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MOVEMENT OF WATER AND
SOLUTES
•The flow of water is in part an inevitable consequence of the
photosynthetic process. With the opening of stomata over the large,
light-capturing surfaces of leaves to allow entry of carbon dioxide, much
water is lost to the atmosphere, down the gradient of water vapor
concentration between the internal spaces of the leaf (where water
vapor concentration is at equilibrium with water inside cells) and the
atmosphere (where water vapor concentration is usually much lower).
Streams followed by water and sucrose in the leaf
Stomata
•The mechanisms that control stomatal
opening reflect the complex and
potentially conflicting demands for
carbon dioxide, for a flow of water
through the plant to maintain
physiological processes (maintainance
of turgor), and for conservation of
water, which is frequently in limiting
supply in the soil.
Stomata
•Guard cells control the opening of stomata
between transpiration and photosynthesis.
Factors regulating the
stomatal aperture:
Stomatal aperture is regulated in response to a host of
environmental factors including:
• Light intensity and quality (receptors on the guard cell
membranes sensitive to light).
• CO2 concentration in the leaf (decrease of CO2 due to
photosynthesis).
• Hydryc stress (release of ABA influencing closing of stomata).
• High temperatures (induce closing of stomata due to the
increase of cell respiration which increases CO2 concentration in
the leaf spaces).
• Water vapor concentration.
• Circadian clock.
•Water enters the roots, moves through the xylem to the leaves, and is
lost to the atmosphere by evaporation. Water in the xylem vessels forms
a continuous column that links water in the soil, at the absorbing surfaces
of the root, to the evaporating surface of the leaf and thus the
atmosphere. This flow of water through the plant is known as the
transpiration stream.
•The main driving force for movement of water through the plant is
the evaporative loss of water from the leaves caused by sunlight.
Cohesion-tension theory
•Evaporation lowers the water potential in the
leaf cells and causes water to move out of the
xylem vessels of the leaf; this reduces pressure in
the xylem and thus “pulls” water up through the
entire length of the xylem from the root. The
distance can be as much as 100 m in the tallest
trees.
•This remarkable phenomenon of long-distance
upward movement of water in a continuous
vertical column, caused by a negative pressure at
the top, is attributable to the cohesive and
adhesive properties of water.
Cohesion-tension theory
•Given the strong mutual attraction between water molecules
(cohesion) and their attraction to solid surfaces (adhesion), a narrow
column of water will not rupture even under considerable negative
pressure (i.e. water has high tensile strength). This hydraulic
explanation for water movement in the plant is called the cohesiontension theory.
Cohesion-tension theory
•Water is taken up from the soil mainly in the
apical regions of the root, where the surface area
is greatly increased by the presence of root hairs.
•There are three routes by which water from the
soil reaches the xylem vessels.
•First, water may move from the soil to the xylem
entirely in the apoplast, that is within cell walls.
Resistance to water movement in this apoplastic
pathway is high, because the walls of the
endodermis contain a band of suberin linked to
the cell wall called Casparian strip. Movement of
water is driven by the hydrostatic pressure
gradient from the soil to the xylem, generated by
the upward “pull” on water in the xylem vessels.
Cohesion-tension theory
•In a second route, water may be taken up into
the symplast in root-hair, epidermal, and
cortical cells, and then pass from cell to cell via
plasmodesmata into the stele, where it moves
out of cells into xylem vessels.
•The third route involves both the symplast
and the apoplast: water passes from cell to cell
via pores in the plasma membrane. Water can
diffuse through lipid bilayers, but its diffusion
through the plasma membrane is greatly
enhanced by pores formed by a family of
proteins called aquaporins.
Cohesion-tension theory
•Water movement in the symplastic
movement is influenced by osmotic
as well as hydrostatic pressure
gradients.
•Osmotic pressure gradients can exist
across the root because mineral
nutrients that enter the symplast of
root cells from the soil are actively
secreted (via plasma mebrane
transporters) from cells of the stele
into xylem vessels.
Root pressure
•When transpiration is minimal (e.g. when stomata are shut at night) and water is
freely available in the soil, a positive pressure may build up in root xylem vessels. This
occurs because water in the xylem is under minimal tension, as described above, so
mineral nutrients secreted into xylem vessels are not carried away rapidly by the
transpiratoin stream. Locally high solute concentrations can develop, drawing water
into the xylem from surrounding cells.
•This phenomenon, known as root pressure, occurs in well-watered plants at night
and under conditions of very high air humidity.
•The positive pressure in the root xylem vessels results in mass flow in the xylem,
which may lead to exudation of xylem fluid from specialized structures on the
margins of leaves known as hydathodes. The dewdrops seen on the tips of leaves as
night ends are formed in this way: a process called guttation.
