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Chapter III. Water And Plant Cells
Water is frequently a limiting resource for plants. Crop yields are directly affected by the number of days
of high water availability during the growing season, or by annual precipitation. Plants must transpire
water at the leaves as a consequence or tradeoff needed for obtaining atmospheric CO2—the H20:CO2ratio
is 500:1 in some healthy well-watered plants. The cell wall is porous to water.
Water moves passively in the plant down gradients of water potential—there are no water “pumps”.
Water: Heat Properties
Water is a polar molecule. The hydrogens form an angle of 105º with the oxygen. The O is more
electronegative than the hydrogens. The weak separation of partial charges and resulting electrostatic
attraction between opposites allows water to form hydrogen bonds. Water is an excellent solvent due to its
small size and polar nature, especially for ionic substances and molecules with -OH or -NH2 groups. The
electrostatic attraction increases the solubility of polar substances in water.
Water requires an unusually large energy input to raise its temperature (high specific heat capacity cp).
Water (liquid) cp = 4.1855 J g–1 K–1 (or about 1 calorie / gram -°C)
Water (liquid) Cp = 75.327 J mol−1 K−1 (or about
The latent heat of vaporization for water33 is c. 44 kJ mol-1 or 2272 J/g, a very high value (e.g., ethyl
alcohol is 855 J/g, and ammonia is 1369 J/g).
These properties help plants buffer temperature fluctuations intrinsic heat capacity and by evaporation.
Water: Cohesion, Adhesion, Surface Tension, and Capillarity
Water has a high cohesivity due to the mutual attraction of its polar molecules—this can be seen with
water droplets.
Specific heat capacity: Specific heat capacity cp is defined as (T/N) (∂S/∂T) where T is the temperature, N
is the number of molecules being heated, and ∂S/∂T is the ratio of heat input to temperature change:
At an air-water interface, there is a greater attraction between water molecules compared to between air
and water molecules, and the lowest energy configuration is one which minimizes the surface area of the
interface.
Water’s adhesivity (attraction to an adjacent solid phase) varies with the solid material and is quantified
via the contact angle. A low contact angle (θ < 90º) indicates relatively high adhesion to a hydrophilic
substance such as glass (higher “wettability”and higher capillarity (i.e., higher level in a capillary tube
compared to the baseline water level). A high contact angle θ > 90º indicates relatively low adhesion to a
hydrophobic substances such as a waxed surface, lower wettability, and lower capillarity (i.e., lower level
in a capillary tube compared to the baseline water level).
In order to overcome the force of cohesion and adhere to a larger curved surface area interface with a
hydrophilic substance, additional energy is required to exceed the minimum energy configuration. This
quantity is called surface tension, and is measured in energy/area J/m2 or in Force/distance N/m)
The surface tension produces a net pressure differential at a curved air-water interface. The pressure is a
function of the surface tension and the radii of curvature of the surface, and is given by
ΔP = γ ( 1/Rx + 1/Ry) where
ΔP is pressure difference in the water
γ is surface tension [72.0 mN m-1 for water at 25 ºC]
Rx and Ry are radii of curvature in each of the axes that are parallel to the surface
and positive when directed into the fluid.
When Rx and Ry are positive (as with hydrophobic surfaces), the internal liquid pressure is increased
allowing the surface to bow out in the center and thereby diminish contact with a hydrophobic surface.
When Rx and Ry are negative, the internal liquid pressure is decreased, allowing the water to extend further
up the hydrophilic surface.
The pressure differential that develops in water from an air-water interface due to surface tension
increases with decreasing radius. For interstices between hydrophilic surfaces (such as inside leaves or
soils), lower water content translates to smaller water collections having smaller radii of curvature, leading
to a greater tension (more negative pressure change, ΔP<0) within this water.
Pressure Unit Comparisons:
1 atmosphere = 14.7 PSI or 0.1013 MPa
1 MPa = 9.9 atmospheres
Tire pressure and home plumbing pressure are c. 0.2 MPa
A water column of 15 feet has positive pressure at the bottom of 0.05 MPa.
