankWelded steel lysimeter provided cooling capacity to

six chambers

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TABLE 1 (continued)General Design Features of Selected Sunlit Growth Chambers

Reference Purpose Enclosure Air-conditioning method

Rafarel Study the effects of 4.4 m diameter with 22 m3 Secondaryet al., 1995 elevated CO2 and volume and 15 m2 floor area

temperature on Usable area about 9 m2 Variable speed fans circulatedplants fresh air via filter through either a

heat exchanger (cooling) forSolardomes constructed of ambient domes or electric heateraluminum frame and glazed (13.5 kW) for elevated temperaturewith 3 to 4 mm thick Sanalux domes, then discharged to theglass attached to a concrete dome from underground duct at thefoundation center, exhausted through 14 near

ground level outlet vents aroundperimeter of the domes

Liu and Study the combination 2.44 × 1.22 × 1.22 m, SecondaryWalker, 1997 effects of air canopy horizontally half plant area

and root zone half equipment area Air temperature controlled with atemperatures on motorized damper to adjust ratioplant growth in Chamber was constructed of of circulating air through agreenhouses 1.3-cm-thick OSB panel on a heat exchanger

2 × 2 wooden frameworkPlant area had transparent Heating by electric heaterpolycarbonate roof

Mist humidifier for humidityChamber constructed in a controlgreenhouse, and had wheelsfor mobility Root zone temperature controlled

through a water-jacketed pot withcirculation of constant-temperaturewater

Tingey et al., Study the effects of 2.0 m wide, 1.0 m front to Secondary1996 environmental back, 1.5 m tall at back and

stresses on a model 1.3 m tall at front An air-handler contained twoecosystem (i.e., squirrel cage blowers, cold andplant and soil Enclosure was an aluminum hot water heat exchanger, anprocesses) frame covered with 3-mil electric heater and a steam humidifier

(0.08 mm) clear Teflon filmexcept the back wall 50-ton chiller, 1600 gal cold water(0.65-cm-thick plexiglass), tank, 800 gal hot water tank,mounted on a lysimeter and two boilers provided controlswith dimensions of for 12 terracosm units2.0 × 1.0 × 1.0 m

resistant to virtually all chemical solvents, andhas low permeability to liquids, gasses, moisture,and organic vapors (DuPont Films, 1997c). Fur-thermore, Teflon FEP film has excellent opticalproperties with 96% solar transmission, and broadspectral transmission, including ultraviolet and

infrared wavelengths from 200 to 7000 nm(DuPont films, 1997c). Transmission of solar UVinto a chamber is desirable because the increasein ambient UV flux is an important aspect ofclimate change (Norris et al., 1996). Teflon FEPfilm is unique among plastics because it also pro-

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TABLE 2Characteristics of Some Transparent Materials

Light transmittance (%)

PAR* UV**Type (400–700 nm) (250–400 nm) Others

Glass 88–92 Resistant Superior heat and scratchresistance

HeavyFragile

Rigid sheet

Acrylic 83–93 Resistant Superior weather resistanceEasy to fabricate on siteEasily scratched

Polycarbonate 79–87 Resistant High impact resistanceLight weightEasily scratched

Plastic films

Polyethylene < 85 82–92a Inexpensive(PE) Easy to install

Available in large sheetShort life

Polyvinylfluoride 92 30–87b Inert to a variety of(PVF) chemicals“Tedlar” Resistant to weathering,

impact and tear, but tearseasily if punctured

Available up to 64′′ widths

Polyester 85–88 Resistantb Good resistance to chemicals(PET) High insulation resistance“Mylar” UV degradable unless treated

Available in 26 to 72′′ widths

Perfluorethenepropene 96 70–92b Chemically inert(FEP/PTFE) Low gas or vapor permeability“Teflon” Wide service temperature

High resistance toweathering, UVdegradation, impact andtearing

Available up to 72” widths

Note: *: Photosynthetically Active Radiation.**: Ultraviolet.

a Norris et al., 1996.b DuPont, 1997.

Modified from Aldrich and Bartok, 1989.

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vides outstanding resistance to weathering andultraviolet degradation and has high resistance toimpact and tearing.

3. Enclosure Shape

There are three basic shapes of chamber en-closures: rectangular, cylindrical, and hemispheri-cal. Cylindrical enclosures are widely used inopen top chambers, for example, Heagle et al.(1973,1989); Musselman et al. (1986); Meyer etal. (1987); Leadley and Drake (1993); and Norriset al. (1996). Hemispherical enclosures were usedfor the solar domes reported by Rafarel et al.(1995) because of their good aerodynamic prop-erties. However, most sunlit chamber enclosureshave been rectangular in shape, for example,Louwerse and Eikhoudt (1974); Phene et al.(1978); Jones et al. (1984); Adaros and Daunicht(1985); Pickering et al. (1994); Liu and Walker(1997); and Tingey et al. (1996). Rectangularchambers are easier to construct and have moreusable inside space with respect to the other shapes.

B. Air-Conditioning Methods

1. Cooling Systems

Techniques of temperature control withingrowth chambers are similar to those in air con-ditioning for human comfort, with cooling (re-frigeration), heating, and ventilation. Typically,the large radiant heat load of sunlit chamber en-closures must be removed by circulating cooled,conditioned air in order to control temperature.

Two types of cooling systems are most widelyapplied for temperature control: direct-expansioncooling and secondary cooling. Direct-expansioncooling systems work in a closed refrigerationcycle where refrigerant absorbs heat from onearea through evaporation, such as the air-condi-tioned space of a growth chamber, and rejects itinto another area through condensation, such asthe outdoors (ASHRAE, 1993). In secondary cool-ing systems, heat is transferred to a refrigerant (orcoolant), which can be any liquid cooled by adirect-expansion refrigeration system and used to

remove heat from the growth chamber withoutphase change. Secondary refrigerants are normallychilled water or water mixed with ethylene glycolor propylene glycol for freeze protection. Thefreezing points of aqueous solutions of glycolsdepend on the glycol concentration and are nor-mally provided by the manufacturers. A completelist of the freezing points of aqueous solutions forglycol is provided by ASHRAE (1993). For ex-ample, for a 30% by volume solution of propy-lene glycol, the freezing point is –13°C. The choiceof glycol concentration depends on the type ofprotection required by the application. If the ap-plication requires that the fluid remains entirelyliquid, a concentration with a freezing point of3°C below the lowest expected temperature shouldbe used (ASHRAE, 1993).

Heat exchangers (cooling coils) are used asthe heat transfer media both in direct-expansionand secondary cooling systems. The heat ex-changer, which contains refrigerant that removesheat as it evaporates in the direct-expansion cool-ing system, is also called an evaporator. Thissystem depends on a thermostatic expansion valveto automatically regulate the rate of refrigerantliquid flow in direct proportion to the evaporationrate to properly control temperature. In secondarycooling systems, the heat exchanger containschilled fluid to remove heat without a change ofstate. Heat exchangers can be placed within theair-conditioned space or in the main air handler.The cooling capacity of a heat exchanger is con-trolled either by varying the rate of secondarycoolant flow or airflow. Coolant flow can be con-trolled by a three-way valve. Airflow is controlledby dampers and multispeed fans. In either case,heat exchangers operate in a counterflow mannerfor air to pass through the colder side and to exitthrough the warmer side.

Direct-expansion cooling systems were usedby eight of the sunlit growth chambers listed inTable 1. Among these chambers, the small green-houses reported by Prudhomme et al. (1984) andTissue and Oechel (1987) had similar coolingsystems with a refrigeration compressor unit at-tached to remote cooling coils inside the green-houses. Fans inside the greenhouses operatedcontinuously to pass the inside air over the coilsfor cooling. The SPAR (Soil-Plant-Atmosphere-

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Research) systems (Phene et al., 1978 and Par-sons et al., 1980) had an integrated air-condi-tioner connected by ducts to the aerial chamber.Conditioned air was continuously delivered intothe chamber from the top, located at the north sideof the chamber, mixed with inside air and re-turned to the air-conditioner via a duct located atthe bottom on the north side of the chamber aswell. The closed canopy system of Egli et al.(1970) also used an air-conditioner system to coolthe plant canopy. A circulation fan in the air-conditioner system provided a constant move-ment of air through the canopy regardless ofwhether the air-conditioner was cooling. The restof the chambers with direct-expansion coolingsystem, for example, Musgrave and Moss (1961),Baker and Musgrave (1964), Hoffman and Rawlins(1970), had remote refrigeration compressorsconnected to evaporative coils inside the air-han-dling systems for cooling.

