by
R. L. CLOSS,
Dominion Physical Laboratory, Lower Hutt, New Zealand
INTRODUCTION
In the Microclimatology Division at the Dominion Physical Laboratory a series of measurements has been undertaken to investigate transpiration. The project is continuing, but some preliminary results have been obtained relating the transpiration rate to the moisture content of the soil.
Under natural conditions, w h e n there is a plentiful supply of water in the soil, it is n o w generally accepted that the transpiration rate is determined by the weather.
Several formulae have been developed to estimate the transpiration rate from standard meteorological data assuming an adequate supply of water. O n e of the best k n o w n of these formulae is that due to P e n m a n [2].1 As the soil dries out, the actual transpiration will, at some stage, fall below the potential rate. The measurements discussed in this paper were undertaken to determine whether or not a fall off in the transpiration rate occurs before the moisture content of the soil reaches the permanent wilting point.
S o m e idea of the conflicting state of the experimental evidence arrived at by various workers can be obtained from the contributed discussion following an article by Veihmeyer and Hendrickson [5]. T h e measurements m a d e b y Veihmeyer and Hendrickson appear to show no decrease in the transpiration rate until the moisture content of the soil reaches the permanent wilting point.
S o m e of the results submitted to the discussions b y other workers support this while others appear directly to contradict it.
The majority of the measurements have been m a d e under natural conditions, where the daily changes of weather m a k e assessment of the results difficult. T o overcome this difficulty the measurements discussed in this paper were m a d e on mustard seedlings growing in a controlled environment. The transpiration rate, the moisture content of the soil and the p F have been m e a -sured; the results indicate that the transpiration rate
begins to fall off before the permanent wilting point is reached.
It is suggested that some light might be thrown on the problem of the effect of the soil moisture on the trans-piration by introducing the concept of "potential water assimilation", which is the m a x i m u m rate at which the plant can obtain water from the soil. The "potential water assimilation" would depend on the amount of water in the soil, the distribution of the roots through the soil and the condition of the water in the roots. This theory is opposed to the explanation put forward by Veihmeyer and Hendrickson to explain their results.
They suggest that, because the free energy in the air surrounding the leaves is very m u c h greater than the free energy in the soil, there is a relatively small change in the free energy gradient from the soil to the air as the soil dries out. The evaporation force tending to remove water from the soil will therefore vary very little as the soil dries from the field capacity to the wilting point.
However, their results do not contradict the idea of
"potential water assimilation", as it is possible that under the conditions of their experiment the potential transpiration did not exceed the "potential water assi-milation" until the permanent wilting point was reached.
Their results might have been different if their expe-riments had been carried out under conditions of higher potential transpiration.
It is possible that some of the very conflicting evi-dence obtained by the various workers in the field could be resolved b y investigating their results in the light of this concept. It might be found that those w h o support Veihmeyer and Hendrickson m a d e their meas-urements under conditions of low potential transpiration or high "potential water assimilation", while those w h o oppose them m a d e their measurements under conditions of high potential transpiration or low "potential water assimilation".
1. T h e figures in brackets refer to the bibliography at the end of this chapter.
EXPERIMENTAL TECHNIQUE
Transpiration from plants with a limited water supply
The controlled environment, necessary to ensure that the potential transpiration was constant, was obtained by growing the plants in pots of soil in a cabinet where the humidity, temperature, and light intensity were controlled. Care was taken that the plant canopy completely covered the soil surface so that when there was a plentiful supply of water the conditions for potential transpiration suggested b y P e n m a n [3] were complied with. The transpiration rate was obtained from the loss in weight of the pots of soil during a run of four hours in the cabinet. The m e a n weight of the pot and contents during each run was later used to calculate the moisture content.
At the beginning of each series of measurements the soil was wetted by immersing the pots in water and allowing them to drain. The series was stopped when the wilting point was approached. The soil was then rewetted and a new series of measurement commenced.
Usually three series were carried out, but if the plants appeared to have suffered from the moisture stress at the end of first or second series no further measurements were made on them. At the end of the final series the dry weight of the soil in each of the pots was determined.
The moisture content during each run in the cabinet was calculated from this dry weight and the actual weight of the soil during the run.
