Water Transport

Cards (45)

  • The movement of water in the plant is primarily determined by two factors
    1. Water potential gradient
    2. Resistance of the path that the water travels
  • Water always moves from high to low water potential (less negative to more negative water potential).
  • Water uptake in the roots is affected by:
    • Soil water content
    • Properties of the soil
    • Architecture of the root systems
    • Presence of root hairs and fungi
    • Conductance of the root system
    • Symbiotic relationships
  • Soil Water Content: the amount of water present in the soil. Depends on the soil type.
    • Clay < Silt < Sand
  • Porosity - spaces formed between irregularly shaped soil particles
    • Large pores (sand) - contain more water but easily drained (low water retention)
    • Small pores (clay) - contain less water but more challenging to drain
  • Gravitational water: water molecules that are retained in larger pores; loosely held; easily drained from the soil; not easily available to plant; found in larger pores; drains out of root zone
  • Capillary water: when gravitational water is all absorbed, this is the available water that plant roots can absorb; water held in micropores; capable of capillary action (water molecules held by each other: cohesion, and by the surface of the soil: adhesion)
  • Hygroscopic water: remaining water adheres to soil particles and is unavailable to plants (high energy requirement is needed for this: similar in towel)
  • Saturation - when all the pore spaces are occupied by water
  • Field Capacity - when all the gravitational water has drained and only the capillary water remains
  • Wilting point - when the soil water constant is so low that the plant can no longer extract water from the soil
  • Solute potential of soil water is generally negligible (except in saline soils).
  • Pressure potential of very wet (saturated soil) is very close to zero and decreases as the soil dries out because of adhesion and surface tension
  • Gravitational potential of soil water is higher at higher elevations and vice versa (easier to get soil water near the roots)
  • Matric potential of soil water decreases as the soil dries out.
  • Overall, the soil water potential decreases as the soil dries. This makes it harder for roots to absorb water.
    • Water moves through the soil by bulk flow driven by pressure gradients. Water flows from regions of higher soil water content to regions of lower soil water content.
  • Root system
    • Soil to Root hairs to cortical cells to vascular tissues
  • Radial Conductance - Water can travel from the soil into root vascular bundle in three ways
    • Apoplastic - through the cell walls (will not enter the cells)
    • Symplastic - through the plasmodesmata of the cells
    • Transcellular - moves through the cell walls and cytoplasm; requires membrane passages
  • Axial Conductance - water encounters the Casparian strip in the endodermis which forces water to cross through the plasma membrane.
  • Casparian Strip - waxy, suberin-impregnated region that forms a hydrophobic belt.
    • Importance: generates pressure; prevent back flow; movement of nutrients; prevents movement of pathogens
  • Aquaporins - channel proteins that transport water molecules and also a range of other substrates (CO, minerals, etc.)
  • Xylem - complex tissue composed of the tracheary elements, xylem fibers, and ray cells
  • Tracheary elements
    • large diameter
    • dead at maturity
    • thickened secondary walls
  • Vessel members - has perforation plates; found in angiosperms
  • Tracheids - a tube with no openings on ends; found in gymnosperms
  • Transpiration
    •  Loss of water vapor from plants through the surfaces of aboveground parts but mainly through the leaves
    • Happens mainly through the stomata
  • Cohesion-Tension Theory
    • The driving force for water movement in the xylem is provided by the evaporation of water from leaf surface which creates tension (pulling)
  • Main driver of transpiration of water in the form of water vapor from the interstitial cells through stomata is simple diffusion
  • Higher humidity = decrease in transpiration
  • Higher temperature = higher humidity
  • Stomatal pore open - turgor pressure in guard cells are high - high solute concentration - low water potential
  • Stomatal pore closes - low pressure in guard cells - low solute concentration - high water potential
  • CO2 - increased CO2 inside the plant, close stomata
    Water status - low water, close stomata
    Light - high light, close stomata
    Air humidity - high humidity, stomata opens
  • Cavitation - Occurs when the tension of water within the xylem becomes so high that dissolved gases within water expands to fill the tracheary element
  • Embolism - the resulting large gas bubble that obstructs the pathway. Pits between vessel elements can be used by water to redirect its travel when specific vessel elements are blocked
  • Guttation - exudation of drops of xylem sap on the tips or edges of leaves of some vascular plants due to high root pressures
  • Hydathode - structures that secrete water through pores in the epidermis or leaf margin, typically at the tip of a marginal tooth or serration
  • Phloem: the vascular tissue in plants which conducts sugars and other metabolic products downwards from the leaves. Complex tissue composed of the sieve tubes, companion cells, fibers and parenchyma cells
  • Sieve elements - living protoplasts at maturity but lack many cellular contents; end walls have sieve plates