Although refrigerant can be transferred tomultiple cooling coils in a direct-expansion cool-ing system, for example, Musgrave and Moss(1961), Hoffman and Rawlins (1964), it is nor-mally desirable to use a secondary cooling systemif more than one chamber is constructed. Seven ofthe sunlit chambers listed in Table 1 used a sec-ondary cooling system. Secondary cooling sys-tems generally include a central refrigeration sys-tem, a coolant pump and distribution system,secondary terminal coils, and air-handlingductworks. The refrigeration system provides coldwater or antifreeze solution, which is circulatedthrough cooling coils. The coils are placed in theair-handling ductwork to cool the air as it passesacross the coils.

A third cooling system, although rarely usedin growth chambers, is evaporative cooling througheither a wetted pad or a fogging system. Evapora-tive cooling is achieved by circulating outdoor aircontinuously through a wetted medium (pad). Theevaporation of water from the pad causes heattransfer from air to the water in latent heat form,thereby lowering the air temperature and increas-ing the air humidity (Albright, 1990). Foggingsystems directly inject atomized water into the airfor cooling and humidifying. When water is at-omized into small droplets, the surface area in-creases tremendously and the evaporation rateincreases proportionately. The effectiveness of

evaporative cooling depends on the ability of theair to absorb water. It is most effective in warm,dry climates. Evaporative cooling is widely usedin agricultural buildings such as greenhouses,barns, and poultry houses for heat rejection whereprecise temperature control is not necessary. Thenatural-light growth chambers constructed insidea greenhouse by Buxton and Walker (1991) werecooled with a continuously ventilated evaporativewetted pad cooling system. The continuouslyventilated system does not require refrigerationand elaborate air recirculation, thus it is simplerand less expensive to construct than closed sys-tems (Buxton and Walker, 1991). Although fewchamber designs explicitly rely on evaporativecooling, this cooling system exists indirectly inall chambers with plants through evaporation inthe form of transpiration by the plant canopy.

Fluid-roof cooling is a fourth cooling systemthat is typically used in greenhouses. The prin-ciple is to circulate a liquid over or through adouble-layered greenhouse cladding to removesome of the radiation heat and thereby reduce theinside air temperature. A fluid-roof system isnormally designed to absorb infrared radiationthat is not used by plants for photosynthesis andto transmit most of the PAR. Chiapale et al. (1983)described a fluid-roof greenhouse constructed withan infrared heat absorbing double-layered glasswith demineralized water flowing through.Feuermann et al. (1997) used double-layered poly-carbonate sheet filled with stabilized water solu-tion of Fe+++ as the liquid radiation filter (LRF) toabsorb the near infrared radiation heat. The fluidwas then cooled down outside the greenhouse ina heat exchanger. If heat is stored, it can be usedat night as a backup heating system (Feuermannet al., 1997).

In summary, when evaluating cooling options,we may consider efficiency and reliability withrespect to project objectives and resources. Directcooling systems tend to be very efficient and costeffective when each chamber has its own coolingsystem and the numbers of chambers is small, forexample, less than five. As researchers use morechambers in order to meet research design criteriafor replication and to study interactions amongvariables, secondary cooling systems become moreattractive because they are less expensive perchamber for both purchase and operation. How-

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ever, the centralized nature of secondary coolingsystems sacrifices some redundancy and exposesresearch to greater risk of failure. Without standbysystems, if a secondary cooling system fails, anentire experiment may be lost, whereas if a directcooling system fails a single treatment may belost.

2. Heating Systems

Enclosures of sunlit growth chambers collectsolar energy during the daytime. Temperaturecontrol is mainly by cooling to balance the solarheat gain. However, air is often overcooled toremove excess moisture by condensation, so heat-ing must be provided to raise overcooled air tomaintain a constant temperature. Even withouthumidity control, heating is often needed at nightto maintain a desired temperature.

Electric heating coils have been used in growthchamber applications. An electric heating coilconsists of a length of resistance wire to which avoltage is applied. Heating coils are normallymounted in the air-handling duct, and they trans-fer heat to the circulating air as it passes over thecoils. Heating coils may be controlled through anon/off switch, or modulated controlled by varyingthe voltage.

Another widely used heating system is thehot-water heat exchanger system. A hot-waterheat exchanger is located in the air-handling duct;cool air gets heated as it passes over the heatexchanger. The heating capacity is normally con-trolled by adjusting the water flow rate. A hot-water heating system is more complicated than anelectric heating system because it needs moreattention for fluid flow. Electric heating systemsand hot-water heating systems have been usedseparately or in combination in most of the sunlitgrowth chambers listed in Table 1. The chambersthat did not include a heating system were devel-oped or used by Musgrave and Moss (1961),Baker and Musgrave (1964), Egli et al. (1970),and Prudhomme et al. (1984).

For sunlit growth chambers that use direct-expansion cooling, the coil temperature can becontrolled by bypassing the hot gas discharged bythe compressor to the evaporator. In this mode thecompressor can heat as well as cool. Moreover,adjusting the ratio of liquid refrigerant to hot gas

as a function of heat load can result in a steadycooling-coil temperature very near the set point ofthe chambers. Such systems are called “two-phasedirect expansion” (Downs, 1975). The chambersdeveloped by Tissue and Oechel (1987) used thistechnique to control the chamber temperature.

Greenhouse surface heating is another type ofheating system with warm water circulated overthe outside surface of the greenhouse. This heat-ing system is not thermally efficient, but can beeconomically feasible when using low-cost heatenergy sources such as power plant cooling water(Walker, 1978, 1979; Walker et al., 1982). Theflowing layer of warm water not only transfersheat into the greenhouse, but also serves as athermal insulating barrier between the greenhouseand outside air (Walker et al., 1982). The waterlayer may also increase PAR light transmissionthrough refraction and prevention of dirt, snow,or ice accumulation, but it greatly decreases trans-mission of wavelengths higher than 800 nm(Heinemann and Walker, 1987). This type of heat-ing system is not applicable for sunlit growthchambers using natural light.

3. Humidification and DehumidificationSystems

Humidity is an indication of the amount ofwater vapor in the air. Humidity can be controlledthrough adding vapor to the air (humidification),or removing vapor from the air (dehumidifica-tion) with a proper set of transducers and control-lers. There are four principal types of humidifica-tion systems: pan, wetted element, atomizing, andsteam (ASHRAE, 1988). Pan-type humidifiersuse a shallow pan filled with water placed in theair-handling unit. Incoming airflow is humidifiedas water evaporates into the air. An electric heatercan be installed in the pan to increase water tem-perature, thus increasing the rate of evaporation.Wetted element humidifiers use an open-textured,wetted media through, or over which air is circu-lated to evaporate water. The evaporative coolingpad used in the natural-light growth chambers ofBuxton and Walker (1991) is an example of wet-ted media for humidification. Atomizing humidi-fiers use a high-speed disk, which slings waterthrough a fine comb to create a fine mist that isintroduced directly into the air, where it evapo-

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rates (ASHRAE, 1988). Misting (Parsons et al.,1980; Tissue and Oechel, 1987; Liu and Walker,1997) is an example of atomizing humidification.Steam humidifiers inject steam directly into theair to be humidified. Steam humidifiers were usedfor humidity control in the sunlit growth cham-bers by Hoffman and Rawlins (1970) and Tingeyet al. (1996).

Steam injection is the most efficient humidi-fication method, provided that the steam is clean.It is commonly recommended for humidificationwhen a supply of steam is available (Downs,1975; Tibbitts, 1978). Steam humidification addsheat, whereas other humidification systems re-move heat through water evaporation. All hu-midification systems affect temperature controls.An integrated control system therefore is neededto control temperature and humidity simulta-neously.

