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Excessive leaf water loss is deadly:
Cuticle seals epidermis.
Most loss through stomata. H20 Opening & Closing of Stomata
668 Would loose too much water if stomata always open.
Need open stomata for CO2 intake.
Stomatal regulation: a balancing act. Open
O2 Closed Regulation of Water Loss
Factors causing stomatal opening
(p.710-11; 670): Light.
Low CO2. Factors causing stomatal closing Darkness.
High temperature (above 30-35o C.)
Water stress. External Environmental Factors
Affecting Transpiration Rates
Temperature: (p. 711-712; 670)
670 Rates double for each 10o C increase. Humidity:
Humidity: Higher evaporation rates at lower humidity Wind:
Wind: Strips off moist boundary layer.
Steepens diffusion gradient.
Increases evaporation rates. Movement of Substances Through
Phloem (P. 720-725; 680-685)
Contents of phloem sap: (after water) Chiefly disaccharide sucrose (90%).
llow concentrations of amino acids.
Inorganic minerals (like in xylem). Sucrose Phloem Flow Patterns:
“From source to sink” Growing Season From leaves to storage
organs & meristems
organs After Dormancy From storage organs
to meristems Phloem Transport:
Osmotically Generated Flow
p. 722-724; 683
683 Active transport of sugars at sources.
Water follows by osmosis: High pressure.
Continuous pipeline of Phloem cells.
Transport of sugars out at sinks
(storage organs, metabolically active
areas, Lower pressure.
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