4 Water Balance of Plants Flashcards
1
Q
the atmosphere is the source of _ , which is needed for photosynthesis.
A
carbon dioxide
2
Q
the atmosphere is
relatively _ and can _ the plant
A
dry
dehydrate
3
Q
_ water loss from
the leaf is driven by a gradient in water vapor concentration.
A
Transpirational
4
Q
Long-distance transport in the
xylem is driven by _ , as is water movement in the soil.
A
pressure gradients
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
6
Q
have a relatively low surface particles may be 1 mm or more in diameter
A
Sandy Soils
7
Q
have a relatively low surface area per gram of soil and have large spaces or channels between particles
A
Sandy Soils
8
Q
have particles that are smaller than 2 µm in diameter
A
Clay Soils
9
Q
have much greater surface areas and smaller channels between particles
A
Clay Soils
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
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
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
13
Q
water does not freely drain from them and is held more tightly
A
clay soils
14
Q
the moisture-holding capacity of soils
A
Field capacity
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
16
Q
clay soils or soils with humus have a _
A
large field capacity
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
18
Q
A Negative Hydrostatic Pressure in Soil Water Lowers _
A
Soil Water Potential
19
Q
2 components of soil water potential
A
- osmotic potential
- hydrostatic pressure
20
Q
is generally negligible because solute
concentrations are low
A
the osmotic potential of soil water
21
Q
but for soils with substantial concentrations of salts, _ is significant
A
osmotic potential Ψs
22
Q
for wet soils, Ψp is very _
A
close to zero
23
Q
as a soil _ , Ψp decreases and can become quite negative
A
dries out
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
25
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
26
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.
very small
27
Water Moves through the Soil by _
Bulk Flow
28
Water moves through soils predominantly by bulk flow driven by a _.
pressure gradient
29
Because the _ in the soil are interconnected, water moves to the root surface by bulk flow through these channels down the pressure gradient.
water-filled pore spaces
30
* 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.
higher
31
2 factors of rate of water flow in soils
1. Size of the pressure gradient through the soil
2. Soil hydraulic conductivity
32
it is a measure of ease with which water moves through the soil
Soil hydraulic conductivity
33
variation of Soil hydraulic conductivity
1. type of soil
2. water content
34
large hydraulic conductivity
Sandy soil
35
low hydraulic conductivity
Clay soil
36
as water content decreases = hydraulic conductivity _ drastically
decreases
37
the decreases in hydraulic conductivity is due primarily to the _ in the soil spaces by air
replacement of
water
38
when air moves into a soil channel previously filled with water, water movement through that channel is _ of the channel
restricted to the periphery
39
as more of the soil spaces become filled with air, water can flow through _ channels, and the hydraulic conductivity falls
fewer and narrower
40
Intimate contact between the surface of the _ is essential for effective water absorption of root.
root and the soil
41
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.
Root hairs
42
Water enters the root most readily in the _ part of the root
apical
43
More mature regions of the root have an outer protective tissue called _ which are impermeable to water.
exodermis or hypodermis
44
Water Moves in the Root via the what pathways
1. apoplast
2. transmembrane
3. symplast pathways
45
water moves exclusively through the cell wall without crossing any membranes
Apoplast Pathway
46
the apoplast is the continuous system of _ in plant tissue
cell walls and intercellular air spaces
47
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
48
in this pathway, water crosses at least two membranes for each cell in its path (the
plasma membrane on entering and on exiting)
Transmembrane Pathway
49
In transmembrane pathway, transport across the _ may also be involved
tonoplast
50
water travels from one cell to the next via the plasmodesmata
symplast pathway
51
the symplast consists of the entire network of cell cytoplasm interconnected by _
plasmodesmata
52
At the endodermis, water movement through the apoplast pathway is obstructed by the _.
Casparian strip
53
Casparian strip is a band of radial cell walls in the endodermis that is impregnated with the wax-like, hydrophobic substance _
suberin
54
acts as a barrier to water and solute movement
Suberin
55
The casparian strip breaks the _ of the apoplast pathway, and forces water and solutes to cross the endodermis by passing through the plasma membrane
continuity
56
water movement across the endodermis occurs through the _
symplast
57
Solute Accumulation in the Xylem Can Generate _
“Root Pressure”
58
Plants sometimes exhibit a phenomenon referred to as _
root pressure.