Movement of mineral nutrients
•Most nutrients (K+, Cl-, HPO4-, Ca2+, Fe2+ etc.)
uptake takes place in the actively growing and
expanding apex of the root and the adjacent
root-hair zone.
•The forward growth of the apical region allows
cells to exploit new sources of soil nutrients, as
nutrients become depleted in the rhizosphere
(the zone of influence of a root in the soil)
adjacent to older parts of the root. Root hairs
provide a very large surface area for nutrient
uptake.
Movement of mineral nutrients
•Nutrients are taken up into epidermal and
cortical cells via specific transporters in the
plasma membrane. The casparian strip in the
walls of the endodermis is highly impermeable to
most nutrients, so movement into cells of the
stele adjacent to the xylem is symplastic. For most
nutrients, export from the stele symplast to the
xylem is brought about by transporters found
specifically in the plasma membrane of cells
adjacent to the xylem.
•Nutrients entering the root xylem are carried to
leaves in the transpiration stream. In the leaves
they are taken up via transporters into the
symplast of cells surrounding the xylem vessels.
Some of the nutrients pass symplastically down
concentration gradients to other leaf cells, to
meet demands for growth and metabolic
processes.
•The remainder are transferred
to phloem elements, which lie
in close proximity to xylem
vessels in the minor vein
endings of leaves. Nutrients
entering the phloem are
carried out in the leaf together
with sucrose and travel to sink
organs.
Mycorrhizae
•The roots of many plants (90% of vascular plants) form symbiotic associations with
fungi, called mycorrhizae (derived from words meaning ‘fungus’ and ‘roots’. The plant
benefits from this association because the fungal hyphae exploit a greater soil volume
than the plant roots, providing the plant with nutrients from this extended region of
the soil. The fungus benefits from access to sugars produced in the plant by
photosynthesis. In addition, colonization of the plant by one species of fungus may
protect against infection by other fungi.
Sucrose transport
Sucrose synthesized in photosynthetic cells is the source of
carbon for all other cells in the plant. It must be transported
from the leaves (source organs) to nonphotosynthetic parts of
the plant (sink organs). Sink organs include meristems, young
leaves that do not yet export sucrose, flowers, seeds and fruits,
and vegetative storage organs such as roots, tubers and
rhizomes.
Sucrose is transported in part of the vascular tissue, the
phloem, that consists of two types of cell: sieve elements and
their companion cells.
Pressure flow
•The contents of the sieve elements in
leaves, where sugars are loaded, are
under high pressure, whereas the
contents in sink organs, where sugars
are withdrawn, are under lower
pressure.
•This pressure gradient in the phloem
causes the movement of phloem
contents from the source to the sink
organs.
•The pressure gradient is generated by
the process of phloem loading in leaves.
Pressure flow
•Sugars are loaded into the phloem from the leaf
mesophyll cells by an energy-consuming process
that results in a large difference in sucrose
concentration between the cytosol of the
mesophyll cells and the phloem.
•The osmotic potential in the sieve element is thus
much higher than that in surrounding tissues.
Water is drawn into the phloem from the adjacent
xylem by this high osmotic potential, creating a
high pressure within the phloem.
•In the sink organs, sugars move out of the phloem
into the surrounding tissue, and water follows.
This means that the pressure in the phloem in sink
organs is much lower than in the leaf.
Phloem loading may be apoplastic or symplastic
•Sucrose moves from the cytosol of the mesophyll cells in which it is synthesized
to mesophyll cells adjacent to the phloem by passive diffusion through
plasmodesmata. The energy-consuming step required to load the phloem against
a concentration gradient occurs within the phloem itself.
•The apoplastic loading occurs through the region outside the plasma membrane,
consisting mainly of cell walls. The symplastic loading occurs through the cellular
volume bounded by the plasma membranes and connected by plasmodesmata. In
most species, one of the two mechanisms predominates.
Phloem loading may be apoplastic or symplastic
Tissues believed to use apoplastic loading have few
plasmodesmatal connections between mesophyll cells and
companion cells
•In apoplastic phloem loading, sucrose moves out of mesophyll cells adjacent to the
phloem and into the apoplast. It is then taken into companion cells or sieve elements
against a concentration gradient by an energy-requiring sucrose transporter protein in the
plasma membrane of these cells.
•In symplastic phloem loading, sucrose moves from mesophyll cells into companion cells via
the plasmodesmata, and in the companion cells it is converted in energy-requiring
reactions to higher-molecular weight sugars, typically the oligosaccharides raffinose,
stachyose, or verbascose. They are too large to pass back through the plasmodesmata into
the mesophyll cells, and thus can only move into the phloem.