During the day, sunlit growth chambers nor-mally tend to have a higher humidity than thehumidity of the outside ambient air because ofplant transpiration and soil evaporation. There-fore, dehumidification is critical for sunlit growthchambers, but it is easier to achieve than humidi-fication. The cooling coils in sunlit chambers thatare used for temperature control can also dehu-midify by cooling the air below its dewpoint(Hoffman and Rawlins, 1970; Louwerse andEikhoudt, 1974; Phene et al., 1978; Parsons et al.,1980; Jones et al., 1984; Pickering et al., 1994;Tingey et al., 1996). The air can then be reheatedto the desired dry bulb temperature. Refrigerationof air below its dewpoint is the most commonmethod of dehumidification in sunlit growth cham-bers. Another method to dehumidify is throughchemical sorption, using chemicals such as silicagel or glycol solution. This method is normallynot used in sunlit growth chambers because of thelarge amount of desiccant needed to remove mois-ture transpired by plants in the chambers.

III. CONTROLLED ENVIRONMENTALVARIABLES

A. Fundamentals of Control Systems

A feedback control system is generally moreaccurate and more desirable for environmental

control than a non-feedback system in sunlitgrowth chambers. In a feedback control system,output conditions, that is, the controlled variables,are compared with predetermined reference in-puts. The difference, or error, between the inputand output activates the control process to reducethe difference and to maintain the output at someprescribed level (Kuo, 1991).

Types of control have been classified as on/off, proportional (P), integral (I), derivative (D),and combinations of the above. In on/off controla binary output either turns control componentson or off. In proportional control, the controlsignal is proportional to the difference betweenthe input and output, the larger the differences,the greater the output control signal. Integral con-trol integrates the deviations of output from theset point over time and brings the average error tozero to obtain a high accuracy of control. Deriva-tive control measures the instantaneous changerate of the error, predicts overshoot, and correctsbefore overshoot actually occurs (Kuo, 1991).

On/off control is the least expensive methodcontrol system. In sunlit chambers it is used mainlyto turn heaters on or off for temperature controland to activate dampers or valves of a ventingsystem for CO2 control (Musgrave and Moss,1961; Egli et al., 1970; Phene et al., 1978;Prudhomme et al., 1984; Jones et al., 1984; Tingeyet al., 1996). Proportional control is often used ina cooling system to regulate the coolant flow ratefor temperature control or to adjust a mass flowcontroller to control CO2 injection in sunlit cham-bers (Hoffman and Rawlins, 1970; Jones et al.,1984; Tingey et al., 1996). There may be consid-erable fluctuations of the controlled variable be-tween on and off cycles. Fluctuations can be re-duced by a proper proportional control, but the setpoint may never be reached exactly with a pro-portional control. Proportional control should becombined with a time integral or time derivativeof the input signal, namely, PI or PD, respec-tively. PI control will improve the steady stateerror, but its response is slow. A PD control canincrease the system response to sudden large de-viations of the controlled variable from the setpoint, but the steady state error is not affected(Albright, 1990; Kuo, 1991). This leads to the useof PID control that combines the best propertiesof each of the PI and the PD controllers. The

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feedback system with a PID controller is one ofthe best-known control systems in practice (Kuo,1991). Sunlit chambers by Adaros and Daunicht(1985) used a PID controller with triac for precisetemperature control. Tingey et al. (1996) imple-mented PID program instructions with only pro-portional devices for the fine control of air anddewpoint temperature.

Feedforward is another control style that pre-dicts the changes of controlled variables in ad-vance and adjusts control processes to reduce thechanges. Feedforward normally uses simulationmodels based on variables that affect controlledvariables, and in combination with a feedbacksystem provides better control. For example, sun-light affects both heat gain and CO2 uptake by theplants in the chamber. By integrating solar radia-tion in the control algorithms, we may be able toadjust to cooling load before temperature risesenough to trigger cooling, or CO2 injection basedon predicted plant assimilation and measured CO2

changes in the air stream. Parsons et al. (1980),Jones et al. (1984), and Pickering et al. (1994)used this technique in their chamber control sys-tems.

B. Temperature

Temperature is an important variable that af-fects plant growth and development. Because ofheat gained from solar radiation, sunlit growthchambers normally have high air conditioningdemands for temperature control. Solar radiationcan change quickly and within seconds full sun-shine can be replaced with clouds. Therefore,control systems must adapt quickly for constantand accurate temperature control. Sunlit cham-bers such as those listed in Table 1 have differenttemperature control capabilities, depending onindividual research specifications. These cham-bers can be divided into two categories accordingto the temperature control range: those that con-trol temperature approximately equal to or a littlehigher than the outside ambient air temperature(Musgrave and Moss, 1961; Baker and Musgrave,1964; Egli et al., 1970; Prudhomme et al., 1984;Tissue and Oechel, 1987; Rafarel et al., 1995) andthose that control temperature above or below theoutside ambient air temperatures (Hoffman and

Rawlins, 1970; Louwerse and Eikhoudt, 1974;Phene et al., 1978; Parsons et al., 1980; Jones etal., 1984; Adaros and Daunicht, 1985; Buxtonand Walker, 1991; Pickering et al., 1994; Tingeyet al., 1996; Liu and Walker, 1997). Environmen-tal variables that have been controlled in the sun-lit growth chambers (Table 1) are listed in Table 3.

Sunlit growth chambers used to measure CO2

gas exchange normally track the outside air tem-perature at about 3 ~ 4°C higher than the insideair temperature. Musgrave and Moss (1961), Bakerand Musgrave (1964), Egli et al. (1970),Prudhomme et al. (1984), and Tissue and Oechel(1987) used direct-expansion cooling systems tocontrol temperature by adjusting the flow of re-frigerant. Unfortunately, there is no informationavailable in the literature with respect to the con-trol precision for temperature in the sunlit cham-bers as well as the uniformity of the temperaturedistribution both horizontally as well as verticallywithin the enclosure. Temperature deviationsgreater than 5°C were reported by Musgrave andMoss (1961).

In the solar-domes the air temperature wascontrolled at ambient or ambient plus 3°C (Rafarelet al., 1995). Ambient air was continuously drawnover either a heat exchanger (cooling coil) for thedomes that maintained ambient temperature, oran electric heater for the domes that maintainedan elevated temperature, then vented out afterpassing through the plant-growing area. Thetemperature data that were reported in the litera-ture were only weekly averages. Weekly meantemperature was about 0.4°C higher than the tar-get temperature on a clear winter day and 0.3°Cabove target on hot summer days. In general, thetarget temperature was maintained to within+0.7°C and –0.5°C in the plant enclosures. Al-though Rafarel et al. (1995) stated that solar heatgain could cause the elevated domes to exceed therequired 3°C above ambient, data were not pro-vided in the literature.

The range of temperature control capabilitiesamong sunlit growth chambers depends on thepossible design as well as the potential applica-tion. Hoffman and Rawlins (1970) designed theirsunlit climate chambers to control temperaturewithin the range from 5 to 50°C to quantify theeffects of environmental factors on various crops.Air temperature was controlled by passing circu-

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TABLE 3Controlled Environmental Variables within the Sunlit Growth Chambers listed in Table 1

CO2 and Gas ExchangeReference Temperature and humidity measurement Others

Musgrave Air temperature controlled to CO2 tracked outside concentration at 20% reduction inand Moss, track outside air temperature, deviation of 10% by injection with intensity of visible1961; but fluctuated with large solenoid valve or air exchange with light range insideBaker and deviations both above and a relay-controlled flap chamberMusgrave, below outside air temperature1964 at times Assimilation and respiration Air velocities

calculated by measuring changes of checked with anNo humidity control; 50–60% CO2 concentration and CO2 injection anemometer; noRH on clear days, near 100% or removal rate within a time period “dead spots”at night and on heavily detected inovercast and rainy days Transpiration rate measured by chambers

collecting condensation water fromcooling coil

Egli et al., Air temperature maintained CO2 controlled at 300, 450, 600 ppm1970 approximately equal to outside within ± 20 ppm

air temperaturePlant assimilation determined bycalculating CO2 balance withinchamber

Transpiration measured by collectingcondensation from the cooling coil

Hoffman Designed air temperature Airflowand range 5 to 50°C with a control downward, 5 airRawlins, accuracy of ±1°C exchanges/ min1970

Dewpoint temperature range 75% incomingdesigned to control RH from solar radiation at15 to 90% and accuracy plant levelof ±1°C

Louwerse Air temperature controlled CO2 controlled and up to 1000 ppm Airflow horizontaland within a range of 8°C below by injection; sub-ambient CO2 below through flexibleEikhoudt, to 15°C above ambient and ambient obtained by depletion of the ducts; air1974 either kept constant or air by crop itself circulated at

following the outside air 0.167 m/stemperature Assimilation and respiration rate

determined by measuring CO2 ofin- and out-going air of enclosed area

Transpiration rate determined bymeasuring humidity of in andout-going air and by collectingcondensation from the coils

