NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW THROUGH PLANTS1"2
R. DUANE JENSEN, STERLING A. TAYLOR, & H. H. WIEBE3
UTAH AGRICULTURAL EXPERIMENT STATION, LOGAN
I. Some investigators have considered the entire
soil-plant-atmosphere system (1, 6, 19, 24). They
applied an analogue of Ohm's law and showed that
water transport is controlled by the potential difference across the section and the resistance within
the segment. This theory also proposes the important consideration that the rate of movement is
governed by the point or region of greatest resistance
in the system. Those who have studied this theory
agree that the greatest resistance under natural conditions is usually located at the leaf-atmosphere interface where the water is converted from liquid to
vapor. Most of these studies seem to be based more
upon theoretical arguments than direct experimental
results.
II. Other scientists have investigated the movement of water in plants by studying some particular
part of the system, such as the flow of water in the
roots, leaves, or stem. Resistance to water flow in
the conducting tissue of the stem is generally considered to be small as compared to other parts of the
plant (5, 13, 15, 17). Some researchers have found
the resistance in the roots is much larger than in the
stems (12, 13, 14). Others have observed that the
resistance in leaves is larger than in stems and roots
(26). The resistance in the vascular elements can
become larger when very small diameters are encountered (7, 27). It has also been indicated that
the resistance to water flow is uniform through the
cell walls, membranes, and vacuoles of plant tissues
(1, 19). The experimental evidence to support these
concepts is meager and inconclusive.
Experimental measurements of the relative magnitude of the resistance of the stem, leaves, and roots
to water flow in the absence of a water phase change
have been made. This gives evidence of the relative
contribution of the several plant parts to water flow
resistance without the complicating factor of vaporization. Once this contribution to water flow resistance is known then studies can be made to combine
the vaporization and vapor diffusion resistance as
well as the soil resistance to water flow to the absorbing root surface. These experiments have also
produced some information about negative transport.
INTRODUCTION
Negative transport is the downward conduction
of water in the plant. This phenomenon has been
studied by several investigators, yet oonsiderable controversy about several aspects of the problem still
exists.
The portion of the leaf through which water enters is obscure. Meidner (16) suggested that specialized epidermal cells of the plant, Chaetachme
aristata were involved in the phenomenon. Gessner
(8) decided that most of the water was absorbed
directly through the cuticle. Most investigators (4,
23) have considered that no water enters through the
stomates (except perhaps a small amount of water
vapor).
Breazeale, McGeorge, and Breazeale (2, 3) investigated the absorption of water by leaves and its
subsequent transport through the plant to the soil
surrounding the roots. They concluded that tomato
plants could grow to maturity, flower, and set fruit
with no other source of water than that absorbed
through the leaves from an atmosphere of 100 %
humidity. They demonstrated that tomato plants
can absorb water from a saturated atmosphere, transport it to the roots, and build up the soil moisture to
or above the field capacity. Other investigators repeated the experiments of Breazeale but could get no
evidence of actual water secretion by roots (9, 10, 25).
Stone, Shachori, and Stanley (22) concluded that
negative transport occurs only when the temperature is allowed to fluctuate and is caused by vapor
pressure gradients and not by any active secretive
force within the plant itself. Slatyer (20, 21), who
reviewed these studies, stated that the main reason
for lack of transport into soil is lack of an adequate
gradient.
The movement of water in plants has been studied
from two different approaches:
'Received Feb. 6,
1961.
2Work reported here was done in cooperation with
the twelve western states and the Agricultural Research
Service, U.S.D.A. through Western Regional Research
Project W-67. Published with approval of the director,
Utah Agricultural Experiment Station as Journal Paper
172.
3 Research assistant, professor of agronomy (soil
physics), and associate professor of botany, Utah State
University, Logan.
MATERIALS & METHODS
A schematical drawing of the apparatus is shown
in figure 1. The apparatus consisted of two cylindrical lucite chambers ( A & B) each 12 inches long
633
634
PLANT PHYSIOLOGY
D----
FIG. 1. Suction apparatus for measurement of water
flow through plants.
and 5 inclhes in diameter. A removable lucite partition (C) containing a 2-inch diameter hole was placed
between the two chambers. Calibrated capillary
tubes (1 mm diameter) (E & E'), 100 cm long,
were fastened to the ends of the cylindrical chambers.
