The Loss of Water

Lecture 2B

Transpiration is the evaporation of water from

aerial parts of plants, especially leaves but also

stems, flowers and fruits.

Transpiration is actually the loss of water from the

leaves by evaporation; this helps keeps water

moving round the plant by sucking water up from

the roots.

It occurs chiefly at the leaves while their stomata

are open for the passage of CO2 and O2 during

photosynthesis.

But air that is not fully saturated with water vapor

(100% relative humidity) will dry the surfaces of

cells with which it comes in contact.

So the photosynthesizing leaf loses substantial

amount of water by evaporation.

This transpired water must be replaced by the

transport of more water from the soil to the leaves

through the xylem of the roots and stem.

Types of foliar transpiration

Stomatal transpiration

Water vapor loss occurs through the stomates

In most species 90% or more of all foliar transpiration is of

the stomatal type.

Cuticular

transpiration

Occurs directly from

the outside surface of

epidermal walls

through the cuticle

Lenticular transpiration

Water vapor loss

occurs through the

lenticells

Importance

Supply photosynthesis ( 1%-2% of the total )

Enables mass flow of mineral

Nutrients from roots to shoots

Cool the plant.

Is a side effect of the plant needing to open its stomates in order to obtain carbon dioxide gas from the air for

photosynthesis.

Transpiration involves two stages:

1. The evaporation of water from cell walls and

2. Its diffusion out of the leaves, chiefly through the stomata.

Stomatal structure

A. Anatomy of a stoma (stomata, plural) - guard cells,

subsidiary cells, substomatal cavity, cuticle, ledge (or lip),

stomatal apparatus.

The subsidiary cells are epidermal cells that may be

specialized and different from the other epidermal cells.

The function of the ledge is to prevent liquid water from

seeping into the pore.

Interestingly, cutin covers most of the cells in the

substomatal cavity; only regions near the actual opening

are free of cuticle and most water is lost from this area. .

Anatomy of a stoma

Stomatal structure

C. Common features :

(1) Thickened inner walls; and

(2) Radial micellation - the cellulose microfibrils radiate out

around the circumference of the pore;

(3) Chloroplasts - these are the only epidermal cells with

chloroplasts;

(4) Connected end-to-end.

Mechanics of guard cell action :

Guard cells open because of the osmotic entry of water into the GC.

In turn, this increases the turgidity (water pressure) in the GC and causes them to elongate.

The radial orientation of cellulose microfibrils prevents increase in girth.

Since GC are attached at the ends and because the inner wall is thicker, the guard cells belly out with the outer wall moving more pulling open the guard cell.

Guard cell closure essential involves reversing this process.

We can summarize the mechanics of GC action as follows:

stoma closed (GC flaccid) water uptake (osmosis) increase pressure stoma open (GC turgid)

Environmental control of GC Action :

Whatever physiological mechanism we finally postulate for the GC, it must also be compatible with the action of various environmental factors that are known to regulate stomatal activity.

Since guard cells respond to their environment, especially any factors that impact the photosynthesis/transpiration compromise.

We expect any factor important in photosynthesis to exert regulatory control on GC. And, we expect water to have the final word on control since if a plant dries out too much it's as good as dead!

Environmental control of GC Action

1-Light • light = open

• dark = closed

2-pH effect • high pH = open

• low pH = closed

3-Oxygen • Low oxygen levels = GC open

4-Carbon dioxide

• high CO2 (i.e., at night, produced during respiration)= closed

• low CO2 (i.e., during the day, used by photosynthesis)= open

5-Temperature • Increased temperatures usually increase stomatal action, presumably

to open them for evaporative cooling.

• If the temperature becomes too high the stomata close due to water

stress and increased CO2 that results from respiration.

6-Wind • Often causes closure because it:

• (a) brings CO2 enriched air; and

• (b) increases the rate of transpiration that causes water stress which

causes the stomata to close.

7- Water :

It protects against excessive water loss.

This is the prevailing and overriding control mechanism.

There are two mechanisms by which water loss regulates

stomatal closure, one of which is active and the other

passive.

Environmental control of GC Action

1. Hydropassive Control

simply put, as the plant looses water, the turgidity of the leaf

cells, including guard cells, decreases and this results in

stomatal closure. The plant is not "intentionally" closing the

stoma, it is simply a consequence of drying out.

2. Hydroactive Control :

this mechanism is one in which the plant actually seems to

monitor its water status. When the water potential drops below

some critical level, it engages a cascade of events that close the

stomata. Presumably the plant is measuring pressure (turgor)

and then synthesizes or releases an anti-transpirant that is

translocated (moved) to the GC to cause closure.

