Plant Water Balance

1

• Plant Water Balance
• Well look at how plants manage water in four
  places
• From the earth Water flows through the soil in
  response to a pressure gradient. Figure 4.1
• To the cross Water moves into the root through
  the hairs, and is drawn into the root xylem by a
  water potential gradient.
• To the grave (?) Water flows up the xylem,
  driven by pressure.

To the sky Water moves into the spaces of the
leafs spongy mesophyll, and out into the air
driven by vapor pressure.
2

• Plant Water Balance
• Hydrostatic pressure of water in the soil is the
  main driving force drawing it toward a plants
  roots.
• A. Soil composition determines its properties.
• 1. Soil is made up of three particles, based
  on diameter.
• a. Sand--the largest particles 2 - 0.02 mm
• b. Silt--intermediate size 0.02 - 0.002 mm
• c. Clay--the smallest particles gt 0.002 mm
• 2. Soil properties are affected by the
  texture--the proportion of each particle
  type.
• a. Soils with lots of clay hold water well
  (have high field capacity), whereas water in
  sandy soils drains
• away rapidly.
• b. The movement of water toward the root is
  hindered in soils with lots of clay, unless the
  clay is aggregated in decomposing material, a
  mixture called humus.

3

• Plant Water Balance

4

• Plant Water Balance
• B. Soil water potential is determined mainly by
  hydrostatic pressure.
• 1. Water in most soils contains very little
  solutes, so solute potential rarely has a huge
  impact on the water potential.
• 2. Hydrostatic pressure in soils is often
  negative.
• a. Very wet soil has ?P
• of about zero.
• b. As soil dries (through
• drainage and
• evaporation), ?P drops.
• This is because of the meniscus
• that forms around soil particles
• as water is replaced by air space.
• This is due to waters ________.

5

• Plant Water Balance
• C. Water moves by bulk flow toward the root.
• 1. Water is absorbed by the root (well see
  why in a minute). Because of the cohesiveness of
  water, this reduces the hydrostatic
  pressure locally around the root.
• Figure 4.2 2. The water in the soil
  stays connected at soil particles
  (unless the soil gets really dry), so
  will move toward the area of lower ?P.

6

• Plant Water Balance
• 3. Besides the strength of the pressure
  gradient, the other factor that determines how
  fast water will move toward the root is the
  soil hydraulic conductivity (how easily water
  can move in the soil).
• This is affected by soil texture and by water
  content itself.
• a. Which type of soil particle would increase
  soil hydraulic conductivity? Which would
  decrease it?
• b. Would water move more easily through soil
  that has more air space or soil that has
  little air space?

7

• Plant Water Balance
• D. If soil gets too dry, the water potential of
  the soil may get equal to or lower than that of
  the plant. This is called the permanent wilting
• point, because
• the plant cannot
• rehydrate itself just by
• closing its stomata to
• limit transpiration.

8

• Plant Water Balance
• So only a certain percentage of water in a field
  is ever really available to a plant.
• This property and others are determined also by
  the water potential of the plant, and it is the
  root where this interaction is felt.

9

• Plant Water Balance
• Movement of water into and through the root.
• A. Root hairs near the apical end the of
  primary and lateral roots do most of the
• absorption of water. Figure 5.8
• 1. Root tip structure
• a. The cells at the very tip
• of the root make up the
• root cap. What might
• this do?
• The mucigel is slimy and lubricates
• the path of the root.

10

• Plant Water Balance
• b. The center of the root tip is the
  quiescent center (QC). This group of cells
  divides very slowly. These are the ancestors
  of
• the whole root. The cells
• at the edges of the QC are
• initials. That means that
• they will give rise to
• specific sets of tissues
• within the roots.
• Can you see where the
• initials are, and what tissues
• they give rise to?

11

• Plant Water Balance
• c. As cells divide out of the QC, they enter
  the zone of division, where they start to
  divide very rapidly,
• producing most of the
• cells that will make up the
• root. This is about 1mm
• long in most plants.

12

• Plant Water Balance
• d. These cells eventually become the cells of
  the
• elongation zone, where the cells elongate
  vertically,
• pushing the root deeper
• into the soil. This zone
• begins at 4 to 15 mm from
• the tip, and is 1 cm long.
• Here the root xylem,
• phloem, pericycle, and
• endodermis form.

13

• Plant Water Balance
• e. As you look further up the root, you see
  that the cells of the elongation zone will
  eventually quit elongating, and that this
• happens in a zone of
• maturation which begins
• at about 1 - 5 cm from the
• tip. Here is where the
• root hairs form. The
• developed epidermal
• cells here are not inhibited
• in their transport of water.

14

• Plant Water Balance
• f. Further up the root, a layer of
  hypodermis forms underneath the epidermis.
  These contain suberin, which blocks water loss
  and uptake. Fig. 4.4
• 2. Root hairs make up as much as 60 of the
  surface area of some roots.
• 3. Root hairs reach into the areas between
  soil particles
• where the film of
• water exists.