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TABLE 3 (continued)Controlled Environmental Variables within the Sunlit Growth Chambers listed in Table 1

CO2 and Gas ExchangeReference Temperature and humidity measurement Others

Phene Air temperature controlled CO2 concentration maintained Airflowet al., from 15 to 35°C within ±0.5°C constant by CO2 injection with a downward;1978; (15-min average basis), with solenoid valve by simulating adjustable baffleParsons ambient temperatures ranging proportional control through an plates mountedet al., from 4 to 32°C on/off duty cycle on air-inlet duct-1980 Temperature distribution within controlled speed

empty chamber was within ±2°C Assimilation calculated from CO2 and mixing of airmeasurements and corrected for

Humidity maintained by leakageinjecting a fine mist of waterinto air duct. RH was 40 to Transpiration measured by collecting75% for temperature control condensation from air-conditioningfrom 10 to 35°C coil

Jones Air temperature controlled to CO2 concentration controlled during Airflowet al., within ±1°C of desired values the day at 330 and 800 ppm within 5% downward; air1984 with discrete CO2 injection flow rate

Dewpoint temperature 0.17 m3/scontrolled to within ±1.5°C

Prudhome Air temperature maintained at CO2 concentrations maintained atet al., ambient levels 330 and 600 ppm by adding pure CO2

or scrubbing greenhouse air throughsoda lime; addition or scrubbing of

1984 CO2 done at a known flow rate andfor a measured amount of time

Tissue and Temperature controlled to CO2 concentrations maintained Acrylic chamberOechel, track ambient or ambient plus 4°C at 340, 510, 680 ppm by adding transmitted1987 pure CO2 or scrubbing greenhouse 90–92% PAR,

Humidity controlled at air through soda lime but attenuated allambient condition by misting of UV lightwhen humidity levels droppedbelow ambient

Adaros and Air temperature differences CO2 exchange measured according AirflowDaunicht, between chamber and ambient to difference at inlet and outlet horizontal; air1985 of 25°C in summer and 40°C in of chamber velocity adjustable

winter maintained within ±0.2°C within 0.08 ~Evapotranspiration measured by 0.74 m/s

RH in the range of 45~90% collecting condensation from heatwithin temperature range of exchanger20~30°C

Buxton and Temperature maintained CO2 concentration maintained by Airflow upwardWalker, uniform within ± 1°C at night regulating pure CO2 injection to at two1991 and during periods of low solar chamber exchanges/min

radiation; during higher solarradiation, control was not Supplementalprecise light available

from overheaddevice

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TABLE 3 (continued)Controlled Environmental Variables within the Sunlit Growth Chambers listed in Table 1

CO2 and Gas ExchangeReference Temperature and humidity measurement Others

Pickering Air temperature controlled CO2 concentration maintained using Airflowet al., within ±0.25°C and dewpoint a proportional mass flow controller for downward1994 temperature within ±0.5°C on a CO2 injection or solenoid-controlled at three

5-min average basis venting for CO2 removal. CO2 could exchanges/minbe controlled within ±5 ppm on a5-min average basis Methane (CH4)

measured usingCO2 exchange rate calculated from gaschange in CO2 concentration and chromatographCO2 injection rate, then correctedfor leakage

Evapotranspiration determined bymeasuring condensation

Rafarel Air temperature controlled CO2 concentration controlled at Airflowet al., at ambient or ambient plus 3°C ambient and ambient plus 340 ppm downward at 3.61995 by CO2 injection through a exchanges/min

proportional mass flow controllerPAR reduced 18%within domes

Liu and Air temperature controlled Airflow upward;Walker, from 10 to 30°C within ±0.2°C air velocity within1997 on a 1-min average plant canopy less

than 1 m/sSoil temperature controlledfrom 10 to 30°C within ±0.5°Con a 1-min average

Tingey Hourly air temperature CO2 concentration maintained using a Airflowet al., controlled within 2°C of target proportional mass flow controller for downward; air1996 temperature for ambient and CO2 injection or solenoid-controlled exchange rate

elevated temperature treatments venting for CO2 removal. CO2 approximately 10concentration maintained within exchanges/min

Vapor pressure deficit ±50 ppm of target concentrations formaintained approximately ambient and elevated treatmentssame in ambient and elevatedtemperature treatments

lation air over a direct-expansion evaporator unitfor cooling, then over a hot water heat exchangerfor heating, and finally discharged to the cham-ber. Air temperature was proportionally controlledfor cooling and heating, and the control accuracywas ±1°C for a set point of 39°C when the outsidetemperature was around 10°C. The maximumspatial variation of air or dewpoint temperatureon a horizontal plane was ±1.5°C, and the maxi-

mum time variation of either air or dewpoint tem-perature was ±3°C in the center of the chamberfor a 5-min period. This temperature control ca-pability was relatively successful for a sunlitclimate chamber that was actually a small green-house with a size of 3.66 m × 2.74 m × 6.10 m.

The mobile laboratory of Louwerse andEikhoudt (1974) included sunlit chamber enclo-sures with temperature control within a range of

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8°C below or 15°C above outside air temperature.Air temperature was controlled by proportionallyadjusting the mixing ratio of bypass air and con-ditioned air within parallel air ducts. A daylightgrowth chamber developed by Adaros andDaunicht (1985) was able to control the differ-ence between ambient air and chamber at 25°Cunder summer irradiation at noon or at 40°C dur-ing the winter. Circulating air was continuouslycooled and reheated by means of a PID controller;no temperature deviations greater than 0.2°C weredetected (Adaros and Daunicht, 1985).

The natural-light growth chambers developedby Buxton and Walker (1991) used evaporativecooling and electrical heating systems for tem-perature control. During cooling, the air movedthrough wet evaporative cooling pads. Duringheating or ventilation, the evaporative pads re-mained dry. Temperature was controlled by ad-justing the time between the different heatingstages. The system was able to maintain tempera-tures within ±1°C of a set point at night andduring periods of low solar radiation. Duringhigher solar radiation periods, the temperaturewas close to ambient air temperature. The maxi-mum variation measured with thermocouplesspaced evenly both horizontally and verticallyin the enclosure was 4°C (Buxton and Walker,1991).

The sunlit growth chamber of Liu and Walker(1997) controlled air temperatures from 10 to30°C with an accuracy of ±0.2°C on a 1-minaverage base. A simple on/off control with motor-ized sliding dampers to adjust the ratio of cold airand bypass air was specially designed for thechamber. Slow movement (0.07 cm/s) of the damp-ers acted similar to a proportional controller togive precise control. Supplemental heat was pro-vided if necessary.

The remaining sunlit growth chambers in-cluded in Table 3 can be called Soil-Plant-Atmo-sphere-Research (SPAR) chambers similar to thosereported by Phene et al. (1978) and Parsons et al.(1980). Based on these SPAR chambers, closedenvironmental plant growth chambers (Jones etal., 1984), outdoor environmental plant chambers(Pickering et al., 1994), and sunlit controlled-environment terracosms (Tingey et al., 1996) weremodified and developed. The purpose of the SPAR

chambers is to control temperature, humidity, andCO2 concentration in the canopy environment, aswell as to control and measure soil water contentand root conditions (Allen et al., 1992). Air isnormally circulated from the north top of thechamber to the plant area and returned throughthe north bottom of the chamber to the air-han-dling duct for reconditioning.