As it is possible that the free energy, rather than the moisture content, controls the uptake of water b y the roots of plants from the soil, the observed moisture contents were converted to free energies. The relation
i i - • • . • • • • — Z-6 3-S 3-0 3-2 3-4 3 * 3-8 4 0 * " * **
FIG. 1. Effect of soil moisture tension on transpiration of mustard plants.
j-o N ^
2-6 • ^ v • " RefaHve humidity 31 por aent
> V O - •• •• 47 ..
k * * ' O O X O x- » - 60 .. :
< »« a o" o > O v
0. H i ^ \
S "* ^ \ . x
i- X , \
s \ ^ X 5 i* \ \ _ >
No. •
i-o ^ ^ . M
04 0-7
a-fi i-e 3-o 3-a 3 * 4 ]•« i-e 4 - 0 4 - 2 4 - 4 pp
FIG. 1. Effect of soil moisture tension on transpiration of mustard plants (averaged points).
between the moisture content and the free energy was determined by a freezing-point technique [1].
RESULTS
Three groups of transpiration measurement were carried out at an air temperature of 27° C . and relative humi-dities of 60, 47 and 31 per cent. The results are shown in Fig. 1 where the transpiration rate in grammes per hour has been plotted against the p F on a scatter diagram.
The pF is the logarithm of the free energy, w h e n the free energy is expressed in centimetres of water below atmosphere pressure, and is a concept introduced by Schofield [4]. T h e straight lines drawn through the points where the transpiration rate is decreasing were fitted by a least squares method and can be expressed by the regression equations: TL = 8.0316 — 1.767 pF;
TM = 9.3305 — 2.114 pF ; TH = 8.252 — 1.757 pF.
Where TL, T M and T H are the transpiration rates at relative humidities of 60,47 and 31 per cent respectively.
The regression lines apply only to the results where transpiration rates are decreasing with increasing pF. In Fig. 2 freehand curves have been drawn to show h o w the transpiration varies over the whole range of pF.
The points shown in Fig. 2 were obtained b y measuring the transpiration rates over narrow ranges of pF and assuming that the m e a n rate corresponded to the trans-piration rate at the m e a n p F value of the range.
Under the controlled conditions of this experiment the transpiration rate decreased as the pF increased and had fallen off to a value measurably less than the potential rate before the permanent wilting point had
Climatology ami microctitnatoiogy / Climatologie et microclimatologie been reached. W h e n the p F exceeded a critical value,
depending on the environment of the plant, the trans-piration rate decreased linearly with increasing p F . S O M E POSSIBLE M E C H A N I S M S C O N T R O L L I N G T H E T R A N S P I R A T I O N R A T E
A possible explanation of the curves in Fig. 2 is that there are two mechanisms controlling the transpiration.
The first is the "potential transpiration", which is determined by the aerial parts of the plant and their environment. It is the transpiration rate which would be estimated from Penman's formulae. The second can be defined as the "potential water assimilation", which is determined b y the roots of the plant and their environ-ment. W h e n there is plenty of water in the soil the
"potential water assimilation" exceeds the "potential transpiration" and the actual transpiration is deter-mined b y the latter. As the soil dries out, the "potential transpiration" continues to determine the actual trans-piration until the moisture content of the soil is such that the "potential water assimilation" equals the transpi-ration. At this stage the "potential water assimilation"
takes control and determines the actual transpiration rate as the soil dries out further. The transpiration-versus-pF curve is therefore divided into two parts.
W h e n the p F is below a certain value the transpiration rate is determined by the potential transpiration. W h e n the p F exceeds the critical value the transpiration Tate depends on the "potential water assimilation" as well as the "potential transpiration".
The "potential water assimilation" is not determined exclusively b y the p F of the soil, as there is a statis-tically significant displacement of the three regression lines in Fig. 1. If the p F had been the only factor deter-mining the "potential water assimilation" then the three regression lines would have been identical.
As the movement of water through the soil-plant system presumably depends on the gradient of the free energy it would be expected that the "potential water assimilation" would be proportional to the difference between the free energy in the roots of a plant and the free energy in the soil. The "potential water assimi-lation", A , can therefore be expressed by the relation:
where p is the free energy in the plant, s is the free energy in the soil and R is the resistance to water movement from the soil to the plant. R and p in equation (1) cannot be constants, otherwise the trans-piration rate would have varied linearly with s. The experimentally determined relation between s and the transpiration rate was approximately logarithmic. This suggests that p and/oT R increase as s increases.
To check equation (1), p and R as well as s must be measured, p can be measured in the roots and leaves of a plant by a freezing-point technique similar to the one UBed to determine the relation between the free energy
and the moisture content of the soil. N o measurements have yet been made on mustard plants, but some results have been obtained using wheat. For these measurements twelve samples of wheat were grown at a temperature of 27° C . and a relative humidity of 47 per cent. At the end of each day the free energies in the soil, the roots and the leaves of one sample were measured.