Water has a high tensile strength arising from its surface tension. This allows water to be pulled up a tube
by tension above. Capillarity in xylem vessels contributes < 1 meter rise, so cannot explain water
transport in tall trees . In a thin capillary, the negative pressure (tension) sustainable
before cavitation (breaking into bubbles) occurs can be as high as -30 MPa. Microbubbles greatly
increase the likelihood of cavitation.
Diffusion Rates of Dissolved Substances
Substances diffuse down a concentration gradients according to Fick’s first law
J = - D ∂φ/∂x where
J is the diffusion flux density (rate of transport) in (amount of substance) length−2 time-1,
D is the diffusion coefficient or diffusivity in length2 time−1
φ (for ideal mixtures) is the concentration in (amount of substance) length−3
Diffusion rate is proportional to concentration gradient. D depends on the substance diffusing, the
medium through which it diffuses, and Temperature. Diffusion can be rapid over short distances, but is
very slow over long distances and not adequate, for instance, to move glucose between parts of a plant
(which would require 32 years to diffuse 1 meter—
Bulk Flow of Water, Water Potential, and Osmosis
Water moves passively in the plant down the gradient of water potential (if semi-permeable membranes are
involved) or the gradient of pressure potential (if bulk flow not involving semi-permeable membranes).
Because diffusion would be so slow, water containing solutes generally moves by bulk (mass) flow. The
volume flow rate is given by Poiseuille’s equation:
Volume flow rate (in m3 s-1) = (πr4/8η
r = radius of tube
η = viscosity
ΔΨp/Δx = gradient of hydrostatic pressure potential
Thus such flow is very dependent on radius of the conducting tube. Bulk flow accounts for long-distance
transport in the xylem.
While chemical potential expresses free energy per mole, water potential is usually used, and is given in
terms of the partial molal volume of water (thus a measure of the free energy of water per unit volume, in
unit of J m-3). 1 mole of water occupies 18x10-6 m3 mol-1. Water potential in plants is primarily a sum
given by
Ψw = Ψs + Ψp + Ψg where
Ψw = water potential
Ψs = solute potential
Ψp = hydrostatic pressure potential
Ψg = gravity potential
The units of these potentials are therefore energy per unit volume J m-3 = N-m m-3 = N m-2 = Pa. (Other
pressure units such as MPa may also be used). The potentials must be given relative to a reference,
usually pure water at ambient temperature and atmospheric pressure and at some defined height relative
to the plant.
Ψg gravity potential is given by
Ψg = ρwgh where
g = gravitational constant = 9.8 m s-2
ρw = water density = 1000 kg/m3
ρwg = water density times gravitational acceleration (9800 Pa = 0.0098 MPa ≈ 0.01 MPa m-1)
h = height (m)
For a height of 1 m, the Ψg is 9800 Pa or 0.0098 MPa or c. 0.01 MPa.
For a height of 10 m, the Ψg is c. 0.1 MPa.
For a height of 100 m, the Ψg is c. 1 MPa.
Solute or osmotic potential Ψs is usually negative because solutes reduce the free energy of water by
diluting it and increasing entropy. It is given by the van ’t Hoff equation:
Ψs = -RTcs where
R = gas constant (8.32 J mol-1 K-1)
T = degrees Kelvin
cs = solute concentration in osmolality (moles of osmotically active solute such as ions
per liter of water, or mol L-1)
Hydrostatic Pressure Ψp: Inside cells it is called turgor pressure and is usually positive. (It might
conceivably be negative at times in lignified living cells, but not in more flexible living cells.) It is often negative in the xylem.
It can be hard to distinguish between solute potential Ψs and hydrostatic potential Ψp in dry soils and dry
plant tissues such as seeds, and these terms are sometimes combined as matric potential Ψm (Ψm = Ψs +
Ψp).
Measure water potential with psychrometer or pressure chambers, solute potential with cryoscopic
osmometer, pressure potential with pressure probes,
Water moves across a semi-permeable (selectively permeable) membranes from areas of higher water
potential to areas of lower water potential—this process is called osmosis. (NOTE: In the absence of the
membrane, the movement of water is controlled simply by hydrostatic pressure differences.) Cells receive
water by reducing their internal water potential.
