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bio 121 plant physio lecture
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Water Loss:
transpiration
Stomatal movement
Direct response to increases or decreases in the osmotic potential of guard cells
Changes in water potential between guard cells and neighboring cells
-if water moves in –guard cells turgid (stoma open)
-if water moves out- guard cells flaccid (stoma closed)
Anatomy and cytology of stoma
Unusual features of guard cell walls:
The cellulose microfibrils making up the wall of a guard cell are arranged radially
Guard cell wall adjacent to the pore is thicker than the outer wall
Shape of guard cells differs
Guard cell with chloroplastids
Anatomy and cytology of stoma
Control of stomatal movement
Potassium, chloride, hydrogen and organic acids
K+ accumulates in the guard cells in the presence of light
K+ accumulation is due to active exchange process in which H+ are pumped out of guard cells
K+ accumulation is accompanied by Cl- (in some) in response to the electrical differential created by the K+ uptake into the guard cells
Potassium, chloride, hydrogen and organic acids
Organic acids (mostly malate) build up in the guard cells as H+ leaves the guard cells; neutralized by K+ influx
Osmotic and water potentials become negative in the presence of K+ , Cl- , organic acids
Water movement towards the guard cells results in increased in turgidity and stomatal movement
Control of stomatal movement
CO2 concentration
Stomata will open when CO2 in leaf intercellular spaces is low
Stomata will close when CO2 in leaf intercellular spaces is high
Factors affecting stomatal movement
Light
Generally opens in the presence of light
Blue light receptor in the plasma membrane of the guard cells perceives the stimulus
Activation of receptors stimulates the activity of ATP-powered proton pumps which promotes uptake of K+
Factors affecting stomatal movement
Factors affecting stomatal movement Water deficit and ABA
If guard cells lose water more than the entry from surrounding epidermal cells- hydropassive closure
If transpiration exceeds water absorption,water deficit is created in the plant- hydroactive closure
ABA produced in the mesophyll of water stressed plants, signals guard cells to close stomates
ABA is the primary regulator of the stomatal apparatus in water-stressed plants
Circadian rhythms- cycles that have intervals
of 24 hours
Rhythmic opening and closing of the
stomata
Presence of internal clock located in the
guard cells
Factors affecting stomatal movement
TRANSPIRATION
Transpiration
Loss of water in vapor form
types :
1. stomatal transpiration
2. cuticular transpiration
3. lenticular transpiration
Stomatal transpiration has the most
significant contribution to loss of water in
plants
Driving force of transpiration
Transpiration is a 2 stage process:
1. Evaporation of water from the moist cell walls into the substomatal air space
2. The diffusion of water vapor from the substomatal space into the atmosphere
Driving force:
Difference in water potential between the substomatal air space and the external atmosphere
Movement of water vapor from a region of
higher vapor density to lower vapor
density
Movement of water vapor from a region of
higher vapor pressure to lower vapor
pressure
Driving force of transpiration
Vapor pressure on a closed container. Initially (A), more molecules escape from the
water surface than condense, filling the air space with water vapor molecules. The
vaporous molecules exert pressure-vapor pressure- against the walls of the chamber
and the water surface. At equilibrium (B), the rate of condensation equals evaporation
and the air is saturated with water vapor. The vapor pressure when the air is saturated
is known as the saturation vapor pressure. At higher temperature (C), a higher
proportion of water molecules have sufficient energy to escape. Both the concentration
of water molecules in the vapor phase and the saturation vapor pressure are
correspondingly higher.
•External Factors affecting rate of
transpiration
Light
Increases transpiration because of its effect on stomatal movement
Humidity of the air
Humidity –actual water content of the air
Relative humidity- ratio of the actual water content of the air to the maximum amount of water that can be held by air at that temperature
-is the ratio of the actual vapor pressure to the saturation vapor pressure
Relative humidity is most commonly expressed as RH x 100, percent relative humidity
Humidity of the air
The internal atmosphere is considered saturated with water vapor.
Water vapor will diffuse from an area of higher vapor pressure to low vapor pressure
The steeper the gradient, the more rapidly transpiration will proceed.
•External Factors affecting rate of
transpiration
Temperature
rise in temperature increases vapor pressure by increasing molecular motion will increase rate of transpiration
effect of temperature on water potential gradient
External Factors affecting rate of
transpiration
Effect of Wind is complex
Air in the immediate vicinity of a transpiring leaf
becomes saturated with water vapor (lowers
vapor pressure gradient and transpiration)
Presence of wind(low speed) disperses water
vapor (increase rate of transpiration)
Cooling effect on an evaporating surface (lowers
vapor pressure gradient and transpiration)
External factors affecting rate of
transpiration