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Water Balance of Plants (water absorption by roots (Root pressure…
Water Balance of Plants
water in soil
soil water potential
osmotic ptential \( \Psi\)s
usually negligible
'> -0.2MPa
saline soils <-0.2MPa
pressure potential \( \Psi\)p
wet soil ~0MPa
soil dries, \( \Psi\)p becomes more negative
=(-2T)/r
T=surface tension of water
7.28 x 10^(-8) MPa m
r=radius of curvature of air-water interface
clay, r <1um
soil particles assumed to be fully wettable
can easily reach -2MPa
gravity potential \( \Psi\)g
roll in drainage
proportional to elevation
higher at higher elevations
lower at lower elevations
bulk flow of water through soil
bulk flow definition
concerted movement of molecules en masse
mass flow
most often in response to pressure gradient
\( \Psi\)p
#
curved air-water interfaces
movement from larger pockets to smaller pockets
some due to diffusion of water vapor
rate of flow
size of pressure gradient
hydraulic conductivity
varies by
water content
decreases with water potential
replace water with air
soil type
clay
minute spaces between particles
small hydraulic conductivity
sand
large spaces between
high hydraulic conductivity
think of tiny strings of water along soil particle surface
influenced by
soil structure
water fill interparticle spaces
by capillary action
small area fills first
may form film on larger particles
after rain
clay retain ~40% water by volume
#
sand retain ~15% water by volume
#
soil type
sandy
#
particle >1mm
small SA:Vol
interparticle space large
clay
particle <2um
large SA:Vol
small interparticle spaces
+hummus \( \uparrow \) crumbiness
#
water absorption by roots
root hairs
increase surface area
only along surface of root
mostly absorbing in growing areas not older ones
Apoplast-Symplast-Transmembrane pathway
apoplast
parts
cells walls
intercellular air spaces
nonliving cell lumens
no membranes to cross
prevented at Casparian strip
composed of
suberin
lignin
hydrophobic
located
nongrowing part of root
distance behind root tip
where xylem starts to mature
symplast
parts
cell cytyoplasm
plasmodesmata
must move so across the endodermis
#
permeability due to aquaporins
genetic mutants low in aquaporins
wilt
larger root systems
respond to intracellular pH
\( \downarrow\) aerobic respiration \( \uparrow\) pH
\( \downarrow\) temp \( \uparrow\) pH
as pH changes, permeability decrease
transmembrane
crosses plasma membrane twice
may move through the tonoplast
in general water follows path of least resistance
Root pressure
due to solute accumulation in xylem
transpiration low or absent
+hydrostatic xylem pressure
roots continue to absorb ions from soil
transport ions to xylem
\( \Psi\)s xylem osmotic potential
probability \( \uparrow\)
soil \( \Psi\) high
E \( \downarrow\)
guttation
due to +\( \Psi\)s
through hydathodes
probability \( \uparrow\)
E \( \downarrow\)
Rel. humidity \( \uparrow\)
adaptive?
consequence of ion accumulation
dissolve gas bubles
mitigate cavitation
water transport through xylem
xylem 2 types of transport cells
tracheids
found in all vascular plants
morphology
torus only found in pines
acts as valve to prevent spreading emboli
structure
elongated
hollow
dead
highly lignified walls
numerous pits
primary cell wall only, no secondary
vessel elements
most angiosperms have
most gymnosperms do not have
structure
dead
connected by perforation plates
tube is a vessel
vessels conected through pits
xylem maturation
secondary walls
cell death
only lignified cell wall left behind
relatively little resistance
via pressure-driven bulk flow
#
independent of solute concentration :warning:
model as if a tube
Poiseuille's equation
r=radius of tube
\( \eta\)=viscosity
\( \Delta\) \( \Psi\)p / \( \Delta\)
x
pressure gradient
smaller necessary through xylem than through living cells
drives flow
m^3 s^(-1)
examples
ideal tube compared to living cells
main point: flow through xylem highly efficient compared to living cells
treetops
tall trees
Sequoia sempervirens
Eucalyptus regnans
minimum of 2MPa needed to move water from base to top
#
Cohesion-tension theory
#
tension at tree top
pulls water through xylem
consequence of E
requires cohesive properties of water
#
water movement leaf to atmosphere
summary pathway
xylem to
cell walls
evaporates into intercellular spaces
through stomata
controlled by gradients in water potential
final part
controlled by concentration gradient of water vapor
transport by diffusion
transport in vapor pahse
transpiration from leaf
depend on
difference in water vapor concentration
between air spaces & external bulk air
\( \Delta\)c\( _{wv(leaf)}\) - \( \Delta\)c\( _{wv(air)}\)
\( \Delta\)c\( _{wv(air)}\) measured easily
\( \Delta\)c\( _{wv(leaf)}\) difficult assess
air space volume
4 more items...
great amount of cell surface area
1 more item...
influenced heavily by temperature
\( \Delta\)c\( _w\)\( _v\)
diffusional resistance
r
2 components
stomatal resistance (r\( _s\))
1 more item...
boundary layer resistance (r\( _b\))
layer of unstirred air next to leaf surface
water vapor diffuse across to reach turbulent air of atmosphere
determined by
2 more items...
water loss to atmosphere
mainly stomata
cuticle <5%
leaf highly resistant
#
~30% of total liquid-phase resistance
aka leaf hydraulic resistance
reflects
xylem
distribution of xylem conduits
vein density
closer spaces veins
2 more items...
size of xylem conduits
number of xylem conduits
hydraulic properties of mesophyll cells
growth conditions
light conditions
sun lower resistance
shade higher resistance
\( \uparrow\) leaf age
\( \downarrow\) \( \Psi\) \( \uparrow\) hydraulic resistance
due to crossing living tissues
stomatal opening
light dependent
water-use efficiency
soil-plant-atmosphere continuum
