A Differential Psychrometer for Continuous Measurements of Transpiration 1
R. 0. Slatyer and J. F. Bierhuizen2
CSIRO Division of Land Research and Regional Survey, Canberra, A.C.T., Australia
Introduction
Transpiration is frequently measured by enclosing
plants, or portions of plants, in transparent chambers
and determining the change of humidity of a stream
of air before and after passing through the enclosure (2, 4). Although this procedure should not
be used to estimate probable transpiration rates under
natural conditions, unless adequate precautions are
taken to ensure that the microenvironment within the
chamber does not differ significantly from that in the
reference locations or that its specific characteristics
can be measured and appropriate corrections made
(9), it is particularly suitable for specific studies of
the transpiration process.
Recently, in developing an apparatus for simultaneous and continuous determinations of transpiration and photosynthesis involving differential airstream measurements of waiter vapor and CO2
concentration, Bierhuizen and Slatyer (1) incorporated a differential psychrometer, based on a design
of Wylie (10). This instrument has been found to
be particularly suitable for transpiration research
(7, 8) and its characteristics and performance are
outlined below.
Materials and Methods
A differential psychrometer can be considered as
a pair of matched wet bulb thermometer elements
enclosed in tubes, through which individual streams
of air, identical except for vapor pressure, are passed
at a common temperature T. Both streams originate
from a single source which provides a nominally
constant water vapor pressure eb by prior vapor
saturation in water at a predetermined temperature.
A sample from the main air stream is passed directly
through 1 tube of the differential psychrometer. The
remainder of the stream first passes through the
transpiration chamber and in the process becomes
enriched with water vapor, emerging with a new
water vapor pressure eaA3 A sample from this stream
is then passed through the other tube of the psychrometer. After leaving the psychrometer the air can
be released or passed through equipment such as an
infra-red gas analyser for differential measurements
of CO2 concentration.
It can be assumed that the pressure drop through
the system is negligible and, since the environments
of the 2 wet bulbs and their rates of ventilation are
practically the same, that the psychrometric constant
will be the same for both. If the actual wet bulb
temperatures are given by Twa and Twb, and the
corresponding saturation vapor pressures by ewa and
ewb, then the standard psychrometric equation can be
written:
I
(ewa-ea) = A ( -Twa)
(eWb - eb) = A (T- Twb)
II
where A is a form of the psychrometric constant proportional to atmospheric pressure. For full ventilation and at a pressure of 755 mm of mercury it has
a value of 0.500 mm of mercury per degree C.
Subtracting equation I from II and substituting
A e for (ea - eb) one can obtain
III
Ae = A (Twa - Twb) + (ewa - ewb)
which gives, on rearrangement
Ae = (A Twa + ewa) - (ATwb + ewb)
IV
It is apparent that equations III and IV provide
2 methods of calculating Ae. From equation III, for
example, by measuring ATw = (Twa - Twb) with a
differential psychrometer and obtaining the saturation
vapour pressures ewa and e, b corresponding to the
actual wet bulb temperatures from meteorological
tables (3, 5, 6) Ae can be readily obtained. Alternatively, from equation IV, by tabulating (ATw + e5,)
as a function of Tw, Ae can be readily calculated
from measurements of Twa and Twb. Typical values
of Ae for TUWb = 20, 25, 30 and 350, and ATw
ranging from 1 to 100 are given in table I. Errors
in the values adopted for psychrometer environment
temperature, T, have been shown by Wylie (10) to
have a small effect (less than 1.0 % per degree error
in T, for T, in the range 0-30o) on the calculated
value of Ae.
Received April 21, 1964.
2 Present address: Institute of Land and Water Management Research, Wageningen, Netherlands.
3 The subscripts b and a refer to the air stream before
and after passing through the test chamber, respectively.
1
1051
Description
The requirements of the apparatus were that it
should be of small size and possess long-term stability
so that it could be incorporated in a gas circuit for
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1052
PLANT PHYSIOLOGY
continuous studies of transpiration and photosynthesis.
