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PLANT MICROCLIMATE
CHAPTER 3
PLANT MICROCLIMATE M.B. JONES
effect of one cannot be known without specifying the state of the others. The productivity of plants is ultimately dependent upon the influence of the microclimate on plant processes such as photosynthesis, respiration, transpiration and translocation. In order to understand how plant processes respond to the microclimate we need to be able to measure the various components of the microclimate in the natural environment. In recent years a whole range of micrometeorological instruments has been designed for this purpose2,3'4, and in the following sections some of these will be described along with the principles upon which the measurements are based. The use of these instruments, and the analysis and interpretation of data collected with them, requires some understanding of environmental physics. Recommended text books on this subject include those by Monteith5, Campbell6, Woodward and Sheeny7 and Jones 8 . Details of the characteristics of these instruments are listed in an Appendix to this book.
3.1 General introduction Microclimate is the complex of environmental variables, including temperature, radiation, humidity and wind, to which the plant is exposed. It is the climate near the surface of the earth and it is different from the weather forecasters' macroclimate or local climate because of the influence of the earth's surface and, most importantly, the presence of vegetation. Plants are "coupled" to their microclimate because a change in one brings about a change in the other, and this results from an exchange of force, momentum, energy or mass1. Two important types of coupling are (i) radiative coupling, where energy is transferred through electromagnetic vibration; and (ii) diffusive coupling, where heat, water vapour and C 0 2 are exchanged across the boundary layer of the plant. The radiant flux incident on a plant is coupled to the temperature of the plant by its absorptivity. If leaf absorptivity is high then the leaf temperature is tightly coupled to incident radiation, and vice versa. Diffusive coupling across the boundary layer can be viewed as an analogue of an electrical circuit where energy in the form of a charge moves from a high to a low "potential" (measured as a voltage) at a rate (the current) which is inversely proportional to the resistance (Figure 3.1). Radiative and diffusive components of the microclimate are themselves coupled so that, for instance, energy from electromagnetic radiation can be consumed in evaporating water in transpiration. Consequently, radiation, air temperature, wind and humidity all interact simultaneously with the plant, so the
3.2 Radiation - solar and long wave 3.2.1 Introduction The ultimate source of energy for photosynthesis and bioproductivity is solar energy. Plants intercept solar energy for photosynthesis but normally less than 5% is used in this process; the rest of this energy heats the plant and surrounding organisms, so that solar energy also determines the temperature at which 26
PLANT MICROCLIMATE
27
H20
Boundary lay«
Epidermis
Mesophyll
Fig.3.1. Diffusive coupling at the leaf surface, showing resistances to gas and heat exchange at the surface of a single leaf. ra is the boundary layer, rs the stomatal, rc the cuticular and rm the mesophyll or residual resistance. physiological processes are functioning. Apart from photosynthesis, solar radiation also influences the plant's growth and development in what are referred to as photomorphogenic and phototropic responses. These normally require only very small amounts of energy to bring about a response, and different discrete parts of the radiation spectrum are involved. About 98% of the radiation emitted by the sun is in the waveband from 0.3 to 3.0 μΐτι. The energy spectrum of this radiation before it reaches the earth's atmosphere peaks at 0.48 μιτι, which is consistent with a radiator or emitter with a temperature of 6000°K (Fig. 3.2). The flux of radiation (φ) follows the Stefan-Boltzmann Law, being proportional to the fourth power of the absolute temperature of the object: φ = σΤ4 where o is the Stefan-Boltzmann constant (5.6 x 10"8 W m~2 K"4) and T is in Kelvin. The units for
radiant fluxes are the units of power (W m - 2 ) where the term irradiance (I) refers to the energy flux incident on unit surface area. Irradiance is the correct radiometrie term for what is commonly called "light intensity". Strictly speaking, "light" is that part of radiation which is visible to humans, so it is not a very appropriate term to use in plant research. Radiant energy can be described either as waves or as discrete packets of energy called photons. When dealing with photochemical processes such as photosynthesis, the number of photons incident in unit time is more relevant than the energy content of the radiation. This is known as the quantum (or photon) flux density (Q) and is measured in units of mol m - 2 s - 1 where a mole is Avogadro's number (6.022 x 1023) of quanta or photons. When measuring rates of photosynthesis, it is most appropriate to express the radiation incident on the plane of the leaf in terms of photon flux density. (See Appendix D). At the earth's surface solar radiation can be divided into two components based on whether
28
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
the radiation comes directly from the sun (direct) or whether it is scattered or reflected by the atmosphere and clouds (diffuse). Diffuse radiation has a different spectral composition from direct radiation because shorter wavelengths are scattered by air molecules more than long ones, giving the blue colour to clear skies. However, larger particles such as dust and water droplets scatter all wavelengths equally, so the sky appears white when cloud-covered. The amount of diffuse radiation varies with sun angle and cloud cover, but even on clear days it contributes 10 - 30% of total solar irradiance. The component of solar radiation used in photosynthesis falls between 400 and 700 nm and is referred to as
photosynthetically active radiation (PAR). Light quanta (photons) within this waveband are almost equally effective in driving the light reactions of photosynthesis9. The proportion of PAR in total (direct + diffuse) radiation is about 50%; this varies little diurnally or seasonally. Plants and any other surface on the earth also emit radiation due to the heating of the sun. According to Wien's displacement law the wavelength at which the maximum amount of radiation is emitted (Am) decreases as the temperature of the body increases: Am —
2897
2000 1000
0.2
0.5
.Shortwave solar
1.0
2.0
5.0
I
20 30 100 Wavelength ( ^ m ) S e n s o r ;
4
Near infrared PAR
10
Solarimeter .
long-wave terrestial
Filtered solarimeter
1
Total radiation1
-·
Silicon celli(quantum sensor) Net radiometer
Fig.3.2. The spectral energy distribution of (i) the solar flux outside the Earth's atmosphere, (ii) the solar flux at ground level after attenuation by gases in the atmosphere, and (iii) the flux of radiation emitted by the Earth's surface (terrestrial flux). Depicted at the bottom of the figure are the typical ranges of instruments used to measure components of the solar and terrestrial fluxes (adapted from Grace10).
PLANT MICROCLIMATE
29
/////////7777777777//>/?/^
Fig.3.3. An illustration of the short wave (φ5) and long wave (ψ,) radiant energy fluxes between a leaf and its surroundings. where λ is in micrometres and T is in Kelvin. Consequently, bodies on the earth's surface emit long wave radiation with a peak at approximately 9.7 μπι (Figure 3.2). There are therefore continuous fluxes of radiation from the sun during the day and between the atmosphere, plants and their surroundings at all times (Figure 3.3). Radiation incident on a leaf or plant canopy can be absorbed, transmitted or reflected. In the PAR region of the spectrum the leaf absorbs 90% of the incident radiation, whilst in the short-wave infra-red region (0.7 - 3.0 μπι) it transmits most of the radiation. The effect of this is to reduce the heat load from wavelengths which are not used in photosynthesis. However, in the far infra-red, leaves are good absorbers; thus (because good absorbers are also good emitters of radiation) they are able to dissipate excess heat very efficiently in the long-wave region of the spectrum. 3.2.2 Radiation measurements Most instruments used for measuring solar and long wave radiation consist of different forms of thermopile arrangement. A thermopile consists of
a series of alternate junctions between two dissimilar metals, e.g. copper and constantan (see Figure 3.4a). When a temperature difference exists between two sets of thermocouples a voltage is generated which is proportional to the temperature difference. When measuring solar radiation the temperature difference is created by embedding one set of junctions in a metal clamp protected from incident radiation and the other in a surface exposed to radiation, or by painting the hot and cold junctions black and white respectively and subjecting them both to the same radiant energy flux. The surface of the sensor is normally protected from wind and rain by glass domes whose transmittance restricts spectral sensitivity to the 0.3 to 3.0 μπι region. An example is the Kipp solarimeter (or pyranometer) using a Moll thermopile, which is the standard instrument in many countries for measuring total (direct + diffuse) and diffuse (using a shade ring) solar radiation. Details of other solarimeters can be found in Monteith 3 , Szeich2 and Fritschen and Gay4. When solarimeters are used with special filters (e.g. Kodak "Wratten" 88), they exclude visible wavelengths, so the energy in the region 0.3-0.75 μηι can be determined by difference.
