Abstract: Research to develop control strategies to implement vapor pressure deficit. (VPD) control for plant propagation is presented. Recently designed and ...
Copyright \0 IFAC Control Applications and Ergonomics in Agriculture, Athens, Greece, 1998
VAPOR PRESSURE DEFICIT CONTROL STRATEGIES FOR PLANT PRODUCTION
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R.S. Gates , S. Zolnier2, J. Buxton3 }.3 Biosystems and Agricultural Engineering and Horticulture and Landscape Architecture, respectively, University of Kentucky Lexington KY 40546-0276 USA 2 Ph.D. candidate at University of Kentucky, Agicultural Engineering, Federal University of Vi~osa , Brazil
Abstract: Research to develop control strategies to implement vapor pressure deficit (VPD) control for plant propagation is presented. Recently designed and constructed lowcost chambers serve as the platform for this discussion, although results can be generalized to any other equipment in which both air temperature and humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods ofVPD control are developed and described. Copyright © 1998 [FAC Keywords : Vapor pressure deficit control, Computer controlled systems, Controlled systems, Environments, Heat flows, Model-based control
1. INTRODUCTION
transpiration. Recent research has suggested that control of vapor pressure deficit might be used to simultaneously reduce water consumption and maintain elevated gas exchange rates, such as CO2, between crop and air (Bates and Bubenheim, 1994). The method requires knowledge of what magnitudes of VPD are appropriate for various plants to maintain open stomata for gas exchange while limiting transpiration by reducing the driving force of VPD.
Propagation and early growth of poinsettia plants in Kentucky is hindered by the need to start cuttings during late summer, when interior greenhouse temperatures and radiant loads are typically quite high. Vapor pressure deficit (VPD) control may offer a means to improve both propagation and growth of poinsettias and other ornamental and bedding plants. Creating an improved environment by using crop-toair vapor pressure deficit (VPDcmp-air) control is now possible as instrumentation for accurate plant temperature measurement has become more cost effective and reliable. An alternative method, sometimes called air vapor pressure deficit (VPD air) control, simply measures air relative humidity and temperature to obtain the vapor pressure, and then computes the difference between this and saturation vapor pressure, thus approximating crop-to-air VPD when plant temperatures are close to air temperatures.
VPD in excess of approximately 2 kPa leads to high transpiration rates and low water potential in leaves of well-watered plants (EI-Sharkaway et al., 1986). In extreme cases, wilting can occur and photosynthesis can be tremendously reduced. Photosynthesis reduction occurs as the canopy stomatal resistance increases, thereby limiting gas exchange. According to Rylski and Spigelman (1986) both high VPD (>2 kPa) and low VPD « O.2kPa) may lead to indirect heat injury as leaf temperatures increase. Transpiration suppression, by means of low VPD, has a significant impact on the energy balance of the canopy since a major fraction of incoming solar radiation must then be accommodated by sensible heat exchange. For constant air temperature this implies a rise in leaf temperature. Thus a VPD control strategy must balance leaf temperature with transpiration.
According to the Penman-Monteith model, the net solar radiation and VPD air largely drive the latent heat flux from crop to environment. Both effects are significant for partially or fully rooted plants with adequate soil water supply. Because the radiation effect is incorporated in the leaf temperature some researchers prefer to use VPDcmp-air to model
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Other transpiration control systems have been developed. Jolliet et al., (1993) related crop requirements to desired transpiration rates over time (integral). Aikman and Houter (1990) utilized a transpiration : crop growth ratio. Transpiration requirements for crops are a key component of these systems, and are still under active research.
where t ~ 0 °C is crop, air or dew point temperature. For a desired VPD setpoint, VPD seb the control error between actual and desired VPDcrop-air is: E
=VPDcrop-air -
(4)
VPD set
Using Equation (1), we can express the error as:
2. THEORY (5)
2.1 Plant-to-Air VPD Alternatively, using Equation (3) for the error:
Vapor pressure deficit between plant leaves and surrounding air can be expressed as (Gaastra (1959), as cited in Agata (1986)): VPDcrop-air = Pws(tcrop) - (j) Pws(tm)
(6)
(1)
A VPDcrop-air control strategy can be developed which manipulates air temperature andlor dew point to drive E to zero.
where: pws =saturation water vapor pressure, Pa tm = air dry bulb temperature, QC; !crop = crop surface temperature, QC; (j) =air relative humidity, decimal
2.2AirVPD In many practical cases it is undesirable or too costly to monitor leaf or canopy temperature. VPDair has been used by many researchers and greenhouse controllers. Its determination requires simple measurement of air temperature and a measure of the moisture in air (e.g. wet-bulb, dew point or relative humidity). For the case where relative humidity and air temperature are known:
The product Pws(tair) is the partial pressure of water vapor, pw, in air surrounding the plant. Expressed in this way, it is clear that VPDcrop-air determination requires independent measurement of three variables, namely air temperature and relative humidity, and leaf temperature. It is assumed that air in leaf stomatal cavities and intercellular spaces is saturated.
