BI111 Lecture Notes - Lecture 21: Root Pressure, Guttation, Xylem
Document Summary
Transpiration: evaporation of water out of plants: greater than water used in growth and metabolism. Cohesion-tension mechanism of water transport: evaporation from mesophyll walls, replacement by cohesion (h-bonded) water in xylem, tension, negative pressure gradient, adhesion of water to xylem wall adds to tension. Tallest trees (>110m) near physical limit of cohesion. Root pressure: positive pressure in roots that forces xylem sap upwards: occurs in high humidity or low light, moves water up short distances. Guttation: when root pressure is strong enough to force water out of leaf openings. Apoplastic pathway: water does not cross cell membrane, diffuses through non- living regions including cell walls and air spaces. Symplastic pathway: water crosses membrane, often uses plasmodesmata, and diffuses through cytoplasm. Transmembrane pathway: water crosses plasma membranes and perhaps tonoplasts (vacuolar membrane). Transpiration losses of water must be regulated to prevent rapid desiccation. Cuticle limits water loss but also prevents carbon dioxide uptake.