30
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Long wave radiation is usually measured using net radiometers which measure the difference between the total incoming and outgoing radiation fluxes at all wavelengths (net)· When net is measured above a canopy, its value is the net radiation absorbed by the canopy. However, the net radiation absorbed by a layer of leaves in the canopy is the difference between net above and below this layer. The main component of a net radiometer is a flat black plate: the temperature difference between the top and bottom surfaces, measured with a thermopile, is proportional to net irradiance. The two sensing surfaces are protected from the wind either by continuous ventilation or by inflatable domes of polythene (which is transparent at all wavelengths). For measurements within plant canopies, where radiation distribution is uneven, averages can be
obtained by moving a small sensor repeatedly along a track or by using a long linear sensor (tube solarimeters and radiometers). These linear sensors are less accurate than flat plate sensçrs because of greater cosine errors (see below), and should be used for relative rather than absolute measurements. Errors can be minimised by taking measurements in two directions at right angles. In addition to the fluxes of energy through the atmosphere, there is also a vertical transfer of energy through the soil which is known as the soilheat-flux. During daylight hours, the soil normally acts as a heat sink, so the soil-heat-flux is positive; at night, it becomes negative and of similar absolute magnitude to daylight values. It may range from 2% of <)>net for dense canopies to more than 30% of <)>net in open canopies. The vertical transfer of heat by conduction through the
Table 3.1. Some Radiation Terms Term
Symbol Meaning
Units
Radiation or Radiant energy
—
Energy transferred through space in the form of electromagnetic waves or quanta
joule (J)
Radiant Flux
—
The amount of radiant energy received, emitted or transmitted per unit time
J s 1 or watts (W)
Radiant flux density
φ
The radiant flux through unit area of a plane surface
Irradiance
I
The energy flux incident on unit area of a plane surface Wm~2
Wm~2
Photon
—
A quantum of light A mole (mol) is 6.022 x IO23 quanta or photons
Quantum flux density
Q
The number of quanta incident on unit area of a plane surface
mol m 2 s '
—
Photosynthetically Active Radiation
PAR
Radiation within the band 400-700 nm
mol m -2 s"1 or Wm"2
Short wave radiation
Radiation in the waveband 0.4 to 3.0 μπι
Wnr 2
Long wave radiation
«h
Net radiation
<)>net
Radiation wavelengths of 3.0 μηι or greater The difference between the downward and upward fluxes of total radiation
Wirr2 WirT2
PLANT MICROCLIMATE
soil is measured with a sensor which is basically similar to a net radiometer4. In practice the plate consists of glass or resin, but errors can be greater than 40% because of poor conductivity matching4.
Hot junctions -
S
radiation
s s
31
by absorbing light quanta which causes a release of electrons and the generation of an electric current (Figure 3.4b). These semiconductors now reasonable accuracy for a relatively low cost but
S
s
radiation
S
S
S
.Sensitive surface
Cold junctions
(a)
(b)
Fig.3.4. Radiation detectors: (a) a thermopile, consisting of a number of thermocouples in series; (b) a silicon semiconductor. V = voltage. I = current.
Another type of radiation sensor which has become increasingly popular in the last decade is the silicon cell which consists of a small chip of silicon that is sensitive to radiation. This functions
100
Actual response—^ /v
-Ideal response
ω50 ω ce
0.3
0.4
0.5 0.6 Wavelength (/am)
0.7
Fig.3.5. The actual and ideal responses of a PAR quantum sensor.
0.8
but unlike unfiltered radiometers, which have a uniform response to all wavelengths, these typically have a peak sensitivity at about 0.85 0.95 μιη with a band width of 0.5 μιη at 50% response. However, their advantage is that they can be constructed using suitable filters so that they measure PAR and give a good approximation to the ' i d e a r ' quantum response (Figure 3.5). The shape of the response is "ideal" because short wavelengths contain more energy and therefore generate more current: the broken line in Figure 3.5 shows the spectral response of the sensor adjusted for tht difterent energy levels at the blue and red ends of the spectrum. Quantum sensors can also be arranged in a linear fashion in a similar way to the tube solarimeters so that they measure Q within canopies. In order for radiation sensors to give accurate readings at all sun angles they should ideally obey Lambert's Cosine Law. According to this law, when radiation is incident at an angle a to the normal the irradiance (I) should be expressed as: I = L cos a where I0 is the irradiance on a surface normal to the sun's rays (Figure 3.6). Different techniques
32
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Zenith
and
<(>n
Tabs
TIrad
Energy storage by the plant, both physical and in the form of chemical bonds associated with photosynthesis and respiration, is generally small in quantitative terms and is generally ignored. The temperature that a leaf will reach in a specified environment can be calculated using an iterative computing procedure10 and the following equation, which describes AE and C in terms of an electrical circuit (Figure 3.1):
IIIIIIIIIÌÌJIÌJÌIÌÌÌÌIÌÌÌIIIIÌÌÌIÌIMÌÌJ:^.:;^^;!!!!;^!
Earth's surface
Fig.3.6. The cosine law, showing the relationship between the angle of the sun's rays (a), the slar zenith angle and the solar irradiance on the Earth's surface (I), where I0is the irradiance on a surface normal to the sun's rays.
are used to achieve good cosine response, but a common method is to use a raised white perspex diffuser on top of the sensor. Most radiation sensors give a millivolt output and they can be connected directly to recorders, data loggers or integrators. If connected to integrators they can give a total of received radiation during a given period of time. This is useful in determining the total radiation interception by a canopy for energy conversion calculations. Daily integrals of irradiance or Q give energy or quantum flux density over a period of time, i.e. J m - 2 d _1 or mol m - 2 d _1 .
3.3 Temperature 3.3.1 Introduction The temperature of the aerial parts of plants is determined by the balance between energy gain by interception of radiation (<|>abs) and the energy losses by re-radiation (rad), convection or ''sensible'' heat loss (C) and transpiration ( AE, where λ is the latent heat of vaporisation), so that tabs
=
<>
t rad
+
AE
+
C
taet =
QCp(es,Ti -
e)
y(rs + ra)H2o)
+
QCpCT) -
Ta)
ra>H
where ρ is the density and cp the specific heat of air, esTi is the saturated vapour pressure of air at the leaf temperature (T^, e is the water vapour pressure of free air, y is the psychrometric constant (66 Pa °C _ 1 ) , and rs, ra>H2o and rai H are , stomatal and boundary layer resistances (reciprocals of conductance). However, a more convenient expression can be derived from this equation; this calculates leaf-air temperature difference from the sum of terms that depend on net radiation and the vapour pressure of air8. Plant temperature is therefore determined by the large number of factors which influence the magnitude of <|>rad, E and C. Units of temperature are degrees Celsius (°C) or Kelvin (K = 273 + °C) and it is generally held that a sensitivity of ± 1 °C is sufficient for analysis of plant growth and development while a sensitivity of ± 0.1 °C is required for calculations of transpiration or heat transfer determinations11. Physiological processes such as seed germination, photosynthesis, respiration and leaf growth all respond to temperature but it is important to be able to measure the temperature which is most relevant to the process being studied. For example, when measuring leaf expansion in grasses the temperature in the meristematic region at the base of the leaf is the most relevant measurement; this may be closer to soil temperature than air temperature because of the location of the meristems in vegetative grasses12. The problem is made more difficult by the fact that plant temperatures can often be several degrees different from air temperature and
PLA«NT MICROCLIMATE
there is also a spatial variation in temperature, often as a result of solar radiation interception at the top of the canopy. In recent years there has been renewed interest in the concept of degree-days or thermal time in controlling plant growth and development. Plant development is assumed to show a linear response to temperature from a threshold (Tb) to an optimum (T0), and the time taken to reach a given phenological stage is related to thermal time, defined as the integral of temperature with time. Units of thermal time (t) are degree-days, calculated as the sum of the differences between daily mean temperature ( T ) and the base temperature for each day beyond a given starting date: n
t = Σ ( f - T„) for T > Tb 0
In many natural environments, it is difficult to uncouple developmental response to temperature from other factors such as irradiance and saturation deficit. This problem can be overcome by calculating a thermal rate (p): p = ζ / ( Τ - Tb) where ζ is the rate of response (e.g. leaf extension in millimetres). The thermal rate is expressed in units of response per unit thermal time (e.g. mm (°C.h) _1 ); it is now possible to correlate this with other environmental variables, although application of this technique to field measurement needs some caution13. It is now possible to carry out temperature integrations to determine degreedays using commercially available transducers and millivolt integrators (Delta-T Devices, Cambridge). 3.3.2 Temperature measurements Temperature is measured by transducers which are based upon temperature effects on expansion, electrical or radiative responses. The two most important sources of error in temperature measurement are the effects of incoming radiation and the effect of the thermal mass of the sensor. Both these effects are more important in air than in water or when measurements are made within the plant tissue.