VPDair =(1 - (j») Pws(tw)
While leaf temperature is directly involved in controlling the rate of photosynthesis, relative humidity of air around the leaves indirectly affects plant growth and development by influencing processes such as transpiration, CO 2 uptake and O 2 exchange. The rate of exchange of water vapor, CO 2 and O2 depends on stomatal aperatures, which are affected by several environmental conditions including humidity, light, etc. Boyer and Nonami (1993) report that for constant conditions of radiation, CO 2 , canopy temperature, and stomatal apertures above a certain threshold, transpiration rate was directly proportional to VPD.
An alternative formulation, explicitly showing dew point, is
(8) Substituting VPDair for VPDcrop-air in Equation (4):
(9) 2.3 Energy Balance for VPDair and VPDcrop-oir Control An energy balance for a crop under steady-state conditions can be calculated by:
An alternative formulation for VPDcrop-air consisting of two truly independent variables is desirable for control purposes. Note that air dew point temperature, t.!p, and pw plot as horizontal lines on a psychrometric chart. Thus we may rewrite Equation (1):
o=(Rn -
(10)
H =sensible heat flux, W m-2 ; LE =latent heat flux, W m·2 ; Rn =net radiation at the canopy level, W m·2 ; G = soil heat flux, W m·2 ;
In this relation, determination of leaf temperature and air dew point is necessary. This is conventient if dew point is a controllable quantity. Saturation partial pressure can be obtained using Tetens' equation (Berry et aI., 1945):
=610.78 xl0[7.5 t I (237.3 + t))
G) + H - LE
where:
(2)
Pws(t)
(7)
The components H, Rn and G are positive downwards while LE is positive upwards. Sensible heat flux can be written as: (11)
(3)
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where:
From Eq. (9) it can be seen that VPDair control is relatively straightforward and requires no feedback from the plant. Assuming a value of VPDset is selected, the control strategy simply manipulates drybulb and/or dew point to satisfy Eq's. (9) and (16).
p =density of air, kg m- ; cp. = specific heat of air at constant pressure, J kg cC-I rh = resistance for sensible heat transfer by convection, srn-I 3
3.2 Crop-to-Air VPD Control Latent heat flux can be written (Monteith, 1965):
In a similar fashion to the previous section, a closed form calculation of necessary dew point, as air temperature and VPD set change, is given by Eq. (16) with tcrop substituted in place of tau-:
where:
t.1p = 237.3 log{ [pws(terop) - VPD se J/61 0.78}
y = "psychrometric constant", Pa cC-I re =canopy surface resistance, srn-I or with the Penman-Monteith equation: LE =6 (Rn - G) + p cp. VPDai/rh
(17)
7.5 -log{ [Pws(tcrop) - VPDseJ/61O.78} The control strategy is thus to manipulate air dew point temperature as leaf temperature varies. However, as will be shown, interactions between drybulb and dew point, implicit in the he!lt balance, are important.
(13)
6 + Y(1 + rr!rh) where:
6
=slope of saturated vapor pressure curve, Pa CC-I
4. EXPERIMENTAL PROCEDURES
Substituting Eq's (11) and (12) for VPDcrop-air control or (11) and (13) for VPD air control into the energy balance (Eq_ 10), gives an explicit model for VPD: VPDerop-air =y* [(tau- - tcrop) + (Rn - G)]
4.1 Propagation Chambers and Plant Material Three growth chambers (0.7 x 0.7 m floor area) with an open base for air outlet were used in this experiment. VPD control was accomplished by a controlled environment unit (PGC Inc, Black Mountain NC) which was connected to the chambers. Dew point and air temperature were adjusted to achieve desired VPD levels. Dew point temperature was manipulated by changing water temperature in a spray chamber upstream of a sensible heater used to adjust dry bulb temperature. The bottom and walls of the chambers were made of insulation board (50 mm thick) and rested on scales for evapotranspiration measurements_ The top of the chambers were light alurninum framing and glazed with acrylic sheets. A population of 49 rooted Poinsettia cuttings (Euphorbia pulcherrima Willd., cultivar Freedom) were used to create a small canopy with area of 0.5 m2 approximately. The propagation medium consisted of expanded plastic foam blocks (Oasis® rootcubes ).