The original SPAR chambers controlled airtemperature, using an air-conditioner connectedto air ducts for cooling (direct-expansion cooling)and an electric heater installed in air inlet ductsfor heating (Phene et al., 1978; Parsons et al.,1980). Temperature control precision of the origi-nal SPAR chambers was ±2.5°C on a 15-minbasis and was achieved by adjusting the speedand mixing of return and ambient air with baffleplates mounted on air-inlet ducts (Phene et al.,1978). Parsons et al. (1980) further developed adata acquisition and control system for the SPARchambers. Their temperature control algorithmwas based on forward projection proportionalcontrol that enabled heaters to be turned on formultiples of 0.016 s up to 4.25 s. The air-condi-tioner ran continuously to maintain a minimumrelative humidity level. Control precision was±0.5°C on a 15-min basis for temperatures rang-ing from 15 to 35°C and with ambient tempera-tures ranging from 4 to 32°C. Temperature distri-bution patterns within the empty SPAR units wereinvestigated on a 3 * 3 grid and the temperaturewithin the aerial portion was controlled within±2°C. The SPAR chambers developed by Jones etal. (1984), Pickering et al. (1994), and Tingey etal. (1996) used secondary cooling systems withan electric heater or a hot-water heat exchangerplus an electric heater to provide aerial tempera-ture control. Hourly values of controlled tempera-tures were within ±1°C of the desired set point(30°C) for the chambers developed by Jones et al.(1984). They used a discrete control algorithm toturn heaters on and off at 10-s intervals. Pickeringet al. (1994) controlled aerial temperature within±0.25°C on a 5-min average basis, using a PIcontrol algorithm to regulate the flow rate of coldwater, hot water, and the output of electric heat-ers. The terracosms developed by Tingey et al.(1996) used a PID algorithm to control air tem-perature within 2°C of the target temperature be-

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tween 85 and 100% of hours for both ambient andelevated temperature treatments.

Technically, any temperature range that mightbe needed for plant research within a growth cham-ber can be provided with available engineering.However, there will be sacrifices to obtain ex-treme conditions. For example, low-temperaturecontrol may be accompanied by large fluctuationsduring defrost cycles. In an air-cooling system,any coil, whether it is a direct-expansion coil or asecondary coil, operating below a surface tem-perature of 0°C accumulates frost or ice. Defrost-ing has to be done either by shutting off thecooling system or by supplemental heating, bothof which hamper the precision of temperaturecontrol. The large temperature deviations in thesunlit chambers by Musgrave and Moss (1961)resulted from the loss of control when ice formedin the expansion valve of the refrigeration equip-ment. When icing occurred, the refrigeration sys-tem had to be defrosted and then reconnected.The deicing process was followed by 2 or 3 h ofgood temperature control, followed by anotherperiod of deviation. Parsons et al. (1980) alsoreported the freezing of the air-conditioner coilsat low temperature controls, that is, below 12°Cair temperature in the SPAR units. The terracosmsdeveloped by Tingey et al. (1996) were unable tomaintain air temperature below 0°C because theheat exchanger would ice up and become ineffec-tive. In fact, the problem with the defrosting cyclecan be overcome by having two cooling coils sothat one can cool while the other defrosts alterna-tively. However, such dual-coil systems have notbeen used in growth chamber applications.

Normally, the temperature control perfor-mance of sunlit growth chambers is evaluatedusing averaged data . Unfortunately, these aver-age temperature data are based on various inter-vals, ranging from weekly (Rafarel et al., 1995),hourly (Tingey et al., 1996), 15-min (Phene et al.,1978; Parsons et al., 1980), 5-min (Pickering etal., 1996), to 1-min (Liu and Walker, 1997) inter-vals. There are no established criterions or proce-dures for temperature measurements in sunlitgrowth chambers. Most researchers have followedpublished guidelines, including those publishedby societies such as ASHS (1972, 1978, 1980),and ASAE (1993). Parsons (1979), Spomer (1980,

1981), Krizek (1982), Krizek and McFarlane(1983) also recommend guidelines for measuringand reporting the environment for plant studieswith indoor growth chambers. These guidelinesrecommend that air temperature should be mea-sured hourly over the period of study. However,hourly (or hourly averaged) data cannot accu-rately describe the performance of a sunlit growthchamber. Because solar radiation changes withinseconds, minute rather than hourly measurementsare necessary to make certain that short periods oflow or high extremes are recognized and aver-aged. These guidelines also recommend that airtemperature in growth chambers be measured witha shielded and aspirated (greater than 3 m/s) in-strument to assure that the sensor accurately de-tects the air temperature. The recommendationsalso include that air temperature be measured atthe top of the plant canopy and averaged horizon-tally over the plant-growing area. This is the re-gion of the exposed leaves where the highestphotosynthetic rates occur. Temperature measure-ments within the canopy are not recommendedbecause they vary with different canopy densitiesand air flow. There are no guidelines regardingmonitoring vertical gradients in the chambers, butreporting the range of spatial variation across thechamber is suggested by Parsons (1979).

C. Humidity

Humidity control is normally considered ofless importance than temperature control for plantgrowth, as long as it varies between some accept-able levels, such as 50 to 90% (Downs, 1975).Because temperature and humidity are directlyrelated, simultaneous control of these two vari-ables is difficult to obtain and to maintain, espe-cially under natural irradiation in sunlit cham-bers. A sudden change of solar radiation not onlyaffects the sensible heat load of the chamber,thereby affecting temperature, but also affects planttranspiration, and therefore the latent load, whichin turn affects humidity. Therefore, many sunlitgrowth chambers normally have temperature con-trol, but no humidity control, for example,Musgrave and Moss (1961), Baker and Musgrave(1964), Egli et al. (1970); Louwerse and Eikhoudt

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(1974); Phene et al. (1978); Prudhomme et al.(1984); Adaros and Daunicht (1985); Buxton andWalker (1991); and Rafarel et al. (1995). If nohumidity control is provided, humidity levels varywidely between light and dark periods and duringheating and cooling cycles (Tibbitts, 1978). Forexample, relative humidity in the sunlit chambersof Musgrave and Moss (1961) fell to 50 to 60%on clear days but tended to be near 100% at nightand on cloudy and rainy days when the refrigera-tion units only operated infrequently. During asunny day, cooling requirements are usually at amaximum and condensation often occurs on thesurface of the cooling coil, thereby decreasing thehumidity of the air circulated within a sunlit growthchamber. Fortunately, this relative humidity fluc-tuation simulates the ambient relative humidity,which normally is lower on clear days and higheron cloudy and rainy days and at night.

The importance of humidity to plant growthhas been well documented (Hoffman, 1979). Low-humidity air leads to plant water stress, and high-humidity air causes pest and disease problems.Both high- and low-humidity levels can restrictplant growth and development. Humidity controlin growth chambers should be provided to ensureuniformity in plant responses for different inves-tigations (Tibbitts, 1978).

Relative humidity (RH), dewpoint tempera-ture, and saturation vapor pressure deficit (SVPD)are three commonly used terms to describe airhumidity in humidity control. RH is the percent-age partial pressure of actual water vapor vs. par-tial pressure at saturation at the same temperatureand pressure (ASHRAE, 1993). Dewpoint tem-perature is the temperature of saturated air at thesame pressure and absolute humidity ratio as thatof the given sample of moist air. The absolutehumidity ratio is a measure of the capacity of airto hold water vapor and is defined as the ratio ofthe mass of water vapor to the mass of dry aircontained in the sample (ASHRAE, 1993). If theabsolute humidity ratio is constant, air at differenttemperatures has a constant dewpoint tempera-ture, but RH can differ significantly. However, ata constant RH, vapor pressure and dewpoint tem-perature vary as dry bulb temperature changes.Identical RH values do not mean the same mois-ture conditions unless the dry bulb temperature is

constant (Downs, 1975). Saturation vapor pres-sure deficit is the difference between saturationvapor pressure and actual vapor pressure of theair (Tibbitts, 1978). The advantage of SVPD isthat it is a more sensitive indicator of the watervapor conditions than RH and varies over a widerrange with temperature change than RH. Themore important advantage of SVPD is that it ismore closely related to VPD (vapor pressuredeficit) that drives plant transpiration. There-fore, SVPD is the most preferable variable whenconsidering plant responses to atmospheric hu-midity (Hoffman, 1979).

As mentioned previously, humidity control isachieved by humidification, dehumidification, ora combination. Dehumidification is normally ob-tained by condensation when moist air passesover a cooling coil. Humidity control in the SPARchambers of Jones et al. (1984) and Pickering etal. (1994) was obtained using this method only,and dewpoint temperature was used as the humid-ity indicator. Humidification was not consideredbecause the chambers already had high humiditydue to plant transpiration. The dewpoint tempera-ture was sensed with a hygrometer and controlledby regulating the flow rate of chilled water throughthe cooling coils. Jones et al. (1984) controlleddewpoint temperature within ±1.5°C by openingappropriate solenoids in a 5-min cycle. Pickeringet al. (1994) used a proportional controller toregulate the chilled water flow rate and controlleddewpoint within ±0.5°C of desired values.