Because it was desirable for some determinations to
be made at very low rates of air flow, and because
transpiration and photosynthesis rates may change
rapidly in response to a change in environmental
conditions, it was also important that the instrument
should be fully sensitive even at flow rates of less
than 0.5 liter/min and that it should have a rapid
response time of significantly less than 1 minute.
A diagrammatic sketch of the instrument (already used by Bierhuizen and Slatyer, 1) is given in
figure 1. The 2 air streams, with water vapor pressure ea and eb are first brought to a common reference temperature T by passing them through a pair
of copper coils, soldered together and immersed in a
thermostatically controlled water bath. The coil
outlets are connected directly to the psychrometer
tube inlets so that the temperature of the air around
the elements is effectively the same as bath temperature T.
Insulation of the psychrometer is also aided by
constructing it from a solid block of acrylic plastic
(plexiglas or perspex). The air streams pass out of
the copper coils, through the psychrometer (entry
is via orifices to induce turbulence in the air flow
past the wet bulb elements) and then through calibrated flowmeters. Flow is regulated by valves on
the inlet side of the transpiration chamber and total
flow through the chamber can be measured by the
differences between the flowmeter reading on the common inlet tube and the amount diverted through the
tube b. Horizontal insertion of the wet bulb elements into the psychrometer was adopted partly for
convenience in manipulation and partly because it
facilitated adequate depth of wet bulb insertion behind the tip of the wick.
Each w-et bulb consisted of a 44 gauge copper-
constantan thermojunction which was threaded
through a central hole in the plexiglas mounting and
allowed to project 2 to 3 mm. The projecting tip
was thinly covered with dissolved plexiglas to protect the junction chemically, seal the hole and provide mechanical support. Fine mesh knitted wet
bulb sleeving, previously boiled in very dilute alkali
solution and then thoroughly rinsed and stored in distilled water, was then drawn over the tip, along the
narrow section of the thermocouple mounting, and
extended into wet bulb reservoirs located directly
below the psychrometer proper. The open end of
the sleeving projected 2 to 3 mm beyond the thermocouple and was closed with a loop of cotton thread.
The use of 0-rings enables each wet bulb unit to be
positioned accurately and simply yet easily removed
for inspection and maintenance.
To facilitate easy threading of the wet bulb sleeving through the access hole leading from the 2 wet
bulb reservoirs to the psychrometer tubes, this part
of the assembly was attached to a brass plate which
could be quickly and easily removed when the wet
bulbs were being introduced or adjusted. Each wet
bulb reservoir could also be removed separately for refilling. The vertical height from water level to
thermocouple was kept as small as possible. It
seldom exceeded 2 cm.
The constantan wires of each thermocouple were
connected together to form the differential psychrometer and, in addition, were connected to a common
constantan reference junction which was located in
the water bath and so maintained at temperature T.
The output of each wet bulb thermocouple and the differential output was then connected to a multichannel
DC potentiometric recorder with a full scale sensitivity of 1 mv. The output of the thermocouples, 43 ,tv
per degree C, permitted output corresponding to a
PERSPEX
BLOCK
THERMOCOUPLE
MOUNTING
BY 2 O-RINGS
'' r
y SECURED
re
E
T R
INL
t~~~~~~~~~~~~~~~~~LA WIRES
gyITHERMOCOUPLE
WATER RESERVOIR FOR WICK
SURROUNDING WET BULB
Fig. 1. Diagrammatic sketch of differential psychrometer. Note that both thermocouple elements are wet bulbs.
One element is shown without wet bulb sleeving to provide details of construction. In use each element is inserted
into the psychrometer until the wick is directly over the water reservoir.
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1053
SLATYER AND BIERHUIZEN-MEASURING TRANSPIRATION
temperature difference of up to 200 to be displayed
on the recorder chart.
