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Significance and Mechanism of Transpiration


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Presentation Transcript

Slide 1: 


Slide 2: 

Transpiration ? Guttation ? Evaporation ?

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Transpiration is necessary evil !!!!!!!!!!!!!

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Importance of Transpiration Cooling effect Suction force Absorption  of water and mineral salts Removal of excess water Gaseous exchange

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Kinds of Transpiration: 1. Cuticular transpiration (up to 20%) 2. Lenticular transpiration ( 0.1%) 3. Stomatal transpiration (80-90%)

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Stomata distribution: 1. Apple or mulberry type: Hypostomatic 2. Potato type : Amphistomatic and anisostomatic 3. Oat type: Amphistomatic and isostomatic 4. Wild lily type: Epistomatic 5. Potomageton

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There are two main types One is typical of monocots and grasses Dumbbell shape with bulbous ends Pore is a long slit The other is typical of dicots Kidney shaped - have an elliptical contour with pore in the center

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-Thickened inner wall Radial micellation Choloroplast Connected end-end Stomatal guard cells

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Mechanism of stomatal opening and closing Stomata closed (GC flaccid) > water uptake (osmosis) > Increase pressure > stomata open (GC turgid)

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Theory of photosynthesis in guard cells: Von Mohl (1956) Light > Photosynthesis > sugar formation > decrease osmotic potential > endosmosis > Turgidity increase > stomata open ? Photosynthesis by guard cell is too slow. Sugar accumulation is not sufficient.

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Theory of starch – sugar interconversion: Stomata open – neutral or alkaline pH ( even in dark) Scarth (1932) CO2 consumed > pH rise > hydrolysis of starch into sugar > decrease OP > water uptake > stomata open ? Insufficient decrease in OP

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Theory of starch – glucose interconversion: After discovery of phosphorylase in guard cells Steward (1964): Light > Photosynthesis in mesophyll cells > CO2 removed from intercellular space > increase in pH in guard cells > starch converted into glucose > decrease OP > endosmosis > guard cell turgid> stomata open

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? Effect of light Some guard cells lack starch

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Theory of proton transport and Hormonal regulation: (Active K+ transport mechanism) Levitt (1974) GC flaccid > K+ and Cl uptake, Malate synthesis > lower solute potential > water uptake > stomata open

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Transpiration: Evaporation from cell wall 2. Diffusion of these water vapours into atmosphere

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Factor affecting transpiration: External factors: Air humidity Temperature Wind Velocity Light, Water supply Internal factors: Stomatal frequency Leaf structure

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1. difference in water vapor concentration between the leaf air spaces and the external air 2. diffusional resistance of this pathway leaf stomatal resistance leaf boundary layer resistance 5. control of stomatal apertures by the guard cells Transpiration from the leaf regulates by

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Thank You

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