Mist humidification was combined with de-humidification by condensation in sunlit growthchambers of Parsons et al. (1980), Tissue andOechel (1987), and Liu and Walker (1997). Par-sons et al. (1980) used an on/off duty cycle tosimulate proportional control by injecting a finemist of water into the duct to maintain RH withinthe range of 40 to 75% for control temperaturesfrom 10 to 35°C. Tissue and Oechel (1987) con-trolled humidity at ambient conditions by mistingwhen the RH dropped below ambient conditions.Liu and Walker (1997) intended to control RHabove 60% by injecting water mist. This controlsystem, however, was not precise.

Steam humidification was applied to correctexcess dehumidification in the chambers devel-oped by Hoffman and Rawlins (1970) and Tingey

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et al. (1996). The quantity of steam entering thehumidifier was proportionally controlled by pneu-matically operated valves. Dewpoint was con-trolled within ±1.5°C spatial variation and ±3°Ctemporal variation and provided an RH controlfrom 15 to 90% (Hoffman and Rawlins, 1970).The humidity control target in the terracosms wasambient VPD (Tingey et al., 1996). The controlobjective was to maintain constant VPD in cham-bers both for ambient and elevated temperaturetreatments, with VPD calculated from air tem-perature and dewpoint temperature, as sensed bya hygrometer. If the chamber VPD was less thenthe target VPD, the chamber air was dehumidi-fied by chilling the air stream to the desireddewpoint temperature. If the chamber VPD wastoo large, the air was humidified to the desiredlevel by steam injection with a PID control algo-rithm.

D. Carbon Dioxide Concentration andGas Exchange

Carbon dioxide (CO2) is an essential factorfor plant growth and development (Tibbitts andKrizek, 1978). Within a closed system controlledenvironment facility, such as sunlit growth cham-bers, the CO2 concentration changes through alight-dark cycle. Plants deplete CO2 during thelight period and respire to produce CO2 during thedark period. Plant growth may be limited by CO2

availability during the light cycle. Therefore, acertain level of CO2 must be maintained in aclosed system sunlit growth chamber for effectiveplant research, even if precise CO2 control is notavailable.

The CO2 level was assumed adequate by leak-age and diffusion through chamber joints andconnections by Liu and Walker (1997). Hoffmanand Rawlins (1970) and Adaros and Daunicht(1985) used fresh air ventilation to maintain CO2

levels for plant growth. Thus, a CO2 control sys-tem and additional CO2 supply or removal werenot included in these chambers. The CO2 concen-tration was controlled in all other sunlit growthchambers, as shown in Table 3. In fact, the advan-tages of these closed system chambers when com-pared with open system chambers are that in closed

system chambers the CO2 concentration can bemuch more accurately controlled and an inte-grated ecosystem gas exchange can be determined(Leadley and Drake, 1993). Sunlit growth cham-bers with precise CO2 control also enable thestudy of the impacts of increased atmosphericCO2 concentration and potential global climatechange on plant growth.

Control of CO2 concentration in a semiclosedcirculation system is commonly obtained by bal-ancing the addition and removal of CO2 withinthe system. The addition of CO2 may be obtainedthrough injection of pure CO2 from gas cylinders,or, less commonly, through the combustion ofmethane. CO2 can be injected at a fixed flow ratewith a modulated frequency, for example,Musgrave and Moss (1961); Baker and Mussgrave(1964); Egli et al. (1970); Louwwerse andEikhoudt (1974); Phene et al. (1978); Parsons etal. (1980); Jones et al. (1984); Prudhomme et al.(1984); Tissue and Oechel et al. (1987), or at acontinuous flow with a modulated flow rate, forexample, Pickering et al. (1994); Rafarel et al.(1995); Tingey et al. (1996). The CO2 can beremoved either through venting to exchange cham-ber air with outside air, for example, Musgraveand Moss (1961); Pickering et al. (1994); Tingeyet al. (1996), or scrubbing CO2 from the chamberair with a soda lime column, for example,Prudhomme et al. (1984); Tissue and Oechel (1987). Precision of CO2 control varies from ±5ppm (Pickering et al., 1994) to ±50 ppm (Tingeyet al., 1996), even though both systems used simi-lar control procedures with a proportional massflow controller for CO2 injection and an on/offcontrolled venting for CO2 removal. The SPARchambers of Jones et al. (1984) maintained a day-time CO2 at 330 ppm or 800 ppm within 5% of thetarget values, using discrete CO2 injection.Musgrave and Moss (1961) reported that theirchambers could track outside CO2 concentrationat a deviation of 10% with CO2 supply manuallyreplenished and a CO2 removal by periodicallyreplacing the air in the chambers with outside air.

A nondispersive infrared gas analyzer (IRGA)was used to measure CO2 concentration in allcited literature. An IRGA operates on the prin-ciple that CO2 molecules absorb energy in theinfrared region of the electromagnetic spectrum.

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An IRGA permits rapid and continuous measure-ment and recording of CO2 concentrations andhas the potential of high-precision measurement(Tibbitts and Krizek, 1978; Pallas, 1979; Allen etal., 1992). The unit of CO2 concentration fromIRGAs or calibration gases is normally ppm (partsper million) or µmol/mol; barometric pressure israrely measured. Therefore, ppm has been widelyused in the cited literature. However, CO2 con-centration should be expressed as moles per cubicmeter (mol/m3) or mmol/m3 or µmol/m3 in SIunits (Krizek, 1979). A pressure transducer canbe easily installed in any IRGA, and the CO2

concentration in SI units can be computed fromppm and the local barometric pressure.

In plant research, CO2 control is often com-bined with CO2 gas exchange measurements, es-pecially the determination of photosynthesis andrespiration rates. The CO2 exchange rate and H2Oexchange rate (transpiration) of plants at elevatedCO2 and temperature are important in predictingchanges of plant growth and productivity, and innet ecosystem carbon storage and water use effi-ciency (Tissue and Oechel, 1987; Allen et al.,1992). As shown in Table 3, CO2 exchange iscommonly calculated from the chamber CO2 bal-ance using measurements of changes in CO2 con-centration, CO2 injection or removal rate, andestimated leakage, for example, Mussgrave andMoss (1961); Egli et al. (1970); Phene et al. (1978);and Pickering et al. (1994). Louwerse and Eikhoudt(1974) and Adaros and Daunicht (1985) calcu-lated CO2 exchanges rate by measuring the CO2

concentration of the in- and outgoing air of thechambers. The transpiration rate was always de-termined by collecting and recording water con-densed from air-conditioning coils. The soil sur-face must be covered to prevent water lost throughsoil evaporation and CO2 lost through soil respi-ration from entering the canopy enclosure(Mussgrave and Moss, 1961; Baker and Musgrave,1964; Egli et al., 1970; Louwerse and Eikhoudt,1974; Phene et al., 1978). Otherwise, evapotrans-piration is measured instead of the transpirationsuch as by Adaros and Daunicht (1985) andPickering et al. (1994). Covering the soil surfacealso prevented CO2 respired by roots from enter-ing the canopy chamber, which can confound theobservations of CO2 assimilation by plants. A gas

diffusion barrier on the soil surface does not stopsoil respiration and may have adverse effects if itrestricts oxygen movement into the root zone. Ifa gas diffusion barrier is used to separate the rootand canopy environments, it is important to pro-vide another avenue for gas exchange of normalbiotic and abiotic soil processes. When it is notfeasible to isolate the aerial environment from thesoil environment, soil respiration can be estimatedseparately by cutting and removing the above-ground biomass and then measuring soil CO2

emission from the bare soil with an intact rootsystem below the soil surface.

E. Air Movement

Air movement in the plant area of a con-trolled environment growth chamber is anotherimportant environmental factor that affects plantgrowth through interaction with temperature, hu-midity, and CO2 exchange rates (Downs, 1975;Krizek, 1978; Duysen, 1979). An air velocity of0.5 m/s is generally considered an optimumspeed for plants grown under controlled envi-ronment conditions (Downs, 1975; Krizek, 1978;ASHRAE, 1993). Air speeds across the leaf of0.03 to 0.1 m/s are needed to facilitate carbondioxide uptake. Air speeds above 1 m/s can in-duce excessive transpiration, reduce carbon di-oxide uptake, and inhibit plant growth in growthchamber studies (ASHRAE, 1993).