59
Roots generate positive hydrostatic pressure by _ from the dilute soil solution and transporting them into the xylem.
absorbing ions
60
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).
xylem osmotic potential
61
This lowering of the xylem Ψw provides a driving force for _, which in turn leads to a positive hydrostatic pressure in the xylem.
water absorption
62
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 _
solutes
63
most likely to occur when soil water potentials are high and
transpiration rates are low
root pressure
64
65
high Ψw and low _ rates = root pressure
transpiration
66
Plants that develop root pressure frequently produce liquid droplets on the edges of
their leaves, a phenomenon known as _.
guttation
67
Xylem constitutes the _ part of the pathway of water transport.
longest
68
xylem is a simple pathway with _ to flow of water unlike the complex pathway across the root tissue
low resistance
69
The conducting cells in the xylem have a specialized anatomy that enables them to
transport large quantities of water with great efficiency
Two Tracheary Elements
70
tracheary element both found in gymnosperms and angiosperms
Tracheids
71
tracheary element only found in angiosperms, Gnetales (a group of gymnosperms) and
some ferns
Vessel Elements
72
The maturation of both tracheids and vessel elements involves the “_ ” of the cell.
death
73
are elongated, spindle-shaped cells that are arranged in overlap- ping vertical files
Tracheids
74
water flows between tracheids by means of numerous pits on their _
lateral walls.
75
are microscopic regions where secondary wall is absent and the primary wall is thin and porous.
Pits
76
Pits of one tracheid are typically located opposite pits of an Adjoining tracheid, forming _
pit pairs.
77
constitute a low-resistance path for water movement between tracheids
Pit pairs
78
a porous layer between pit pairs, consisting of two primary
walls and a middle lamella
Pit membrane
79
In some species of conifers, pit membranes have a central thickening known as _
torus
(pl. tori).
80
acts like a valve to close the pit by lodging themselves on the circular wall thickenings bordering the pits
Torus
81
Tori act thereby prevents dangerous gas bubbles from forming (process called
_ ) and invading neighboring tracheids
cavitation
82
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
perforiation plate
83
like trachieds, vessel elements had _ on their lateral walls
pits
84
unlike tracheids, the perforiated end walls allows vessel elements to be stacked end to end to form conduit called _
vessel
85
The _ Theory Explains Water Transport in the Xylem
Cohesion – Tension
86
The Cohesion – Tension Theory, the pressure gradients needed to move water through the xylem could result
from the generation of
1. positive pressures at the base of the plant; or
2. negative pressures at the top of the plant.
87
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.
high
88
the water at the top of a tree develops a large tension (a _ ), and this tension pulls water through the xylem
negative hydrostatic
pressure
89
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
cohesion–tension theory of sap ascent
90
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
collapse
91
The _ of tracheids and vessels
are adaptations to offset this tendency to collapse.
secondary wall thickenings and lignifications
92
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
93
the phenomenon of air formation and expansion
Cavitation
94
cavitation aka
also known as embolism
95
once a gas bubble has formed within the water column under tension, it will
expand because gases cannot resist tensile forces
cavitation/ embolism
96
_ breaks the continuity of the water column and prevents water transport in
the xylem
Cavitation
97
Cavitation breaks in xylem water continuity, if not repaired, would be _ to the plant
disastrous
98
by blocking the main transport pathway of water, such embolisms would cause
the _ of the leaves
dehydration and death
99
Because the tracheary elements in the xylem are interconnected, one gas bubble
might, in principle, expand to fill the _
whole network
100
In practice, gas bubbles do not spread far because the expanding gas bubble cannot
easily pass through the small pores of the _.
pit membranes
101
Since the capillaries in the xylem are interconnected, one gas bubble does _ water flow.
not
completely stop
102
water can detour around the blocked point by traveling through neighboring,
connected _
conduits
103
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.
104
many plants have secondary growth in which _ each year.
new xylem forms
105
The new xylem becomes functional before the old xylem ceases to function,
because of _ or by substances secreted by the plant.
occlusion by gas bubbles
106
Water Evaporation in the Leaf Generates a _ Pressure in the Xylem
Negative
107
The tensions needed to pull water through the xylem are the result of _ from leaves.
evaporation of
water
108
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
vascular bundle
109
The negative pressure that causes water to _ through the xylem develops at the surface of the cell walls in the leaf
move up
110
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
interstices
111
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
112
Thus, the motive force for xylem transport is generated at the _
air-water interfaces
within the leaf.