15

• Plant Water Balance
• B. Why is water drawn into the root?
• C. Inside the root, water can move by three
  pathways until it reaches the endodermis.
• 1. Trans- membrane (not
• shown)--Water
• can cross cell
• walls, membranes, and go through
  cytoplasm to get to the
  endodermis.

16

• Plant Water Balance
• 2. Apoplast pathway--Water can skirt around
  cells altogether, by staying in the cell wall
  matrix and middle lamellae. But this pathway is
  blocked at the endodermis by
• the Casparian strip, a thick band of suberized
  cell wall (contains lots of that waxy suberin),
  so here water must enter a cell and go through
  the
• 3. Symplastic pathway--Here, water flows
  through the plasmodesmata that connect the cells.
  As with the transmembrane pathway, water may
  encounter organelles along the way, so it may
  still do a bit of membrane- crossing. By forcing
  water to take this pathway at the endodermis,
  water loss is inhibited.

17

• Plant Water Balance
• D. Root pressure
• 1. Roots actively accumulate solutes, mostly
  ions, from the soil water around them even
  though they are very dilute. These ions are
  taken to the xylem.
• 2. This lowers the roots solute potential.
  If the soil and the root were to come to
  equilibrium, the hydrostatic pressure in the
  root would be measured as being 0.1 MPa or
  higher (about one atmosphere of pressure above
  the ambient pressure.) But usually the root and
  soil never actually come to equilibrium (the
  pressure is never allowed to build up)why not?

18

• Plant Water Balance
• because water flows up the
• plant instead of hanging
• around to build up pressure.
• The pressure that would build up
• is just part of the pressure gradient
• between the root and the leaves
• (where the pressure is negative due
• to so much transpiration) that pulls
• the water up the plant.
• But there is a time when the
• root and soil can come to
• equilibriumwhen would that
• happen?

19

• Plant Water Balance
• When there is no net transpiration, there is no
  negative pressure to pull the water up. Pressure
  in the roots can then build up, and that pressure
  can actually be strong enough to push water up
  the plant and cause it to leak out of the ends of
  the xylem, modifed for just that purpose, called
  hydatodes. This causes the phenomenon of
  guttation, and is seen when the relative humidity
  is 100 (when the air temperature is at the dew
  point).

20

• But I said that the
• root otherwise
• doesnt build up enough pressure because the
  water is being
• pulled up the plant through the xylemwhats it
  like in there?

21

• Plant Water Balance
• The xylem provides a low-resistance path from
  root to leaf, in which a pressure gradient pulls
  water up the plant.
• A. Review Water conducting cells (tracheary
  elements)
• 1. They are all dead at
• maturation--no nucleus, organelles,
  or membranes.
• 2. Two types of cells
• a. Tracheids--long narrow overlap each
  other, where water flows between through pit
  pairs (areas where there is no secondary
  wall, but only a thinned out and porous
  primary wall and middle lamellae,
  collectively called the pit membrane.)
• Figure 4.6A

22

• Plant Water Balance
• The xylem provides a low-resistance path from
  root to leaf, in which a pressure gradient pulls
  water up the plant.
• A. Review Water conducting cells (tracheary
  elements)
• 1. They are all dead at Figure 4.6c
• maturation--no nucleus, organelles,
  or membranes.
• 2. Two types of cells
• a. Tracheids--long narrow overlap each
  other, where water flows between through pit
  pairs (areas where there is no secondary
  wall, but only a thinned out and porous
  primary wall and middle lamellae,
  collectively called the pit membrane.)
• Some have a moveable thickening called a
  torus,
• which slides over to block an air bubble from
  spreading

23

• Plant Water Balance
• b. Vessel elements short wider compound or
  simple perforation plates at the end allow
  them to be stacked, forming a continuous tube
  called a vessel.
• These also have pits to allow water to
• move out laterally. Figure 4.6B
• B. The pressure needed to move
• water at 4 mm per second in
• xylem is about 0.02 MPa per meter.
• This is about ten billion-fold less
• pressure than it would take to
• water that fast through living cells!
• This only takes into account the friction in the
  system, not the gravity.

24

• Plant Water Balance
• C. How do those really tall trees do it?
• How much pressure would it take to get water
  to the top of a 100 meter tree?
• 1. The pressure ? needed to overcome the
  friction is
• 0.02 MPa per meter, times 100 meters, or 2
  MPa.
• 2. Gravity, as we saw earlier adds another
  0.01 MPa per meter, increasing the required
  pressure ? by 1 MPa.
• So we need 3 MPa in pressure difference to get
  water from the ground to the top of the tallest
  tree. Thats about 30 atmospheres of pressure
  difference.
• 3. Root pressure cant do it, it only
  generates 0.1 MPa or so at most.
• 4. Research suggests that it is done by
  negative pressure that is generated as water
  transpires from leaves.
• Can a water column and a plant handle that kind
  of pressure?

