Plant Water Relations

Osmosis

Movement of water through a semipermeable membrane from higher water potential to lower water potential; passive process.

Diffusion

Movement of molecules from higher concentration to lower concentration; semipermeable membrane not required.

Imbibition

Absorption of water by dry seeds and colloidal substances causing swelling.

Root Hair

Absorbs water from the soil by osmosis.

Root Pressure

Positive pressure developed in roots due to water absorption; maximum at night/early morning; responsible for guttation.

Transpiration

Loss of water as water vapour from aerial parts of plants, mainly through stomata (90–95%); creates transpiration pull, the main force for ascent of sap.

Guttation

Loss of liquid water through hydathodes due to high root pressure; occurs at night or early morning.

Hydathodes

Special pores at leaf margins through which guttation occurs.

Stomata

Tiny pores in leaves responsible for transpiration and gaseous exchange; regulated by guard cells.

Guard Cells

Specialized cells that control the opening and closing of stomata.

Xylem

Conducts water and minerals from roots to shoots; transport is unidirectional.

Phloem

Conducts prepared food from source to sink; transport is bidirectional.

Ascent of Sap

Upward movement of water and minerals through xylem, mainly due to transpiration pull (Cohesion-Tension theory).

Cohesion

Attraction between water molecules that maintains a continuous water column in xylem.

Adhesion

Attraction between water molecules and xylem walls, aiding water movement.

Plasmolysis

Shrinkage of the cell protoplast due to water loss in a hypertonic solution.

Water Potential

Tendency of water to move; water moves from higher to lower water potential.

Active Transport

Movement of substances against the concentration gradient using ATP.

Passive Transport

Movement of substances along the concentration gradient without ATP.