All living organisms have a characteristic feature of irritability or sensitivity and they respond to external and internal stimuli. The responses and needs of the living body resulted in a number of metabolic processes which are interwoven. Some of these metabolic functions are carried out at the same time or one after the other. They require some degree of internal coordination and control to ensure the maintenance of steady state and survival of the organism.
There are two major types of control mechanisms found in living organisms. One is the chemical control mechanism, which is slower in action. Its effects are diffused and rely on chemical transmission through the circulatory system or diffusion process. The other mechanism of control and coordination is nervous control mechanism, found only in multicellular animals. It is faster in action, its effects are localized and it involves electrical and chemical transmission.
The multicellular organisms have division of labour among their organs. Some of their organs have special cells and tissues which detect different stimuli like water, light, temperature, chemical, etc. and respond in special way. Such as eyes detect visible light; ear detect sound waves; taste buds detect some chemicals etc. As a result of these detections body responds in a particular and set manner.
Living organisms also control and coordinate their functions by producing some chemicals. These chemicals usually play controlling function in those parts of the body which are far away from their site of synthesis. Transport between these sites and the sites of their action takes place by vascular system in higher plants and circulatory system in higher animals. They are effective in very low concentration.
*Hormones are chemical substances that act like messenger molecules in the body. After being made in one part of the body, they travel to other parts of the body where they help control how cells and organs do their work. For example, insulin is a hormone that's made by the beta cells in the pancreas.
3.1 CONTROL IN PLANTS
Control through plant hormones:
Plants are the major part of our environment, they are sessile living organisms. Though remain fixed at one place, they respond to a variety of stimuli like animals. Some of their parts having special tissues, which produce special chemicals for control mechanism and also function as coordination chemicals. Plants usually respond to these chemical messages by adjusting their pattern of growth and development. These chemical messages are recognized as hormones, a Greek word meaning "to excite". The plant hormones commonly called phytohormones are also known as growth regulators.
* Phytohormones/ Plant hormones are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development.
The concept of chemical messengers in plants emerged from a number of common observations for examples phototropism which shows that when the growing tip of a plant is illuminated from one side it will bend towards light. It was found that this response to light was due to the presence of a hormone in its growing tip. This hormone was named auxin by F.W. Went.
Plant hormones control and coordinate the plant responses in two ways:
(ii) By showing movement and regulating various metabolic functions.
(ii) By controlling and initiating growth at various regions of plant.
3.1.1 Plant movement:
Movement is one of the properties of living organisms. Animals show noticeable movement in the form of locomotion, which is not observed in plants. The movements in higher plants are chiefly in the form of bending, twisting and elongation of certain parts or organs. These movements occur in response to certain stimuli and the direction of response is related to the direction of stimulus. Such responses are called tropisms (Tropos means 'turn').
Tropisms are growth responses that result in curvatures of whole plant organs away or towards stimuli. There are three stimuli which can induce tropism i.e. light (Phototropism); gravity (Geotropism) and touch (Thigmotropism). You have already studied different types of plant movement in previous chapter (Chapter 2).
3.1.2 Biological clock and circadian rhythms:
Plants and animals have developed within themselves a wonderful mechanism to measure the passage of time and the means to regulate their physiological and metabolic activities which means that a clock system is present in a living organism. It has long been known that the leaflets of certain plants open during the daytime and close at night but these rhythmic movements have been seen to continue even if the plants are kept in total darkness. It shows that the rhythmic movement of plant was not only controlled by light intensity and temperature changes but was due to an independent time measuring system called biological clock.
Living organisms when repeat their biological or behaviourial activities at regular intervals, this behaviour is called biological rhythms or biorhythms. When these biorhythms occur with a frequency of about 24 hours they are called circadian rhythm (L. Circa=about, approximately; dies=day).
Circadian rhythms also take place if organism is taken away from environmental factors e.g. a bean plant will continue its sleep movements even if kept in constant light or constant darkness, the leaves are not simply responding to sunrise or sunset.
Recently such rhythmic movements have been investigated in the growth rates of young oat seedlings, photosynthesis and luminescence in algae and marine dinoflagellates and in the CO2 metabolism in Bryophyllum.
These rhythms have seen to be controlled internally by an efficient time measuring system and are independent of light and temperature effects. These types of rhythms are endogenous. This is a fact that an independent oscillating (fluctuating) system is present in all individuals and is inherited from one generation to the next generation. This system does not alter unless stimulated by a sudden change in the natural environment.
3.1.3 Photoperiodism:
The length of day light period has marked influence on the behaviour of Plants particularly on their flowering. The phenomenon in which the influence of day length on plants is studied is called photoperiodism. It may also be defined as response of plants to relative length of the day and night".
