Hales established the view of a pulled fluid movement in plants on
the basis of experiments with plant cuttings. The concept of water
transport within vessels and tracheid’s - considered as tubes - of
the xylem of intact plants was mainly developed by Boehm, Dixon
and Joly and Renner on plant cuttings and samples. The textbook
opinion of plant moisture movement is based on an assumed,
hydraulic volume flow in the xylem. It is supposed to take place in
continuous: "... water threads hanging from the water evaporating
leaf cell,” (Translated from German). In order to ensure the
cohesion of the fluid even under a presumed high tensile stress,
e.g., at growth heights above 10 meters, the tensioned water threads
must be in a metastable state, since under these conditions
(superheated state) they should physically boil with bubble
formation (“embolism”) already at ambient temperature and
tearing off. According to cohesion theory there should not form
bubbles (“emboli”) filled with gas or water vapor, and should not
be already contained in the xylem which would prevent the
suspected hydrodynamic fluid flow. In fact, gases or water vapor
bubbles are ubiquitously found in living plants [1-5]. The hydraulic of physics is part of fluid mechanics and the Hagen/Poiseuille law
applies to hydraulic volume flow in ideal capillary pipes or tubes.
The vessels and tracheid's of the xylem do not possess ideal
properties of pipes or tubes (Compare section 2.2), as required by
the hydraulic cohesion theory. Unlike ideal pipes, the walls of the
vessels and tracheid's interact with the transported water. The latter
penetrates (imbibes) the dead cell walls and the cells can shrink and
swell. Due to the interaction with water, the cells serve as source
or sink for the transported fluid.2Therefore, since the properties of
natural xylem do not conform to the basic law of fluid flow, the
continuity equation (No reaction of the wall material with the
moved fluid), is not satisfied by the cohesion theory [6-8]. The nonexistence of ideal capillary pipes/tubes as hydraulic pathways of
water, can be considered an excluding criteria for long-distance
fluid-flow transport in plants according to cohesion theory. Many
plant physiologists nevertheless consider the theory to be valid, but
it does not go unchallenged. See for example: and [9-12]. Textbook
authors as and express doubts regarding the current doctrine of
water transport in plants. Reject the cohesion theory and conclude:
"... that the arguments of the proponents of the cohesion theory are
completely misleading".
Controversies with the concept of long-distance water
transport, the cohesion-tension theory
“The cohesion-tension theory has been a controversial subject for
more than a century and continues to generate lively debate.” [13-
16]. The theory tries to give an answer to the question, according
to which principle the water transport in intact, living plants from
the root to the leaf takes place. It is supposed to be a hydraulic
suction driven flow in pipes, but this is not true.
In xylem there is no hydraulic suction driven flow
The transpiration of plant as well as many other diffusion
processes, can be physically described by Fick's laws.
Referring to the first law the authors write: „This equation accounts
only for movement in response to a concentration gradient, and not
for movement in response to other forces (e.g., pressure, electric
fields, and so on)”. So not in response to a pressure- driven volume
flow as one suspects with the hydraulic cohesion theory. Later in
their book, the authors contradict themselves on this point and
incorrectly assume that in plant water transport a pressure driven
volume flow according to Poiseuille would result from a diffusion
process, which is not true. An attempt is made to construct such a
process: “As water evaporates from the surface film that covers the
cell walls of the mesophyll water withdraws farther into the
interstices of the cell wall, and surface tension causes a negative
pressure of the liquid phase.”. This assumption of a withdrawal of
the water away from its destination the atmosphere, is contradicted
by several researchers. Contradicts this assumption: "... that even
under extreme evaporation conditions no withdrawal could be
expected”. contradicts this as well: “The large tensions that
potentially could be present in the cell wall generally do not occur
in living cells since water is usually available and gets ‘pulled’ into
the interstices, thus filling them.” notes for the above-described
reversal of the transport direction of water (“water withdraws”)
from the atmosphere to the root, that such a reversal of movement
is unlikely but could occur when the relative humidity of the
surrounding atmosphere exceeds RH = 0,98. However, this does
not describe moisture release (transpiration), but rather moisture
uptake via the leaf [17-19].
