CHAPTER 4: Water balance of plants Flashcards
The water content and the rate of water movement in soils depend to a large extent on (2)
soil type and soil structure
Particle Diameter (µm)
-Coarse sand
-Fine sand
-Silt Clay
-Clay
-Coarse sand 2000-200
-Fine sand 200-20
-Silt Clay 20-2
-Clay <3
Difference of Sandy Soils to Clay Soils
-diameter
-surface area
-channels
Sandy soils
diameter- 1 mm or more
surface area- low surface area
channels- low surface area
Clay Soils
diameter- 2 µm
surface area-greater surface areas
channels- smaller channels
(decomposing organic matter)
humus
help improve soil aeration and infiltration
of water
crumbs
clay crumbs help improve (2)
-soil aeration
-infiltration of water
the spaces in between particles are large that water drains from them;
water remains only on the soil particle surfaces and at interstices between soil
particles
sandy soil
Sandy soils, the spaces in between particles are __ that water ___ from them;
water remains only on the ____ and at ____
large
drains
soil particle surfaces
interstices between soil particles
Characteristics of Sandy Soil and Clay Soil in terms of
-water in soil
- surface of the soil particles
Sandy Soil- the spaces in between particles are large that water drains from them;
water remains only on the soil particle surfaces and at interstices between soil
particles
Clay Soil- water does not freely drain from them and is held more tightly
water does not freely drain from them and is held more tightly
clay soils
Clay soil, water ____ from them and is held ___
does not freely drain more tightly
he moisture-holding capacity of soils.
field capacity
water content of a soil after it has been saturated with water and
excess water has been allowed to drain away.
field capacity
clay soils or soils with__ have a large field capacity
humus
clay soils might retain ___water by volume ____ after being saturated
sandy soils, which retain __ water by volume after saturation
40%
a few days
3%
Soil has water potentials and can be dissected into two components:
o Osmotic Potential
o Hydrostatic Pressure
A Negative Hydrostatic Pressure in Soil Water Lowers__
Soil Water Potential
Lowers Soil Water Potential
Negative Hydrostatic Pressure
the osmotic potential of soil water is generally negligible because
solute concentrations are low
for soils with __ concentrations of salts, Ψs is significant
substantial
osmotic pressure
-soil water
-soil
Osmotic Potential (Ψs)
▪ the osmotic potential of soil water is generally negligible because solute concentrations are low
▪ but for soils with substantial concentrations of salts, Ψs is significant
Hydrostatic Pressure (Ψp)
▪ for wet soils, Ψp is very __
▪ as a soil dries out, Ψp ___ and can become __
close to zero
decreases, quite negative
reason why Ψp decreases and can become quite negative
soil dries out
Hydrostatic Pressure (Ψp)
-difference in wet soil and dry soil
▪ for wet soils, Ψp is very close to zero
▪ as a soil dries out, Ψp decreases and can become quite negative
Where does the negative pressure in soil water came from?
-water is removed from the soil
As the water content of the soil decreases, the water recedes into the interstices
between soil particles, and the air–water surface develops
curved air–water interfaces
The value of Ψp in soil water can become ___ because the __ of curvature of air–water surfaces may become___ in drying soils.
quite negative
radius
very small
Water moves through soils predominantly by bulk flow driven by a
pressure gradient
Water Moves through the Soil by
Bulk Flow
As plants absorb water from the soil, they deplete the soil of water near the
surface of the roots.
b. The rate of water flow in soils depends on two factors:
▪ Size of the pressure gradient through the soil
▪ Soil hydraulic conductivity
it is a measure of ease with which water moves through the soil and it varies with the: 1) type of soil; and
2) water content
Soil hydraulic conductivity
Soil hydraulic conductivity it is a measure of ease with which water moves through the soil and it varies with the:
1) type of soil
2) water content
Soil hydraulic conductivity
o Sandy soil
o Clay soil
o Sandy soil- large hydraulic conductivity
o Clay soil- low hydraulic conductivity
hydraulic conductivity as water content decreases
decreases drastically
the decreases in hydraulic conductivity is due primarily to the
replacement of water in the soil spaces by air
when air moves into a soil channel previously filled with water, water
movement through that channel is ____ of the channel.
restricted to the periphery
as more of the soil spaces become filled with air, water can flow through fewer and narrower channels ___
hydraulic conductivity falls
microscopic extensions of root epidermal cells
root hais
Rot hair helps in increasing the
surface area of the root
Increase in the surface area of the root provide
greater capacity for absorption of ions and
water from the soil.