33
Liquid (normally mercury)-in-glass thermo meters are the most common instruments used for measuring temperature. They are widely used as accurate devices in meteorological stations, but they have no facility for recording. However the less accurate bimetallic strips used in thermographs do register on a dial or strip chart through a series of levers. Many temperature sensors depend on the fact that a change in temperature can alter the electrical properties of certain materials. These electrical temperature transducers are either thermocouples which generate a flow of electrons between two junctions of dissimilar metals if their temperatures are different, or resistance thermometers and thermistors where resistance changes with temperature. Thermocouples are widely used for temperature measurements in biology because they are small, easy to construct and cheap. A number of types of thermocouple can be purchased or made from combinations of different metals. They have different electrical and physical properties which influence their sensitivity and suitability for different uses. Characteristics of the more common thermocouples are shown in Table 3.2. When two thermocouple junctions are joined the voltage (V) generated is proportional to the difference in temperature between the measuring junction (sensor) and a reference junction: V = k(T - T0) where T is the sensor temperature, T0 the reference temperature and k the temperature coefficient (the change in e.m.f. per unit change in temperature at the reference temperature). It is common practice to assume a linear relationship between thermocouple e.m.f. and temperature, but for more accurate work the relationship is more precisely described by a quadratic regression equation 7 . Normally thermocouples are used with the reference junction maintained at a constant temperature; this is most conveniently an icewater mixture contained in a Dewar (vacuum) flask, which has a temperature of 0°C. Calculated values of e.m.f. for a range of temperatures with the reference junction at 0°C are given in Table 3.2. Alternatively, soil temperature at a depth of
34
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Table 3.2. Typical electrical properties and characteristics of thermocouples. (Adapted from Woodward and Sheeny7). <)>mni is the smallest practicable thermocouple diameter. Thermocouple
Type
Uniformitymin(mm)
Temperature °C 0°
10°
20°
30°
40°
50°
Copper-constantan
T
Low
0.2
0
0.39*
0.79
1.19
1.61
2.03
Chromel-alumel
K
Low
0.1
0
0.40
0.80
1.20
1.61
2.02
Chromel-constantan
E
Medium
0.05
0
0.61
1.23
1.85
2.48
3.08
Iron-constantan
J
Low
0.05
0
0.52
1.05
1.58
2.12
2.66
Plantinum-platinum/10% rhodium
S
High
0.025
0
0.06
0.11
0.17
0.24
0.30
one metre is quite stable; this can be used as reference if its temperature is measured with a thermometer. More convenient than either of these references are the electronic references now available on many meters; when used in this way only one thermocouple is required to measure temperature. Thermocouples can be easily constructed, taking care to ensure a good junction between the two metals. Tin or silver can be used to solder the junctions, but silver gives the smallest junctions using borax as a flux. After soldering, the junctions can be cut with a blade under a binocular microscope to make them as small as possible. Ideally, all thermocouples should be individually calibrated because of small variations in characteristics of the wires and junctions. Wire resistance thermometers are most often constructed from platinum, nickel or copper. Usually the commercially available platinum resistance thermometers are quite bulky, being typically 20 mm long and 3 mm in diameter. They are therefore only useful for measuring temperatures of large volumes but they are often favoured for long term use because of high stability, resistance to weathering and an almost linear change in resistance with temperature. However, the change in resistance with temperature is relatively small, so a circuit to read voltage output must be designed with care to avoid large error resistances2,4. Thermistors are semiconductors, composed of sintered mixtures of metallic oxides. The resistance of thermistors decreases exponentially
with temperature but with about ten times the sensitivity of resistance thermometers. They are available in a range of sizes down to miniature bead types of 0.2 mm diameter, and the circuitry required to give a readout is relatively simple and robust. The only non-contact method of measuring temperature is by using the infra-red thermometer, which is based upon the principle that all surfaces emit energy11. The flux of radiation follows the Stefan-Boltzmann law and is proportional to the fourth power of the absolute temperature of the object (Section 3.2.1). As the temperature of vegetation is about 290 K it emits long wave radiation with a peak of emission at about 10 μιτι. Infra-red thermometers are typically fitted with filters which allow only radiation in the range 8 - 13 μιη to pass to the detector. They are expensive, difficult to calibrate and prone to errors if reflected long wave radiation is detected. When used correctly, errors are between 0.1 and 0.5°C. They are intrinsically preferable to contact methods because the latter can alter surface temperature during measurement by simultaneous conduction between the thermometer, surface and air, possibly resulting in large errors. However, infra-red thermometers cannot be used for measuring "sky" temperature. 3.3.3 Use of thermometers Before use, thermometers should be calibrated over the expected range of temperature. The simplest method is to immerse the sensors in a
PLANT MICROCLIMATE
water bath whose temperature is controlled and compare temperatures with an accurate mercuryin-glass thermometer. The infra-red thermometer cannot be calibrated in this way, but it can be set up to receive radiation from the inside of a blackened sphere immersed in a water bath whose temperature is known. Temperatures measured are usually of air, surface, soil and tissue. The latter two are less prone to difficulties because the thermometer is immersed in the material it is sensing. For air temperatures to be measured accurately the absorption of solar and long wave radiation should be prevented by use of a radiation shield and possibly also ventilation. The ideal shield should have a high reflectivity for solar radiaton and a high emissivity for long wave radiation. Aluminised "Mylar" and clear matt white paint have been found to be the most suitable shield coverings. Surface temperature measurements are the most difficult to make accurately because they depend on a good thermal contact between the sensor and surface being measured. Clips, springs or tapes are often used to make contact but their presence can lead to errors. Further details on the use of thermocouples and an analysis of the errors which might be experienced can be found in Perder 11 .
3.4 Humidity 3.4.1 Introduction The water content of air is known as the absolute humidity (χ) and is the density of water vapour in the air in g m 3. The importance of humidity to a plant's functioning is twofold. Firstly, it determines the rate of water lost in transpiration (E) because: E
=
g (Xair -
Xleaf)
where g is the conductance for water vapour transfer between the evaporating surfaces within the leaf and the air. Secondly, humidity has a direct effect on the stornata of many plants, so that stornata tend to close in dry air restricting water loss but also reducing C 0 2 assimilation.
35
3.4.2 Definitions Because water vapour is a gas, its pressure contributes to the total measured atmospheric pressure and its potential pressure is called vapour pressure (e). When air above water has no extra capacity for holding water vapour the partial pressure of the water vapour is the saturated vapour pressure (es) measured in kPa and its density the saturation density (g m~ 3 ). The saturation vapour pressure increases with temperature (Figure 3.7). If air is cooled without change in water content, condensation occurs at its dewpoint temperature (Td), when e = es. When water evaporates into less than saturated air then the temperature of the air decreases up to a point. This is the wetbulb temperature (Τ'), the temperature to which the wet bulb falls in a psychrometer (Section 3.4.3). Its value is given by the intercept of a line of slope - y (where y is the psychrometric constant), passing through the vapour pressure of the air at the dry bulb temperature, with the curve of saturated vapour pressure against temperature (Figure 3.7). The slope of the line differs according to whether the wet bulb is ventilated or not. Relative humidity is the ratio of the actual vapour pressure (e) to the saturated vapour pressure (es) at the dry bulb temperature (T). It is usually expressed as a percentage. However the use of this term should be discouraged as plants do not respond directly to relative humidity. Saturation deficit or vapour pressure deficit (òe) is the difference between the saturation vapour pressure and the actual vapour pressure at the same temperature (Figure 3.7). It is an index of the drying power of the air; the higher the deficit the greater the evaporation rate. 3.4.3 Measurements Many different devices can be used to measure the humidity of the air. They are based on several principles including the electrical properties of sulphonated polystyrene or thin-film solid state semiconductors; wet-bulb depression; condensation of water vapour on a surface cooled to the dew-point; and infra-red absorption. Some
36
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
of the instruments more widely used in the field are considered here. A psychrometer is a pair of identically shaped thermometers, one of which is covered with a wet sleeve. Evaporation cools the wetted sensor to the wet-bulb temperature, and the vapour pressure (e) is calculated as: e = es
-y(T
-
Τ')
where T ' and T are the wet- and dry-bulb temperatures respectively, e s / r is the saturated vapour pressure at the wet-bulb temperature, and
y is the psychometric constant (equal to 66 Pa ° C - 1 at sea level in a ventilated psychrometer). Several types of psychrometers are available as commercial units, the best of which ensure efficient radiation shielding of the thermometers and minimise heat conduction along the stem of the thermometer 2 . The Assman psychrometer is a ventilated psychrometer containing matched thermometers; it is used for standard humidity measurements. Smaller ventilated psychrometers are now available for use above and within plant canopies (DeltaT Devices, Cambridge). The hand held whirling or sling psychrometers are the
3.0r
130 Temperature (°C)
Fig.3.7. The influence of temperature on the saturated water vapour pressure of water. The point X represents air at 18°C and 1.0 KPa vapour pressure (e). The line Y-X-Z, with a slope of - y , gives the wet-bulb temperature (Τ') where it intercepts the curve at Y (12°C). The water vapour pressure deficit (de) is the difference between X and W (the saturated vapour pressure at 18°C). The dew-point (Td) is the point at which the saturated vapour pressure is equal to X.