The results are given in Table 1 where the free energies are expressed in ergs per g r a m m e .
T A B L E 1. Corresponding free energies in the soil, roots and leaves of wheat
The interesting point arising out of these results is that the free energy in the roots does increase as the free energy in the soil increases. Further measurements are being made on mustard plants and it is hoped that these will be suitable for checking equation (1), and further investigating the concept of "potential water assimilation".
D I S C U S S I O N
Under the conditions of the experiment the ration rate begins to fall below the "potential transpi-ration" before the soil dries to the permanent wilting point. The actual transpiration decreases linearly with p F when the p F exceeds a critical value depending on the potential transpiration. It has been suggested that the decrease in the transpiration rate can be explained in terms of the "potential water assimilation" and an attempt has been made to give a physical meaning to this concept in terms of the free energies in the soil and the roots. Sufficient evidence has not yet been obtained to develop a complete theory, but it seems possible that the "potential water assimilation" can be expressed by an equation of the form
where p — s is the difference in free energy between the roots and the soil and R is the resistance to water movement across the soil-root boundary.
Stomatal control of the transpiration and the move-ment of the roots through the soil have not been inves-tigated in detail. A single series of measurements of the stomatal effect has been m a d e by measuring the
transpi-Transpiration from plants with a limited water supply T A B L E 2. Effect of light on the transpiration rate
Day Transpiration
1 light 2 dark 3 light 4 dark
2.8 3.0 3.7 3.9
1.45 0.90 1.20 0.45 ration rate, in the light a n d in the dark, o n alternate days. T h e results are briefly s u m m a r i z e d in Table 2 .
It can b e seen from Table 2 that the transpiration rate decreases with increasing p F w h e n there is n o light shining o n the plants. This suggests that the decrease in the transpiration as the soil dried w a s not caused b y stomatal closure.
T h e g r o w t h of the roots through the soil has not b e e n investigated, but it is possible that it is a factor in determining the "potential water assimilation".
These results indicate the lines along w h i c h future w o r k should proceed. M o r e versatile g r o w t h cabinets are n o w being constructed w h i c h will b e able to simulate a range of climates extending from the extremely arid to the tropical. T h e experiments planned for this n e w e q u i p m e n t include g r o w t h studies as well as the conti-nuation of the investigation of transpiration. It is pro-posed to m a k e use of m e a s u r e m e n t s of photosynthesis to investigate the relation b e t w e e n g r o w t h a n d soil moisture. T h e first series of m e a s u r e m e n t s will b e directed towards determining at w h a t moisture content g r o w t h begins to fall off, a n d w h e t h e r or not there is a relation b e t w e e n the fall off in g r o w t h a n d the decline in transpiration. T h e m e a s u r e m e n t s of free energies in the roots a n d leaves of the plants will also b e continued in a n attempt to obtain a better understanding of the m e c h a n i s m of the "potential water assimilation".
R É S U M É
La transpiration des plantes disposant d'une quantité d'eau limitée ( R . L . Closs).
Les taux de transpiration d e plants d e m o u t a r d e cultivés en milieu expérimental et leur variation en fonction de la tension superficielle d e l'eau dans le sol ont fait l'objet de mesures. Les résultats montrent que la transpi-ration réelle est égale à la transpitranspi-ration potentielle
jusqu'au m o m e n t o ù le dessèchement d u sol atteint u n e valeur correspondant approximativement à u n jpF de 3 , 4 . Ensuite, la transpiration réelle décroît linéai-r e m e n t e u fonction d e l'acclinéai-roissement d u j>F. L'auteulinéai-r propose u n e explication d u fait q u e la transpiration d é p e n d , lorsque le pF est supérieur à 3 , 4 , d e la tension superficielle de l'eau dans le sol.
BIB LIO G R A P H Y /BIBLIOGRAPHIE
1. C L O S S , R . C . Soil Sci., vol. 78, 1954, p. 333.
2. P E N M A N , H . L . Proc. roy. Soc, 193 A , 1948, p. 120.
3. . Netherlands J. agrie. Sci., vol. 4, 1955, p. 9.
4. S C H O F I E L D , R . K . Trans. Third Intern. Congr. Soil Sci., vol. 2, 1935, p. 37.
5. V E I H M E Y E R , F. J.; H E N D R I C K S O N , A . H . Trans. Amer, geophys. Un., vol. 36, 1955, p. 425.