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Plant Physiology > 4 Water Balance of Plants > Flashcards

4 Water Balance of Plants Flashcards

1
Q

the atmosphere is the source of _ , which is needed for photosynthesis.

A

carbon dioxide

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2
Q

the atmosphere is
relatively _ and can _ the plant

A

dry
dehydrate

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3
Q

_ water loss from
the leaf is driven by a gradient in water vapor concentration.

A

Transpirational

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4
Q

Long-distance transport in the
xylem is driven by _ , as is water movement in the soil.

A

pressure gradients

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5
Q

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

A

soil type and soil structure

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6
Q

have a relatively low surface particles may be 1 mm or more in diameter

A

Sandy Soils

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7
Q

have a relatively low surface area per gram of soil and have large spaces or channels between particles

A

Sandy Soils

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8
Q

have particles that are smaller than 2 µm in diameter

A

Clay Soils

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9
Q

have much greater surface areas and smaller channels between particles

A

Clay Soils

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10
Q

with the aid of organic substances such as _ (decomposing organic matter), clay particles may aggregate into “crumbs” that help improve soil aeration and infiltration
of water

A

humus

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11
Q

Water in the soil may exist as a _ adhering to the surface of the soil particles, or it may fill the entire channel between particles.

A

film

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12
Q

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

A

sandy soils

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13
Q

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

A

clay soils

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14
Q

the moisture-holding capacity of soils

A

Field capacity

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15
Q

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

A

Field capacity

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16
Q

clay soils or soils with humus have a _

A

large field capacity

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17
Q

clay soils might retain _ % water by volume a few days after being saturated unlike sandy soils, which retain _ % water by volume after saturation

A

40
3

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18
Q

A Negative Hydrostatic Pressure in Soil Water Lowers _

A

Soil Water Potential

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19
Q

2 components of soil water potential

A
  1. osmotic potential
  2. hydrostatic pressure
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20
Q

is generally negligible because solute
concentrations are low

A

the osmotic potential of soil water

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21
Q

but for soils with substantial concentrations of salts, _ is significant

A

osmotic potential Ψs

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22
Q

for wet soils, Ψp is very _

A

close to zero

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23
Q

as a soil _ , Ψp decreases and can become quite negative

A

dries out

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24
Q

As a soil dries out, water is first removed from the center of the largest spaces between particles. Because of _ , water tends to cling to the surfaces of soil particles, so a large surface area between soil water and soil air develops.

A

adhesive forces

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25
Q

As the water content of the soil decreases, the water recedes into the interstices between soil particles, and the air–water surface develops _.

A

curved air–water interfaces

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26
Q

The value of Ψp in soil water can become quite negative because the radius of curvature of air–water surfaces may become _ in drying soils.

A

very small

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27
Q

Water Moves through the Soil by _

A

Bulk Flow

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28
Q

Water moves through soils predominantly by bulk flow driven by a _.

A

pressure gradient

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29
Q

Because the _ in the soil are interconnected, water moves to the root surface by bulk flow through these channels down the pressure gradient.

A

water-filled pore spaces

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30
Q
  • As plants absorb water from the soil, they deplete the soil of water near the surface of the roots.
  • This depletion reduces Ψp in water near the root surface and establishes a pressure gradient with respect to neighboring regions of soil that have _ Ψp values.
A

higher

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31
Q

2 factors of rate of water flow in soils

A
  1. Size of the pressure gradient through the soil
  2. Soil hydraulic conductivity
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32
Q

it is a measure of ease with which water moves through the soil

A

Soil hydraulic conductivity

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33
Q

variation of Soil hydraulic conductivity

A
  1. type of soil
  2. water content
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34
Q

large hydraulic conductivity

A

Sandy soil

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35
Q

low hydraulic conductivity

A

Clay soil

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36
Q

as water content decreases = hydraulic conductivity _ drastically

A

decreases

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37
Q

the decreases in hydraulic conductivity is due primarily to the _ in the soil spaces by air

A

replacement of
water

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38
Q

when air moves into a soil channel previously filled with water, water movement through that channel is _ of the channel

A

restricted to the periphery

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39
Q

as more of the soil spaces become filled with air, water can flow through _ channels, and the hydraulic conductivity falls

A

fewer and narrower

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40
Q

Intimate contact between the surface of the _ is essential for effective water absorption of root.

