Transpiration
Water
absorbed by roots ultimately reaches the leaf and gets released into the
atmosphere in the form of vapour. Only a small fraction of water (less than 5%)
is utilized in plant development and metabolic process.
The loss
of excess of water in the form of vapour from various aerial parts of the plant
is called transpiration.
Transpiration is a kind of evaporation but differs by the involvement of
biological system. The amount of water transpired is astounding (Table 11.4).
The water may move through the xylem at a rate as fast as 75cm /min.
Activity
Select a leafy twig of fully grown plant in your school campus.
Cover the twig with a transparent polythene bag and tie the mouth of the bag at
the base of the twig. Observe the changes after two hours and discuss with your
teacher
Transpiration
is of following three types:
Stomata
are microscopic structures present in high number on the lower epidermis of
leaves. This is the most dominant form of transpiration and being responsible
for most of the water loss (90 - 95%) in plants.
In stems
of woody plants and trees, the epidermis is replaced by periderm because of
secondary growth. In order to provide gaseous exchange between the living cells
and outer atmosphere, some pores which looks like lens-shaped raised spots are
present on the surface of the stem called Lenticels.
The loss of water from lenticels is
very insignificant as it amounts to only 0.1% of the total.
The
cuticle is a waxy or resinous layer of cutin,
a fatty substance covering the epidermis of leaves and other plant parts. Loss
of water through cuticle is relatively small and it is only about 5 to 10 % of
the total transpiration. The thickness of cuticle increases in xerophytes and
transpiration is very much reduced or totally absent.
The
epidermis of leaves and green stems possess many small pores called stomata. The length and breadth of
stomata is about 10-40µ and 3-10µ respectively. Mature leaves contain between
50 and 500 stomata per mm2. Stomata are made up of two guard cells, special semi-lunar or kidney-shaped living epidermal
cells in the epidermis. Guard cells are attached to surrounding epidermal cells
known as subsidiary cells or accessory cells. The guard cells are joined together at
each end but they are free to separate to form a pore between them. The inner
wall of the guard cell is thicker than the outer wall (Figure 11.14). The stoma
opens to the interior into a cavity called sub-stomatal
cavity which remains connected with
the intercellular spaces.
Stomatal
movements are regulated by the change of turgor pressure in guard cells. When
water enters the guard cell, it swells and its unevenly thickened walls stretch
up resulting in the opening of stomata. This is due to concave non-elastic
nature of inner wall pulled away from each other and stretching of the convex
elastic natured outer wall of guard cell.
Different
theories have been proposed regarding opening and closing of stomata. The
important theories of stomatal movement are as follows,
1. Theory of Photosynthesis in guard cells
2. Starch – Sugar interconversion theory
3. Active potassium transport ion concept
Von Mohl (1856) observed that stomata open in light and close in the night.
According to him, chloroplasts present in the guard cells photosynthesize in
the presence of light resulting in the production of carbohydrate (Sugar) which
increases osmotic pressure in guard cells. It leads to the entry of water from
other cell and stomatal aperture opens. The above process vice versa in night leads to closure of stomata.
Demerits
1.
Chloroplast of guard cells is poorly developed and
incapable of performing photosynthesis.
2.
The guard cells already possess much amount of
stored sugars.
i.
According to Lloyd (1908), turgidity of guard cell depends on
interconversion, of starch and sugar. It was supported by Loftfield (1921) as he found guard cells containing sugar during the daytime when they are open and starch
during the night when they are closed.
ii.