The transport flow rate of water across a membrane Jv is given by
Jv = Lp ΔΨw where
Jv = transport flow rate of water across a membrane (m3 m-2 s-1 or m s-1)
Lp = hydraulic conductivity of the membrane (m s-1 MPa-1)
ΔΨw = transmembrane water potential difference ΔΨw
Osmosis across semi-permeable membranes is driven by water potential gradients, combining both the
solute potential Ψs and the hydrostatic pressure Ψp. Plants do not have active transmembrane water
pumps, but can change intracellular (or intraluminal xylem) solute potentials in order to influence water
flow. Water can move in a plant against its water potential only when coupled to the movement of solutes,
in which case the decrease of free energy of the solute exceeds the gain in the water’s free energy—the
overall net change of free energy is negative as expected. Flow across membranes is passive in
response to water potential differences, and occurs primarily through water-selective aquaporins (integral
membrane proteins) rather than directly through the otherwise nearly impermeable membrane.
Aquaporins can be gated reversibly, so that plants may be able to control their plasma membrane water
permeability.
A wilted or flaccid plant cell has low turgor(hydrostatic) pressure, though its negative solute potential contributes
to a typically negative water potential. The cell wall collapses along with the plasma membrane and remains in contact with it.
A flaccid cell with high internal solute content placed in 0.1 M sucrose will attract water to its interior and
into the vacuole, and may end up with increased hydrostatic pressure as a result of distension, especially
of the vacuole, pressing against the restraining cell membrane. Cells placed in unusually high
concentrations of glucose may exhibit plasmolysis from water loss—a separation of the plasma membrane
from the cell wall with interposed fluid [or air, somewhat like a pneumothorax]. (This phenomenon rarely
occurs in nature except in extreme conditions such as air entry through the cell wall. )
External pressure applied to a cell may squeeze water out of it, make its internal solute concentration
higher and its solute potential more negative. Small changes in water content of a cell may result in large
changes in hydrostatic pressure, as depicted in Höffler diagrams. If cell volume decreases by only 10-
15%, the turgor pressure may drop to 0.
In cacti, the water-storing cells are much more distensible than the photosynthetic cells, due to more
flexible walls, so are less prone to have rising hydrostatic pressure.These cells give up their water
more readily, allowing the photosynthetic cells to maintain higher water content during water stress.
The Ψs of the apoplast is usually very low.
Well watered plants have water potentials of 0 to -1 MPa, but with higher degrees of water stress develop
water potentials down to -3 of -4 MPa, with extreme cases down to -10. Halophytes, which grow in saline
conditions, are able to survive by generating a very low intracellular Ψs, allowing them to maintain a lower
intracellular water potential to attract water. Water stress affects many processes as follows (listed in
order from earlier to later to occur based on degree of water stress):
Cell expansion < Wall synthesis < Protein synthesis < Stomatal Conductance <
Photosynthesis < Solute accumulation < Abscisic acid accumulation
Positive turgor pressure is important to stretch the cell walls, provide support for nonlignified tissues, and
facilitate plant growth.
Chapter IV. Water Balance Of Plants
The need for CO2 gas exchange exposes plants to risk of dehydration. The plant does not use metabolic
energy to pump water in from the soil, but rather uses spontaneous movement of water to regions of lower
free energy and water potential.
Soil Water
Soils range from sandy soils (with c. 1 mm particles, low overall surface area, and low “field capacity” to
retain water) to clays (with particles < 1 μm, high overall surface area, and high capacity to retain water).
Soil has air in between the particles. Soils typically have a nearly zero water potential Ψw due to low
concentrations of dissolved solutes (but may be more negative if solutes are high). In addition, Ψp is near
zero for wet soils, but more negative for dry soils. The negative potential arises from the relationship of
surface tension and curvature of the water/air interface in the soil interstices as previously discussed. Soil
water tension may be estimated at
Ψp = -2T/r where
Ψp = hydrostatic pressure (tension in this case)
T = surface tension of water (7.28 x 10-8 MPa m)
r = radius of curvature at air/water interstitial interfaces
As the soil dries out and water recedes into the smallest cracks, the radii of curvature become smaller (c. 1
μm) and soil Ψp becomes more negative to as low as -1 to -2 MPa.