A

root and the soil

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41
Q

microscopic extensions of root epidermal cells that greatly increase the surface area of the root, thus providing greater capacity for absorption of ions and water from the soil.

A

Root hairs

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42
Q

Water enters the root most readily in the _ part of the root

A

apical

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43
Q

More mature regions of the root have an outer protective tissue called _ which are impermeable to water.

A

exodermis or hypodermis

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44
Q

Water Moves in the Root via the what pathways

A
  1. apoplast
  2. transmembrane
  3. symplast pathways
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45
Q

water moves exclusively through the cell wall without crossing any membranes

A

Apoplast Pathway

46
Q

the apoplast is the continuous system of _ in plant tissue

A

cell walls and intercellular air spaces

47
Q

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

A

Transmembrane Pathway

48
Q

in this pathway, water crosses at least two membranes for each cell in its path (the
plasma membrane on entering and on exiting)

A

Transmembrane Pathway

49
Q

In transmembrane pathway, transport across the _ may also be involved

A

tonoplast

50
Q

water travels from one cell to the next via the plasmodesmata

A

symplast pathway

51
Q

the symplast consists of the entire network of cell cytoplasm interconnected by _

A

plasmodesmata

52
Q

At the endodermis, water movement through the apoplast pathway is obstructed by the _.

A

Casparian strip

53
Q

Casparian strip is a band of radial cell walls in the endodermis that is impregnated with the wax-like, hydrophobic substance _

A

suberin

54
Q

acts as a barrier to water and solute movement

A

Suberin

55
Q

The casparian strip breaks the _ of the apoplast pathway, and forces water and solutes to cross the endodermis by passing through the plasma membrane

A

continuity

56
Q

water movement across the endodermis occurs through the _

A

symplast

57
Q

Solute Accumulation in the Xylem Can Generate _

A

“Root Pressure”

58
Q

Plants sometimes exhibit a phenomenon referred to as _

A

root pressure.

59
Q

Roots generate positive hydrostatic pressure by _ from the dilute soil solution and transporting them into the xylem.

A

absorbing ions

60
Q

The buildup of solutes in the xylem sap leads to a decrease in the _ (Ψs) and thus an decrease in the xylem water potential (Ψw).

A

xylem osmotic potential

61
Q

This lowering of the xylem Ψw provides a driving force for _, which in turn leads to a positive hydrostatic pressure in the xylem.

A

water absorption

62
Q

In effect, the whole root acts like an osmotic cell; the multicellular root tissue behaves
as an osmotic membrane does, building up a positive hydrostatic pressure in the
xylem in response to the accumulation of _

A

solutes

63
Q

most likely to occur when soil water potentials are high and
transpiration rates are low

A

root pressure

64
Q
A
65
Q

high Ψw and low _ rates = root pressure

A

transpiration

66
Q

Plants that develop root pressure frequently produce liquid droplets on the edges of
their leaves, a phenomenon known as _.

A

guttation

67
Q

Xylem constitutes the _ part of the pathway of water transport.

A

longest

68
Q

xylem is a simple pathway with _ to flow of water unlike the complex pathway across the root tissue

A

low resistance

69
Q

The conducting cells in the xylem have a specialized anatomy that enables them to
transport large quantities of water with great efficiency

A

Two Tracheary Elements

70
Q

tracheary element both found in gymnosperms and angiosperms

A

Tracheids

71
Q

tracheary element only found in angiosperms, Gnetales (a group of gymnosperms) and
some ferns

A

Vessel Elements

72
Q

The maturation of both tracheids and vessel elements involves the “_ ” of the cell.

A

death

73
Q

are elongated, spindle-shaped cells that are arranged in overlap- ping vertical files

A

Tracheids

74
Q

water flows between tracheids by means of numerous pits on their _

A

lateral walls.

75
Q

are microscopic regions where secondary wall is absent and the primary wall is thin and porous.

A

Pits

76
Q

Pits of one tracheid are typically located opposite pits of an Adjoining tracheid, forming _

A

pit pairs.

