BM

Water and Solute Movement in Plants

The Photosynthesis-Transpiration Compromise

  • Plants face a trade-off between gaining carbon dioxide for photosynthesis and losing water vapor through transpiration.
  • The challenge is to maximize CO2 intake while minimizing H2O loss.
  • H2O diffuses 1.6 times faster than CO2.
  • The rate of water loss influences the rate of CO2 intake.

Transpiration-to-Photosynthesis Ratio

  • Expressed as kgs of H2O lost per kgs of dry material produced.
  • C3 plants: 600:1
  • C4 plants: 300:1 (or less), indicating greater assimilation efficiency.
  • CAM plants: 150:1, with stomata open only at night, leading to greater assimilation efficiency.

Leaf Structure in C3, C4, and CAM Plants

  • C3 plants: possess a waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, vascular bundles (xylem and phloem), lower epidermis, and stomata.
  • C4 plants: similar to C3 but include bundle sheath cells around the vascular bundles.
  • CAM plants: possess waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, aquiferious parenchyma, vascular bundles, lower epidermis, and stomata.

Mechanism of Stomatal Opening and Closing

  • Guard cell walls adjacent to the pore are thicker.
  • Cellulose microfibrils are radially arranged.
  • Changes in turgor pressure in guard cells control the opening and closing of stomatal pores.
  • Low turgor pressure (flaccid cells) = stomata closed.
  • High turgor pressure (turgid cells) = stomata open.
  • Turgor pressure changes due to shifts in solute concentrations via active transport between epidermal cells and guard cells, and osmosis.
  • Active transport of K+ is coupled to H+ pumping, driven by voltage, causing K+ to enter the cell.

Stomatal Movement Response to Light

  • In well-watered plants, light is the primary signal controlling stomata.
  • Pumping of K+ into guard cells is triggered by blue light receptors.
  • Phototropins (proteins) and Zeaxanthin (carotenoid pigment) are involved.
  • Circadian rhythms (24-hour clock) control stomatal movement.
  • K+ is pumped in at dawn, followed by osmosis.
  • K+ leaves the cell via passive transport at night, and water follows.

Stomatal Movement Response to CO2

  • Increased CO2 concentrations cause stomata to close, with the threshold varying by species.
  • Higher CO2 concentrations lead to a decreased number of stomata.
  • There has been a 40% reduction in stomata over the last 200 years.
  • Atmospheric CO2 has increased from 280 to approximately 412 µmol/mol.

Stomatal Movement Response to Temperature

  • The rate of water evaporation doubles for every 10°C increase.
  • Water stress leads to the production of Abscisic Acid.
  • Increased temperature can elevate respiration and CO2 levels within the leaf.
  • Evaporation cools the leaf surface, thus surface temperature rises more slowly than ambient air temperature.

Transpiration - Other Factors

  • Humidity: Higher ambient humidity slows water loss.
  • Leaf size: Can reflect average humidity levels.
  • Shady forest understories (higher humidity): often Broad leaves. Cuticle thickness can determine a plant's transpiration rate.
  • Exposed grasslands (lower humidity): often Narrower leaves with thicker cuticles and higher stomatal density.
  • Air Currents: A dry breeze increases transpiration, while a humid breeze may decrease it.

Water Potential Gradient

  • Water moves from areas of high water potential to areas of low water potential.
  • Example gradient: Soil (-0.3 MPa) -> Trunk xylem (-0.6 to -0.8 MPa) -> Leaf cell walls (-1.0 MPa) -> Leaf air spaces (-7.0 MPa) -> Outside air (-100.0 MPa).

Transpiration Cohesion-Tension Theory

  • Water molecules are polar, leading to cohesion (attraction to each other) and adhesion (attraction to other substances).
  • Oxygen has a slight negative charge, and hydrogens have a slight positive charge.
  • Cohesion-tension Theory: A model for water transpiration in vascular plants.
  • Water molecules stick to each other and to vessel surfaces.
  • Tension (upward pull) is caused by water loss in leaves.
  • There is a continuous water potential gradient between the uppermost leaves and the soil solution surrounding the roots.

Transpiration – A Closer Look

  • The atmosphere has the lowest water potential (\Psi).
  • Water evaporates from the cell wall surface bordering the air space inside the leaf.
  • Evaporated water is replaced by water from inside the cell.
  • Water diffuses freely across the plasma membrane, but solutes do not.
  • Water moves from an adjacent cell with higher water potential into the cell with lower water potential.
  • This chain reaction continues down the plant.
  • Tension is ultimately transmitted to the roots, which now have a lower water potential than the soil, causing them to draw water from the soil.

Cavitation and Embolism

  • Cavitation: Rupture of the water columns in xylem due to bubble formation.
  • Embolism: Filling of the tracheary element with air or water vapor following rupture.
  • Embolized xylem cells cannot conduct water.
  • Causes include freezing of vascular fluids, transpiration and dehydration, and cuttings.
  • The largest pores (vessel perforations) are most vulnerable to embolisms.
  • When an embolism forms, it may expand through perforations until the entire vessel is emptied.
  • Often occurs during freezing when air is not soluble in ice, leading to air bubbles when xylem sap freezes.

Strategies/Adaptations to Prevent Cavitation and Embolism

  • Smaller Pit Pores: Plants with smaller pores in their pit membranes are less vulnerable to cavitation because air bubbles are less likely to enter and disrupt the water flow.
  • Thicker Pit Membranes: Thicker pit membranes also help to reduce the entry of air bubbles and maintain the integrity of the water column.
  • Xylem Fiber Support: The mechanical support provided by xylem fibers can help to minimize cavitation fatigue and prevent vessel implosion.
  • Vessel Diameter: Plants can also evolve to have smaller vessel diameters, which can reduce the risk of cavitation.

Root Pressure

  • When transpiration is absent (or very low), the water potential gradient is driven by ions.
  • Ions accumulate in the xylem, decreasing water potential.
  • Water moves into the xylem from surrounding cells.
  • This creates positive pressure (root pressure).
  • Root pressure forces water and dissolved ions upward via the xylem.

Hydraulic Redistribution

  • Passive movement of water from wet to dry soil via roots.
  • Hydraulic redistribution is enhanced when transpiration is not occurring or is occurring at low rates.
  • Benefits:
    • Water moved from deep soil to the surface is available to neighboring plants with shallow roots.
    • Water transferred from the surface to deeper layers reduces water logging.
    • Provides moisture to mutualistic partners during drought.

Nutrient Uptake by Roots

  • Absorption of inorganic ions occurs via the epidermis of young roots and is often enhanced by mycorrhizal fungi and rhizobia bacteria associations.
  • Enhancement occurs because these associations increase the surface area for absorption and provide access to nutrients that would otherwise be unavailable.

Nutrient Uptake by Roots via Active Transport

  • Concentration inside is higher in root cells than in the soil solution.
  • Diffusion into root cells requires energy.
  • Absorption decreases when roots are deprived of oxygen (respiration impeded) or when the plant is deprived of light.

Nutrient Uptake by Leaves

  • Inorganic ions can also be absorbed in small amounts through the leaf epidermis.
  • This enables foliar application of fertilizers and herbicides.