Transpiration

• The loss of water (vapour), by evaporation, from the upper parts of the plant, such as the leaves via the stomata, the lenticels from the stem or waxy cuticle on the top layer of the leaf
• <90% of water enters a plant passes into leaf air spaces and evaporates through stomata; >5% of water escapes through the cuticle (water is moving by passive transport of osmosis down a water potential gradient)
• Once roots take up water, it crosses the cortex and enters the xylem in the centre of the root; 2 routes to get through:

  • Apoplast pathway: Through the fully permeable cellulose cell wall of the parenchyma cells
  • Symplast pathway: Through the cytoplasm of parenchyma cells, water and dissolved solutes must pass through the selectively permeable membrane
• The pathway water takes will depend on the plant species and environmental conditions, movement across the root is driven by active transport which occurs at the endodermis
• Endodermis cells move minerals by active transport from the cortex into xylem lowering water potential; they have special cotransporter proteins, to pump mineral ions from cytoplasm of cortex cells into the medulla and xylem
• Water moves over the Cortex until the Casparian strip; made of suberin (waterproof) which blocks the apoplast pathway between the cortex and medulla so water and dissolved mineral ions are forced across a plasma membrane through the cytoplasm and into the xylem
• Xylem in the roots have more pits in their lignified walls, allowing the flow of water, which makes water potential of the medulla and xylem more negative and the hydrostatic pressure in the xylem increase (root pressure)
• Casparian Strip Importance: Prevents harmful substances entering the xylem, and water leakage from xylem vessels also aid the development of root pressure (force pushing water up the stem)
• Water diffuses from root hair cells, across the root to xylem vessels in the central vascular bundle, down a water potential gradient (xylem vessels have the lowest water potential)
• Capillary Action: The narrower the tube the more effect the adhesion properties of water and Root Pressure: Higher hydrostatic pressure in roots required for mass flow of water up xylem vessels, into stem
• The water potential is higher in the root xylem vessels, lower in the leaves and even more negative in the atmosphere around the leaves so water molecules are pulled up in the stem into the leaves; xylem vessels is under tension when transpiration is occurring
• Water transport in a vessel against gravity requires a continuous column of water formed by cohesion between polar water molecules and adhesion of the molecules to the polar lignin molecules in the walls of the xylem vessels
• The adhesion of water molecules to lignin in the vessel is responsible for the capillary movement of water up the thin vessels, against gravity