Water relations

Question 1

Water relations of plants

Answer 1

Diffusion of water into cells = concept of water potential
Absorption of water by roots and transport across root to vascular system
Upward transport in the xylem
Stomata and transpiration

Question 2

Chemical potential

Answer 2

All chemicals have capacity to do work = free energy
Chemicals will diffuse from areas of high chemical potential to areas of low chemical potential

Question 3

Water potential

Answer 3

Diffusion of water, chemical potential of water
Water diffuses from areas of high wp to low wp
Pure water at atmospheric pressure has wp of 0
Wp in a system can be increased or decreased in a number of ways
Anything that increases the water potential to diffuse in a system increases water potential
Wp is expressed in pressure units (Pa, mbar)

Question 4

Things affecting water potential

Answer 4

Increased by heat, pos pressure
Decreased by dissolved solutes, adhesion

Question 5

Components of water potential

Answer 5

Osmotic potential – from solutes in vacuole
Pressure potential – from cell wall
Matric potential – from soil
w =  + p + m

Question 6

Properties of plant cells

Answer 6

Properties of plant cells
Bound by a selectively permeable cell membrane and surrounded by a cell wall
Contain dissolved solutes – ions, sugars, amino acids, etc
Cell membrane permeable to water but largely impermeable to solutes
Cell wall permeable to water and largely permeable to solutes
Vacuole bound by selectively permeable tonoplast
Wp varies from 0 to <0
Averge wp for plant tissues = -0.1 - -2.0 MPa

Question 7

Osmotic potential of plant cells

Answer 7

Water moves across semi-permeable membrane from a region of high op (low dissolved solute conc, high op) to low op (higher dissolved solute conc, low op)
If no pressure potential or matric potential involved, op is the water potential of the cell
If this continues the cell will take up a large amount of water and burst
Cells in water swell up and become turgid
Cells in conc salt solution shrink and shrivel
At 25oC for a non-ionic solution (e.g. sucrose)
* 0.01 mol L-1 water = -0.0248 MPa
* 0.10 mol L-1 water = -0.248 MPa
* 1.00 mol L-1 water = -2.48 MPa
Seawater = -2.8 MPa
* For an ionic substance e.g. NaCl  Na+ + Cl-
* Effectively twice as many dissolved particles
* i.e. 0.5 mol L-1 water NaCl  1.0 mol L-1 sucrose
Hypotonic – solution which contains a lower conc of dissolved solutes = higher op
Hypertonic – solution which contains a higher conc of dissolved solutes = lower op
Isotonic – same conc of dissolved solutes

Question 8

Pressure potential

Answer 8

When plant cells are placed in pure water they take up water until they become fully turgid
Plant cell walls have a high elastic modulus and they can develop high turgor pressures
When plant cells are fully turgid, the cell wall exerts a back pressure which cancels op
p +  = 0
In a fully turgid plant cell, wp = 0 MPa

Question 9

Plasmolysis

Answer 9

If plant cell placed in hypertonic solution with low op water will move from cell into solution, decreased pp and the cell eventually plasmolysis (cell membrane pulls away from cell wall) = pp becomes 0

Question 10

Estimating osmotic potential

Answer 10

1. Plant tissue is placed in solutions of varying osmotic potential ()
2. Degree of plasmolysis is recorded
3. Solution in which plasmolysis is 50% is estimated from graph (plot of % plasmolysis/)
4. This gives approximate (average)  of cells in tissue

Question 11

Movement of water in whole plant

Answer 11

Absorbed by roots
Moves across root to xylem of vascular cylinder
Ascends plant via xylem to the shoot and eventually to leaves
Lost from leaves to atmosphere via stomata by transpiration = transpiration stream
Transpiration stream also transports minerals in xylem

Question 12

Water in soil

Answer 12

Water content and water movement in soil depends on the type of soil
Soil = mixture of minerals (clay and sand) and organic particles, water, solutes and air
In clay soils water is help tightly (low m) by particles – e.g adsorption, capillary forces, surface tension
When saturated with water a soil is at field capacity after water is allowed to drain (by gravity)
Water in soil spaces more readily available
As water is depleted wp decreases

Question 13

Absorption of water by roots

Answer 13

Root system may be >50% of plant body
Most water absorbed through the younger parts of root through epidermis
Roots hairs = extensions of epidermis providing enormous surface area for absorption

Question 14

Water across roots

Answer 14

Root hairs – cortex – endodermis – xylem
Apoplastic pathway – cell walls and intercellular spaces
Symplastic pathway – protoplast to protoplast via plasmodesmata
Transcellular – from cell to cell through vacuoles

Question 15

Endodermis in root

Answer 15

Layer of cells that separates the cortex of the root from the central stele
Cells have a band of suberin called casparian strip on their walls
Water cannot follow apoplast rout into the stele across endodermis so has to cross barrier via symplast or transcellular route

Question 16

Xylem

Answer 16

All vascular plants have tracheids
Flowering plants have tracheids and vessel elements
Water moves from tracheid to tracheid via pits
Vessels have perforation plates at their ends
Water can move from vessels to tracheids and vice versa

Question 17

How water moves upwards from root to top of shoot

Answer 17

Possibilities = root pressure, capilliarity, transpiration (cohesion theory)

Question 18

Transpiration

Answer 18

1. Evaporation of water from moist cell walls in the leaf to substomatal space
2. Diffusion of water vapour from the substomatal space via stomatal pore, into atmosphere
   Transpiration through cuticle is usually verry low, depends of thickness
   As water evaporates from wet cell walls in the leave a neg pressure (surface tension) develops which pulls water from adjacent cells
   Tension is transmitted back to vascular xylem and pulls water up
   Creates neg pressure (tension) which is transmitted all the way to the root and soil

Question 19

Embolisms and cavitation

Answer 19

Gas bubbled (embolisms) can from when dissolved gases come out of solution in a water column under tension which results in cavitation (break in water column)
Xylem cells have a torus which seals off bordered pits
Embolism is contained
Surface tension helps to prevent gas bubbles from passing through perforations inside or end walls of vessles
Water follows detoured routes through pits avoiding embolism
Gas bubbles cannot move through small pores of pit membranes
Since vessels are interconnected water flow is not stopped but moves through neighbouring vessels

Question 20

Stomata

Answer 20

May be on both or one surface of leaf (usually only on underside)
Stomatal pore regulated by guard cells on either side
Two types – elliptic and dumbbell (depends on shapes of guard cells)
Potassium levels of open guard cells are very high but very low in closed guard cells
Active import/export of K+ regulated op of guard cells
When K+ is high (low op) water diffuses in along wp gradient, guard cells become turgid and stomata open
When K+ is low (higher op) guard cells lose turgidity and stomata close