25

• Plant Water Balance
• D. Physical constraints
• 1. Thickened secondary walls with lignin in
  tracheary elements make them strong enough to
  take the pressure (so they wont collapse
  inward), however...
• 2. Columns of water that have no dissolved
  gasses can handle up to 30 MPa pressure without
  breaking, but
• 3. The negative pressure
• sometimes pulls air into
• a vessel by opening
• microscopic holes. Air Figure 4.7
• bubbles like this block
• water flow, and could
• kill the plant if not fixed
• or if they spread too far.

26

• Plant Water Balance
• 3. Cavitation, continued
• a. Spreading in vessels is minimized due to
  fact that vessels eventually end in a wall
  without perforations, where water has
  to flow through pits, but air has a harder
  time of it.
• b. Air bubbles can be repaired at
• Figure 4.7 night when the pressure becomes
• less negative, as the bubble
  dissolves into the water. Fairly
• recent studies show that bubbles
• can be fixed even when the pressures
  are quite negative, but how this works isnt
  understood.

27

• Plant Water Balance
• Transpiration of water from leaves is a
  multi-stage process dependent on the differences
  in water vapor content of the inner leaf, the
  spaces on either side of the stomata, and the
  bulk air in the atmosphere itself.
• Figures 4.10, 4.8

28

• Plant Water Balance
• A. Evaporation of water from curved surfaces
  within the cell walls of spongy mesophyll cells
  creates negative pressure.
• 1. A thin film of
• water coats the mesophyll
• cells, and evaporates into
• the air space inside the Figure 4.9
• leaf. Net evaporation
• occurs as water molecules
• in the film escape as vapor
• at a faster rate than the
• opposite process occurs.
• The temperature, purity,
• and shape of the water film
• are important to this process, and we define
  relative humidity.

29

• Plant Water Balance
• 2. As water evaporates from the curved surface
  (curved for the same reasons as in the soil, only
  this time is it cellulose microfibrils the water
  clings to), it stretches the surface to maintain
  the curve. Remember, water hates interfaces
  with air. This stretching produces negative
  pressure, so all the Figure 4.9 spongy
  mesophyll acts like a wick to draw water up.

30

• Plant Water Balance
• B. The diffusion of water vapor out of the leaf
  is driven by differences in water vapor
  concentrations of the air, and regulated by
  certain diffusional resistances along the way.
• 1. Diffusion drives water vapor from the high
  concentration inside the leaf toward areas of
  low concentration, namely the outside air.
• a. Water vapor concentration can be measured
  at various points along the way
  Table 4.2 from the inner leaf to the
  outside air.
• b. The water vapor in the air also has
• a water potential. Potential decreases with
  concentration.

31

• Plant Water Balance
• c. Water vapor concentration (and therefore
  potential) usually decreases from inner leaf
  to outside air (unless what?), but the
  decrease is not continuous.
• 2. Resistances to diffusion--Two resistances
  impede the possible routs of diffusion for water
  (a) a layer of still air just around the leaf,
  where water vapor content is higher than the bulk
  air and (b) the size of the stomatal aperture.
• Figure 4.10

32

• Plant Water Balance
• a. Boundary layer
• resistance is due to the still
• air just surrounding the leaf
• (especially the bottom side).
• Figure 4.12
• The most important thing that
• determines the effect of this
• resistance is the ____.
• Trichomes
• (leaf hairs)
• can also
• slow the
• wind down.

b. Stomatal resistance is based on the size
of the stomatal aperture. When the wind is
strong, this is about the only thing that
matters in controlling transpiration rates.
33

• Plant Water Balance
• C. The size of the stomatal aperture is
  controlled by increasing or decreasing turgor
  pressure in the guard cells, and is regulated to
  balance water loss against the need for the leaf
  to take in CO2 for photosynthesis.
• 1. Stomatal guard cells act like the guys by
  the door at a club (not that I would know from
  first hand experience.) At the same time,
  they are also like the door itself.

34

• Plant Water Balance
• a. Notice how thick is the wall at the part
  that faces into the stomatal cavity of the leaf,
  toward the inside of the
• stoma.
• b. Look at the cells on the left (dicot
  stoma). The cellulose microfibrils radiate out
  from the center of the pore. What would happen
  if the turgor pressure on the inside of this
  cell was increased?
• Figure 4.15
• Now the cells on the right, like those of
  grasses? Here the cellulose microfibrils are
  arranged longitudinally, so that the length is
  maintained.
• What happens if the pressure is increased?

A cross-section through tobacco guard cells.
35

• Plant Water Balance
• c. Turgor pressure is increased by pumping in
  ions from outside the cell. This lowers water
  potential inside the cell, and causes water to
  flow in, in order to come to equilibrium. The
  lack of plasmodesmata means that there is nothing
  for the pressure to do but to stay in the guard
  cells.
• 2. The plant needs to have its stomata open
  for taking in CO2, but limits the opening to
  prevent too much water loss. a. There are
  usually about 500 molecules of water that leave
  for every one molecule of CO2 that is fixed.
  This is the transpiration ratio.
• b. Why such a difference in movement?