3.1.4 Responses to environmental stress:
Changes in environmental conditions are the big threats for living organisms especially for plants. These factors which change the normal conditions of light, CO2, nutrients, temperature, etc. cause severe stresses on plant. This stress can be defined as an environmental factor that cause adverse effects on growth, reproduction and survival of an organism. Plants either die due to these adverse conditions or cope with these stresses by evolutionary adaptations that enable certain plant, to survive in the stressful conditions. The common environmental stresses for plants are:
(i) Water shortage (Drought condition)
(ii) Less oxygen supply
(iii) High concentration of salt in the soil
(iv) High temperature .
(v) Low /Cold temperature
(vi) Herbivory / over grazzing
(i) Response to drought condition:
A condition in which water content of soil is very low, a plant may be stressed by shortage of water because plant loses its water by high rate of transpiration. Plants growing in drought condition have control system to cope with this drought stress. Many plants respond to water deficit, help the plant, to conserve water by reducing rate of transpiration. Water deficit in a leaf causes guard cells to lose turgor, a simple control mechanism that shows transpiration by closing stomata. It also stimulates increased synthesis and release of abscisic acid from mesophyll cells in leaf, this hormone helps in keeping stomata closed. Leaves respond to water deficit in several other ways. Root growth also responds to drought by developing deeper root system to enable their exposure with maximum soil water and inhibiting growth of shallow roots.
(ii) Response to oxygen deficiency:
Some plants are structurally adapted to very wet habitat or marshes. They have developed aerial roots that provide access to oxygen. Another structural adaptation is the development of air tubes that provide oxygen to submerged roots.
(iii) Response to salt stress:
High salt of soil stresses the plant by lowering the water potential of soil due to which exosmosis occurs. The plants especially halophytes have salt glands in their leaves where desalination occurs. As a result salt is pumped out from plants. However, except halophytes, other plants can not survive in salt stress for long.
(iv) Response to heat stress:
High heat in the environment can harm and ultimately kill a plant by denaturing its enzyme and damaging its metabolism. One way to cope with this stress is the transpiration, which produces cooling effect in plants by evaporation. Another way is backup response that enables plant to survive in heat stress. Above 40"C plant cells start synthesizing relatively large quantities of special proteins called heat-shock-proteins, which prevent enzymes and metabolic proteins from getting denatured.
(v) Response to cold stress:
When the temperature of environment falls, a change in the fluidity of cell membrane occurs, it loses its fluidity as the lipids become locked into crystalline structures. This alters solute transport. Plants respond to cold stress by altering the lipid composition of membrane. In freezing condition, changes in solute composition of cells by producing different polymers of fructose ( fructans), which allow the cvtosol to super cool without ice forming, though ice crystals may form in the cell-walls.
(vi) Responses to herbivory:
Herbivore- is the process of eating of plant by herbivorous animals. It is an especial stress for plants being commonly eaten up by animals. Plants overcome excessive herbivory by developing thorns and production of distasteful or toxic compounds.
3.1.5 Defence against pathogens:
Plants are exposed to pathogens like viruses. bacteria and spores of fungi, which can cause different plant diseases. We have already studied some pathogenic plant diseases in class XI biology. Plants normally not get affected by such organisms because they have their own defence system against the pathogens.
Like an animals skin the epidermal layer of plant serves as first line of defence against pathogens. But some pathogens become successful to enter through wounds or through natural openings like stomata. Once a pathogen invades, the plants use chemicals to attack on pathogen. This chemical attack is the second line of defence. The infected plant produces a variety of compounds called phytoalexins an antibiotic, which destroys or inhibits the growth of microorganisms.
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References:
hormone definition
https://kidshealth.org/en/teens/hormones.html#:~:text=Hormones%20are%20chemical%20substances%20that,beta%20cells%20in%20the%20pancreas.
IAA
http://animations.liquidjigsaw.com/index.php/2017/07/25/plant-hormones-how-iaa-the-most-common-form-of-auxin-works/
Drought-Tolerant Plants
https://www.smithsonianmag.com/smart-news/scientists-can-program-plants-be-more-drought-tolerant-180954159/
Rooting and aeration system of some mangrove trees.
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/pneumatophores
Salt Crystals on Mangroove
https://asknature.org/strategy/glands-remove-excess-salt/attachment/salt-crystals-on-mangrove-leave/
Transpiration
https://www.freepik.com/free-vector/diagram-showing-transpiration-plant_5982932.htm
modified leaves
https://courses.lumenlearning.com/boundless-biology/chapter/plant-defense-mechanisms/
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3.2 PLANT HORMONES
Certain chemicals produced by plants have profound effect on their subsequent growth and development. Such chemicals are called plant hormones or Phytohormone. They are synthesized by in minute concentration and men their effect either by altering gene expression. activating or inhibiting enzymes or changing properties of membrane. They are produced in young embryonic tissues as there is no specific organ for their production in plants.