Results
Instrument performance was checked in several
different ways, the most important being the examination of the effect of rate of air flow on wet bulb
depression, and of the accuracy of the observed wet
bulb temperature at full ventilation. The sensitivity
of the instrument in terms of thermocouple output
was quite satisfactory since differential temperature
could be measured to better than 0.10 and, consequently, differences of less than 0.05 mg water per
liter of air could be determined. For single leaves of,
say, 100 cm2 area this corresponds to about 1 % of
typical transpiration rates.
ASPIRATION
SPEED
20
10
9
The calibration data are shown in figures 2 and 3.
In figure 2 rate of air flow past the wet bulb elements
was calculated from the flowmeter readings and effective cross sectional area, in each tube, available for
flow. Since the diameter of the element plus sleeve
was 3 mm and the internal diameter of the tube 6
mm the cross sectional area was approximately 0.20
cm2. A flow rate of 500 cm3/minute in each tube was
therefore represented by an air speed past the elements
of the order of 25 m/minute. The figure shows that
with humid air (T 30', TWeb 24.60 relative humidity
63 %) maximum wet bulb depression occurred at
flow rates less than 100 cm3/minute and even with
relatively dry air (T 30°, T,,b 13.30 relative humidity
4 %) it was obtained at flow rates of less than 250
cm3/minute.
(cm / sec)
40
30
18-
R.H. =4%
0
60
14-
U
0
12-
z
o 0
(n
(n
w
a.
(Li
ox 8OD
-J
6-
R.H. =63%
3
4-
2-
0]
6
1
i6o
I
200
T
I
300
400
I
500
RATE OF AIR FLOW
(cc/ min)
Fig. 2. Effect of ventilation rate on wet bulb depression using airstreams of 4 % R.H. (hollow circles) and 63 %o
R.H. (solid circles).
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1054
PLANT PHYSIOLOGY
*,
0
w
z 15
:3
w
a.
w
04
cr
w
I-
3:
m
z
w
z
a.
3.
w
a
30
25
2'0
DEWPOINT GIVEN BY THERMOCOUPLE
0b
1'5
MEASUREMENTS
(0c)
Fig. 3. Relation between observed dewpoint temperature, from thermocouple measurements, and the value giverr
by the temperature of the water in which the airstream was equilibrated. Hollow circles refer to thermocouple tube
a and solid circles to thermocouple tube b.
In figure 3 the observed wet bulb depression, taken
from recorder readings, has been used to calculate
dewpoint temperature and is plotted against a range
of water temperatures, Tb, in which the air stream
was equilibrated. The 2 estimates agreed closely
and zero stability, measured as the difference between
the 2 psychrometer elements when identical streams
of air were passed through both tubes of the psychrometer, was high.
At the extreme relative humidities obtainable
(4 % and 89 % at 300) differential output did not
exceed +0.5 ,uv over a test period as long as 40 minates.
The speed of response to changed humidity in
the tubes was also rapid; in almost every case the rate
of equilibration of the air itself appeared to be the
limiting factor.
An example of the use of the instrument in practice is given in figure 4. These data were collected
for cotton leaves in transpiration experiments similar
to those described by Slatyer and Bierhuizen (10, 11).
Environmental conditions of light (6,000 ft-c), bulk
air temperature (30°), windspeed (3.1 cm sec'1),
and bulk CO2 concentration (0.03 %) were kept constant. Bulk air relative humidity was increased
abruptly on 2 occasions from the initial value of
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SLATYER AND BIERHUIZEN-MEASURING TRANSPIRATION
2
TIME
(Hours)
Fig. 4. Change of transpiration rate of an enclosed
cotton leaf exposed to constant environmental conditions
except for abrupt changes in bulk relative humidity.
about 20 % so that the effective leaf-air vapor pressure difference [calculated from additional leaf
temperature measurements (1)] was reduced by
about 25 % and 50 % respectively. The rapid change
in transpiration rate as the leaf adjusts to a new
energy balance is well illustrated as is the constancy of the transpiration rate at any one humidity
level. Numerous other examples of the use of the
instrument can be found in the papers already cited
(10, 11).