Sayre (1920) observed that the opening and closing of stomata
depends upon change in pH of guard cells. According to him stomata open at high
pH during day time and become closed at low pH at night. Utilization of CO2
by photosynthesis during light period causes an increase in pH resulting
in the conversion of starch to sugar. Sugar increase in cell favours endosmosis
and increases the turgor pressure which leads to opening of stomata. Likewise,
accumulation of CO2 in cells during night decrease the pH level
resulting in the conversion of sugar to starch. Starch decreases the turgor
pressure of guard cell and stomata close.
iii. The
discovery of enzyme phosphorylase in
guard cells by Hanes (1940) greatly
supports the starch-sugar interconversion theory. The enzyme phosphorylase hydrolyses starch into
sugar and high pH followed by
endosmosis and the opening of stomata during light. The vice versa takes place during the night.
iv. Steward (1964) proposed a slightly modified
scheme of starch-sugar interconversion theory. According to him,
Glucose-1-phosphate is osmotically inactive. Removal of phosphate from Glucose-
1-phosphate converts to Glucose which is osmotically active and increases the
concentration of guard cell leading to opening of stomata (Figure 11.15).
i. In
monocots, guard cell does not have starch.
ii.
There is no evidence to show the presence of sugar at a time when starch
disappears and stomata open.
iii. It
fails to explain the drastic change in pH from 5 to 7 by change of CO2.
This
theory was proposed by Levit (1974)
and elaborated by Raschke (1975).
According to this theory, the following steps are involved in the stomatal
opening:
i. In
guard cell, starch is converted into organic acid (malic acid).
ii.
Malic acid in guard cell dissociates to malate
anion and proton (H+).
iii.
Protons are transported through the membrane into
nearby subsidiary cells with the exchange of K+ (Potassium ions) from
subsidiary cells to guard cells. This process involves an electrical gradient
and is called ion exchange.
iv.
This ion exchange is an active process and consumes
ATP for energy.
v.
Increased K+ ions in the guard cell are balanced by
Cl– ions. Increase in solute concentration decreases the water potential in the
guard cell.
vi.
Guard cell becomes hypertonic and favours the entry
of water from surrounding cells.
vii. Increased
turgor pressure due to the entry of water opens the stomatal pore (Figure
11.16).
i. In
dark photosynthesis stops and respiration continues with accumulation of CO2
in the sub-stomatal cavity.
ii.
Accumulation of CO2 in cell lowers the pH level.
i.
Low pH and a shortage of water in the guard cell
activate the stress hormone Abscisic
acid (ABA).
iv. ABA
stops further entry of K+ ions and also induce K+ ions to leak out to
subsidiary cells from guard cell.
v. Loss
of water from guard cell reduces turgor pressure and causes closure of stomata
(Figure 11.17).
The
factors affecting the rate of transpiration can be categorized into two groups.
They are 1. External or Environmental factors and 2. Internal or plant factors.
i. Atmospheric humidity: The
rate of transpiration is greatly
reduced when the atmosphere is very humid. As the air becomes dry, the rate of
transpiration is also increased proportionately.
ii. Temperature: With the increase in atmospheric temperature, the rate
of transpiration also increases. However, at very high-temperatures stomata
closes because of flaccidity and transpiration stop.
iii. Light: Light intensity increases the temperature. As in temperature,
transpiration is increased in high light intensity and is decreased in low
light intensity. Light also increases the permeability of the cell membrane,
making it easy for water molecules to move out of the cell.
iv. Wind velocity: In
still air, the surface above the
stomata get saturated with water vapours and there is no need for more water
vapour to come out. If the wind is breezy, water vapour gets carried away near
leaf surface and DPD is created to draw more vapour from the leaf cells
enhancing transpiration. However, high wind velocity creates an extreme
increase in water loss and leads to a reduced rate of transpiration and stomata
remain closed.
Activity
What will happen if an indoor plant is placed under fan and AC?
v. Atmospheric pressure : In low atmospheric pressure, the rate of
transpiration increases. Hills favour high transpiration rate due to low
atmospheric pressure. However, it is neutralized by low temperature prevailing
in the hills.
vi. Water: Adequate amount of water in the soil is a pre-requisite for
optimum plant growth. Excessive loss of water through transpiration leads to
wilting. In general, there are three types of wilting as follows,
a.
Incipient wilting : Water content of plant cell decreases but
the symptoms are not visible.
b.
Temporary wilting: On hot summer days, the freshness of herbaceous
plants reduces turgor pressure at the day time and regains it at night.
c.