Water moves in soil by bulk flow along pressure gradients ΔΨp, and also by diffusion, especially in drier
soils. Plants deplete the water (and nutrients) next to the roots and root hairs, locally reducing Ψp near the
root surface. This water deficit is replenished at a rate dependent on the hydraulic soil conductivity.
This quantity decreases as water content decreases, due primarily to presence of soil air
and narrowing of conduction channels. In very dry soils, the Ψw may fall to the permanent wilting point
(c. 1.5 MPa) at which point the plant cannot maintain turgor pressure.
Root Absorption And Water Transport
Root hairs greatly increase the surface area available for absorption. Water enters primarily near the root
tip. Older root areas are sealed off by hydrophobic materials and protective outer tissues (exodermis,
etc.)—this facilitates water entry preferentially from new regions that have not already been depleted of
nutrients. The delicate connections between new root tips and hairs and soil water are easily disrupted, as
with transplanting seedlings.
Water enters the root via both apoplastic and symplastic pathways under the driving force of differences in
Ψp. In the apoplast pathway, water travels across the epidermis and along the root cortex. At the
endodermis (surrounding the vascular bundle), apoplastic water encounters the Casparian strip, an
impervious barrier of suberin, forcing this water to cross a cell membrane (through aquaporins) and enter
the symplast. This transport process requires actively metabolizing root cells and is inhibited when these
cells are not receiving adequate O2 (such as when the root is submerged, since water diffusion of oxygen is
much slower). The permeability of aquaporins may decrease from increased pH [sic, may actually be
acidosis] resulting from decreased rates of respiration. Thus shoots may paradoxically wilt when roots
are flooded with water and hypoxic.
Water crossing a cell membrane and entering directly into the symplast traverses the cytosol of cells via
their desmoplasts until the xylem conduits are reached. There is also a possible “transmembrane”
pathway involving multiple membrane crossings, including the tonoplasts. Both of these pathways depend
on well-oxygenated root cells that can be impaired by root hypoxia/anoxia.
Roots can generate “root pressure”, a positive Ψp in the root xylem as high as 0.05 to 0.5 MPa. This
occurs primarily at night and early morning when soil water potential is high, humidity is high and
transpiration rates are low—absorbed solutes build up in the xylem, lowering Ψs and decreasing Ψw in the
xylem, providing a driving force like an osmotic cell for water absorption. Positive root pressure is
associated with guttation or “dewdrops” of xylem sap forming at hydathodes at vein endings at the tips of
leaves. Positive root pressure may help to dissolve xylem bubbles developed during the day.
Water Transport Through The Xylem
Xylem consists of dead lignified (and thus rigid) tracheary elements: tracheids and vessel elements .
The simple pits connect adjacent tracheids and vessel elements, and the
latter also have connecting end wall perforations (which provide a larger and lower resistance
connection). The simple pits have a porous pit membrane (with a thickened torus in conifers) that acts as
a safety valve which closes in the event of gas bubbles and air embolism.
Water movement in xylem is by bulk flow along pressure gradients ΔΨp, and is facilitated by the relatively
low resistance. A velocity of 4 mm s-1 and vessel radius 40 μm requires by Poiseuille’s equation a gradient
of 0.02 MPa m-1, but actual resistance may be twice as high. To move water across cell membranes at this
rate would require 2x108 MPa m-1—thus xylem transport is vastly more efficient.
The pressure differential required to lift water 100 m from the base to the top of a tall tree is estimated at 2
MPa, or 3 MPa allowing for friction—the top of the tree must have a water potential of c. -3 MPa.
The still somewhat debated cohesion-tension theory of sap ascent explains how this
pressure differential is achieved . Root pressure (0.05 to 0.5 MPa maximal under optimal
conditions) is not sufficient, and most of the pressure differential arises from tension (negative pressure)
that develops in the leaves and that “pulls” the water up. The water in the interstices between the
hydrophilic mesophyll cell cell walls is like a fine capillary network, forming curved air-water interfaces
which induce a tension transmitted to the cells. The sun provides the energy by heating and evaporating
water that is transpired.