The xylem is not composed of pipes/tubes
The proponents of cohesion theory mistakenly consider the vessels
and tracheid of the xylem as tubes in which the Hagen/Poiseuille
flow law is supposed to apply, which is not established. Plants are
cut open from experimenters and plumbing suction or pressure is
applied to the samples open on both sides [20,21]. Samples cut
from plants are spun in centrifuges by Ziegler et al to impose a flow
on the moisture present in the xylem.
Figure 1: The rise height for water measured in vessels of Aristolochic
siphon and that calculated for ideal capillaries of the same inner
diameter according to [23] differ significantly.
The aim is to obtain results with specimens identical to the
behavior of rooted, intact plants, but this is already prevented by
the destructive experimental design. Strasburg devoted himself
to answering the question: is a volume flow-wise movement
according to Poiseuille possible? Consists the xylem of ideal
tubes/capillaries, yes, or no? He writes about his experiments on
Aristolochic Sipho and Vitus vinifera, among others: "Since
capillarity has played and continues to play such an important role
in explaining water ascension in plants, it is worthwhile to examine
directly what the plant waterways are capable of achieving as
capillaries. This is all the more worthwhile because the walls of
these waterways are in an imbibed state, and this raises the question of whether this state does not influence their conductivity”. And
concludes from this further: "For the tubular system of the plant, in
which the water moves, the same laws must also apply as for other
tubular systems,”. As a result of his experiments on the
"capillarity" of plant waterways, published with measured values
(Figure 1), Strasburger writes: " ... that the capillarity level for
water in the vessels of plants is considerably lower than in glass
capillaries ...", “. Where the measured value ... was often half that
calculated for glass capillaries." (The passages in quotation marks
are translated from German). From the finding in that the vessels
of the xylem do not behave like glass capillaries he concludes:
"There remains almost only the assumption that an interaction
between the imbibed wall and the content of the vessels influences
the shape of the meniscus, flattens it and thus reduces its loadbearing capacity" (Translated from German). In other words, at
least the investigated vessels of the xylem of Aristolochic Sipho
and of Vitis vinifera, do not correspond to the properties of
impenetrable glass capillaries on which the equation of capillary
fluid flow was empirically derived. See e.g., in. Thus, the need
mentioned above: “. The same laws must also apply as for other
tubular systems, ..." does not fit the presumed “tubular system” of
the plant. The results of the experiments speak against tubular
systems assumed in plants and thus against the cohesion theory.
The elimination of a fluid mechanical water transport in plants in
the sense of the cohesion theory - the non-existence of pipes/tubes
- is also described in: “. by the walls of the trachea which, as is well
known, though very permeable to water.". Referring to doubts
about the cohesion theory in: "A more serious difficulty is the fact
that wood does not contain ideal capillaries ... ", and Site et al point
out: " ... so it is clear that the pathways do not correspond to ideal
capillaries:” (Translated from German). Thus, the questioning of
the cohesion theory is confirmed by several researchers [22-26].
Therefore, the cohesion theory with its hydraulic view cannot be
accepted for water movement in living plants. How is water really
moved?
The diffusive/adsorptive principle of water transport at
the xylem/wood matrix
Physicochemical relationships between water and wood that
are important for long-distance moisture transport
Since water is transported from the root to the leaf in the xylem
(Figure 2 at b b) and there is a high affinity between water and
xylem, some physicochemical relationships of wood that are
important for understanding water transport are then discussed in
more detail in 3.1. Moist wood can be considered as an anisotropic
gel “In the case of wood, an analogy exists to an aqueous solution.
The wood can be thought of as being partially dissolved in the
sobbed water. As with a true solution this water is held at a vapor
pressure p, lower than the saturated vapor pressure p0 of pure
water" [27]. Damp wood is in equilibrium at a defined water
content, depending on the type of wood, with a certain relative
humidity RH. The relative humidity RH of the air is defined as p/p0
where p means the existing water vapor pressure is generally lower
than the saturation water vapor pressure p0
RH = p/p0 (T, p = constant) 1)
(RH multiplied by 100 is the percentage relative humidity)
In the wood of living plants, water may be present in three main
forms:
? Easily movable liquid water completely or partially filling
the cell cavities in Figure 3 at a and b.