7- Water :

The anti-transpirant is abscisic acid (ABA), one of the major plant growth regulators.

It is active in very low concentration (10-6 M) and appears very rapidly after water stress (within 7 minutes).

After 4-8 hours, the [ABA] increases nearly 50x.

ABA comes from two sources:

(a) root –in response to water stress, the xylem sap pH increases which in turn stimulates the release of ABA into the xylem sap for transport to the leaves. This seems to be a root signal to the leaves that "water stress is coming"; and

(b) leaves – water stress stimulates a synthesis of new ABA and redistribution of existing ABA.

7- Water :

Density of stomata :

The density of stomata produced on growing leaves varies

with such factors as:

The relationship is inverse; that is, as CO2 goes up, the

number of stomata goes down, and vice versa.

Temperature Humidity Light

Intensity CO2

Concentration

Environmental or external factors affect transpiration:

Environmental or external factors affect transpiration:

1-light

Light affects directly by heating the leaf so it causes rise in

temperature and which indirectly favors opening of

stomata.

The evaporation of water from the outer surface of the cell

walls of mesophyll cells into the intercellular spaces

followed by diffusion through pores spaces is accelerated.

In the dark, stomata are closed and transpiration ceases.

Environmental or external factors affect transpiration:

2-Saturation pressure deficit or vapor pressure deficit

The rate of diffusion of vapour through the Stomatal pores

depends upon the difference between the V.P, inside and

outside the leaf. Transpiration rate increases, if the

difference (VPD) is high.

Environmental or external factors affect transpiration:

3-Temperature

The rate of transpiration increase by any rise in temperature

of the leaf or both leaf and air.

This is due to the effect of temperature on stomatal

movement and VP gradient.

Environmental or external factors affect transpiration:

4-Wind

Wind increases evaporation and consequently transpiration by removing the moist air from the surface of the leaf and replacing it by dry air.

When plants are suddenly exposed to wind, there is sharp rise in the rate of transpiration followed by gradual decline following this increase.

This is because of cooling effect which wind produces on the evaporating surfaces lowering their vapour pressure gradient. Wind of high velocity may cause stomatal closure.

Environmental or external factors affect transpiration:

5-Availability of soil water

If the supply of water to the leaves is not adequate the rate of transpiration decreases.

If this condition is prolonged, a water deficit will result and the plant will appear wilted.

The supply of water can be inadequate due to the following reasons:

(I) water in soil about the root reaches coefficient.

(II) root system is not adequate to supply the top.

(III) a low soil temperature of soil solution.

(IV) a high concentration of soil solution.

Significance of Transpiration

The Stomatal connects the inner air spaces of the leaf with

the atmosphere and thus ventilate the internal tissues.

The transpiration as previously mentioned causes passive

absorption of water, due to the transpiration pull originated

in leaves.

The transpiration stream is accompanied by ions from soil

sap and hormones synthesized in roots.

These ions and hormones will aid in the growth and

development of the shoot.

Guttation is the appearance of drops of xylem sap on the tips or

edges of leaves of some vascular plants, such as grasses.

At night, transpiration usually does not occur because most

plants have their stomata closed at night.

When there is a high soil moisture level, water will enter plant

roots at night because the water potential of the roots is lower

than in the soil solution.

The water will accumulate in the plant creating a slight root

pressure.

The root pressure forces some water to exude through special

leaf tip or edge structures, hydathodes, forming drops.

Root pressure provides the impetus for this flow, rather than

transpirational pull.

Guttation on a strawberry leaf

Guttation on a prayer plant

Guttation fluid may contain a variety of organic compounds,

mainly sugars, and mineral nutrients, mainly potassium. On

drying, a white crust remains on the leaf surface.

Guttation is not to be confused with dew, which condenses

from the atmosphere onto the plant surface.

In several plants the root pressure is manifested as guttation which is the exudation of droplets of water from the margin and tips of leaves.

The exudation takes place through groups of cell called hydathodes.

A hydathodes is an opening or pore in the leaf epidermis around which several thin walled parenchyma cells are situated.

Guttation water is easily observed in the young barley seedlings. Guttation water is rich with organic and inorganic solutes.

This is the process of slow exudation of a watery solution

from an incision made in the plant tissue.

There are four types of bleedings from them the exudation

of xylem sap from an incision made in the stem of a well-

watered plant. This takes place due to root pressure.