PLANT MICROCLIMATE
simplest and cheapest ventilated units. In order to achieve an aspiration rate of 3 m s"1 they have to be rotated at about two revolutions per second. Many materials show a change of physical dimensions when they absorb water, and this property can be used to make instruments that measure humidity. For example, the length of animal hair increases as the air becomes wetter and decreases as the air dries; this property is used in simple hygrometers. Provided an allowance is made for the effect of temperature, hair hygrometers are usually accurate to within 5% over most of the humidity range. The change in electrical properties of materials as they absorb water is used in several humidity sensors. Until recently the lithium chloride sensor was the most common type of electrical sensor. Lithium chloride is hygroscopic and the moisture content of the air determines how much water is absorbed, which in turn influences the AC resistance of the sensor. This type of sensor is susceptible to contamination by dust and other hygroscopic particles, and it suffers from a certain amount of hysteresis when wetting or drying. More recently, capacitance hygrometers, which measure the change in electrical capacitance caused by water-absorption into a dielectric, have become commercially available (Humicap, manufactured by Vaisala, Helsinki, Finland) and are less temperature sensitive and show less hysteresis than other electric sensors. Dewpoint meters measure the temperature at which dew forms on a cooled surface. Dewpoint is usually determined by cooling a surface to below the point of saturation, allowing water to condense onto it, and then gradually raising the temperature until the film of condensation starts to evaporate. The temperature at which this change occurs is taken as the dewpoint temperature, and the presence of the film can be detected optically or electrically. Dewpoint temperatures must be corrected for changes in atmospheric pressure if they are converted into vapour pressure. Infra red gas analysis can measure water vapour concentration of air as well as C 0 2 (Chapter 6). The instruments are expensive but they are accurate and respond quickly. With suitable switching systems this type of instrument is almost
37
always employed to measure concentration differences, making it very suitable for profile studies. 3.5 Wind Wind is the large-scale transport of air masses resulting from differences in air pressure. It is directly involved in heat and mass transfer by forced convection, so it is very important in influencing heat and gas exchange across the boundary layers of plants. Increase in wind speed decreases the boundary layer resistance over leaves (see Figure 3.1); this tends to increase evaporation and bring leaf temperature closer to air temperature. Wind is also important because it causes mechanical deformation of plants (due to the frictional drag of moving air) and because it disperses pollen, seeds and aerial pollutants. However, of all the elements of the microclimate, wind is the most spasmodic. Short term variations in wind speed are described by the intensity of turbulence; this value represents the standard deviation of the instantaneous values, divided by the mean wind speed14. Turbulent air moves in packets or eddies; these are important for the movement of C0 2 , H 2 0 and other gases in and above plant canopies. The meteorologists' measurements of wind speed are normally made 10 m above the ground, but wind speed decreases rapidly as the plant surface or ground is approached. Wind speeds near or within vegetation can therefore be much lower than at 10 m. The analysis of the profile of mean wind speed above the canopy can be used to derive coefficients for calculating the flux of C 0 2 and H 2 0 between the canopy and the atmosphere5,7. However, the principles of environmental physics required to make these calculations are beyond the scope of this discussion. Furthermore, the number of environmental sensors and the size of data recording and processing facilities required are beyond the budget of most research groups. 3.5.1 Measurement A complete picture of air movements requires continuous recording of instantaneous wind speed measured in three directions; the vertical, the
38
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
horizontal lateral and the horizontal perpendicular. However, these are difficult to measure, so it is generally sufficient to determine the mean value in one direction over the measurement period. The most commonly used instrument is the cup anemometer. This normally consists of three hemispherical or conical cups mounted on arms and attached to a central vertical spindle, so that they are free to rotate in the wind. The number of rotations of the cup assembly is usually measured in metres (the "run of the wind"), and can be divided by the elapsed time to give the mean wind speed. An alternative mechanical anemometer is the vane or propellor type. The vane anemometer is simply a miniature windmill, consisting of a number of light vanes radially mounted on a horizontal spindle. The main sources of error with mechanical anemometers are firstly, that they have a threshold below which the friction of the system prevents rotation, and secondly, that their inertia makes them over-run when the wind speed drops. Additionally, the vane anemometer is directional; it must be aligned to the wind direction. A third type of instrument is the hot-wire anemometer, which estimates wind speed by measuring the rate of cooling of a heated wire in moving air. These can be made very small, and are useful for measuring the rate of air flow around leaves14, but they are often delicate and easily broken. For this reason they are not used routinely for field measurements. Further details of instruments for wind measurement can be found in Grace14.
3.6 Automatic weather stations In many studies, a rather broad description of the climate experienced by the plants is sufficient to begin untangling plant/climate relationships. For these purposes, information obtained from standard meteorological sites located close to the vegetation under investigation is useful. However, in recent years, automatic weather stations have been increasingly used. These stations can be set up on experimental sites to provide detailed
measurements of the weather including solar radiation, net radiation, wind run, wind direction, air temperature, wet-bulb temperature and rainfall. The output from these instruments is recorded on standard cassette tapes, used in conjunction with battery operated data loggers. Modern data loggers often incorporate many channels, enabling the recording of additional data such as soil temperature.
3.7 Recording The simplest method of recording the output from micrometeorological instruments in the field is using a pencil and note-pad; and in many cases this is all that is necessary. There are, of course, many advantages in adopting automatic recording but the methods used must be considered in relation to the use to which the measurements will be put. For example, detailed measurements of vertical profiles of temperature, wind speed, humidity, radiation and C 0 2 concentration can be used to estimate canopy évapotranspiration and C 0 2 exchange, but they require complex recording facilities; these techniques are normally only possible when resources of equipment and manpower are extremely good. However, less intensive measurements using limited recording facilities can still tell us a lot about the relationship between the plant and its environment. Micrometeorological measurements are not an end in themselves and usually we need to relate them to plant physiological responses such as photosynthesis, stomatal movement, water potential and leaf expansion. These responses have different time scales (e.g. photosynthesis and leaf expansion) and measurements should be recorded accordingly. The simplest automatic method of accumulating output from instruments is using analogue recorders. The most suitable of these are galvanometer recorders which can be multi channel, and either use pen and ink as tracer or record on pressure-sensitive paper using a chopper bar. Analogue integrators can be used where detailed chart records are not necessary as they integrate small currents and voltages and can be
PLANT MICROCLIMATE
39
used to determine characteristics such as degreedays and daily solar radiation integrals. Digital data logging is perhaps the most convenient way of collecting micrometeorological data, especially where a large number of measurements are involved which can subsequently be handled by a computer. Here the analogue input from the instruments is converted into a number (by an analogue-digital converter) and recorded on a magnetic tape. Usually a large number of inputs can be scanned in sequence to give discontinuous but frequent records of a large number of measurements. For further discussion of this topic see Woodward and Sheehy7.
the temperature of the undersurface of a leaf and of the air below the leaf. Repeat the measurements at a number of heights between the top of the canopy and the soil and also measure soil temperature by carefully pushing the thermocouple into the soil to a depth of 1 cm. (d) Wet- and Dry-bulb temperature - using the Assman or Delta-T ventilated psychrometers to make measurements at a number of heights in the plant canopy. Calculate the saturation deficit of the air (kPa) from the wet- and dry-bulb temperatures using the tables or slide-rule provided.
3.8 Experimental work
References
The objective of this experiment is to measure the vertical profiles of different environmental factors in canopies of two crops of contrasting structure (e.g. maize and bean). These are carried out in conjunction with measurements of stomatal conductance and leaf water potential in order to determine which factors control stomatal activity. Measure the following parameters at five different positions in the crop. (a) Photosynthetically Active Radiation (PAR) using the Lambda linear quantum sensor. These measurements should be made at right angles to the rows. If the incident radiation is constant, measure above the crop and then at progressively lesser heights down to the soil surface. However, if the incident radiation is changing, measure first above the canopy and then at a lower height, then above the canopy again before measuring the next lowest position. Express the value of quantum flux at a particular height as a percentage of the incident flux. (b) Short- Wave and Visible Radiation - using two tube solarimeters, one of which is fitted with a filter to eliminate the visible wavelengths (400-700 nm) to measure the non-visible component of short-wave radiation. The difference between the two gives the value for visible radiation. Express the value at a given height as a percentage of the incident radiation. (c) Leaf and air temperature - using a WESCOR thermocouple thermometer to measure
1. Monteith, J.L. (1981) Coupling of plants to the atmosphere. In: Plants and their Atmospheric Environment, 21st Symposium of the British Ecological Society (Grace, J., Ford, E.D. and Jarvis, P.G. eds.) pp. 1-29. Blackwell Scientific Publications, Oxford. 2. Szeich, G. (1975) Instruments and their Exposure. In: Vegetation and the Atmosphere, Vol. 1: Principles (J.L. Monteith ed.) pp. 229-273. Academic Press, London. 3. Monteith, J.L. (1972) Survey of Instruments for Micrometeorology. IBP Handbook No. 22. Blackwell Scientific Publications, Oxford. 4. Fritschen, L.J. and L.W. Gay, (1979) Environmental Instrumentation. Springer-Verlag, New York. 5. Monteith, J.L. (1973) Principles of Environmental Physics. Edward Arnold, London. 6. Campbell, G.S. (1977) An Introduction to Environmental Biophysics. Springer-Verlag, New York. 7. Woodward, F.I. and J.E. Sheehy, (1983) Principles and Measurements in Environmental Biology. Butterworths, London. 8. Jones, H.G. (1983) Plants and Microclimate. Cambridge University Press. 9. McCree, K.J. (1976) A rational approach to light measurements in plant ecology. In: Commentaries in Plant Science (H. Smith ed.). Pergamon Press, Oxford. 10. Grace, J. (1983) Plant-Atmosphere Relationships. Outline Studies in Ecology. Chapman and Hall, London. 11. Perrier, A. (1971) Leaf temperature measurement. In: Plant Photosynthetic Production, a Manual of Methods, (ed. Z. Sestâk, J. Catsky and P.G. Jarvis) pp. 632-671. Dr. W. Junk, The Hague.
