THE EXTERNAL CONDITIONS ON THE WILTING
COEFFICIENT OF PLANTS
BY
CHARLES ORLANDO PEAK
THESIS
For the
DEGREE OF BACHELOR OF ARTS
PLANT PHYSIOLOGY
COLLEGE OF LIBERAL ARTS
UNIVERSITY OF ILLINOIS
1921
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UNIVERSITY OF ILLINOIS
June 191?.^..
THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
Char 1 e s . . 0 r ,1 an.do Peak
ENTITLED Thg,..Ext e;rnal.
Co e f f i c i e n t _ 0 f _ P
IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE
DEGREE OF B.,... A,.
Instructor in Charge
Approved :
HEAD OF DEPARTMENT OF .BqMXIY,
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Table of Contents.
Page.
I. Introduction. 1.
II. Methods. 7.
III. Results. 11.
IV. Conclusions. 12.
V. Literature Cited 14.
Digitized by the Internet Archive
in 2016
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- 1 -
I. INTRODUCTION.
In the complex factors which enter into the make
up of the environment of the plant the available soil water
is of great importance. The question has often arisen as to
just what portion of the soil moisture is available to the plant.
The quantity available varies with the nature and extent of the
root system, and the environmental factors surrounding the shoot.
The water of the soil cannot be removed completely by the plant.
As the available water is reduced through absorption by the roots
the physical attraction of the soil particle for the remaining
water becomes greater and greater until finally an equilibrium
is reached between the absorptive powers of the root and the
attractive force of the soil for the water. When this stage is
reached wilting because of lack of sufficient water to cover
that lost by evaporation ensues.
In the attempt to determine the wilting coefficient,
described later, of different plants, or of the same plant under
different conditions, numerous factors of the environment must
be taken into consideration. Most of the water taken up by the
plant through its roots is lost by transpiration. Thus we see
that conditions that increase the rate of transpiration call for
an increase in the absorption of water from the soil. The effioi
enoy of the roots of different species, or of different individu-
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als of the same species growing under identical conditions can
be tested relative to their absorptive powers. The measure of
the ability to absorb water under trying conditions of soil and
of atmosphere is the point of wilting of the young shoots.
The non-available moisture left in the soil at the
time of wilting of the plant has been termed "the wilting coeffi-
cient". This method was first used by Sachs (l) in 1859 on
tobacco plants grown in various soils, and has since been used
and considerably extended by plant physiologists, physicists,
and ecologists. The work has been done entirely on plants grow-
ing in soils of definite moisture content and under normal at-
mospheric conditions. No attempt has been made to find the
effect that a change in the atmospheric conditions - wind, mois-
ture, etc. - have upon the wilting coefficient. In dealing with
the question of wilting in plants one is facing the rates or
proportion between two physiological processes, namely those of
transpiration and of absorption. These two processes are affected
a great deal by external conditions. As an example, on a hot
day a plant will wilt for a time because the absorption of water
by the roots in insufficient to cover that lost by transpiration.
In 1912 Briggs and Shantz (2) carried on extensive ex-
periments with 20 different kinds of soils and one hundred differ-
ent plants, including hydrophytes and xerophytes. They took a
long series of readings, but their conclusions are unusual, and
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warrant further investigation. They concluded that for any
given texture of soil the wilting coefficient remains a constant,
regardless of the kind of plant or the conditions under which
the plant was grown. Instead of considering all the factors
which cause wilting, they have only considered one factor, and
that is the moisture in the soil.
V. H. Blackman (3) , working with plants under desert
conditions reached very different results from those of Briggs
and Shantz. Blackman found a wide variation under different
environmental conditions. Some of his readings on plants grown
under lath shelter, and in the open, varied as much as 40^ in
the moisture content of the soil at the permanent wilting stage.
A low rate of transpiration for a mesophyte, may be
very high for a zerophyte. Yet it has been found that mesophytic
and zerophytic plants transpiring at different rates and grow-
ing in soil of the same texture, will reduce the soil to the
same degree of dryness. Blackman gives as a possible reason for
this that the more actively transpiring plant has a larger ab-
sorbing surface with which to supply the plant with the necessary
water.