77
Q

constitute a low-resistance path for water movement between tracheids

A

Pit pairs

78
Q

a porous layer between pit pairs, consisting of two primary
walls and a middle lamella

A

Pit membrane

79
Q

In some species of conifers, pit membranes have a central thickening known as _

A

torus
(pl. tori).

80
Q

acts like a valve to close the pit by lodging themselves on the circular wall thickenings bordering the pits

A

Torus

81
Q

Tori act thereby prevents dangerous gas bubbles from forming (process called
_ ) and invading neighboring tracheids

A

cavitation

82
Q

vessel elements this tends to be shorter and wider than tracheids and have perforiated end walls that form a _ at each end of the cell

A

perforiation plate

83
Q

like trachieds, vessel elements had _ on their lateral walls

A

pits

84
Q

unlike tracheids, the perforiated end walls allows vessel elements to be stacked end to end to form conduit called _

A

vessel

85
Q

The _ Theory Explains Water Transport in the Xylem

A

Cohesion – Tension

86
Q

The Cohesion – Tension Theory, the pressure gradients needed to move water through the xylem could result
from the generation of

A
  1. positive pressures at the base of the plant; or
  2. negative pressures at the top of the plant.
87
Q

root pressure is typically less than 0.1 MPa and disappears when the transpiration rate is _ , so it is clearly inadequate to move water up a tall tree.

A

high

88
Q

the water at the top of a tree develops a large tension (a _ ), and this tension pulls water through the xylem

A

negative hydrostatic
pressure

89
Q

First proposed toward the end of the 19th century, is called
the _ because it requires the cohesive properties of water to sustain large tensions in the xylem water columns

A

cohesion–tension theory of sap ascent

90
Q

Water under tension transmits an inward force to the walls of the xylem and if cell
walls are weak, they would _ under the influence of this tension

A

collapse

91
Q

The _ of tracheids and vessels
are adaptations to offset this tendency to collapse.

A

secondary wall thickenings and lignifications

92
Q

this is the increased tendency for air to be pulled through microscopic
pores in the xylem cell walls due to increasing water tension

A

Air seeding

93
Q

the phenomenon of air formation and expansion

A

Cavitation

94
Q

cavitation aka

A

also known as embolism

95
Q

once a gas bubble has formed within the water column under tension, it will
expand because gases cannot resist tensile forces

A

cavitation/ embolism

96
Q

_ breaks the continuity of the water column and prevents water transport in
the xylem

A

Cavitation

97
Q

Cavitation breaks in xylem water continuity, if not repaired, would be _ to the plant

A

disastrous

98
Q

by blocking the main transport pathway of water, such embolisms would cause
the _ of the leaves

A

dehydration and death

99
Q

Because the tracheary elements in the xylem are interconnected, one gas bubble
might, in principle, expand to fill the _

A

whole network

100
Q

In practice, gas bubbles do not spread far because the expanding gas bubble cannot
easily pass through the small pores of the _.

A

pit membranes

101
Q

Since the capillaries in the xylem are interconnected, one gas bubble does _ water flow.

A

not
completely stop

102
Q

water can detour around the blocked point by traveling through neighboring,
connected _

A

conduits

103
Q

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
_

A

restrict cavitation.

104
Q

many plants have secondary growth in which _ each year.

A

new xylem forms

105
Q

The new xylem becomes functional before the old xylem ceases to function,
because of _ or by substances secreted by the plant.

A

occlusion by gas bubbles

106
Q

Water Evaporation in the Leaf Generates a _ Pressure in the Xylem

A

Negative

107
Q

The tensions needed to pull water through the xylem are the result of _ from leaves.

A

evaporation of
water

108
Q

water is brought to the leaves via the xylem of the leaf _ which branches into an intricate network of veins throughout the leaf from the xylem, water is drawn into the cells of the leaf and along the cell walls

A

vascular bundle

109
Q

The negative pressure that causes water to _ through the xylem develops at the surface of the cell walls in the leaf

A

move up

110
Q

initially water evaporates from a thin film lining these air spaces as water is lost to the air, the surface of the remaining water is drawn into the _ of the cell wall, where it forms again, curved air-water interfaces

A

interstices

111
Q

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 _

A

decreases
negative

112
Q

Thus, the motive force for xylem transport is generated at the _

A

air-water interfaces
within the leaf.

Plant Physiology (4 decks)

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