? water vapor in the cell cavities at c and d and
? Bound water in the cell wall at a, b, c and d.Figure 2: Reduced tangential section of a coniferous plant in [28].
Legend: " a) The parenchyma covering the root tip. b b) The wood body
composed of tracheids in tangential section. The tracheids ...; their upper
part is filled with air, the lower part with water. All conducting structures,
which are in contact with the parenchyma of the root or the leaves, show
a spiral pattern on the wall, so that the water can easily pass through the
large thinned surfaces. ......, c) The leaf parenchyma, which gets the water
from the spiral vessels and loses it by transpiration." (Translated from
German). The outward epidermis with cuticle and stomata in the leaf at c)
and the cambium, cortex and bark of the woody body at bb), are not drawn
in [28].
"The liquid water in the cell cavities is sometimes called ‘free’
water to distinguish it from the cell-wall water which may then be
called ‘bound’ water. This is because it is attracted or bound to the
wood with stronger forces than those which hold the so-called
‘free’ water. “. The terms "free" and “bound” are relative because
the water is subjected to more or less large adsorptive van der
Figure 3: Schematic, idealized representation of the moisture distribution in the cross-section of a wood cell: a above the fiber saturation point Mf at
the maximum water content Maxx, b above the fiber saturation point Mf, c at the fiber saturation point Mf and d below Mf. Based on [27]
The maximum moisture content is rarely reached in living wood.
Even at maximum water content Mmaxmoisture is retained almost
without tension by the adsorption force fields of the wood at any
growth height. Only when a drop of water is placed on the upper
interface of a branch cut off at two points and held vertically with
maximum water content in the woody part, does a drop escape at
the bottom. At this high moisture value Mmax it can additionally
no longer be held by the adsorption force in the wood compared to
the gravitational force and drips off. To maintain the vital water
transport in the apoplast from the root via the stem to the leaf, a
continuous liquid film must be present in the xylem, the moisture
content in the xylem must be greater than the fibre saturation Mf.
Below this value, water transport is only possible in insufficient
amount in vapor form. Trapped air can make the matrix more
hydrophobic, its possible rehydration therefore more difficult.
Among other things, it follows: "Already the conduction channels
of the seedlings are filled with water and remain filled accordingly
..." (Translated from German). A main task, that the pathways are
not emptied too far in the course of transpiration but remain "filled
accordingly", comes to the closing movements of the stomata and
the almost hermetic closure of the conducting tissue to the outside.Waals force fields depending on the water content in the cell
cavities and is therefore, not in the same thermodynamic state as
ordinary liquid water in a large container.
The point of water content at which a given xylem cell has lost its
cavity water or "free" water and contains only water vapor in the
lumen, but the cell walls are still fully saturated, is called the fibber
saturation point Mf (Figure 3 at c). Near the fibre saturation point
(sometimes called the: fibber saturation range), several technical
wood properties change in addition to physicochemical ones.
Cambium, cortex and bark form the almost hermetic closure of the
xylem to the outside. In fact, according in, one of the foci of the
water transport principle is based on the "hermetic closure" of its
tracheal pathways to the outside for different species, the moisture
content of fiber saturation point Mf is generally between 25% and
35%, based on dry wood. The maximum moisture content Mmax
in Figure 3 at a, can range from about 30% (Pock wood: 31%) to
more than 230% (Balsa wood: 767%). The nearly airtight closure of the wood could be measured by using
absolute pressure sensors in sapwood. While July/August in stem
of Pendula pendula, the moisture/vapor pressure of the wood rarely
dropped to 0.0043 MPa absolute pressure (corresponding to the
boiling range of water at 30 °C). In contrast, the pressure more
often rose to more than 0.8 MPa. The maintenance of these
significant deviations from normal pressure indicates a high
airtight closure of the stem [28-30].
Water transport is driven by transpiration, a diffusional
process
Just before sunrise and adequate water supply, the plant should be
almost completely hydrated. There is no net movement of
moisture. From this state of equilibrium, the stomata open in the
leaf after sunrise, and transpiration starts. During it, water
evaporates from a thin film covering the walls of the leaf
intercellular surface belonging to the apoplast. The phase transition
from water film to vapor during evaporation, represents a diffusive
process. The latter is part of transpiration. The reason for
evaporation is the clear concentration gradient for water between
the liquid phase of the intercellular walls with high water concentration and the water vapor phase of the outside air with low
water concentration. The atmosphere is relatively dry and can: "...
dehydrate the plant". Without any energy input of their own, rooted
land plants exploit the external concentration gradient between
moist soil and relatively dry atmosphere for water transport from
root to stem to top.