Water enters the root most readily in the
apical part
More mature regions of the root have an outer protective tissue called
exodermis or hypodermis
characteristic exodermis or hypodermis
impermeable to water
Water Moves in the Root via(3)
the Apoplast, Transmembrane, and Symplast Pathways
In the symplast pathway, water flows between cells through the ____without crossing the plasma membrane.
plasmodesmata
In the symplast pathway, water flows between cells through the plasmodesmata _____
without crossing the plasma membrane
water flows between cells through the plasmodesmata without crossing the plasma membrane
symplast pathway,
, water moves across the plasma membranes, with
a short visit to the cell wall space.
In the transmembrane pathway
. In the transmembrane pathway, water moves across the plasma membranes, with
a short visit to the ___
cell wall space.
In the transmembrane pathway, water moves _____, with a short visit to the cell wall space.
across the plasma membranes
the apoplast pathway is blocked by the
Casparian strip at the
endodermis
Difference between the water movement in
-Apoplast Pathway
-Transmembrane Pathway
-Apoplast Pathway, water moves without crossing any membranes
-Transmembrane Pathway, water crosses at least two membrane
water moves exclusively through the cell wall without crossing any membranes
Apoplast Pathway
continuous system of cell walls and intercellular air spaces in plant
tissues
Apoplast Pathway
the route followed by water that sequentially enters a cell on one side, exits the cell on the other side, enters the next in the series, and so on
Transmembrane Pathway
water crosses at least two membranes for each cell in its path (the
plasma membrane on entering and on exiting)
Transmembrane Pathway
transport across the tonoplast may also be involved
Transmembrane Pathway
Transmembrane Pathway transport across the ______ may also be involved
tonoplast
water travels from one cell to the next via the plasmodesmata
Symplast Pathway
entire network of cell cytoplasm interconnected by plasmodesmata
Symplast Pathway
Symplast Pathway water travels from one cell to the next via the
plasmodesmata
symplast consists of the _____ interconnected by plasmodesmata
entire network of cell cytoplasm
At the endodermis, water movement through the apoplast pathway is obstructed by the
Casparian strip
is a band of radial cell walls in the endodermis that is impregnated with the wax-like,
hydrophobic substance suberin
Casparian strip.
Casparian strip is a band of radial cell walls in the endodermis that is impregnated with the wax-like, hydrophobic substance
suberin
acts as a barrier to water and solute movement
suberin
breaks the continuity of the apoplast pathway, and forces water
and solutes to cross the endodermis by passing through the plasma membrane.
casparian strip
The casparian strip breaks the continuity of the apoplast pathway, and forces water
and solutes to cross the ___ by passing through the ___.
endodermis
plasma membrane
water movement across the endodermis occurs through the
symplast
Solute Accumulation in the Xylem Can Generate
“Root Pressure”
What can Generate “Root Pressure”
Solute Accumulation in the Xylem
Roots generate____ by absorbing ions from the dilute soil solution and transporting them into the xylem.
positive hydrostatic pressure
Roots generate positive hydrostatic pressure by absorbing ions from the dilute soil solution and transporting them into the
xylem.
The buildup of solutes in the xylem sap leads
-osmotic potential
-water potention
-decrease in the xylem osmotic
potential (Ψs)
-decrease in the xylem water potential (Ψw).
