Water Transport

• 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