Discussion
1055
e8 is the saturation vapor pressure (at temperature
T) in the same units as Ae, and p is the density of
water vapor in saturated air, also at temperature T.
Alternatively, the initial calculation of Ae can be
eliminated, and the difference between the relative
humidity appropriate to (T - T,,) multiplied directly by the density of water vapor in saturated air
(at temperature T) to give C. The value of C,
multiplied in turn by the rate of air movement
through the test chamber gives transpiration rate.
The alternative procedure can be recommended
for many experimental applications, especially when
a limited number of values of T are used as experimental parameters. It, too, can be simplified but
at the risk of introducing errors. The simplification
takes advantage of the fact that the relationship between relative humidity and (T - T,) is approximately linear for moderate differences in temperature. The slope of this relationship (per cent relative
humidity/degree C temperature difference) for
any 1 value of T, can be obtained from standard psychrometric charts or tables. It is then multiplied by
the density factor as above, and the product of the
value so obtained with A/T and with rate of air movement gives the transpiration rate to within ±5 %.
This error can be tolerated for many purposes and
even for accurate measurements the calculation provides a useful rapid checking procedure.
Summary
Table I. Valutes of Ae in mm for 4 levels of TWb anid
Values of ATw Ranging from 1 to 10°
A differential psychrometer is described which is
particularly suitable for continuous transpiration
measurements of plants in experimental enclosures.
The instrument is small, inexpensive to construct and
maintain, and has high accuracy and precision, together with good long term stability. Response time
of the instrument appears to be quicker than the
equilibration of the airstream being monitored. A
thermostated equilibration water bath is used so that
nominally similar dry bulb temperatures exist in both
the test and reference airstreams. This makes the
measurements relatively tolerant of fluctuating environmental temperature and pressure. Also the use
of small psychrometer tube dimensions and turbulence
inducing entry orifices enable maximum wet bulb
depression to be achieved at apparent aspiration
speeds of < 20 cm/second corresponding to airflow
rates of < 250 cm3/minute.
Twb
250
Acknowledgments
In order to calculate transpiration rate the values
of Ae obtained from equations III or IV must be
converted to water vapor concentration (mg water/
liter air) and multiplied by the rate of flow of air
through the test chamber.
The conversion from Ae in mm of mercury can be
made using the expression
C=p (Ae/e8)
V
where C is the water vapor concentration in mg/liter,
200
A~Tia
1
2
3
4
5
6
7
8
9
10
1.61
3.29
5.03
6.76
8.72
10.67
12.70
14.81
17.00
19.28
1.95
3.98
6.09
8.28
10.56
12.94
15.40
17.97
20.64
23.42
300
2.38
4.84
7.41
10.08
12.86
15.74
18.75
21.87
25.12
28.50
350
2.88
5.89
9.01
12.26
15.64
19.20
22.82
26.62
30.58
34.70
The authors acknowledge the considerable amount of
advice and assistance provided by Dr. R. G. Wylie and
his colleagues from CSIRO, Division of Physics, Sydney,
throughout the period of development of this instrument.
They also acknowledge the assistance of Mr. J. V. Savage
who fabricated both the prototype and final version.
Literature Cited
1. BIERHUIZEN, J. F. AND R. 0. SLATYER. 1964. An
apparatus for the continuous and simultaneous
measurement of photosynthesis and transpiration
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Copyright © 1964 American Society of Plant Biologists. All rights reserved.
1056
2.
3.
4.
5.
PLANT PHYSIOLOGY
under controlled environmental conditions. C.S.I.R.O. Div. Land Res. Tech. Paper 24.
DECKER, J. P., W. G. GAYLOR, AND F. D. COLE.
1962. Measuring transpiration of undisturbed
Tamarisk Shrubs. Plant Physiol. 27: 393-97.
DEUTSCHEN, W. 1955. Aspirations-PsychrometerTafeln (Friedr, Viewg, and Sohn, Braunschweig,
Germany).