Mercury manometers (D & D') constructed from 1
cm diameter glass tubing, were joined to the capillary
tubes. The apparatus was suspended in a constant
temperature water bath after the plants were properly
placed in the chambers. The temperature could be
quickly changed and maintained at any desired value.
A coil of %4 inch copper tubing was placed inside
each chamber through which the bath water was circulated, thus reducing the temperature lag inside the
chambers. Tomato (Lycopersicont esculentum Mill.)
and sunflower (Helianthus annuus L.) plants, grown
in Hoagland and Arnon's number two nutrient solution (11) until they were between 10 and 14 inches
high, were used for the experiment. The plant stem
was placed through the hole of the partition (C) and
sealed with Armstrong's Adhesive A-1 (Armstrong
Products Co., Warsaw, Ind.) a short distance above
the roots. When the adhesive had hardened sufficiently, the partition was placed between the chambers
so that the roots protruded into one chamber and the
leaves into the other. The chambers were then bolted together and filled with air-free distilled water to
give a continuous water system from each chamber
through the capillary tubes to the mercury columns
of the manometers. At the beginning of each run,
suctions of equal magnitude were applied to both
chambers simultaneously to remove the air from the
plant tissue. Preliminary studies showed that unless
this air was removed, the water measurements were
jumpy and unpredictable.
FIG. 2.
differences
FIG. 3.
differences
FIG. 4.
differences
FIG. 5.
The amount of water flowing through the plant
was determined by the movement of mercury droplets
in the water filled capillary tubes. To make sure
that water was moving from the one chamber
through the plant into the other chamber, the mercury
droplets had to show movements in both tubes before
the flow was recorded. The system was designed so
that the roots or leaves or both, could be cut from
the plant with a razor blade through the stopper filled openings (H, H', H", & H"') in the cylinders
without removing the plant. This made it possible
to see how the rate of water flow differed when either
the roots or leaves or both were removed. The
leaves were severed at the point where they joined
the petioles; the roots were cut off just above the upper roots. All experiments were performed with the
entire system below atmospheric pressure, consequently the term suction is used rather than pressure.
Suction differences of 15, 25, 35, and 45 cm of
mercury and 20 C were used.
The apparatus was tested by sealing a glass rod
in place of the plant stem, the system was then placed
under suction to be sure that there were no leaks in
the system. With no leaks in the system any transfer of water from one chamber to the other would be
through the plant.
RESULTS & DISCUSSION
Water flowed equally well through plants in both
the normal and negative directions. This observation was further confirmed by the use of dyes. A
water solution of Red Shilling U.S. Certified Food
Color (McCormick & Co., Inc., Baltimore Md.) was
used. The color solution was tested to see that it did
not move through the plant tissue to any considerable
extent unless it moved with the water when a proper
gradient was established, (table I). With both sunflower and tomato leaves the food color solution
streamed from the leaf veins which extended to the
leaf edge. The coloring appeared as small streams or
rivers flowing into the water filled cylinders. The
solution apparently was escaping through the hydathodes of the leaves. In not a single instance was the
solution observed to escape from any other part of the
tissue. When the flow was in the negative direction,
the solution escaped near the tips of the roots much
the same as described for the leaves, except that the
dye streams were considerably smaller.
The relationship between the quantity of water flowing through plant tissue and time at a series of suction
for a whole sunflower plant No. S-14.
The relationship between the quantity of water flowing through plant tissue and time at a series of suction
for sunflower stem plus leaves No. S-14.
The relationship between the quantity of water flowing through plant tissue and time at a series of suction
for sunflower stem No. S-14.
The water flux as a function of the suction difference for sunflower tissues at 20 C.
635
JENSEN ET AL.-NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW
TIME (min)
I
-18
4
2
5 -
0.14
161
2
0.12
141_
E i2
-
0.10
4
E 10
*E 0.08
U-
UJ
E
8
D 0.06
-J
U-
6
I 45 cm Hg
2 35 cm Hg
3 25 cm Hg
4 IS cm Hg
41
Q04_
0.02-
2
f
I
2
.
