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I. Introduction
II. The Water Molecule
III. Hydrogen Bonding
IV. Diffusion and Osmosis
V. Water Potential
VI. Xylem Tissue
VII. Pathway of Water Flow
VIII. Cohesion Tension Theory
IX. Hydraulic Conductance
X. Some Factors Affecting Water Flow
- air vapor pressure
- water stress and embolism
- freezing
- over fertilizing
- CO2 concentration
- pollution and stomata
- pathogens
Introduction
Why study water
flow through plants and plant water relations?
Because we are utterly dependent upon plants.
The growth of plants is largely dependent upon
water availability. In many environments
water is scarce and a limiting factor for plant
growth. Plants have a difficult task of
obtaining water from the environment and
retaining water. Water is a critical
factor in plant growth. Plants require
water for photosynthesis, elongation growth of
cells, and distribution of materials. If
we can better understand water flow through
plants and plant water relations we can better
change the situation when there is a problem, or
change the situation to reduce plant water loss,
increase yield, or even improve the appearance
of plants.
The Water
Molecule
A water molecule
consist of two hydrogen atoms and one oxygen
atom. Each hydrogen atom is covalently
bonded to oxygen, electron pairs shared.
Strongly electronegative oxygen, polar molecule
with partial charges.
Hydrogen
Bonding
Attraction
between partial charges of polar molecules.
Weak bond and short lived but cumulative effect
is great. Cohesion and adhesion.
Gives water certain physical properties.
Diffusion and
Osmosis
Osmotic
concentration, diffusion and osmosis
Water
Potential
Free energy is
the energy of a system that is available to do
work, such as to move water. Water
potential is a measure of the free energy of the
system. Pure water has a maximum free
energy of zero. Water potentials measured
in plants are usually negative values.
Water potential = pressure potential + osmotic
potential + Matric potential + gravitational
potential. Pressures above atmospheric
increase the free energy of water, pressures
below atmospheric decrease the free energy of
water. Osmotic potential is the effect of
dissolved solutes in decreasing the free energy
of water. Matric potential is the effect
of structural elements that bind to water
molecules and decrease the free energy of water.
Gravitational potential is the effect of gravity
in lowering the free energy of water.
Water moves from a higher to a lower water
potential
Xylem Tissue
Water, minerals,
and some organic compounds move in xylem.
Complex tissue comprised of many cell types.
Tracheary elements are water conducting cells.
Two types include tracheids and vessel members.
Conducting cells are dead at maturity, hollow
tubes with mostly cellulose cell walls.
Pathway of
Water Flow
Soil solution
> root epidermal call > root xylem tissue
> stem xylem tissue > leaf. Xylem
tissue > leaf mesophyll cell > leaf air
space > out stomata > atmosphere.
Soil plant atmosphere continuum (SPAC)
Cohesion
Tension Theory
Cohesion-tension
theory as the mechanism of sap ascent was
proposed by Dixon and Joly in 1894 and expounded
by Dixon in 1914. It has withstood the
test of time. The sun supplies the energy.
Water evaporates from moist mesophyll cell
walls. Water diffuses from mesophyll cell
a to cell wall. Osmotic concentration of
mesophyll cell increases, water concentration
decreases, water potential decreases.
Water diffuses from mesophyll cell b to
mesophyll call a from higher to lower water
potential. Water diffuses from mesophyll
cell c to mesophyll call b from higher to lower
water potential. Water from xylem replaces
water in mesophyll cell c decreasing the water
potential in the xylem. Decrease of water
potential is transmitted all the way to the
roots. Decrease of water potential in
roots helps roots get water from soil.
Gradient of water potential drives flow of water
through the xylem. Water in xylem is under
tension. Water potential gradients
necessary to move water to the top of tall trees
have been verified by a variety of methods.
Hydraulic
Conductance
Conductance is
the reciprocal of resistance. Hydraulic
conductance is the ease of water flow through
the xylem. Hydraulic conductivity is the
hydraulic conductance per unit length.
Hagen-Poisuille equation adopted for botanical
use assumes laminar flow through cylindrical
horizontal conduits with smooth walls.
Some Factors
Affecting Water Flow
Air vapor
pressure: Water vapor pressure difference
between inside of leaf and atmosphere drives
transpiration. Higher air vapor pressures
decrease transpiration rate. Lower air
vapor pressures increase transpiration rate.
Water stress and
embolism: Water stress may be caused by
reduced supply of water to roots or by excessive
transpiration. Sever water stress places
the water column in tracheary elements under
greater tension. The water column may
break leading to air embolism (bubble
formation). Sometimes the tracheary
element may refill as pressure increases during
the evening. Often the tracheary element
becomes non-functional.
Freezing induced
embolism: Freezing may induce air
embolisms in the xylem
Over fertilizing:
As fertilizer concentration of soil solution
increases the water concentration of the soil
solution decreases. If the fertilizer
concentration of the soil solution is too high
water diffuses out of the roots and the plant
may wilt or even die.
CO2
concentration: Increasing CO2
concentration of air around C3 plants may
increase photosynthetic rate. If CO2 is
increased too much the stomata close.
Pollution and
stomata: Particulates may plug up stomata
decreasing transpiration. Some chemicals
may cause the stomata to close permanently (e.g.
abscisic acid) Some chemicals may cause
stomata to remain permanently open.
Pathogens:
Root parasites such as nematodes can decrease
the water supply to leaves leading to air
embolisms. Infection may cause the release
of a senescence inducing substance into the
transpiration stream. Some pathogen
metabolites such as fusicoccin cause stomata to
remain open and may lead to wilting, air
embolism formation or permanent blocking of
xylem.
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