The diffusive evaporation of water during transpiration physically
follows Fick's first law:
J = - D. A. ?c/?x (2)
In equation (2) J [mol/s] means the quantity of of a substance diffused
per unit time, D its diffusion coefficient [m2
/s], A the crossed area
[m2] and ?c/?x the concentration gradient [mol/m3
. m] of the
substance. The quantity of a substance diffused per unit time J
[mol/s], is directly proportional to the size of the diffusion
coefficient D [m/s], der crossed area A [m2
] and the driving
concentration gradient ?c/?x [mol/m3
. m]. The minus sign occurs
because the direction of net diffusion is towards regions of lower
concentration. The movement of water vapor from respiratory
cavities outward to the atmosphere within the transpiration process
also occurs by diffusion. The moist intercellular area A of the
respiratory cavities, adjacent to the outside air when the stomata
are open, is huge. By additional action of wind, a large
concentration gradient ?c/?x of water vapor between leaf interior
and atmosphere may be maintained at the leaf surface. Wind
amplifies the quantity of total stomatal and cuticular transpiration.
The transpiration- or diffusion- rate E [mol/m2
. s] of water between
foliage and atmosphere measured in many studies reflects about
70-75% of the amount of water taken up by the root, moved and
released to the atmosphere by transpiration (diffusion) [31]. For
example, the transpiration- diffusion- rate of a Tradescantia zebrine
plant in still air E = 1.5. 10-3[mol/ m2
. s] and for strongly moving
air almost three times the amount (= 4.4. 10-3[mol/ m2
. s]) was
measured (Experimental conditions: 23°C ±2°C, relative humidity
of ambient air RH = 0.65, stomata width = 15µm). Transpiration
requires a high amount of energy, which can dehydrate the plant.
The energy must be supplied by the environment of the plant. The
molar heats of vaporization of water ?H0 at standard pressure and
30°C is: ?H0 = 43,7 kJ. For comparison: the enthalpy changes by
warming-up one mole of water ?Hw from 30 °C to 31°C needs
significantly less energy: ?Hw = 0,075 kJ [43]. Transpiration
involves a simultaneous transfer of energy and mass. In the
apoplast too, a concentration gradient ?c/?x develops between the
water-bearing root xylem which is higher in moisture to the waterreleasing intercellular walls in the leaves which have lower
moisture concentration but without phase change. Spontaneously,
the water diffuses down the physiological concentration range from
the root to the intercellular walls in the leaf. A purely diffusive
water transport is superimposed on its path in the xylem by a
desorption/adsorption process triggered by the partial dehydration
of the plant via the effect of van der Waals forces in the apoplast
(Compare section 3.4). The huge outer and inner surface A of root
and leaf, as well as the huge inner apoplectic surface of root, stem
and leaf, allow - in addition to the factors described above - a
potentially high thermodynamic mass movement triggered by
diffusive transpiration.
Support for the diffusive plant water transport in
vascular bundle with heavy water HDO under noninvasive conditions
Heavy water HDO, diluted with normal water, has almost the same
physiological properties as normal water and can be used in plants,
for example, as a tracer for the movement of water. HDO can be
quantitatively determined within rooted plants by mass
spectroscopy. discovered on young Vicia fava plants, which had
been grown in nutrient solution and then adjusted in diluted heavy
water HDO, that the tracer, only after about 25-30% of the total
water of the plant had evaporated, could be detected in the
transpiration water after about five to eight hours (Depending on
the transpiration rate). During the transport process in the plant,
there was a measurable exchange between vascular water and the
cellular water of the surrounding parenchyma tissue. This shows
that during its movement in the plant transversely to the shoot axis,
the water shifts molecularly beyond the xylem conducting bundles
by de- and re-wetting, so to speak "on a broad front", and does not
flow in ideal capillaries according to Hagen/Poiseuille (Cohesion
theory). Result: "The water in the vascular bundle of the plant is in
diffusion equilibrium with the total water of the plant"(Translated
from German).