In the experiments carried on thus far in regard to the
wilting coefficient of plants, the investigators have been unable
to find any marked difference in regard to a difference in the
absorptive efficiency of roots of different plants. One would
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expect to find in different plants differences in the poxer to
absorb water from the soil and thus reduce soil moisture. Black-
man (3) says in his article on "The Wilting Coefficient of the
Soil" - "Whether absorption is considered to be mainly a proto-
plasmic process or as mainly controlled by the osmotic pressure
of the cells and the condition of aggregation of the cell colloids,
there must be marked differences in such factors in different
plants which one would expect to affect the wilting coefficient."
That such differences have not been noted is due mainly to the
conditions under which the experiments have been carried out.
The soil moisture is reduced to a large extent before it becomes
the controlling factor in absorption. This would affect the
plant at a very late and critical stage.
For a thorough study of the wilting coefficient of
plants in nature, we must consider along the line of changing
environment. There may be as many wilting points as there are
factors in the environment. Caldwell (4) points out that wilt-
ing, brought about by high rate of transpiration of short duration
is less serious than wilting brought about by a long period of
slow transpiration. In the first case the root hairs are only
plasmolyzed, while in the second they are killed, and the recovery
of the plant takes place only after new root hairs are formed.
The relations of the wilting coefficient to the differ-
ent soils have been brought out very well by the work of V. H.
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Elackman (3). The coefficient is related directly to the physi-
cal characteristics of the soil. Thus the smaller the particles
of soil the lower will be the rate of root absorption. The
conducting property of the soil must needs play an important
part, for even in a soil through which the roots are well distrib-
uted only a small portion of the soil is in actual contact with
the roots.
Crump -(5), in his paper on moorland soils, brought out
the relation of water content to the humus content of the soil.
He showed that the roots of a plant may occupy several different
kinds of soil layers, varying in moisture content. Thus he found
that the amount of water in the soil was directly related to the
amount of hurnus. This water content he called the coefficient of
humidity. The soil around the roots of a heather moor, though it
varied largely in water content, its coefficient of humidity varied
but very little and could be taken as characteristic of its asso-
ciation. The ground societies of nearly all the plants on the
moorland could be distinguished by means of this coefficient. \
\
He showed clearly that the wilting coefficient of Eriophorum
angustifolium and Colleraa vulgaris were closely related to the
humus content of the soil. Thus Crump’s coefficient of humidity
may be taken as the standard index in soils where humus is the
dominant constituent.
One of the phases of the problem of wilting coefficient
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- 6 -
is a study which shall lead to a formula that will apply to
show the relation between wiltings at various rates of transpira-
tion on the one hand and the water content and mechanical composi-
tion of the soil on the other.
It has been long maintained by plant physiologists,
ecologists and agriculturists, that plants differ in their power
to withdraw water from the soil. Until of late no definite ex-
periments have been carried on where the conditions have been
definitely under control. Hedgcock (6) has found that the
amount of moisture in the soil at the time of wilting is most in
the case of hydrophytes and decreases through xerophytes. Living-
ston (7), comparing the zerophytes - Allonia, Bouhavia and Eu-
phorbia with the mesophytes - Phaseolus, Helianthus and Vicia, -
that the desert forms show an adaptation to live in drier soils
than plants of a humid climate.
It has been found by SchroSder that the water content
of the plant is the same during the growing time regardless of
the conditions under which it was grown or the water content of
the soil. It has been thought that a relationship existed be-
tween the water content of the soil and that of the leaves. In
plants the water content may vary within considerable limits
without affecting in any marked way its life processes. The
water content of the soil so long as it affects the water content
of the plant within the limits of this critical range are without
m£ 3>^
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-7-
material effect on the vegetal covering. A lowering of the
water content of the soil that will affect the water content of
the plant within the limits of its critical range, at once brings
about changes in growth and finally wilting in the plant.
II. METHODS.
The plants were grown in pure quartz sand obtained
from Ottawa, 111. It had a water holding capacity of 21.- grams
per 100 grams. The sand was used in glazed earthenware and glass
containers. The earthenware jars were 5 inches in diameter, by
3-^ inches deep; the glass Petri dishes were 7 inches in diameter
and 1 inch deep.
The containers were weighed and the weight recorded.