But can xylem cells really maintain such high tensions without cavitation? Apparently so, though some
vessels do cavitate .The lignin prevents collapse of the xylem vessels under tension, and
denser wood is required to sustain higher tensions. The tension creates a metastable state in the xylem,
since the saturated vapor pressure of water is c. 0.002 MPa at 20 ºC. Why does the water not boil?
The cohesion and adhesion make the activation energy of the
transition to the vapor phase very high, and the xylem structure minimizes the existence of nucleating
sites for gas bubbles. [In contrast, the highest sustainable water column at 1 atmosphere (0.1013 MPa) is
about 10.3 m =33.9 ft in larger-diameter non-biological pipes—above this height, the water boils or
“cavitates”.]
Water Movement From The Leaf To The Atmosphere
The waxy impervious cuticle prevents water diffusion across much of the leaf surface—95% of water exits
by diffusion via the stomata. The factors affecting this transpiration movement are
(1) a gradient in water vapor concentration between the internal leaf air spaces (5-40% of the leaf) and the
air in the boundary layer immediately outside the stomata, and
(2) Diffusion resistance
The internal surface area inside the leaf and from which water evaporates is quite large, 7-30x the leaf
surface area. This allows water potential equilibration inside the leaf between the air spaces and the cell
wall spaces. The water potential for inner air is c. -1.38 MPa, for inside air next to the pore is -7 MPa, and
for air just outside the pore is -103 MPa. Thus water vapor wants to move down this potential gradient to
the outside.
The boundary layer water content, and thus the potential gradient, can be strongly influenced by wind
speed (“still air” diminishes exchange), leaf size and shape, fluttering of leaves (as in quaking aspen), leaf
hairs, and sunken configuration of stomata. The opening of the stomata is done to lower resistance to CO2
diffusion and increase uptake, but water loss inevitably accompanies this. When a plant attempts to
prevent dehydration by keeping its stomata closed, the availability of CO2 inside the leaf diminishes, and
therefore photosynthesis diminishes. Movement of water in outside air away from the boundary layer is by
convection, a type of bulk flow.
The stomata pore size is regulated by guard cells, which are found in liverworts, mosses, and higher
plants. They guard cells are flanked by subsidiary cells. The cellulose microfibrils of guard cells vary with
species but tend to fan out radially from the pore [or are oriented like barrel hoops per LVV], so that when
the cells enlarge from increased turgor pressure, they primarily elongate Since the ends are tethered
together, and the inner and outer cell wall may differ in thickness, elongation results in bowing outward,
opening the pore. Swelling results from lowering of Ψw inside the cell compared to adjacent cells resulting
from lowering of Ψs by ion uptake and biosynthesis of organic molecules in the guard cell. (Guards cells
close in water stress conditions under the influence of the stress hormone, abscisic acid ABA.)
The transpiration ratio expresses water moles transpired / CO2 moles assimilated. This ratio is up to 500
in C3 plants, as a result of:
• higher water than CO2 gradients
• lower CO2 concentrations compared to water
• slower rate of CO2 diffusion
• CO2 must cross several membranes to be assimilated.
The C4 plants have more favorable transpiration ratios of c. 250, and CAM desert plants achieve much
more efficient use of scarce water, with ratios of c. 50.
Summary Of Water Movement In The Soil-Plant-Atmosphere Continuum
A tug-of-war for water exists in the soil-plant-atmosphere continuum as follows:
• Soil: Dry soils have very negative water potential Ψw and may attract water from the roots.
• Root xylem: Lowered solute potential Ψs and lowered overall Ψw by solute accumulation in the root
xylem attracts water into the roots (sometimes increasing xylem Ψp in the roots under optimal
conditions).
• Shoot xylem and leaves: Tension (low Ψp) and resulting low leaf Ψw draw water up the xylem to the
leaves.
One useful tip :
More N (dry mass / plant cell proteins ) assimilated by the plant(s) ,more energy (number of photons to drive PS ,thus red.. ) needed...
