Transpiration

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Dr. Kennard's note from MS3, summer of 2007

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What does water do in a plant? : 

What does water do in a plant? 85-95% of plants is water 95% of H20 taken up by plant transpired back into atmosphere (transpiration) formation of organic compounds (sugars) through photosynthesis support (turgor pressure) Solvent - medium to transport other chemicals

How does water move in a plant? : 

How does water move in a plant? Diffusion of water and solutes is adequate for single-celled organisms Multicellularity and invasion of land required specialized tissues for movement of water and solutes Xylem: carries water and solutes from roots to leaves Phloem: carries photosynthates from leaves to roots and back up

How does water move in a plant? : 

How does water move in a plant? Hydrogen bonding in water leads to:  cohesion - H2O molecules stick together  adhesion - molecules stick to hydrophilic molecules

Slide 4: 

Water molecules Water molecules are polar with local regions of + and – charge Water forms weak hydrogen bonds with itself and other molecules 1/16 strength of covalent bonds Hydrogen bonds allows for high cohesion between molecules of water

Water Potential : 

Water Potential Movement of water in xylem doesn’t require energy, but it moves along an energy gradient Flows passively due to differences in potential energy Water potential (, psi) measured in MPa (MegaPascals) Pure water:  = 0 everything else:  < 0 Water flows from high water potential (less negative) to low water potential (more negative) Or from hypotonic to hypertonic….

Osmosis : 

Osmosis Osmosis = diffusion of water through a selectively permeable membrane Hypotonic sol’n = low solute concentration Hypertonic sol’n = high solute concentration Water moves from hypotonic sol’n to hypertonic sol’n until equivalent concentrations are reached Still moving from greater to lower conc - of water Hypotonic  Hypertonic

Diffusion : 

Diffusion Net movement of solutes from area of greater concentration to an area of lower concentration

Soil-Plant-Atmosphere Continuum : 

Soil-Plant-Atmosphere Continuum Water pulled up plants by evaporation evaporation of water leads to dry cell walls in mesophyll cells further from site of evaporation have  so water moves toward mesophyll replace lost water from xylary cells water in roots even  so it replaces water in xylem Continuous stream due to cohesion forces

What happens when soils are salty? : 

What happens when soils are salty? Adding solutes (salts in this case) make the water potential of a soil lower (more negative) So a water potential may be -1.2 MPa instead of -0.6MPa A plant root at -0.9 MPa will no longer be able to take that water up and the plant can wilt and die

How can some plants can live in salty soils? : 

How can some plants can live in salty soils? Such plants are known as halophytes One response of plants is to move solutes (sugars and other solutes into the cells in their roots) Drops the water potential in their roots (~ -1.5MPa) so roots are able to take up water from soil even if it’s at ~1.2MPa

Transpiration and Productivity : 

Transpiration and Productivity Leaves adapted for photosynthesis abundant stomata large number vein endings Tradeoff of CO2 for photosynthesis and H2O loss through transpiration H2O limits biomass production in most areas

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