40
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
12. Peacock, J.M. (1975) Temperature and leaf growth in Lolium perenne. II. The site of temperature perception. J. Appi. Ecol. 12, 115-123. 13. Ong, C.K. (1983) Response to temperature in a
stand of pearl millet (Pennisetum typhoïdes) 1. Vegetative development. J. Exp. Bot. 34, 322-336. 14. Grace, J. (1977) Plant Response to Wind. Academic Press, London.
PLANT MICROCLIMATE M.B. JONES
effect of one cannot be known without specifying the state of the others. The productivity of plants is ultimately dependent upon the influence of the microclimate on plant processes such as photosynthesis, respiration, transpiration and translocation. In order to understand how plant processes respond to the microclimate we need to be able to measure the various components of the microclimate in the natural environment. In recent years a whole range of micrometeorological instruments has been designed for this purpose2,3'4, and in the following sections some of these will be described along with the principles upon which the measurements are based. The use of these instruments, and the analysis and interpretation of data collected with them, requires some understanding of environmental physics. Recommended text books on this subject include those by Monteith5, Campbell6, Woodward and Sheeny7 and Jones 8 . Details of the characteristics of these instruments are listed in an Appendix to this book.
3.1 General introduction Microclimate is the complex of environmental variables, including temperature, radiation, humidity and wind, to which the plant is exposed. It is the climate near the surface of the earth and it is different from the weather forecasters' macroclimate or local climate because of the influence of the earth's surface and, most importantly, the presence of vegetation. Plants are "coupled" to their microclimate because a change in one brings about a change in the other, and this results from an exchange of force, momentum, energy or mass1. Two important types of coupling are (i) radiative coupling, where energy is transferred through electromagnetic vibration; and (ii) diffusive coupling, where heat, water vapour and C 0 2 are exchanged across the boundary layer of the plant. The radiant flux incident on a plant is coupled to the temperature of the plant by its absorptivity. If leaf absorptivity is high then the leaf temperature is tightly coupled to incident radiation, and vice versa. Diffusive coupling across the boundary layer can be viewed as an analogue of an electrical circuit where energy in the form of a charge moves from a high to a low "potential" (measured as a voltage) at a rate (the current) which is inversely proportional to the resistance (Figure 3.1). Radiative and diffusive components of the microclimate are themselves coupled so that, for instance, energy from electromagnetic radiation can be consumed in evaporating water in transpiration. Consequently, radiation, air temperature, wind and humidity all interact simultaneously with the plant, so the
3.2 Radiation - solar and long wave 3.2.1 Introduction The ultimate source of energy for photosynthesis and bioproductivity is solar energy. Plants intercept solar energy for photosynthesis but normally less than 5% is used in this process; the rest of this energy heats the plant and surrounding organisms, so that solar energy also determines the temperature at which 26
PLANT MICROCLIMATE
27
H20
Boundary lay«
Epidermis
Mesophyll
Fig.3.1. Diffusive coupling at the leaf surface, showing resistances to gas and heat exchange at the surface of a single leaf. ra is the boundary layer, rs the stomatal, rc the cuticular and rm the mesophyll or residual resistance. physiological processes are functioning. Apart from photosynthesis, solar radiation also influences the plant's growth and development in what are referred to as photomorphogenic and phototropic responses. These normally require only very small amounts of energy to bring about a response, and different discrete parts of the radiation spectrum are involved. About 98% of the radiation emitted by the sun is in the waveband from 0.3 to 3.0 μΐτι. The energy spectrum of this radiation before it reaches the earth's atmosphere peaks at 0.48 μιτι, which is consistent with a radiator or emitter with a temperature of 6000°K (Fig. 3.2). The flux of radiation (φ) follows the Stefan-Boltzmann Law, being proportional to the fourth power of the absolute temperature of the object: φ = σΤ4 where o is the Stefan-Boltzmann constant (5.6 x 10"8 W m~2 K"4) and T is in Kelvin. The units for
radiant fluxes are the units of power (W m - 2 ) where the term irradiance (I) refers to the energy flux incident on unit surface area. Irradiance is the correct radiometrie term for what is commonly called "light intensity". Strictly speaking, "light" is that part of radiation which is visible to humans, so it is not a very appropriate term to use in plant research. Radiant energy can be described either as waves or as discrete packets of energy called photons. When dealing with photochemical processes such as photosynthesis, the number of photons incident in unit time is more relevant than the energy content of the radiation. This is known as the quantum (or photon) flux density (Q) and is measured in units of mol m - 2 s - 1 where a mole is Avogadro's number (6.022 x 1023) of quanta or photons. When measuring rates of photosynthesis, it is most appropriate to express the radiation incident on the plane of the leaf in terms of photon flux density. (See Appendix D). At the earth's surface solar radiation can be divided into two components based on whether
28
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
the radiation comes directly from the sun (direct) or whether it is scattered or reflected by the atmosphere and clouds (diffuse). Diffuse radiation has a different spectral composition from direct radiation because shorter wavelengths are scattered by air molecules more than long ones, giving the blue colour to clear skies. However, larger particles such as dust and water droplets scatter all wavelengths equally, so the sky appears white when cloud-covered. The amount of diffuse radiation varies with sun angle and cloud cover, but even on clear days it contributes 10 - 30% of total solar irradiance. The component of solar radiation used in photosynthesis falls between 400 and 700 nm and is referred to as
photosynthetically active radiation (PAR). Light quanta (photons) within this waveband are almost equally effective in driving the light reactions of photosynthesis9. The proportion of PAR in total (direct + diffuse) radiation is about 50%; this varies little diurnally or seasonally. Plants and any other surface on the earth also emit radiation due to the heating of the sun. According to Wien's displacement law the wavelength at which the maximum amount of radiation is emitted (Am) decreases as the temperature of the body increases: Am —
2897
2000 1000
0.2
0.5
.Shortwave solar
1.0
2.0
5.0
I
20 30 100 Wavelength ( ^ m ) S e n s o r ;
4
Near infrared PAR
10
Solarimeter .
long-wave terrestial
Filtered solarimeter
1
Total radiation1
-·
Silicon celli(quantum sensor) Net radiometer
Fig.3.2. The spectral energy distribution of (i) the solar flux outside the Earth's atmosphere, (ii) the solar flux at ground level after attenuation by gases in the atmosphere, and (iii) the flux of radiation emitted by the Earth's surface (terrestrial flux). Depicted at the bottom of the figure are the typical ranges of instruments used to measure components of the solar and terrestrial fluxes (adapted from Grace10).
PLANT MICROCLIMATE
29
/////////7777777777//>/?/^
Fig.3.3. An illustration of the short wave (φ5) and long wave (ψ,) radiant energy fluxes between a leaf and its surroundings. where λ is in micrometres and T is in Kelvin. Consequently, bodies on the earth's surface emit long wave radiation with a peak at approximately 9.7 μπι (Figure 3.2). There are therefore continuous fluxes of radiation from the sun during the day and between the atmosphere, plants and their surroundings at all times (Figure 3.3). Radiation incident on a leaf or plant canopy can be absorbed, transmitted or reflected. In the PAR region of the spectrum the leaf absorbs 90% of the incident radiation, whilst in the short-wave infra-red region (0.7 - 3.0 μπι) it transmits most of the radiation. The effect of this is to reduce the heat load from wavelengths which are not used in photosynthesis. However, in the far infra-red, leaves are good absorbers; thus (because good absorbers are also good emitters of radiation) they are able to dissipate excess heat very efficiently in the long-wave region of the spectrum. 3.2.2 Radiation measurements Most instruments used for measuring solar and long wave radiation consist of different forms of thermopile arrangement. A thermopile consists of
a series of alternate junctions between two dissimilar metals, e.g. copper and constantan (see Figure 3.4a). When a temperature difference exists between two sets of thermocouples a voltage is generated which is proportional to the temperature difference. When measuring solar radiation the temperature difference is created by embedding one set of junctions in a metal clamp protected from incident radiation and the other in a surface exposed to radiation, or by painting the hot and cold junctions black and white respectively and subjecting them both to the same radiant energy flux. The surface of the sensor is normally protected from wind and rain by glass domes whose transmittance restricts spectral sensitivity to the 0.3 to 3.0 μπι region. An example is the Kipp solarimeter (or pyranometer) using a Moll thermopile, which is the standard instrument in many countries for measuring total (direct + diffuse) and diffuse (using a shade ring) solar radiation. Details of other solarimeters can be found in Monteith 3 , Szeich2 and Fritschen and Gay4. When solarimeters are used with special filters (e.g. Kodak "Wratten" 88), they exclude visible wavelengths, so the energy in the region 0.3-0.75 μηι can be determined by difference.