(14)
VPD air =(,M y*) (tau- - terop) + y* rh (Rn-G)/ P cp. (15) where:
y* =Y(1 + rr!rh) y* = modified "psychrometric constant", Pa CC-I Eq's (14) and (15) show that, for constant VPD, if the available energy (Rn - G) increases, then the value (tau- - tcrop) must decrease_ 3. TWO CONTROL STRATEGIES Two control schemes are suggested from equation (I), namely control of air temperature and humidity (VPDair) , or control of plant-to-air vapor pressure deficit (vpDerop-air).
4.2 Environmental Measurements A data acquisition board and accessory (Keithley MetraByte, Cleveland OH, models 1601 and EXP-I6) were used to collect all environmental data. Artificial radiation was measured with silicon-cell pyranometers (LI-COR Inc., Lincoln, NE, model LI 200 SA), installed in the center of each propagation chamber, and calibrated in the chamber with an Eppley pyranometer for three sources of artificial radiation (Fluorescent + High Pressure Sodium from o to 150 W m-2 and Fluorescent + High Pressure Sodium + Incandescent from 150 to 300 W m-2). A
3.1 VPDair Control Equation (8) shows how dew point affects VPD air, and Eq. (9) describes how the difference between actual and desired VPD is affected by dew point. At control equilibrium, E=O. Substituting Eq. (3) into Eq. (9), and solving for dew point yields:
t.1 p =237.3log{[pw.(twr) - VPD set]/61O.78}
(16)
7.5 -log{[Pws(tair) - VPDseJ/6IO.78}
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~r----------------~
spatial calibration was performed by measuring the radiation at five locations in each chamber. Air temperature and relative humidity were measured using a temperature and relative humidity probe (Rotronic AG, Basserdorf Switzerland) placed in an aspirated radiation shield. Crop temperature was measured with 150 field-of-view infrared temperature transducer (Everest Interscience, model 4000.4GL, Tucson, AZ). Heat flux into the propagation medium was measured with soil heat flux plates (Radiation and Energy Balance Systems, Inc. , model HFf-3 .1, Seattle W A), placed at a depth of 0.5 cm. Evapotranspiration was measured by scales (Ohaus Corp., model B50S05, Florham Park, NI) placed beneath of the base of the propagation chambers.
.."
~
0..
l5
Q.
~
Artificial Radiation (W m.2)
Fig. 2. Changes in VPDcrop-air as a Function of Artificial Radiation with VPD oir Maintained Approximately Constant at Levels of 1500, 2000 and 2500 Pa. Three Replicates Of Each Treatment Combination Are Shown.
5.1. Energy Balance The linear variation in sensible heat flux as a function of artificial radiation for three levels of VPD air control (1500, 2000 and 2500 Pa) is shown in Fig. 1. Sensible heat flux was obtained as the residual of Eq (10). For higher levels of radiation, the crop is a source of sensible heat (H VPDair •
5.3.
The variation of VPDcrop-air as a function of radiation when VPD air is maintained constant at levels of 1500, 2000 and 2500 Pa is illustrated in Figure 2. At low levels of radiation, the crop is a sink for sensible heat and the crop temperature is lower than the air temperature; consequently, based on Eq's (2) and (8), VPDCTOP-air is less than VPD air. For higher levels of
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Figures 4 and 5 show the canopy, air and dew point temperatures plotted on a psychrometric chart for VPD air control at 1500 Pa and VPDcroP-oir control at 1900 Pa, respectively. It can be observed that to maintain control, the temperature triangle (crop, air and dew point) slides up the chart as radiation increases. Both VPDair and VPDcrop-oir control for o2 radiation levels increasing from 100 to 300 W m are accomplished by increasing the dew point temperature according to Eq's (16) and (17), respectively. When VPDair control was used at 100 W o2 m radiation, the air-dew point temperature difference o2 was 11 °C and decreased to 9.5 °C at 300 W m (Figure 4). For VPDCTOp-oir control this difference was 15 °C and reduced to 9.5 °C (Figure 5).
-----------;=====::::;-] • o
VPO.. s 2000 Pa
...
VPO • ., 1500 Pa
Leaf, Air and Dew Point Temperature Interactions for VPD air and VPDcrop-air Control
Figure 3 shows a simple schematic representation of equipment necessary for VPD control. Dew point can be modified by changing water temperature in a spray chamber. Growth chamber air temperature can be maintained constant by providing sensible heat during low levels of radiation. To control VPD at higher radiation levels, it is necessary to increase dew point temperature and decrease sensible energy supplied by the heater, because more sensible heat via radiation is supplied at the crop level (Eq's 14 and 15).
5.2. VPD air and VPDcrop.air Interactions
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