Permanent wilting: The absorption of water virtually
ceases because the plant cell does not get water from any source and the plant
cell passes into a state of permanent wilting.
i. Leaf area: If the leaf area is more, transpiration is faster and so
xerophytes reduce their leaf size.
ii. Leaf structure: Some
anatomical features of leaves like
sunken stomata, the presence of hairs, cuticle, the presence of hydrophilic
substances like gum, mucilage help to reduce the rate of transpiration. In
xerophytes the structural modifications are remarkable. To avoid transpiration,
as in Opuntia the stem is flattened to look like leaves called Phylloclade. Cladode or cladophyll in
Asparagus
is a modified stem capable of limited growth looking like leaves. In some
plants, the petioles are flattened and widened, to become phyllodes example Acacia melanoxylon.
The term
antitranspirant is used to designate any material applied to plants for the
purpose of retarding transpiration. An ideal antitranspirant checks the
transpiration process without disturbing the process of gaseous exchange. Plant
antitranspirants are two types:
Colourless plastics, Silicone oil and low viscosity waxes are sprayed on
leaves forming a thin film to act as a physical barrier (for transpiration) for
water but permeable to CO2 and O2. The success rate of a
physical barrier is limited.
Carbon-di-oxide induces stomatal closure and
acts as a natural antitranspirant. Further, the advantage of using CO2
as an antitranspirant is its inhibition of photorespiration. Phenyl Mercuric
Acetate (PMA), when applied
Use of abscisic
acid highly induces the closing of
stomata. Dodecenyl succinic acid also
effects on stomatal closure.
•
Antitranspirants reduce the enormous loss of water by transpiration in crop
plants.
• Useful
for seedling transplantations in nurseries.
During
high humidity in the atmosphere, the rate of transpiration is much reduced.
When plants absorb water in such a condition root pressure is developed due to
excess water within the plant. Thus excess water exudates as liquid from the
edges of the leaves and is called guttation.
Example: Grasses, tomato, potato, brinjal and Alocasia. Guttation occurs through stomata like pores called hydathodes generally present in plants
that grow in moist and shady places. Pores are present over a mass of loosely
arranged cells with large intercellular spaces called epithem (Figure 11.18). This mass of tissue lies
The liquid coming out of hydathode is not pure
water but a solution containing a number of dissolved substances.
Ganongs
potometer is used to measure the rate of transpiration indirectly. In this, the
amount of water absorbed is measured and assumed that this amount is equal to
the amount of water transpired.
Apparatus consists of a horizontal graduated tube which is bent in opposite directions at the ends. One bent end is wide and the other is narrow. A reservoir is fixed to the horizontal tube near the wider end. The reservoir has a stopcock to regulate water flow. The apparatus is filled with water from reservoir. A twig or a small plant is fixed to the wider arm through a split cock. The other bent end of the horizontal tube is dipped into a beaker containing coloured water. An air bubble is introduced into the graduated tube at the narrow end (Figure 11.19). keep this apparatus in bright sunlight and observe.As transpiration takes place, the air bubble will move towards the twig. The loss is compensated by water absorption through the xylem portion of the twig. Thus, the rate of water absorption is equal to the rate of transpiration.
Select a
healthy dorsiventral leaf and clean its upper and lower surface with dry
cotton. Now place a dry Cobalt chloride (CoCl2)
strips on both surface and immediately cover the paper with glass slides and
immobilize them. It will be observed after some time that the CoCl2
strip of lower epidermis turns pink. This indicates that CoCl2
becomes hydrated (CoCl2.2H2O or CoCl2.4H2O)
due to water vapours coming out through stomata. The rate of transpiration is
more on the lower surface than in the upper surface of the dorsiventral leaf.
Transpiration
leads to loss of water, as stated earlier in this lesson 95% of absorbed water
is lost in transpiration. It seems to be an evil process to plants. However,
number of process like absorption of water, ascent of sap and mineral
absorption directly relay on the transpiration. Moreover plants withstand
against scorching sunlight due to transpiration. Hence the transpiration is a “necessary evil” as stated by Curtis.
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