Inside xylem, a de- and an adsorptive water transport
superimposes the diffusive moisture movement
In the hermetically sealed, porous system of the xylem, a diffusive
movement of water in response to a concentration gradient (3.2)
can be superimposed on additional physicochemical transport
processes, such as desorption/dehydration (3.4.1) and subsequent
adsorption/rehydration (3.4.2). These sorption processes between
wood and water are based on physisorption. The holding and
moving adsorption forces are relatively weak induced dipole and
van der Waals forces respectively, acting from wood to water and
from water to water. Phys sorption is often characterized by the
formation of multiple adsorption layers (“multilayers”). The
moisture de- and adsorption overlap reversibly in the physiological
moisture range of the xylem between Mph and Maxx, where water
transport takes place [32-34].
Desorption (Dehydration)
Based on the assumed moisture saturation of a land plant in the
morning, after opening the stomata first the imbibed mesophyll cell
walls lose water in vapor form to the atmosphere. Endergonic ally ("energy-consuming") this is 7 controlled by the prevailing
environmental conditions. The extent of desorption of the leaves
discussed here can be quantified, for example, by measuring their
temporally decreasing moisture content (Figure 4).
Figure 4: Daily variations of the moisture content (% of dry weight) of
Pinus ponderosa needles (dry site, summer). Chandler et al. [35].
The investigation of the daily water content variation of the needles
of Pinus ponderosa according to Figure 4, shows a maximum at a
moisture content of about 155% in the night hours and a minimum
of 125% in the afternoon, both related to dry weight. The daily
water loss by desorption in this study is about 30%. From the
measurements it is clear from this example that the water loss is
not immediately compensated. The observed daily fluctuations
occur. It is not until the following night at 24 o'clock that the
approximate 155 % of the initial value is reached again after water
adsorption from the root. As a result of the concept of local
equilibrium it can be concluded that the measured moisture content
of the total needles corresponds approximately with the xylem
moisture content which was not measured. This is because: "...
most cells in a typical leaf are within 0.5 mm of a minor vein"
[35,36]. After long lasting moisture loss of the leaves, the water
loss continues also in the wood at the trunk down to the root: As
daily maximum of the water content in the sapwood was measured
in the outer 10 annual rings of the trunk of three different Picea
abies trees in with 4 samples each, on average = 175 % moisture
content and as daily minimum also in the three spruces with the
same sample size on average = 150% (Late summer 1967, moisture
content measured at 1,3 m stem height, tree age approximately 75
years with a mean height of approximately 27 m, pure Norway
spruce [Picea abies] forest near Munich/Germany). The daily water
loss of the wood in this study is 25%. For the fibre saturation point
Mf of the sapwoods of Picea abies, a value of 35% is given in
tables, and for its maximum moisture content Maxx = 201% in
[37]. Here between both values the moisture content of the wood
remained under favourable growing conditions. Using hydroponics
was able to measure the transpiration losses and water supply of
trees simultaneously and separately. On a 3,12 m tall Canadian
poplar (Populus x canadensis), it was shown that the tree began to
take up water through the root only after reaching a saturation
deficit after dehydration of the total plant of about 3 kg of water.
The externally forced dehydration of plants described above
changes not only the moisture content but also the energy ratio of
the water to the wood. This is crucial for water transport in the
xylem: dehydration leads to new endergonic ("energy-consuming")
formation of free adsorption sites, or free interfaces for water. They
are energy-rich and form a thermodynamic energy store.
Depending on the temporal adsorptive reoccupation with water
(rehydration), the storage can be maintained for more or less time.
Adsorption (Rehydration)
With sufficient water supply from the root, the energy storage
wood (formed after desorption of water) will exergonic ally
("energy-releasing") and spontaneously, adsorb water and move it
almost without tension. For water involved in this physical
reaction, the so-called free energy G is a measure of the driving
force of the reaction. The decrease in molar free energy ?Gs of
water, associated with the adsorption of liquid water by a
hygroscopic material such as wood is given by the following
equation
?Gs = - R · T · ln (RH) (3)
Where ?Gs [J/mol] is the molar free energy of water, R [J/mol ×
K] is the gas constant, T [K] is temperature (in degrees Kelvin) and
RH the relative humidity of the atmosphere surrounding the moist
hygroscopic material. Equation (3) is valid for reversible processes.