I
4
I
6
8
TIME (min)
a
I
I
10
12
1D
IC
.
--
.
.
0% 1c,
SUCTION DIFFERENCE
636
PLANT PHYSIOLOGY
TABLE I
TIME REQUIRED
PLANT
PART
FOOD COLOR SOLUTIONS TO MOVE THROUGH PLANT TISSUES
NEGATIVE DIRECTIONS UNDER DIFFERENT SUCTION DIFFERENCES
FOR
DIRECTION
OF
FLOW
SUCTION
SUNFLOWER
TISSUE
DIFFERENCE
LENGTH
(cm)
TIME
SUCTION
IN
NORMAL &
TOMATO
TISSUE
TIME
( min )
DIFFERENCE
LENGTH
(cm Hg)
(cm)
6.6
46
5.6
6.0
52
4.8
(min)
Nor.
(cm Hg)
47
Neg.
50
Stem
&
Roots
Nor.
46
16
3.8
Neg.
54
13
3.4
46
16
3.4
Stem
&
Leaves
Nor.
49
2.8
50
15
1.9
Neg.
47
20
21
3.2
Nor.
54
45
26
0.3
42
20
0.3
21
0.4
43
15
0.25
Whole
Stem
Neg.
No reports from the literature have been found
that mention the possibility of the hydathodes functioning in the movement of water into and out of the
leaf except in the process of guttation. It has also
been observed that the drops of guttation water are
sometimes reabsorbed through these pores (18).
This evidence, coupled with the observation that the
water moved into and through the hydathodes of the
leaf equally well in both the normal and negative
directions, indicates that these pores might be involved in water absorption through the leaf.
The resistance to water movement in the various
tissues was calculated by A. directly measuring the
water flow as a function of time at a series of suction differences; B. evaluating the slope of the plot
water flow versus time to obtain the flux; C. obtaining the apparent conductivity from an evaluation of
the slope of the graph water flux versus the suction
difference, and D. applying the fact that resistance
and conductivity are reciprocals.
The plot of amount of water moved through the
plant versus time for the several plant tissues at a
series of suction differences shows a linear relationship (typical examples in figs 2, 3, & 4). The water
flux versus the suction difference relationships also
are linear, (typical samples of plots in fig 5).
The resistance to water flow was 67 % greater in
whole sunflower plants than in tomatoes when no
water phase change was involved. The resistance to
fluid transfer was 66 % greater in sunflower roots
and 92 % greater in tomato roots than in leaves of
the respective species. The resistance in the stem
plus leaves, stem plus roots, and the whole sunflower
plant was 101, 234, and 438 % larger than the stem
resistance. The same comparisons with the tomato
plant yielded percentages of 75, 238, and 483, thus
confirming that the conducting vessels of the stem offer little resistance to water transfer. The rate of
liquid water movement through the leaves is consider-
ably faster than through the roots. The differences
between the magnitudes of the resistances in the various sunflower tissues and the tomato tissues may
have resulted from the difference in the age and size
of the plants and the structural differences in the
conducting tissue of the two plant species.
The sum of the resistances for the stem, leaves,
and roots does not equal the resistance obtained for
the whole plant. The total of the resistances of the
several parts is somewhat smaller. It was impossible to obtain measurements on the leaves or roots
alone. The plant stem was included and had to be
subtracted to obtain the resistance in the roots or
leaves. If there was an error which increased the
stem resistance, the error would have a double effect. The stem resistance could easily have been increased if some of the vessel elements were crushed or
plugged when the roots and leaves were cut from the
stem.
The resistance in a system is usually proportional
to the length of the conductor and inversely proportional to its cross section. This raises the logical
question: Is the resistance in plant tissue the same
per unit length of tissue? If so, the larger resistances in the leaves and roots could be caused by the
longer channels involved in these tissues. Even
though the length and cross section of the conducting tissues could not be determined, the resistance
per unit of root and leaf tissue was calculated from
the root volume and leaf surface area measurements
that were obtained from the plants used. The resistance was 0.14 millibar seconds per gram of water
per cubic centimeter of sunflower leaf tissue. The
values were 0.075 and 0.0011 for tomato roots and
leaves, respectively. These results confirm that the
resistance per unit is greater for root than for leaf
tissue. No phase change was involved in water
movement out of the leaf. Any phase change would
undoubtedly (lecrease the flow rate considerably.
