Water Balance

1

• Lecture 6
• Water Balance
• of Plants

2
Water Movement from the Leaf to the Atmosphere
Transpiration the evaporation of water from
leaf surfaces
3
The Driving Force for Water Loss Is the
Difference in Water Vapor Concentration

• Transpiration from the leaf depends on
• 1) difference in water vapor concentration
  between the leaf air spaces and
• the external air cwv(leaf) cwv(air)
• 2) the diffusional resistance (r) of this
  pathway
• cwv(air) water vapor of bulk air
• cwv(leaf) water vapor of leaf
• Internal leaf surface from which water evaporates
  is 7-30 times the air space volume
• Concentration of water vapor changes along the
  transpiration pathway

4
Factors affecting transpiration

• Environmental factors
• Light stimulates the stomata to open allowing
  gas exchange for photosynthesis ? transpiration
  increases (plants may lose water during the day
  and wilt)
• Temperature High temperature increases the rate
  of evaporation of water from the spongy cells,
  and reduces air humidity, so transpiration
  increases.
• Humidity High humidity means a higher water
  potential in the air, so a lower water potential
  gradient between the leaf and the air, so less
  evaporation.
• Wind Blows away saturated air from around
  stomata, replacing it with drier air, so
  increasing the water potential gradient and
  increasing transpiration.

http//www.biologymad.com/master.html?http//www.b
iologymad.com/PlantTransport/PlantTransport.htm
5
Factors affecting transpiration

• Plant factors
• Leaf structure
• Leaf area
• Shoot-Root ratio

Adaptations to dry habitats Plants in different
habitats are adapted to cope with different
problems of water availability.
Mesophytes plants adapted to a habitat with
adequate water Xerophytes plants adapted to a
dry habitat Halophytes plants adapted to a salty
habitat Hydrophytes plants adapted to a
freshwater habitat
http//www.biologymad.com/master.html?http//www.b
iologymad.com/PlantTransport/PlantTransport.htm
6
Factors affecting transpiration
Adaptations to dry habitats Some adaptations of
xerophytes are
7
Water Loss Is Also Regulated by the Pathway
Resistances
Water loss also determined by the diffusional
resistance of the transpiration pathway,
consisting of rs leaf stomatal resistance
(resistance associated with diffusion through the
stomatal pore) rb boundary resistance
(resistance due to the layer of unstirred air
next to the leaf surface through which water
vapor must diffuse to reach the turbulent air of
the atmosphere)
Dependence of transpiration flux on stomatal
aperture of zebra plant
8
Pathway Resistances
9
Stomatal Control Couples Leaf Transpiration to
Leaf Photosynthesis

• Problem
• Plants have to take up CO2 from atmosphere, but
  simultaneously need to
• limit water loss
• Cuticle protects from desiccation
• However, plants cannot prevent outward diffusion
  of water without
• simultaneously excluding CO2 from leaf and
  concentration gradient for
• CO2 uptake is much smaller than concentration
  gradient that drives water
• loss
• Solution Temporal regulation of stomatal
  apertures
• Closed at night no photosynthesis ? no demand
  for CO2 ? stomatal
• aperture kept small ? preventing unnecessary
  loss of water
• Open during day sunny morning water abundant,
  light favors
• photosynthesis ? large demand for CO2 ? stomata
  wide open ?
• decreased stomatal resistance to CO2 diffusion
  ? water loss by
• transpiration also substantial, but water
  supply is plentiful, i.e. plant trades
• water for the product of photosynthesis needed
  for growth