This lowering of the xylem Ψw provides a driving force for water absorption, which
in turn leads to a____ in the xylem.
positive hydrostatic pressure
In effect, the whole root acts like an osmotic cell; the multicellular root tissue behaves as an osmotic membrane does, building up a____ in the xylem in response to the ___
positive hydrostatic pressure
accumulation of solutes
is most likely to occur when soil water potentials are high and
transpiration rates are low
root pressure
Plants that develop root pressure frequently produce liquid droplets on the edges of their leaves, a phenomenon known as
guttation
constitutes the longest part of the pathway of water transport.
xylem
xylem is a ___ with __ to flow of water unlike
the ____across the root tissue.
simple pathway
low resistance
complex pathway
WATER TRANSPORT THROUGH THE
XYLEM
Xylem Consists of Two Tracheary Elements
Tracheids
Vessel Elements
only found in angiosperms, Gnetales (a group of gymnosperms) and
some ferns
Vessel Elements
both found in gymnosperms and angiosperms
Tracheids
Difference between
-tracheids
-vessel elements
Tracheids – both found in gymnosperms and angiosperms
Vessel Elements – only found in angiosperms
The maturation of both tracheids and vessel elements involves the “___” of the cell.
death
are elongated, spindle-shaped cells that are arranged in overlap- ping vertical files
tracheids
Tracheids-water flows between tracheids by means of numerous pits on their
lateral walls
are microscopic regions where secondary wall is absent and the primary
wall is thin and porous.
Pits
Pits of one tracheid are typically located opposite pits of an Adjoining
tracheid, forming
pit pairs
constitute a low-resistance path for water movement between
tracheids.
Pit pairs
a porous layer between pit pairs, consisting of two primary
walls and a middle lamella.
Pit membrane
In some species of conifers, pit membranes have a central thickening known as
torus
acts like a valve to close the pit by lodging themselves on the circular
wall thickenings bordering the pits
Torus -
dangerous gas bubbles formation
cavitation
this act thereby prevents dangerous gas bubbles from forming and invading neighboring tracheids
torus
this tends to be shorter and wider and have perforiated end walls that form a perforiation plate at each end of the cell
vessele elements
perforiated end walls that form
perforiation plate
Similarity between
-tracheids
-vessel elements
like- pit on their lateral walls
unlike- perforiated walls
tracheids- allow movement
vessel elements- allow stack
Tension Theory Explains Water Transport in the Xylem
cohesion
the pressure gradients needed to move water through the xylem could result
from the generation of (2)
o positive pressures at the base of the plant; or
o negative pressures at the top of the plant.
the pressure gradients needed to move water through the
xylem
root pressure is typically less than 0.1 MPa and disappears when the transpiration rate is
high
Instead, the water at the top of a tree develops a _____, and this tension pulls water through the xylem.
large tension (a negative hydrostatic pressure)
This mechanism, first proposed toward the end of the 19th century, is called
cohesion–tension theory of sap ascent
Xylem Transport of Water in Trees faces
physical challenges
Water under tension transmits an ___ force to the walls of the xylem and if cell
walls are weak, they would __ under the influence of this tension.
inward
collapse
adaptations to offset this tendency for tracheids and vessels to collapse.
-secondary wall thickening
-lignification
- this is the increased tendency for air to be pulled through microscopic
pores in the xylem cell walls due to increasing water tension.
Air seeding
- the phenomenon of air formation and expansion
Cavitation
Cavitation is also known as
embolism
once a gas bubble has formed within the water column under tension, it will
expand because gases cannot resist tensile forces
cavitation
breaks the continuity of the water column and prevents water transport in
the xylem.
cavitation
by blocking the main transport pathway of water, cause the dehydration and death of the leaves
cavitation
Since the capillaries in the xylem are interconnected, one gas bubble___
completely stop water flow.
does not
Thus, the finite length of the tracheid and vessel conduits of the xylem, while
resulting in an increased resistance to water flow, also provides a way to
restrict cavitation
Finally, many plants have _____ in which new xylem forms each
year.
secondary growth
Water Evaporation in the Leaf Generates a ____ in the Xylem
Negative Pressure
The tensions needed to pull water through the xylem are the result of
evaporation
The negative pressure that causes water to move up through the xylem develops at the
surface of the cell walls in the leaf
as more water is removed from the wall, the radius of curvature of the air
water interfaces ___ and the pressure of the water becomes more ___
decreases
negative
the motive force for xylem transport is generated at the ____ within the leaf
air-water interfaces