Reversibility applies in the physiological, liquid water containing
region above Mph for the de- and adsorption processes considered
here, and thus for the region of water transport [34]. Equation (3)
can therefore be applied in this moisture concentration range. In
principle, a spontaneous change going toward equilibrium, such as
the adsorption of water, can be used to do work, including water
transport against gravity. With the adsorption process there is - at
constant temperature and pressure - a relationship between the
maximum (non-expansion) work waxes and the change in free
energy ?Gs.
Wax’s = ?Gs (T, p = constant) (4)
The desorption isotherms of moist wood of Pinus tied and
Que
The desorption isotherm of water-saturated wood, respectively of
a coniferous tree Pinus tied and a deciduous tree Quercus rubric, is
shown in Figure 5. crus rubric to the aim of their work the authors point out in “Specifically, it should provide insight into the
fundamental driving force for the transport of water in wood ...". In
similar studies on Pica Mariana are performed. Using special
relative humidity measurement methods beyond the isopiestic
method above RH = 0.955, characteristic desorption curves are
obtained over the entire range of moisture content. These are the
tension plate, the pressure plate and the pressure membrane
methods. For undisturbed water transport in the xylem from root to
apex, the continuous presence of liquid water, i.e., above the fibre
saturation concentration Mf, is crucial. According to, "liquid"
water is present in the physiological range of the wood of Pinus
teed and Quercus rubra above a relative humidity of about RH =
0.98 (Figure 5). This region is above a moisture content
corresponding to the fibre saturation point Mph (Figure 3 in a and
b). For comparison with the leaf moisture conditions (at the inner
side of the stomata), the relative humidity amounts to RH = 0.95
(25 °C) The authors in mention:”. a value of nearly 1 ..." for the
relative humidity inside a leaf. During the desorption (dehydration)
of the wood, starting from the maximum water content Maxx (for
Pinus taeda = 160%, for Quercus rubra = 120% in), the range
around Mf coincides with the loss of liquid water in the cell cavities
of the xylem (Figure 3 at c). Mf is assumed to be = 30% for Pinus
tied and Mf = 25% for Quercus rubra. The abrupt transition of
water content at Mf from the so-called over-hygroscopic- (above
Mf) to the hygroscopicrange (below Mf) in the coniferous wood of
Pinus tied appears remarkable. In the hardwood of Quercus rubra,
the decrease in concentration from the over-hygroscopic moisture
range to the hygroscopic range is much less abrupt than in the
softwood plant. This may be related to the fact that there is always
more living parenchyma in hardwood than in softwood.
Figure 5: Comparison of the de-sorption isotherms of Pinus taeda and Quercus rubric, measured at 30 °Cover the entire range [40]. For clarity, the
range of fiber saturation point Mph (? 30%) and maximum water content Maxx (?160%), is plotted for Pinus teed only.
Table 1: Measurements and calculations belonging to the desorption isotherm between Mf and Maxx of Picea taeda wood (Ambient
temperature 30°C) [40].Energetic consideration of sportive plant water transport
on moist wood of Pica tied
Using the measurement results from in column 1 (“Moisture
content”) and 2 (“Relative humidity”) of Table 1, it is possible to
calculate the decrease in molar free energy ?Gs, according to (3),
listed in column 3 (“Calculated”), made during spontaneous
adsorptive rewetting of the wood from Mf to Maxx. During the
adsorption of one mole of water mw (mw = 0,018 kg) for example
from Mf to Maxx of the wood of Pica tied, ?Gs = 7,1 [J/mol] free
energy is released (Table 1). According to (4), this value
corresponds to molar (lifting-) work wax’s ? 7,1 [N × m/mol].
Without loss, one can adoptively raise the mass mw of one mole of
water by the height h = wax’s / (mw × g) ? 40 m (with: g = 9,81
[m/s2
] and: N = kg ×m/s2
). Using measurements of the desorption
isotherm on Pica Mariana in, similar calculations are made for Pica
Mariana in [38-42].