10
The cell walls of guard cells have specialized
features
Two main types of guard cells -
kidney-shaped stomata for dicots, monocots,
mosses, ferns, gymnosperms - grass-like
stomata grasses and a few other monocots
(e.g. palms)
Kidney-shaped stomata
Grass-like stomata
11
An increase in guard cell turgor pressure opens
the stomata
- guard cells function as multisensory hydraulic
valves - guard cells sense changes in the
environment light intensity, light quality,
temperature, leaf water status, intracellular CO2
- this process requires ion uptake and other
metabolic changes in the guard cells (discussed
later) - due to this ion uptake ? ?s decreases ?
?w decreases ? water moves into guard cells ?
?p (turgor pressure) increases ? cell volume
increases ? opening of stomata (due to
differential thickening of guard cell walls)
12
The cell walls of guard cells have specialized
features
Portions of the guard cell wall are substantially
thickened (up to 5 µm across) Alignment of
cellulose microfibrils are essential for opening
and closing
Atmosphere
Pore
Nucleus
Plastid
Vacuole
Substomatal cavity
Inner cell wall
13
Stomata
Stomata from sedge (Carex)
Cytosol and vacuole
Pore
Heavily thickened guard cell wall
Guard cells
Subsidiary cells
Stoma from a grass
Epidermal cell
14
Stomata
Stomata from onion epidermis
Outside surface of the leaf with stomatal pore
inserted into the cuticle
Guard cells facing the stomatal cavity, toward
the inside of the leaf
Guard cell
Stomatal pore
15
The Transpiration Ratio Measures the Relationship
between Water Loss and Carbon Gain

• Transpiration ratio the effectiveness of plants
  in moderating water loss while allowing
  sufficient CO2 uptake for photosynthesis
• Transpiration ratio Amount of water lost by
  transpiration Amount of
  CO2 fixed by photosynthesis
• Also termed Water Use Efficiency (WUE) inverse
  of transpiration ratio (e.g. plant with
  transpiration ratio of 500 has a WUE of 1/500
  0.002)
• Generally, H2O efflux is more than CO2 influx
  this is due to
• Concentration gradient driving water loss is 50
  times larger than driving the influx of CO2 (due
  to low concentration of CO2 in air) and the
  relatively higher water vapor in leaf
• CO2 diffuses 1.6 times more slowly through air
  than water does (CO2 molecule is larger than H2O
  and has a smaller diffusion coefficient)
• CO2 uptake must cross the plasma membrane,
  cytoplasm, chloroplast envelope before it is
  assimilated in the chloroplast ? these membranes
  add to the resistance of CO2 diffusion pathway

16
The Plant Soil Atmosphere Continuum
Movement of water from soil through plant to
atmosphere involves different mechanisms of
transport In soil and xylem, water moves by
bulk flow in response to a pressure gradient
(??p). In the vapor phase, water moves by
diffusion until it reaches outside air (and
convection, a form of bulk flow, becomes
dominant). For water transport across
membranes, water potential difference across
membrane is driving force (osmotic flow, e.g.
when cells absorb water and roots
transport water from soil to xylem). In all
cases water moves toward regions of low water
potential.
17
Water moves toward regions of low water potential
Overview of water potential and its components at
various points in the transport pathway from soil
through plant to atmosphere.
18
Water and Plants Summary
- water is the essential medium of life - plants
gain energy from sunlight and need to have an
open pathway for CO2 - plants have large
surface area that is not differentially
permeable to CO2 vs. water vapor ? conflict need
for water conservation and need for CO2
assimilation - the need to resolve this conflict
determines structure of plants - extensive root
system to extract water from the soil - a
low-resistance pathway through the xylem vessels
and tracheids to bring water into the
leaves - a hydrophobic cuticle covering the
plant to reduce evaporation - stomata to
allow gas exchange - guard cells to regulate
stomatal aperture (opening)
19
Water and Plants Summary
- plants transport water from soil to atmosphere
through diffusion, bulk flow, osmosis -
capillarity is key element allowing water
movement from soil to leaves - transpiration is
regulated by guard cells - water evaporation from
leaf mesophyll cells generates negative
pressure (i.e. suction or tension) in the
apoplastic water by capillarity
(inflation/deflation apparatus supplying
positive pressure for expanding a balloon
during inflation and negative pressure for
contracting the balloon during deflation) -
negative pressure is transmitted to the xylem,
pulling water through the long xylem elements -
when transpiration is high ? large negative
pressures in xylem water ? cavitation
(embolism) ? block water transport ? water
deficit in leaves