30
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Long wave radiation is usually measured using net radiometers which measure the difference between the total incoming and outgoing radiation fluxes at all wavelengths (
obtained by moving a small sensor repeatedly along a track or by using a long linear sensor (tube solarimeters and radiometers). These linear sensors are less accurate than flat plate sensçrs because of greater cosine errors (see below), and should be used for relative rather than absolute measurements. Errors can be minimised by taking measurements in two directions at right angles. In addition to the fluxes of energy through the atmosphere, there is also a vertical transfer of energy through the soil which is known as the soilheat-flux. During daylight hours, the soil normally acts as a heat sink, so the soil-heat-flux is positive; at night, it becomes negative and of similar absolute magnitude to daylight values. It may range from 2% of <)>net for dense canopies to more than 30% of <)>net in open canopies. The vertical transfer of heat by conduction through the
Table 3.1. Some Radiation Terms Term
Symbol Meaning
Units
Radiation or Radiant energy
—
Energy transferred through space in the form of electromagnetic waves or quanta
joule (J)
Radiant Flux
—
The amount of radiant energy received, emitted or transmitted per unit time
J s 1 or watts (W)
Radiant flux density
φ
The radiant flux through unit area of a plane surface
Irradiance
I
The energy flux incident on unit area of a plane surface Wm~2
Wm~2
Photon
—
A quantum of light A mole (mol) is 6.022 x IO23 quanta or photons
Quantum flux density
Q
The number of quanta incident on unit area of a plane surface
mol m 2 s '
—
Photosynthetically Active Radiation
PAR
Radiation within the band 400-700 nm
mol m -2 s"1 or Wm"2
Short wave radiation
Radiation in the waveband 0.4 to 3.0 μπι
Wnr 2
Long wave radiation
«h
Net radiation
<)>net
Radiation wavelengths of 3.0 μηι or greater The difference between the downward and upward fluxes of total radiation
Wirr2 WirT2
PLANT MICROCLIMATE
soil is measured with a sensor which is basically similar to a net radiometer4. In practice the plate consists of glass or resin, but errors can be greater than 40% because of poor conductivity matching4.
Hot junctions -
S
radiation
s s
31
by absorbing light quanta which causes a release of electrons and the generation of an electric current (Figure 3.4b). These semiconductors now reasonable accuracy for a relatively low cost but
S
s
radiation
S
S
S
.Sensitive surface
Cold junctions
(a)
(b)
Fig.3.4. Radiation detectors: (a) a thermopile, consisting of a number of thermocouples in series; (b) a silicon semiconductor. V = voltage. I = current.
Another type of radiation sensor which has become increasingly popular in the last decade is the silicon cell which consists of a small chip of silicon that is sensitive to radiation. This functions
100
Actual response—^ /v
-Ideal response
ω50 ω ce
0.3
0.4
0.5 0.6 Wavelength (/am)
0.7
Fig.3.5. The actual and ideal responses of a PAR quantum sensor.
0.8
but unlike unfiltered radiometers, which have a uniform response to all wavelengths, these typically have a peak sensitivity at about 0.85 0.95 μιη with a band width of 0.5 μιη at 50% response. However, their advantage is that they can be constructed using suitable filters so that they measure PAR and give a good approximation to the ' i d e a r ' quantum response (Figure 3.5). The shape of the response is "ideal" because short wavelengths contain more energy and therefore generate more current: the broken line in Figure 3.5 shows the spectral response of the sensor adjusted for tht difterent energy levels at the blue and red ends of the spectrum. Quantum sensors can also be arranged in a linear fashion in a similar way to the tube solarimeters so that they measure Q within canopies. In order for radiation sensors to give accurate readings at all sun angles they should ideally obey Lambert's Cosine Law. According to this law, when radiation is incident at an angle a to the normal the irradiance (I) should be expressed as: I = L cos a where I0 is the irradiance on a surface normal to the sun's rays (Figure 3.6). Different techniques
32
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Zenith
and
<(>n
Tabs
TIrad
Energy storage by the plant, both physical and in the form of chemical bonds associated with photosynthesis and respiration, is generally small in quantitative terms and is generally ignored. The temperature that a leaf will reach in a specified environment can be calculated using an iterative computing procedure10 and the following equation, which describes AE and C in terms of an electrical circuit (Figure 3.1):
IIIIIIIIIÌÌJIÌJÌIÌÌÌÌIÌÌÌIIIIÌÌÌIÌIMÌÌJ:^.:;^^;!!!!;^!
Earth's surface
Fig.3.6. The cosine law, showing the relationship between the angle of the sun's rays (a), the slar zenith angle and the solar irradiance on the Earth's surface (I), where I0is the irradiance on a surface normal to the sun's rays.
are used to achieve good cosine response, but a common method is to use a raised white perspex diffuser on top of the sensor. Most radiation sensors give a millivolt output and they can be connected directly to recorders, data loggers or integrators. If connected to integrators they can give a total of received radiation during a given period of time. This is useful in determining the total radiation interception by a canopy for energy conversion calculations. Daily integrals of irradiance or Q give energy or quantum flux density over a period of time, i.e. J m - 2 d _1 or mol m - 2 d _1 .
3.3 Temperature 3.3.1 Introduction The temperature of the aerial parts of plants is determined by the balance between energy gain by interception of radiation (<|>abs) and the energy losses by re-radiation (
=
<>
t rad
+
AE
+
C
taet =
QCp(es,Ti -
e)
y(rs + ra)H2o)
+
QCpCT) -
Ta)
ra>H
where ρ is the density and cp the specific heat of air, esTi is the saturated vapour pressure of air at the leaf temperature (T^, e is the water vapour pressure of free air, y is the psychrometric constant (66 Pa °C _ 1 ) , and rs, ra>H2o and rai H are , stomatal and boundary layer resistances (reciprocals of conductance). However, a more convenient expression can be derived from this equation; this calculates leaf-air temperature difference from the sum of terms that depend on net radiation and the vapour pressure of air8. Plant temperature is therefore determined by the large number of factors which influence the magnitude of <|>rad, E and C. Units of temperature are degrees Celsius (°C) or Kelvin (K = 273 + °C) and it is generally held that a sensitivity of ± 1 °C is sufficient for analysis of plant growth and development while a sensitivity of ± 0.1 °C is required for calculations of transpiration or heat transfer determinations11. Physiological processes such as seed germination, photosynthesis, respiration and leaf growth all respond to temperature but it is important to be able to measure the temperature which is most relevant to the process being studied. For example, when measuring leaf expansion in grasses the temperature in the meristematic region at the base of the leaf is the most relevant measurement; this may be closer to soil temperature than air temperature because of the location of the meristems in vegetative grasses12. The problem is made more difficult by the fact that plant temperatures can often be several degrees different from air temperature and
PLA«NT MICROCLIMATE
there is also a spatial variation in temperature, often as a result of solar radiation interception at the top of the canopy. In recent years there has been renewed interest in the concept of degree-days or thermal time in controlling plant growth and development. Plant development is assumed to show a linear response to temperature from a threshold (Tb) to an optimum (T0), and the time taken to reach a given phenological stage is related to thermal time, defined as the integral of temperature with time. Units of thermal time (t) are degree-days, calculated as the sum of the differences between daily mean temperature ( T ) and the base temperature for each day beyond a given starting date: n
t = Σ ( f - T„) for T > Tb 0
In many natural environments, it is difficult to uncouple developmental response to temperature from other factors such as irradiance and saturation deficit. This problem can be overcome by calculating a thermal rate (p): p = ζ / ( Τ - Tb) where ζ is the rate of response (e.g. leaf extension in millimetres). The thermal rate is expressed in units of response per unit thermal time (e.g. mm (°C.h) _1 ); it is now possible to correlate this with other environmental variables, although application of this technique to field measurement needs some caution13. It is now possible to carry out temperature integrations to determine degreedays using commercially available transducers and millivolt integrators (Delta-T Devices, Cambridge). 3.3.2 Temperature measurements Temperature is measured by transducers which are based upon temperature effects on expansion, electrical or radiative responses. The two most important sources of error in temperature measurement are the effects of incoming radiation and the effect of the thermal mass of the sensor. Both these effects are more important in air than in water or when measurements are made within the plant tissue.