They were then filled with the quartz sand and again weighed,
and the ?/eights recorded. This sand was then poured out into a
pan and the amount of water added to give the desired per cent of
soil moisture. This was mixed thoroughly so as to insure an even
distribution of the water. The moist sand was now replaced in
the container and beans, sunflower, or diseased and disease free
corn, were planted.
The containers were weighed at twelve hour intervals
and the necessary water added to bring the weight back to normal.
This was kept up until the plants were put under the fan for the
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purpose of testing the wilting coefficient. At first the plants
were grown in the green house hut due to the dry hot air, those
in the 10^ and 20^ moisture content wilted. This wilting would
inhibit the normal growth of the plant by reducing for a time the
normal turgor of the plant cells. To prevent this the plants
were grown in a trough 2-J- ft. x 4 ft. x 8 inches, and covered
with glass. The relative humidity in this chamber was between
30*^^ and 40'^, as compared with the 20^ and 25*^ in the laboratory.
The earthen crocks used at first proved unsatisfactory
because of the time necessary to bring the sand down to the wilt-
ing coefficient in the tests. To avoid this time factor, flat
granite pans 7 in. x 11 in. x 1 in. were used. After running
several series in the pans it was found that the pans were too
shallow, that is, they exposed too much surface for evaporation.
Preliminary experiments clearly demonstrated that the plants
grown in these pans showed a wide difference in the time and
degree of wilting an consequently made it impossible to determine
the residual moisture in the sand at the time of wilting of the
different individual plants. In the experiments that followed
single plants were grown in glass Soyka dishes 3 inches in dia-
meter and 1-J inches in depth.
The moisture content of the sand used in the different
series was 10^, 20^, 40'^, and 60'^ of its water holding capacity.
Beans, corn, and sunflowers planted in these dishes were allowed
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to grow from a week to ten days before they were placed at a
definite distance from an electric fan, and in the path of the
air current. When the plants were placed before the fan, water
was withheld. They were weighed every three hours and the rate
of loss in moisture was determined. When permanent wilting took
place, the plant was removed, and the sand was air ‘dried and
weighed.
In another series corn was grown under glass jars,
at a relative humidity of about 80f^ or 90fo, where the rate of
transpiration was low. The plants grew rapidly and were from
one to two days ahead of the corn grown otherwise. These seed-
lings, when placed in the air current from the fan showed a very
marked difference in the wilting coefficient. In place of the
plants wilting and falling over as in the first series, they dried
retaining the green color and not "firing" in any way. The plants
were permanently wilted in from 5 to 7 hours after being placed
before the fan.
In nearly all cases, the leaves of the diseased corn
when placed in the air current would begin to "fire" at the tips
and this would gradually enlarge, and at the time of wilting ex-
tended half way dom the leaf. The stalks of the diseased com
would wilt and bend over at the sand, while in the disease free
corn they would wilt at the insertion of the first leaves. In
the sunflower the youngest plants ivould be the first to wilt.
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while in the bean the oldest plants would wilt first. In the
sunflov/er there was considerable loss of water from the stems
as shown by the gradual shriveling. In the bean seedlings the
cotyledons showed a similar shrinkage due to shriveling. The
leaves seemed to retain their turgidity until the cotyledons
were dried up, and then they rapidly wilted. The bean never
fell over after permanent wilting.
There was little difference between the root develop-
ment of the disease free and diseased corn. The most marked
difference was in sand of saturation. Here the root develop-
ment of the diseased corn was very weak. The length of the roots
in the disease free corn was from 1^ to 2 times that of the
diseased. In sand of 60^ saturation molds would form around the
shoot of the diseased corn and the mycelium would branch out
over the sand and grow up the stalk.
In the carrying out the experiments considerable
difficulty was met with in keeping the water content of the sand
uniform throughout the dish. This was practically impossible to
do for the roots would have the bottom of the sand dry, while
evaporation from the surface would keep this below the moisture
content. The decrease I got in the ability of my corn to take
moisture from the soil was due to the physiological effect of
moisture upon the fungus which causes root rot of corn.
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-11
III. RESULTS.
Table 1.
Average results obtained on series including 46 beans.