HEATH, 0. S. AND H. MEIDNER. 1961. The influence of water strain on the minimum intercellular
space carbon dioxide concentration, r, and stomatal
movement in wheat leaves. J. Expl. Botany 12:
226-42.
HODGMAN, C. D., R. C. WEAST, AND S. M. SELBY.
1962. Handbook of Chemistry and Physics.
Chemical Rubber Publishing Company, Cleveland,
Ohio.
6. LIST, R. J. 1958. Smithsonian Meteorological Tables. Smithsonian Institute, Washington, D. C.
7. SLATYER, R. 0. AND J. F. BIERHUIZEN. 1964.
Transpiration from cotton leaves under a range of
environmental conditions in relation to internal and
external diffusive resistances. Australian J. Biol.
Sci. 17: 115-30.
8. SLATYER, R. 0. AND J. F. BIERHUIZEN. 1964. The
influence of several transpiration suppressants on
transpiration, photosynthesis and water use efficiency of cotton leaves. Australian J. Biol. Sci.
17: 13146.
9. SLATYER, R. 0. AND I. C. MCILROY. 1961. Practical Microclimatology. UNESCO, Paris.
10. WYLIE, R. G. 1963. A new tool for transpiration
experiments. C.S.I.R.O. Annual Report 1962-63,
32-3.
Cation, Organic Acid, and pH Relationships in Peel Tissue of Apple Fruits
Affected with Jonathan Spot 1. 2
A. E. Richmond,3 D. R. Dilley, and D. H. Dewey
Department of Horticulture, Michigan State University, East Lansing, Michigan
Jonathan spot is a physiological disorder of the
epider,mal and hypodermal tissues of Jonathan and
other apple varieties. Pentzer (8) observed that the
pH of the affected tissue was higher than that of
adjacent normal tissue and suggested that the spot
symptom resulted from the conversion of anthocyanin
color from red to blue. The affected tissue is also
characterized by a marked accumulation of K, Ca and
Mg (1).
Wilkinson (10) suggested that K, the dominant
fruit cation, may serve in a buffer system with malic
acid, the dominant fruit acid, for maintaining tissue
pH. Several investigators (2, 3, 10) have observed a
direct relationship between the K content and titratable acidity of apple fruits. It is generally accepted
that organic acids are synthesized in the cells to compensate ion balance when cation absorption exceeds
anion absorption (4, 9). Recently, Johnson et al. (5)
provided additional evidence that organic acid synthesis is the result rather than the cause of excess
cation absorption.
The relationship between cation accumulation
and organic synthesis and the development of Jonathan
spot on apple fruits is herein reported.
1
Received April 27, 1964.
2 Journal Article 3350 of the Michigan Agriculture
Experiment Station.
3 Current Address: Department of Plant Biochemistry,
University of California, Los Angeles, California.
Materials and Methods
Mature Jonathan apples (Malus sylvestris, Miller)
were harvested and maintained in storage at approximately 00 for several months to obtain fruits with
varying degrees of spot development. Discs (approximately 5 mm in diameter and 2 mm thick) of
spotted tissue and adjacent normal tissue were obtained from sections of fruit peel with a cork borer.
Samples of spotted and normal peel tissue were analyzed for mineral composition as described by Kenworthy (6). Quantitative analysis of the nonvolatile
organic acids of spotted and normal tissue was determined by the ion exchange column chromatographic
procedure of Markakis et al. (7) as modified by
Dostal (2).
A pH titration profile was obtained for spotted and
normal peel tissue employing a 95 % ethanol extract
of 2 g of tissue. Prior to titration, the extract was
evaporated on a hot plate, the residue dissolved in
20 ml of water, and the solution clarified by centrifugation. Half of the supernatant solution was titrated
with 0.0354N HCI to pH 2.5, while the other half
was titrated to pH 8.0 with 0.0280 N NaOH. pHTitration profiles were also obtained for solutions of
those acids present in 1 g of spotted and normal peel
tissue after adjustment of the solution to the pH
observed in the respective tissue homogenates.
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