33
Liquid (normally mercury)-in-glass thermo meters are the most common instruments used for measuring temperature. They are widely used as accurate devices in meteorological stations, but they have no facility for recording. However the less accurate bimetallic strips used in thermographs do register on a dial or strip chart through a series of levers. Many temperature sensors depend on the fact that a change in temperature can alter the electrical properties of certain materials. These electrical temperature transducers are either thermocouples which generate a flow of electrons between two junctions of dissimilar metals if their temperatures are different, or resistance thermometers and thermistors where resistance changes with temperature. Thermocouples are widely used for temperature measurements in biology because they are small, easy to construct and cheap. A number of types of thermocouple can be purchased or made from combinations of different metals. They have different electrical and physical properties which influence their sensitivity and suitability for different uses. Characteristics of the more common thermocouples are shown in Table 3.2. When two thermocouple junctions are joined the voltage (V) generated is proportional to the difference in temperature between the measuring junction (sensor) and a reference junction: V = k(T - T0) where T is the sensor temperature, T0 the reference temperature and k the temperature coefficient (the change in e.m.f. per unit change in temperature at the reference temperature). It is common practice to assume a linear relationship between thermocouple e.m.f. and temperature, but for more accurate work the relationship is more precisely described by a quadratic regression equation 7 . Normally thermocouples are used with the reference junction maintained at a constant temperature; this is most conveniently an icewater mixture contained in a Dewar (vacuum) flask, which has a temperature of 0°C. Calculated values of e.m.f. for a range of temperatures with the reference junction at 0°C are given in Table 3.2. Alternatively, soil temperature at a depth of
34
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
Table 3.2. Typical electrical properties and characteristics of thermocouples. (Adapted from Woodward and Sheeny7). <)>mni is the smallest practicable thermocouple diameter. Thermocouple
Type
Uniformity
Temperature °C 0°
10°
20°
30°
40°
50°
Copper-constantan
T
Low
0.2
0
0.39*
0.79
1.19
1.61
2.03
Chromel-alumel
K
Low
0.1
0
0.40
0.80
1.20
1.61
2.02
Chromel-constantan
E
Medium
0.05
0
0.61
1.23
1.85
2.48
3.08
Iron-constantan
J
Low
0.05
0
0.52
1.05
1.58
2.12
2.66
Plantinum-platinum/10% rhodium
S
High
0.025
0
0.06
0.11
0.17
0.24
0.30
one metre is quite stable; this can be used as reference if its temperature is measured with a thermometer. More convenient than either of these references are the electronic references now available on many meters; when used in this way only one thermocouple is required to measure temperature. Thermocouples can be easily constructed, taking care to ensure a good junction between the two metals. Tin or silver can be used to solder the junctions, but silver gives the smallest junctions using borax as a flux. After soldering, the junctions can be cut with a blade under a binocular microscope to make them as small as possible. Ideally, all thermocouples should be individually calibrated because of small variations in characteristics of the wires and junctions. Wire resistance thermometers are most often constructed from platinum, nickel or copper. Usually the commercially available platinum resistance thermometers are quite bulky, being typically 20 mm long and 3 mm in diameter. They are therefore only useful for measuring temperatures of large volumes but they are often favoured for long term use because of high stability, resistance to weathering and an almost linear change in resistance with temperature. However, the change in resistance with temperature is relatively small, so a circuit to read voltage output must be designed with care to avoid large error resistances2,4. Thermistors are semiconductors, composed of sintered mixtures of metallic oxides. The resistance of thermistors decreases exponentially
with temperature but with about ten times the sensitivity of resistance thermometers. They are available in a range of sizes down to miniature bead types of 0.2 mm diameter, and the circuitry required to give a readout is relatively simple and robust. The only non-contact method of measuring temperature is by using the infra-red thermometer, which is based upon the principle that all surfaces emit energy11. The flux of radiation follows the Stefan-Boltzmann law and is proportional to the fourth power of the absolute temperature of the object (Section 3.2.1). As the temperature of vegetation is about 290 K it emits long wave radiation with a peak of emission at about 10 μιτι. Infra-red thermometers are typically fitted with filters which allow only radiation in the range 8 - 13 μιη to pass to the detector. They are expensive, difficult to calibrate and prone to errors if reflected long wave radiation is detected. When used correctly, errors are between 0.1 and 0.5°C. They are intrinsically preferable to contact methods because the latter can alter surface temperature during measurement by simultaneous conduction between the thermometer, surface and air, possibly resulting in large errors. However, infra-red thermometers cannot be used for measuring "sky" temperature. 3.3.3 Use of thermometers Before use, thermometers should be calibrated over the expected range of temperature. The simplest method is to immerse the sensors in a
PLANT MICROCLIMATE
water bath whose temperature is controlled and compare temperatures with an accurate mercuryin-glass thermometer. The infra-red thermometer cannot be calibrated in this way, but it can be set up to receive radiation from the inside of a blackened sphere immersed in a water bath whose temperature is known. Temperatures measured are usually of air, surface, soil and tissue. The latter two are less prone to difficulties because the thermometer is immersed in the material it is sensing. For air temperatures to be measured accurately the absorption of solar and long wave radiation should be prevented by use of a radiation shield and possibly also ventilation. The ideal shield should have a high reflectivity for solar radiaton and a high emissivity for long wave radiation. Aluminised "Mylar" and clear matt white paint have been found to be the most suitable shield coverings. Surface temperature measurements are the most difficult to make accurately because they depend on a good thermal contact between the sensor and surface being measured. Clips, springs or tapes are often used to make contact but their presence can lead to errors. Further details on the use of thermocouples and an analysis of the errors which might be experienced can be found in Perder 11 .
3.4 Humidity 3.4.1 Introduction The water content of air is known as the absolute humidity (χ) and is the density of water vapour in the air in g m 3. The importance of humidity to a plant's functioning is twofold. Firstly, it determines the rate of water lost in transpiration (E) because: E
=
g (Xair -
Xleaf)
where g is the conductance for water vapour transfer between the evaporating surfaces within the leaf and the air. Secondly, humidity has a direct effect on the stornata of many plants, so that stornata tend to close in dry air restricting water loss but also reducing C 0 2 assimilation.
35
3.4.2 Definitions Because water vapour is a gas, its pressure contributes to the total measured atmospheric pressure and its potential pressure is called vapour pressure (e). When air above water has no extra capacity for holding water vapour the partial pressure of the water vapour is the saturated vapour pressure (es) measured in kPa and its density the saturation density (g m~ 3 ). The saturation vapour pressure increases with temperature (Figure 3.7). If air is cooled without change in water content, condensation occurs at its dewpoint temperature (Td), when e = es. When water evaporates into less than saturated air then the temperature of the air decreases up to a point. This is the wetbulb temperature (Τ'), the temperature to which the wet bulb falls in a psychrometer (Section 3.4.3). Its value is given by the intercept of a line of slope - y (where y is the psychrometric constant), passing through the vapour pressure of the air at the dry bulb temperature, with the curve of saturated vapour pressure against temperature (Figure 3.7). The slope of the line differs according to whether the wet bulb is ventilated or not. Relative humidity is the ratio of the actual vapour pressure (e) to the saturated vapour pressure (es) at the dry bulb temperature (T). It is usually expressed as a percentage. However the use of this term should be discouraged as plants do not respond directly to relative humidity. Saturation deficit or vapour pressure deficit (òe) is the difference between the saturation vapour pressure and the actual vapour pressure at the same temperature (Figure 3.7). It is an index of the drying power of the air; the higher the deficit the greater the evaporation rate. 3.4.3 Measurements Many different devices can be used to measure the humidity of the air. They are based on several principles including the electrical properties of sulphonated polystyrene or thin-film solid state semiconductors; wet-bulb depression; condensation of water vapour on a surface cooled to the dew-point; and infra-red absorption. Some
36
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
of the instruments more widely used in the field are considered here. A psychrometer is a pair of identically shaped thermometers, one of which is covered with a wet sleeve. Evaporation cools the wetted sensor to the wet-bulb temperature, and the vapour pressure (e) is calculated as: e = es
-y(T
-
Τ')
where T ' and T are the wet- and dry-bulb temperatures respectively, e s / r is the saturated vapour pressure at the wet-bulb temperature, and
y is the psychometric constant (equal to 66 Pa ° C - 1 at sea level in a ventilated psychrometer). Several types of psychrometers are available as commercial units, the best of which ensure efficient radiation shielding of the thermometers and minimise heat conduction along the stem of the thermometer 2 . The Assman psychrometer is a ventilated psychrometer containing matched thermometers; it is used for standard humidity measurements. Smaller ventilated psychrometers are now available for use above and within plant canopies (DeltaT Devices, Cambridge). The hand held whirling or sling psychrometers are the
3.0r
130 Temperature (°C)
Fig.3.7. The influence of temperature on the saturated water vapour pressure of water. The point X represents air at 18°C and 1.0 KPa vapour pressure (e). The line Y-X-Z, with a slope of - y , gives the wet-bulb temperature (Τ') where it intercepts the curve at Y (12°C). The water vapour pressure deficit (de) is the difference between X and W (the saturated vapour pressure at 18°C). The dew-point (Td) is the point at which the saturated vapour pressure is equal to X.
