transpiration-Stomata-Guttation | uksir-notes |Plant- Physiology
Plant Physiology:
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| Transpiration |
Want to know about transport of water, osmosis in plants, Click the Link below:
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https://uksirnotes.blogspot.com/2022/01/Water-Absorption-ascent-of-sap-uksir.html
Transpiration:
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Transpiration may be defined as the loss of water form
aerial part of plant in the form of water vapor.
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Although huge
amount of water is absorbed by plants, but only 1-2% used, up to 98-99% lost in
transpiration.
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However transpiration follows 2 steps.
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At first- water loss occurs from the mesophyll cells to the inter cellular space.
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2ndly water diffuse through the stomata.
Types of transpiration:
Cuticular transpiration:
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There is loss of minute quantity of water through the
cuticle.
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Cuticle is one waxy coat on leaf epidermis.
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| cuticle |
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Cuticle may found in stem and leaf of some plants.
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The cuticular transpiration is around 20% of total
transpiration.
Lenticular transpiration:
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Loss of water vapor through lenticels of woody plants,
is called as lenticular transpiration.
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These lenticels
are found on stem and fruit of woody plants.
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| lenticel |
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The water loss is around 1-5% only.
Stomatal transpiration:
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If water loss occurs through stomata, then it is called
as stomatal transpiration.
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How ever water loss occurs in water vapor form.
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Here water loss amount is around 80%.
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So maximum water loss occurs by stomatal transpiration.
Structure of stomata:
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Stomata are minute pores present in leaf surface and
some time young past of stem.
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The distribution of stomata in monocot and dicot leaf,
are different. Like wise the shape also varies.
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Also the numbers varies according to plant species.
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| structure of stomata |
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Stomatal pore if mainly formed by 2 guard cells (kidney
shape in dicot and dumbbell shape in monocot.) and many subsidiary cells.
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The guard cell is mainly responsible for opening and
closing of stomata. It has a thick and inelastic inner wall towards the pore
and thin elastic wall towards outer side.
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The cytoplasm contain many cytoplasmic lining, a
nucleus, a central vacuole and many chloroplasts (non functional)
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The opening and closing of stomata is generally
dependent on the water transport.
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If the guard cell become turgid, then stomata open.
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Where as, if cell become flaccid then stomata closed.
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Subsidiary cells also help in stomatal opening and
closing.
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Normal distribution- 1000- 60000/ cm2
Type of Stomata:
- On the basis of position and
Acessory cell
Anomocytic- Accessory cell
absent (Malvaceae)
Diacytic: 2 Accessory cells
perpendicular to pore (Acantheceae)
Paracytic: Accessory cells
parallel to stoma (Rubiaceae)
Anisocytic: 3 accessory
cells are found (Cruciferae, solanum)
Actinocytic: at least five
radiating cells forming a star-like circle (musaceae)
Cyclocytic: form a ring like structure (cyclanthaceae)
Gramineous: dumbbell shape
stomata (monocots)
Type of stomata
On the basis of
Distribution:
Hypostomatic: stomata lower
side (apple)
Anisostomatic: more on
lower, few at upper (Potato, Tomato )
Isostomatic: Equal on both
surface (wheat, maize)
Epistomatic: only on upper
surface (water lily)
Astomatic: (Potamogiton
type) stomata absent
Basis- opening and closing
1. Alfa- alfa type: open all day (pea, bean)
2. Potato type: open throughout day and night, but close at evening. (Alium, tulip)
3. Barley type: open for Few hour in a day (wheat, maize)
Mechanism of stomatal opening and closing:
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Some theories discussed below to explain the mechanism
of stomatal opening and closing:
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| opening and closing of stomata |
1. Theory of starch-
glucose inter conversion:
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Proposed by Yin and Tund (1948) and Steward (1964)
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When phosphrylase enzyme was found in guard cell,
scientists strongly supported this theory.
During day time :
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When the pH is
high, starch of guard cell convert to glucose-1-phosphate by using
phosphorylase enzyme.
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Later glucose-1- PO4 covert to Glucose-6-PO4
and finally to glucose with the help of enzyme phosphoglucomutase and
phosphatase respectively.