Air movement is also essential to provideuniformly controlled variables within the enclo-sure to prevent spatially biased treatment effects.Uniformity of controlled temperature, humidity,and CO2 concentration in a chamber is main-tained by a turbulent airflow that is created bymixing conditioned air with chamber air by en-trainment. It is more critical to have a relativelyuniform airflow within controlled environmentsunlit growth chambers than within artificiallylighted growth chambers, because high solar ra-diation can cause extremely high temperatures atdead flow spots. We use the term “relatively uni-form” because absolute uniformity of the con-trolled chamber variables is impossible. For ex-ample, according to the energy balance, thetemperature of conditioned air must be increased

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to absorb sensible heat as it flows across a sunlitgrowth chamber. The function of precise tem-perature control is to keep the temperature in-crease as small as possible and to ensure tempera-ture uniformity through a horizontal plane.

There are three direction patterns of airflowin a growth chamber: horizontal (Louwerse andEikhoudt [1974] and Adaros and Daunicht [1985]),upward (Buxton and Walker [1991] and Liu andWalker [1997]), and downward. Horizontal (side-to-side) airflow is generally undesirable for largegrowth chambers, because a temperature gradientdevelops between intake and exhaust sides of achamber (Morse, 1963). This gradient may belarge in a sunlit growth chamber, as the condi-tioned air flowing across a chamber accumulatesheat from solar radiation.

There is no obvious difference with respect tocontrol systems between upward (bottom-to-top)and downward (top-to-bottom) flows. The advan-tage of upward flow is that a perforated floor (Liuand Walker [1997]) can be arranged for uniformairflow, although this pattern is only applicablefor chambers with potted plants. However, a per-forated ceiling is not desirable for downward flowdue to light shading. Downward flow is normallyfrom the top of one side to the bottom of the sameside or the opposite side of a growth chamber.Downward flow provides a means of carrying theexcess heat away from the top and avoids largetemperature gradients within the plant canopy(Morse, 1963; Downs, 1975; Duysen, 1979;Krizek, 1978;). Moreover, the most physiologi-cally active part of plants is the top of the canopy,where sunlight and CO2 enter from the outside ofthe system. If we can accurately control condi-tions at the top of a canopy, then downward flowgives greater precision because there is less pos-sibility for interference between the air inletand the control point. Thus, a downward airflowpattern was adopted by most sunlit growth cham-bers (e.g., Hoffman and Rawlins [1970]; Phene etal. [1978]; Parsons et al. [1980]; Jones et al. [1984];Pickering et al. [1994]; and Tingey et al. [1996]).

The average and range of air velocity in cham-ber spaces should be reported in controlled envi-ronment plant studies (Duysen, 1979). Becauseplant size and leaf shape influence air movement,Duysen (1979) also suggested that the air velocity

also be measured both at the beginning and at endof studies. Unfortunately, none of the sunlit cham-bers in Table 3 followed these recommendations.Some articles reported only overall air velocity(Louwerse and Eikhoudt, 1974; Adaros andDaunicht, 1985). Some reported total air exchangerate or air flow rate (Hoffman and Rawlins, 1970;Jones et al., 1984; Buxton and Walker, 1991;Pickering et al., 1994; Rafarel et al., 1995; Tingeyet al., 1996). Although average air velocity can becalculated from air exchange rate and chambervolume, information on air flow distribution anduniformity is unknown. Still others did not reportany air flow data (Musgrave and Moss, 1961;Baker and Musgrave, 1964; Egli et al., 1970;Phene et al., 1978; Parsons et al., 1980;Prudhomme et al., 1984; Tissue and Oechel, 1987).One method to determine uniformity of air flowwithin a growth chamber is to measure the tem-perature distribution patterns with spatially ar-ranged temperature sensors. This temperature dis-tribution should be measured while the chamberis occupied with plants (Parsons et al., 1980;Buxton and Walker, 1991).

IV. FURTHER NEEDS

A. Portability and Movability

The controlled environment sunlit growthchambers reviewed above have different designsand environmental control approaches based onthe individual research applications. Althoughsome chambers were typically designed for useinside greenhouses with potted plants (Buxtonand Walker [1991] and Liu and Walker [1997]),the more general application of the sunlit cham-bers is to obtain near natural solar radiation underfield conditions. Field sunlit chambers have beenequipped with CO2 monitoring and control sys-tems to study the impact of global climate changeon plant growth. Global climate change is becom-ing one of the most important issues facing themodern society today. Controlled environmentsunlit growth chambers are important tools forglobal change studies. Research has been done todetermine the relationships between CO2 concen-tration and plant growth in sunlit chambers (Allen

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et al., 1992). However, scientists are still trying toimprove the predictions of the potential impact ofclimate change on agricultural production. Oneobjective is to eventually adapt plants and mini-mize the effects of global change on agriculture(Stelljes et al., 1997). Therefore, there is a needfor further improvement of in-field sunlit cham-bers.

One of the important features of the in-fieldsunlit chambers is portability or movability, whichprovides the ability to relocate the controlled en-vironment to different terrestrial landscapes andplace it on natural soil. Realistic soil conditionsare normally unable to be created, even whensoils are brought in from the field. The portabilityof the sunlit chambers makes it easy to be used forin field-based experiments, using natural soils butproviding controlled environmental conditions andalso to provide a greater cost effectiveness.

Most sunlit chambers lack portability. Thesunlit climate chambers developed by Hoffmanand Rawlins (1970) are much like small-scalegreenhouses and include a concrete basement asa machine room. The solar dome developed byRafarel et al. (1985) was attached to a concretefoundation with underground ducts for conditionedair circulation. Adaros and Daunicht (1985) re-ported a movable daylight growth chamber, butthe chamber was connected to an air-conditioningand control system that was placed inside a build-ing; movability was limited to a small range fororientation adjustment. The SPAR chambers(Phene et al., 1978; Parsons et al., 1980) and theirlater generations (Jones et al., 1984; Pickering etal., 1994; Tingey et al., 1996) have steel soil binsor soil lysimeters for root zone control. Control ofshoot and root environments of the SPAR cham-bers makes it possible to study interactions be-tween whole plant responses and elevated CO2

concentrations and temperature and a moisturestress (Allen et al., 1992). However, they requireheavy-duty environmental control equipment,such as two 70-kw chillers, one 97-kw naturalgas heater, and a 300-L water storage tank(Pickering et al., 1994) or a 50-ton chiller, 1600-gallon cold water storage tank, 800-gallon hotwater storage tank, and two boilers (275000 BTU)(Tingey et al., 1996). This type of environmentalcontrol equipment makes it impractical, if not

impossible, to move the chambers to differentlocations.

Several portable sunlit chambers have beenreported in the literature list and are included inTable 1. The assimilation chambers of Musgraveand Moss (1961) were designed to be portable formeasuring net assimilation of maize grown in afield. The air-conditioner unit was mounted onportable skids to facilitate portability. However,temperature and humidity inside the chamber couldnot be controlled precisely. Temperature was con-trolled to be the same as the outside temperature,but fluctuated with large deviations. The small in-field greenhouses of Prudhomme et al. (1984) andTissue and Oechel (1987) were installed directlyabove the soil for elevated CO2 studies. Althoughthe authors did not report on portability, thesegreenhouses seemed to be movable without mucheffort. Temperature and humidity levels in thegreenhouses were regulated to track ambient con-ditions, but the acrylic plastic attenuated all natu-ral ultraviolet radiation. The mobile laboratory ofLouwerse and Eikhoudt (1975) includes two trans-parent plant chambers for continuous measure-ment of photosynthesis, respiration, and transpi-ration of field crops, while control equipment anda power generator were housed in a mobile van.The laboratory could be transferred easily fromone plot to another during the growing seasonwith little interference to the crops. However, thehorizontal airflow pattern was not ideal for tem-perature and humidity control in a sunlit growthchamber.

Therefore, we propose that further develop-ment is needed for relatively precisely controlledenvironment sunlit growth chambers that haveportability or movability (easy to install, disas-semble, and transport) for applications with avariety of field crops conducted at different sitesin either experimental or farmers’ fields. Suchportable sunlit growth chambers should provideuniform control of air temperature, humidity, andcarbon dioxide concentration and be equippedwith instrumentation to continuously measurecanopy CO2 exchange and H2O exchange rates.These chambers are most suitable for studies thatdetermine the interaction of varying CO2 concen-trations with other environmental variables, suchas temperature and humidity, under field condi-

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tions. Although the development of these cham-bers is expensive, portability makes it cost effec-tive for use over a short time period or through anentire growing season.