63 7
JENSEN ET AL.-NEGATIVE TRANSPORT & RESISTANCE TO WATER FLOW
TABLE II
AVERAGE CONDUCTANCE & RESISTANCE TN VARIOUS PLANT TISSUES
CONDUCTANCE
cm3 H2
(cm Hg) (min)
PLANT PART
5.2±0.2
8.4±0.3
14.0±0.3
28.1±0.4
Whole plant
Stem & roots
Stem & leaves
Stem
8.7±0.3
15.1±0.3
29.0±0.3
51.2±0.3
Whole plant
Stem & roots
Stem & leaves
Stem
a mb represents millibar.
In moving from the root surface to the xylem,
water must traverse the protoplasm or move along
the walls of the several layers of cells comprising the
epidermis, cortex, endodermis, and pericycle. It is
likely that these tissues account for most of the resistance in the root, and that the resistance per unit
length along the xylem of the roots is no greater than
the resistance in the xylem of stems or leaves. In
the leaves the water moved out through the hydathodes, in which the cells are loosely arranged, and
which apparently presented much less resistance than
the root tissues.
Water transfer through the roots and leaves was
further studied by altering the experimental apparatus so that the roots, a small section of the stem,
and the leaves of intact plants were sealed in separate
adjoining chambers. To measure the water flow, a
mercury manometer was fastened to the central
chamber, and the plant stem in this chamber was
severed. The amount of water moving into and
through the roots was measured with the capillary
tube fastened to the root chamber. The other capillary tube showed the water flow through the leaf tissue. This system provided a method of measuring
TABLE III
AVERAGE WATER FLUXa THROUGH SUNFLOWER & TOMATO
ROOTS & LEAVES AT SUCTION DIFFERENCES OF 25 & 35
CENTIMETERS OF MERCURY
PLANT
TISSUE
SUNFLOWER
Leaves
Roots
% Greater
in leaves
a
TOMATO
25 cm Hg 35 cm Hg 25 cm Hg 35 cm Hg
cm3/min.
0.0222
0.0134
64
0.0340
0.0207
65
0.0302
0.0181
64
0.0466
0.0271
69
RESISTANCE
(cm Hg) (min)
cm3 H)O
(mb) (sec)a
g H20
Sunflower plant
1,928
1,195
719
358
Tomato plant
1,150
666
345
197
2.4
1.5
0.88
0.44
1.4
0.81
0.42
0.24
the movement of water through the roots and leaves
of the same plant, at the same time, and under identical conditions. This appeared to be the best method
of comparing the rate of water movement in the
leaves with the flow in the roots.
The average water flow per minute (the slope of
the plot water flow versus time) through the roots
and leaves of each plant are recorded in table TII.
The movement of water through the sunflower leaves
was 64 and 65 % greater than in the roots for suction
differences of 25 and 35 cm of mercury, respectively.
The same comparisons with tomato plants yielded
percentage values of 63 and 69. Thus, the flow of
water was always considerably greater through the
leaves than through the roots of both sunflower and
tomato plants. These results confirm the conclusion
that the resistance to water movement in the roots is
considerably larger than that in the leaf tissue.
SUMMARY & CONCLUSIONS
Experiments, conducted under conditions which
eliminated the leaf-atmosphere interface and substituted a leaf-water interface, confirmed that water
can move into and through plants equally well in both
the normal and negative directions when the proper
gradient is established. Water flows through the
aerial parts of the plant more easily than through the
root tissue and appears to escape through hydathodes
of the leaves. This might explain the lower resistance observed in the leaf tissue since the water needs
to pass through only a few layers of loosely arranged
cells in order to escape from the leaf. On entering
the root, in comparison, the water probably encounters
most of the resistance on traversing the epidermis,
cortex, endodermis, and pericycle before reaching the
xylem. The greatest resistance was found in the
roots, followed by the leaves, with resistance to water
flow very much lower in the stem.
638
PLANT PHYSIOLOGY
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