CHAPTER 4: Water balance of plants Flashcards by jy d

The water content and the rate of water movement in soils depend to a large extent on (2)

soil type and soil structure

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Particle Diameter (µm)
-Coarse sand
-Fine sand
-Silt Clay
-Clay

-Coarse sand 2000-200
-Fine sand 200-20
-Silt Clay 20-2
-Clay <3

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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

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(decomposing organic matter)

humus

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help improve soil aeration and infiltration
of water

crumbs

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clay crumbs help improve (2)

-soil aeration
-infiltration of water

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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

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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

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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

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water does not freely drain from them and is held more tightly

clay soils

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Clay soil, water ____ from them and is held ___

does not freely drain more tightly

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he moisture-holding capacity of soils.

field capacity

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water content of a soil after it has been saturated with water and
excess water has been allowed to drain away.

field capacity

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clay soils or soils with__ have a large field capacity

humus

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clay soils might retain ___water by volume ____ after being saturated

sandy soils, which retain __ water by volume after saturation

40%
a few days

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Soil has water potentials and can be dissected into two components:

o Osmotic Potential
o Hydrostatic Pressure

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A Negative Hydrostatic Pressure in Soil Water Lowers__

Soil Water Potential

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Lowers Soil Water Potential

Negative Hydrostatic Pressure

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the osmotic potential of soil water is generally negligible because

solute concentrations are low

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for soils with __ concentrations of salts, Ψs is significant

substantial

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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

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Hydrostatic Pressure (Ψp)
▪ for wet soils, Ψp is very __
▪ as a soil dries out, Ψp ___ and can become __

close to zero
decreases, quite negative

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reason why Ψp decreases and can become quite negative

soil dries out

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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

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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