Three portable sunlit growth chambers (Liuet al., 1997, 1998) have been designed and con-structed at the University of Georgia, College ofAgricultural and Environmental Science campusin Griffin. These chambers are portable and canbe moved to different field plots. They includeenvironmental equipment to provide precise anduniform control of temperature, humidity, andCO2 to determine whole plant responses to envi-ronmental factors. The combination of high-levelcontrol precision and movability make these cham-bers unique to study plant interactions with tem-perature, humidity, and CO2 under field condi-tions.

B. Guidelines

As mentioned previously, guidelines havebeen established particularly for controlled envi-ronment plant studies in indoor artificially lightedgrowth chambers. Two groups, the ASHS Com-mittee on Growth Chamber Environments and theNorth Central Region NCR101 Committee onGrowth Chamber Use, are working closely toprovide these guidelines (Tibbitts and Krizek,1997). Guidelines for reporting studies conductedin controlled environment chambers were firstpublished by an ASHS committee (1972). Theywere then revised, reviewed, and published in anextensive literature series (ASHS, 1978, 1980;Parsons, 1979; Tibbitts and Kozlowski, 1979;Spomer, 1980, 1981; McFarlane, 1981; Krizek,1982; Krizek and McFarlane, 1983; ASAE, 1993).These guidelines are equally applicable to sunlitgrowth chambers and can be modified for use byresearchers who are working with these cham-bers. However, sunlit chambers differ from in-door chambers and special guidelines should beprovided for their use. Moreover, there is a needfor a modification of the guidelines for indoorchambers as well, because of the lack of detailprovided in the current guidelines.

For example, guidelines for air temperaturemeasurement and reporting in controlled environ-

ments are (1) use a shielded and aspirated (greaterthan 3 m/s) device; (2) measure temperature atthe top of plant canopy and average over plantgrowing area; (3) measure temperature hourlyover the period of study; and (4) report hourlyaverage values for light and dark periods of thestudy with range of variation over the growingarea. Hourly temperature data are insufficient toquantify variations within short periods for a sun-lit growth chamber under unpredictable changesof solar radiation, and thus cannot adequatelyrepresent the control performance of a sunlitgrowth chamber. There are no recommendationson reporting temperature vertical gradients, whichmay be very large if airflow is too slow or nonuni-form. Furthermore, guidelines should providequantitative allowable spatial variations both hori-zontally and vertically. Nevertheless, environmen-tal control precision and accuracy should be statedin order for others to repeat the experiments.

Precision and accuracy are two terms to de-termine performance of a control process. Preci-sion is the repeatability of measurements of thesame quantity under the same conditions, andaccuracy is the capability of an instrument toindicate the true value of measured quantity(ASHRAE, 1993). Obtaining a certain degree ofcontrol precision is necessary, but accuracy mustbe indicated to obtain meaningful results. Accu-racy in measurement makes it possible to com-bine the data, results, and conclusions of experi-ments conducted in various places at various timesto arrive at generally valid concepts (Langhansand Urquhart, 1979; van Bavel, 1979). Expectedtypical instrument precision and measurementaccuracy are given by ASAE (1993) and shouldbe used as guidelines in controlled environmentstudies to improve experiment precision and ac-curacy. Certainly, precision of a control processalso depends on other factors such as controltechniques and human errors.

Control precision is often given in terms ofdeviation (temporal variation) from a desiredvalue. For example, Parsons et al. (1980) con-trolled the SPAR chambers from 15 to 35°C within±0.5°C on a 15-min average basis, and Pickeringet al. (1994) controlled air temperature within±0.5°C of designed values on a 5-min averagebasis.

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The spatial variation of environmental vari-ables is not provided as often as the temporalvariation of the variables in controlled environ-ment chambers. However, it is an important indi-cation of the quality of controlled environmentconditions. Poor spatial distribution of controlledparameters may confound experimental results orintroduce a level of uncontrolled error that makesinterpretation of results difficult or impossible.Lack of spatial uniformity with indoor growthchambers, primarily temperature and radiance,has been reported by Hammer and Langhans(1972, 1978), Langhans and Urquhart (1979), Leeand Rawlings (1982). It is more difficult to main-tain a horizontal spatially uniform environmentwithin sunlit growth chambers than with indoorchambers. Vertical spatial uniformity within sun-lit growth chambers is impossible due to the na-ture of high solar radiation, but efforts for rela-tively uniform control should be stated andmeasurement of variation is necessary to explainexperimental error. The purpose of precision con-trol with sunlit growth chambers should reducespatial variation vertically and should ensure uni-formity horizontally.

Another method to reduce experimental errorwith growth chambers is to use proper random-ized experimental design and replications. This isalways recommended, no matter how preciselyand accurately the environment is controlled(Hammer and Langhans, 1972, 1978; Langhansand Urquhart, 1979; van Bavel, 1979; Lee andRawlings, 1982). Researchers should at least ran-domize experimental plants within a chamber ifmultiple units are not available, so that unwantedspatial variation of controlled environment willbe equally distributed over the plant material.Additionally, there is generally no assurance thatthe nature or condition of test plants can beduplicated to a similar degree. For this consider-ation a replication in time is needed (van Bavel,1979).

V. SUMMARY AND CONCLUSIONS

Controlled environment sunlit growth cham-bers are widely used to study the effects of cli-matic factors on plant growth under nearly natu-

ral solar radiation conditions. There are no gen-eral criteria on the specifications of sunlit growthchambers because of the different applications ofindividual researchers. Sunlit growth chambersdiffer from each other in size, structure, material,and environmental control systems.

Sunlit chambers may have different ap-proaches in design and control, but the sunlightcondition within the chamber enclosures shouldbe uniform and maintained as close as possible tonatural light. Transparent materials such as rigidAcrylic sheet, Mylar film, and Teflon film arenormally used for the sunlit chamber enclosuresdue to high light transmissivity. Clear Teflon filmis considered to be the best transparent materialfor sunlit growth chambers, because of its opticalclarity, broad spectral transmission, and chemicalinertness. While chamber shape could be cylin-drical or hemispherical, most sunlit chamber en-closures are rectangular for easier constructionand more usable inside space compared with theothers.

The engineering aspects of environmentalcontrol in sunlit growth chambers follow the prin-ciples of heating, ventilating, and air-condition-ing (HVAC) engineering. Environmental variablescontrolled inside sunlit growth chambers can beair temperature, air humidity, concentrations ofcarbon dioxide and other pollutant gases, soiltemperature, soil moisture, and combinations ofthe above. The simplest sunlit chambers controlonly air temperature. Because solar radiation maychange rapidly, effective control systems must beimplemented in order to achieve accurate control.Control of the CO2 concentration in sunlit growthchamber is commonly obtained by CO2 injectionand removal. CO2 control in plant research isoften combined with CO2 gas exchange and watervapor exchange measurement in order to studychanges of plant growth and productivity, and innet ecosystem carbon storage and water use effi-ciency. Air movement is also an important factorin sunlit chambers to provide a uniform control ofthe aerial environment, especially temperature,humidity, and CO2 concentration. Uniformity isnormally established by circulation and/or venti-lation of air throughout the chamber using fans,the position and pattern of the air inlet and outlet,and airflow velocity.

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Lack of detailed information with respect tothe performances of the control system is a prob-lem common to most sunlit chambers reported inthe literature. For example, some reports onlypresent averaged data of controlled variables. Inmost cases, the spatial variations of the controlledvariables within each chamber are not reported.Other articles only include averaged data on anhourly, daily, or even weekly basis. These datacannot represent the correct performance of asunlit growth chamber because short periods oflow or high extremes resulting from dramaticsolar radiation changes are not recognized.

Controlled environment sunlit growth cham-bers have been and will continue to be used tostudy the interactions of plants with their environ-ment. Further progress in development of pre-cisely controlled environment sunlit growth cham-bers should include portability, or movability, sothese chambers can be used at various field sitesor even in farmers’ fields for greater cost effec-tiveness.

Further needs in controlled environment stud-ies also include modifications of current guide-lines to specify allowable spatial variations and todefine control precision and accuracy. Moreover,there is a need to provide special guidelines forstudies with sunlit growth chambers.

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