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The presence of glucose in cytoplasm increases conc. of
guard cell.
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Now endosmosis occurs, for which cell become turgid.
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Due to turgor pressure stomata open.
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| starch sugar inter conversion |
At
night:
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reverse process occurs by formation of glucose-1-PO4
and lastly to Starch. This needs ATP.
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The cells become flaccid and stomata close.
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However there are some objections:
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No sugar accumulation in guard cell.
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Guard cell of monocots do not show starch synthesis.
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Chloroplasts of guard cells are rudimentary.
Potassium Ion Theory:
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It was observed by Imamura and Fujino (1959)
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It was observed that there is K+ ion
increase leads to stomatal opening despite of CO2 conc. and Light.
During
day time- H+ pumps activation can be seen in guard cells.
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Now a hyper polarized situation seen with inner –ve
charge and outer + charge (H+ ions).
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Now k+ ions enters the cell (activation of potassium
pump due to blue light).
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There will be
entry of Cl- ions from the surrounding cells.
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Malic acid forms malate.
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Potassium combine with malate forming potassium malate.
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It increases the conc. for which endosmosis occurs.
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Thus stomata
open.
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| potassium ion theory |
During night:
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Reverse occurs to close the stomata.
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During night photosynthesis stops, now k+ ion efflux
from guard cell.
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Potassium malate convert to mallic acid.
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Thus conc. of guard cell decrease.
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Water comes out of the cell, closing the stomata.
Factors for Rate of Transpiration:
Internal:
- Root- shoot ratio: the absorption surface
is more important.
- Leaf area: more leaf area, more transpiration.
- Stomatal frequency: directly proportional to
transpiration rate.
- Structure of leaf: tick cuticle, wax layer,
sunken stomata etc. decreases the rate of transpiration.
External:
- Light: optimal temperature
increases transpiration rate.
- Wind: increase in wind velocity increases rate of
transpiration.
- Temperature: optimal temperature
,maximum transpiration.
- Humidity of air: inversely proportional
to transpiration.
Significance of Transpiration:
Beneficial role:
- Gaseous exchange: it helps in absorption
of carbon dioxide and release of oxygen to environment.
- Cooling effect: transpiration helps in
surface cooling as well as cools the photosynthetic system in plants.
- Mineral transport and water movement: transpiration is the
main cause for transport of water, from root to shoot. How ever minerals also transported
through the transport of water.
- Mechanical tissue development: more transpiration
develops more mechanical tissue, for which plant become more healthy and
resistant.
- Turgidity: it is maintained by transpiration. It helps
in better growth and gives mechanical support.
Harmful role:
- Large amount of water is
lost as compare to absorption.
- Waste of huge amount of
energy.
- Deficit of water due more transpiration,
may cause less metabolic activity.
- Xerophytic plants show
modifications to check transpiration.
Thus transpiration may be described as necessary evil for plants.
Anti transpirants:
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There are some factors or materials which slow down the
rate of transpiration.
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Colorless plastids, silicon oil, low viscosity wax etc.
spread on leaf to prevent water.
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Rise in CO2 conc. results in partial closure of
stomata.
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Phenyl mercuric acetate, ABA etc are also in this
category.
Guttation:
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It is the loss of water in the form of liquid from uninjured
part of plant.
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Generally it occurs in tip and margin of leaves.
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| Guttation |
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The epidermal layers of leaf contain grooves in which
hydathodes can be found.
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It mostly occurs during early morning or in night time.
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Mostly present in herbaceous plants.
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It do not control the leaf temperature and occurs due to
root pressure.
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| Hydathode |
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Hydathodes are openin both day and night.
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They contain pore ending with vascular supply
surrounded by parenchyma.
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It exuded water with some dissolve inorganic and
organic substance.
Want to know about transport of water, osmosis in plants, Click the Link below:
Want to know about Ascent of sap in plants, Click the Link below:
https://uksirnotes.blogspot.com/2022/01/Water-Absorption-ascent-of-sap-uksir.html
Thank you for visiting.. feel free for asking doubt in comment session.... UK Sir
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