57 sunflowers, and
93 diseased
and disease free
(Root Rot)
Plant
Soil
Moisture
'
lOfo
20fo
40^
eofo
Disease free corn
.27
.309
.73
.45
Diseased corn
.894
1.08
.999
.495
lOfo
20fo
ZOfo
40fo
Bean
.27
1.46 -
6.37
10.31 -
Sunflower
.462
1.12
1.48
8.26
Table 2.
Results on bean and sunflower seedlings.
Number
Dry weight
(grams)
Plant
Original water
content
Percent of non-
avail able water
16
1052
S. F.
40f
10.6
18
1083
n
30f
2.06
6
1092
tt
40^
11.7
3
1101
H
lOf.
.27
5
1007
M
30>
1.03
4
1061
((
20%
1.03
10
1007
II
20%
.69
59
1073
Bean
20%
1.3
60
1048
It
30^
1.3
68
1093
II
30fo
6.69
58
1134
II
20%
2.06
72
1070
t!
i0%
10.3
56
1120
II
10%
.27
55
1073
II
10%
.27
57
1065
II
10%
.27 -
67
1079
II
00%
6. 55
70
1079
II
Wfo
10.69
15
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6.89
71
1047
II
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-13-
Table 3. (continued).
Number
Dry weight
Plant
Original water
Percent of non-
(grams)
content
available water
69
1038
Bean
30^0
5.8
61
1107
n
30fc
5.8
63
1090
ti
20i
6.5
66
1114
II
40‘fo
13.0
64
1067
II
40i
10.0
53
1116
II
30i
1.7
65
1041
ti
40i
10.3
63
1075
It
30i
6.3
53
1103
It
30%
1.03
IV. CONCLUSIONS.
Individual plants show a wide variation to drought
resistance. This is due in part to the more efficient absorptive
power of the roots of some which can overcome to a greater degree
than others the physical forces that hold the water of the soil.
In general it may be stated that under similar conditions differ-
ent plants have different wilting coefficients in the same texture
of soil.
In making wilting coefficient determinations the follow-
ing precautions should be observed:
1. The soil used should be of uniform texture.
3. The soil should be brought to a uniform water content before
being used.
3. The moisture lost by transpiration should be replaced at
frequent intervals.
f: ’ *. . . V'
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4. All sudden fluctuations in temperature should be avoided.
5. The moisture determination should be made as soon as the
plant has reached the stage of permanent wilting.
There is a marked difference in the root development
of different plants. This difference may become more marked
under certain soil moistures. Thus in a soil of 10^, 20*^, 30^,
and 40^ moisture content the root systems of the bean and sun-
flower seedlings showed essentially the same development, but
the bean seedling had a lower wilting coefficient, than the sun-
flower seedling at 10^, while at 20/^, 30^, and 40'^ moisture con-
tent the sunflower had the lower wilting coefficient.
This shows conclusively that the wilting is dependent
on the absorptive power of the roots rather than on root develop-
ment. This was well brought out in the case \7here sunflower and
bean seedlings were grown in the same pot under identical condi-
tions. IIThere the plants were of uniform height and vigor the
bean seedlings under certain conditions, would wilt from one to
three hours before the sunflower. This may in part be due to the
difference in structure. In the case of the bean the cotyledons
would dry up before the leaves would show signs of wilting.
One may conclude that the difference in the wilting
coefficients of plants is due to a difference in the absorptive
powers of the roots rather than to differences in development,
which in a way may affect the wilting coefficient.
i:
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V. LITERATURE CITED.
1. Sachs. Tobacco plants in different soils. 1859.
2. Briggs, L. J. & Shantz, H. L, The wilting coefficient for
different plants and its indirect determination.
U. S. Dept, of Agr. Bur. of Plant Industry. Bull. 230. 1912
3. Blackman, V. H. The wilting coefficient of soils.
Jour, of Ecology. Vol. 2. 1914.
4. Caldwell, J. S. The relation of environmental conditions to
the phenomenon of permanent wilting in plants.
Physiological Researches. No. 1. 1913.
5. Crump, W. B. The coefficient of humidity.
New Phytologist 12:125. 1913.
6. Hedgcock, G. C. The relation of the water content of the
soil to certain plants. Bot. Survey of Nebr. Vol. 6.
7. Livingston, B. E. & Brown, W. H. Relation of the daily march
of transpiration to variations in the water content of
foliage leaves. Bot. Gaz. 52:309. 1912.