PLANT MICROCLIMATE
simplest and cheapest ventilated units. In order to achieve an aspiration rate of 3 m s"1 they have to be rotated at about two revolutions per second. Many materials show a change of physical dimensions when they absorb water, and this property can be used to make instruments that measure humidity. For example, the length of animal hair increases as the air becomes wetter and decreases as the air dries; this property is used in simple hygrometers. Provided an allowance is made for the effect of temperature, hair hygrometers are usually accurate to within 5% over most of the humidity range. The change in electrical properties of materials as they absorb water is used in several humidity sensors. Until recently the lithium chloride sensor was the most common type of electrical sensor. Lithium chloride is hygroscopic and the moisture content of the air determines how much water is absorbed, which in turn influences the AC resistance of the sensor. This type of sensor is susceptible to contamination by dust and other hygroscopic particles, and it suffers from a certain amount of hysteresis when wetting or drying. More recently, capacitance hygrometers, which measure the change in electrical capacitance caused by water-absorption into a dielectric, have become commercially available (Humicap, manufactured by Vaisala, Helsinki, Finland) and are less temperature sensitive and show less hysteresis than other electric sensors. Dewpoint meters measure the temperature at which dew forms on a cooled surface. Dewpoint is usually determined by cooling a surface to below the point of saturation, allowing water to condense onto it, and then gradually raising the temperature until the film of condensation starts to evaporate. The temperature at which this change occurs is taken as the dewpoint temperature, and the presence of the film can be detected optically or electrically. Dewpoint temperatures must be corrected for changes in atmospheric pressure if they are converted into vapour pressure. Infra red gas analysis can measure water vapour concentration of air as well as C 0 2 (Chapter 6). The instruments are expensive but they are accurate and respond quickly. With suitable switching systems this type of instrument is almost
37
always employed to measure concentration differences, making it very suitable for profile studies. 3.5 Wind Wind is the large-scale transport of air masses resulting from differences in air pressure. It is directly involved in heat and mass transfer by forced convection, so it is very important in influencing heat and gas exchange across the boundary layers of plants. Increase in wind speed decreases the boundary layer resistance over leaves (see Figure 3.1); this tends to increase evaporation and bring leaf temperature closer to air temperature. Wind is also important because it causes mechanical deformation of plants (due to the frictional drag of moving air) and because it disperses pollen, seeds and aerial pollutants. However, of all the elements of the microclimate, wind is the most spasmodic. Short term variations in wind speed are described by the intensity of turbulence; this value represents the standard deviation of the instantaneous values, divided by the mean wind speed14. Turbulent air moves in packets or eddies; these are important for the movement of C0 2 , H 2 0 and other gases in and above plant canopies. The meteorologists' measurements of wind speed are normally made 10 m above the ground, but wind speed decreases rapidly as the plant surface or ground is approached. Wind speeds near or within vegetation can therefore be much lower than at 10 m. The analysis of the profile of mean wind speed above the canopy can be used to derive coefficients for calculating the flux of C 0 2 and H 2 0 between the canopy and the atmosphere5,7. However, the principles of environmental physics required to make these calculations are beyond the scope of this discussion. Furthermore, the number of environmental sensors and the size of data recording and processing facilities required are beyond the budget of most research groups. 3.5.1 Measurement A complete picture of air movements requires continuous recording of instantaneous wind speed measured in three directions; the vertical, the
38
TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
horizontal lateral and the horizontal perpendicular. However, these are difficult to measure, so it is generally sufficient to determine the mean value in one direction over the measurement period. The most commonly used instrument is the cup anemometer. This normally consists of three hemispherical or conical cups mounted on arms and attached to a central vertical spindle, so that they are free to rotate in the wind. The number of rotations of the cup assembly is usually measured in metres (the "run of the wind"), and can be divided by the elapsed time to give the mean wind speed. An alternative mechanical anemometer is the vane or propellor type. The vane anemometer is simply a miniature windmill, consisting of a number of light vanes radially mounted on a horizontal spindle. The main sources of error with mechanical anemometers are firstly, that they have a threshold below which the friction of the system prevents rotation, and secondly, that their inertia makes them over-run when the wind speed drops. Additionally, the vane anemometer is directional; it must be aligned to the wind direction. A third type of instrument is the hot-wire anemometer, which estimates wind speed by measuring the rate of cooling of a heated wire in moving air. These can be made very small, and are useful for measuring the rate of air flow around leaves14, but they are often delicate and easily broken. For this reason they are not used routinely for field measurements. Further details of instruments for wind measurement can be found in Grace14.
3.6 Automatic weather stations In many studies, a rather broad description of the climate experienced by the plants is sufficient to begin untangling plant/climate relationships. For these purposes, information obtained from standard meteorological sites located close to the vegetation under investigation is useful. However, in recent years, automatic weather stations have been increasingly used. These stations can be set up on experimental sites to provide detailed
measurements of the weather including solar radiation, net radiation, wind run, wind direction, air temperature, wet-bulb temperature and rainfall. The output from these instruments is recorded on standard cassette tapes, used in conjunction with battery operated data loggers. Modern data loggers often incorporate many channels, enabling the recording of additional data such as soil temperature.
3.7 Recording The simplest method of recording the output from micrometeorological instruments in the field is using a pencil and note-pad; and in many cases this is all that is necessary. There are, of course, many advantages in adopting automatic recording but the methods used must be considered in relation to the use to which the measurements will be put. For example, detailed measurements of vertical profiles of temperature, wind speed, humidity, radiation and C 0 2 concentration can be used to estimate canopy évapotranspiration and C 0 2 exchange, but they require complex recording facilities; these techniques are normally only possible when resources of equipment and manpower are extremely good. However, less intensive measurements using limited recording facilities can still tell us a lot about the relationship between the plant and its environment. Micrometeorological measurements are not an end in themselves and usually we need to relate them to plant physiological responses such as photosynthesis, stomatal movement, water potential and leaf expansion. These responses have different time scales (e.g. photosynthesis and leaf expansion) and measurements should be recorded accordingly. The simplest automatic method of accumulating output from instruments is using analogue recorders. The most suitable of these are galvanometer recorders which can be multi channel, and either use pen and ink as tracer or record on pressure-sensitive paper using a chopper bar. Analogue integrators can be used where detailed chart records are not necessary as they integrate small currents and voltages and can be
PLANT MICROCLIMATE
39
used to determine characteristics such as degreedays and daily solar radiation integrals. Digital data logging is perhaps the most convenient way of collecting micrometeorological data, especially where a large number of measurements are involved which can subsequently be handled by a computer. Here the analogue input from the instruments is converted into a number (by an analogue-digital converter) and recorded on a magnetic tape. Usually a large number of inputs can be scanned in sequence to give discontinuous but frequent records of a large number of measurements. For further discussion of this topic see Woodward and Sheehy7.
the temperature of the undersurface of a leaf and of the air below the leaf. Repeat the measurements at a number of heights between the top of the canopy and the soil and also measure soil temperature by carefully pushing the thermocouple into the soil to a depth of 1 cm. (d) Wet- and Dry-bulb temperature - using the Assman or Delta-T ventilated psychrometers to make measurements at a number of heights in the plant canopy. Calculate the saturation deficit of the air (kPa) from the wet- and dry-bulb temperatures using the tables or slide-rule provided.
3.8 Experimental work
References
The objective of this experiment is to measure the vertical profiles of different environmental factors in canopies of two crops of contrasting structure (e.g. maize and bean). These are carried out in conjunction with measurements of stomatal conductance and leaf water potential in order to determine which factors control stomatal activity. Measure the following parameters at five different positions in the crop. (a) Photosynthetically Active Radiation (PAR) using the Lambda linear quantum sensor. These measurements should be made at right angles to the rows. If the incident radiation is constant, measure above the crop and then at progressively lesser heights down to the soil surface. However, if the incident radiation is changing, measure first above the canopy and then at a lower height, then above the canopy again before measuring the next lowest position. Express the value of quantum flux at a particular height as a percentage of the incident flux. (b) Short- Wave and Visible Radiation - using two tube solarimeters, one of which is fitted with a filter to eliminate the visible wavelengths (400-700 nm) to measure the non-visible component of short-wave radiation. The difference between the two gives the value for visible radiation. Express the value at a given height as a percentage of the incident radiation. (c) Leaf and air temperature - using a WESCOR thermocouple thermometer to measure
1. Monteith, J.L. (1981) Coupling of plants to the atmosphere. In: Plants and their Atmospheric Environment, 21st Symposium of the British Ecological Society (Grace, J., Ford, E.D. and Jarvis, P.G. eds.) pp. 1-29. Blackwell Scientific Publications, Oxford. 2. Szeich, G. (1975) Instruments and their Exposure. In: Vegetation and the Atmosphere, Vol. 1: Principles (J.L. Monteith ed.) pp. 229-273. Academic Press, London. 3. Monteith, J.L. (1972) Survey of Instruments for Micrometeorology. IBP Handbook No. 22. Blackwell Scientific Publications, Oxford. 4. Fritschen, L.J. and L.W. Gay, (1979) Environmental Instrumentation. Springer-Verlag, New York. 5. Monteith, J.L. (1973) Principles of Environmental Physics. Edward Arnold, London. 6. Campbell, G.S. (1977) An Introduction to Environmental Biophysics. Springer-Verlag, New York. 7. Woodward, F.I. and J.E. Sheehy, (1983) Principles and Measurements in Environmental Biology. Butterworths, London. 8. Jones, H.G. (1983) Plants and Microclimate. Cambridge University Press. 9. McCree, K.J. (1976) A rational approach to light measurements in plant ecology. In: Commentaries in Plant Science (H. Smith ed.). Pergamon Press, Oxford. 10. Grace, J. (1983) Plant-Atmosphere Relationships. Outline Studies in Ecology. Chapman and Hall, London. 11. Perrier, A. (1971) Leaf temperature measurement. In: Plant Photosynthetic Production, a Manual of Methods, (ed. Z. Sestâk, J. Catsky and P.G. Jarvis) pp. 632-671. Dr. W. Junk, The Hague.
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TECHNIQUES IN BIOPRODUCTIVITY AND PHOTOSYNTHESIS
12. Peacock, J.M. (1975) Temperature and leaf growth in Lolium perenne. II. The site of temperature perception. J. Appi. Ecol. 12, 115-123. 13. Ong, C.K. (1983) Response to temperature in a
stand of pearl millet (Pennisetum typhoïdes) 1. Vegetative development. J. Exp. Bot. 34, 322-336. 14. Grace, J. (1977) Plant Response to Wind. Academic Press, London.