Water Flashcards
Importance of water in plants
Solvent in which all biochemical reactions take place
Gives structure to plants (turgor)
Medium for transporting metabolites (xylem/phloem)
Water requirements
80-95% mass of non-woody tissues is water
99% of water taken by roots is lost by transpiration
Water potential
force that causes water to move from where it is relatively more pure or it is under pressure, to locations where it is under less pressure or has higher solute content.
Measured in mega Pascals (MPa).
1 Pa = 1 Newton / m2 1 MPa = 106 Pa 1 Mpa ~ 10 atm
Pure water at 24.85°C (298°K) and atmospheric pressure (sea level, 1 atm) is, by definition, 0 MPa. Inside plant cell ~ -0.5 MPa.
Water potential inside a plant cell
-0.5 MPa
Ψ = Ψs + Ψp
Ψs: solute potential (osmotic potential). Always has negative value. As solute concentration increases, Ψs becomes more negative
Ψp: hydrostatic pressure potential. Physical pressure on a solution:
Turgor pressure has positive values
Suction pressure has negative values
Ψs
solute potential (osmotic potential).
Always has negative value.
As solute concentration increases, Ψs becomes more negative
Ψp
hydrostatic pressure potential.
Physical pressure on a solution:
Turgor pressure has positive values
Suction pressure has negative values
How is water moved short distances
Osmosis
How does water move long distances
Mass or bulk flow
difference in Ψ
determines the direction of water.
Water molecules can diffuse through the phospholipid bilayer, but there are helped by Aquaporins, channels in the plasma membrane that can close or open, allowing rapid movement of water molecules.
Soil and root cells
As soil dries, cells adjust their water status by accumulating osmotically active compounds (inorganic ions and organic acids) such as glycinebetaine, sorbitol, and proline.
Different types of transport between cells
Apoplastic
Via vacuoles
Symplastic (cytoplasm and plasmodesmata)
Casparian strip
Belt of waxy material that blocks the passage of water and dissolved nutrients
Only by crossing the plasma membrane water and nutrients can get into the stele
Transpiration
Transpiration draws water from the leaf
Cohesion and adhesion draw water up the xylem
Negative water potential draws water into the root
Xylem
Root pressure accumulated cause the influx of water into the xylem by osmosis.
Capillary action: water adheres to the wall of tubes, elevating it.
Cohesion-tension: due to transpiration (water loss), water is moved upwards to the leaves.
Water stress
When transpiration rate is higher than absorption rate
Potometer
Used to measure transpiration rate
Drought avoidance
Complete life cycle during a wet period (often <6 weeks)
Leaf fall under drought
Deep roots with access to ground-water
Characteristics of drought avoidance
High photosynthetic rates
High leaf area
High stomatal conductance and transpiration rates
Strategies to balance with water stress
Drought avoidance
Drought tolerance
Drought tolerance
try to minimise the water loss and cope with its effects
At low water potential: survive with minimum metabolism (e.g. resurrection plants).
Even after drying up, they can almost fully restore their metabolism when water is available
At high water potential: maintain tissue hydration:
Regulation of water loss
Regulation of water absorption
Production of protection compounds (e.g. carotenoids, osmolytes, stress, proteins, antioxidants)
Response to water deficit
ABA synthesised in roots is translocated to leaves where it closes stomata and inhibits plant growth
ABA and stomata
ABA creates rapid changes in the osmotic potential of guard cells: guard cells release K+ and Cl- and water exits, loosing turgor and thus closing.
High salinity problems
Low water potential => water stress
Toxicity due to Na+ or Cl-
