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