There are five kinds of plant hormones. These are:
(i) Auxins
(ii) Gibberellins
(iii) Cytokinins
(iv) Abscisic acid
(v) Ethene
1.Auxins (Gr. Auxano= To increase):
Auxins are a class of plant growth substances both natural and synthetic first revealed by Fritz-Went (1926).
They were the first of the major plant hormones to be discovered and major plant-hormones to be discovered and are a major co-ordinating Signal in plant development. Indole-acetic acid (IAA) is the principal type of auxin of higher plants, synthesized at the apices of stem and root (apical meristem) In addition to (IAA) other naturally occurring auxins are : 4-chloro, indole acetic acid, phenyl acetic acid (PAA) and indole-3 butyric acid (IBA) where is synthetic auxins include nephtaline acetic acid (NAA), 2,4- dichcloro:- phenoxy acetic acid (2, 4-D) and others. Auxins co-ordinate development at all levels in plants, from the cellular level to organs and ultimately the whole plant.
Role of Auxin
(a) Cell division and cell- enlargement:
It stimulates cell division, cell enlargement and brings about increase is length of plant. It stimulates wall loosening factor, for example, elastin to looses the cell-walls. If Gibberellins are also present, the effect is stronger. It also stimulates cell-division if cytokinins are present. Xylem tissues can be generated when the auxin concentration is equal to the cytokinins.
(b) Initiation of roots:
Auxin also initiates development of adventitious roots when applied at the cut base of stem.
(c) Abscission:
In mature leaves and fruits when auxin production diminishes, a layer of thin-walled cells is formed at the base of petiole and stalk of fruit. This layer is called abscissic layer and causes fall of leaves and fruit with slight jerk.
(D) Growth of fruit:
Auxins produced in young embryo promotes the growth of fruit
(e) Parthenocarpy:
*parthenocarpy is the development of fruit without prior fertilization.
Use of auxin also helps in producing parthenocarpic of seedless fruits.
(f) Apical dominance:
*apical dominance is the phenomenon whereby the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems
Besides growth promoting function, auxin also ho. inhibitory effect on growth. Growth of apical bud inhibits growth of lateral buds beneath the stem. This phenomenon is termed as apical dominance. Removal all apical bud initiates growth of lateral buds with more leaves and axillary buds.
(g) Weedicide:
Auxins are selective weed killer. 2-4-dichlorophenoxy acetic acid (2-4-D) is used to kill weeds in lawns and cereal crops.
*Auxins are toxic to plants in large concentrations; they are most toxic to dicots and less so to monocots. Because of this property, synthetic auxin herbicides, including 2,4-D and 2,4,5-T, have been developed and used for weed control.
(h) Flowering:
Auxin plays a minor role in the initiation of flowering. It can delay the senescence of flowers in low concentrations.
*Plant senescence is the process of ageing in plants.
2. Gibberellins:
It is a group of chemicals that promote cell division and cell elongation. First noticed in Gibberella fujikuroi fungus which infected rice seedlings and produced a disease called bakanae (Foolish seedling). The infected seedlings elongated and ultimately fell over without producing grains. Even extract from the fungus when applied to rice seedlings produced the same disease indicating that a definite chemical compound is responsible for the disease. T.Yabuta and T. Hayashi succeeded in isolating the active substance from the fungus and was named Gibberellin, after the name of the genus. Its ability to induce growth attracted the scientists who have isolated 70 different types of Gibberellin many of them occur naturally in higher plants. Major sites of their production are roots, stems and leaves.
Role of Gibberellin:
Gibberellin produces wide variety of effects. One of their effects, like auxin, is to stimulate cell division and cell elongation and prevent genetical and physiological dwarfism. For instance, dwarf pea plant used by Mendel in his experiment when treated with gibberellin attained normal size. They also mobilize food stored in endosperm by producing enzyme (amylase) that converts starch into sugar which is made available to developing embryo.
They also stimulate flowering, fruit development, bud sprouting, growth of pollen tube and parthenocarpy.
3. Cytokinins:
These are a group of substances both natural and synthetic, which react with auxin to induce cell division. Originally obtained from coconut milk, the other source is Herring sperm DNA and Yeast extract. One of the naturally occurring cytokinins is zeatin, which was obtained in pure crystalline form from immature corn grains. Kinetin, a synthetic cytokinin has the same effect as that of zeatin.
Role of Cytokinin:
They initiate rapid cell division but only in presence of auxin. They also cause delayed senescence (old age). Detached leaves which would normally lose chlorophyll are prevented from becoming decolourized by their application. They also break seed dormancy and promote fruit development in some species.
4. Abscisic acid:
In contrast to growth-promoting hormones like auxins, gibberellins and cytokinins, abscisic acid (ABA) is a growth inhibitor produced by plant during adverse environmental conditions such as drought condition and at the onset of winter. It induces dormancy in buds and seeds, causes stomata to close, turns leaf primordia (tissue in its earliest recognizable stage of development.) into scale which protects the buds and promotes senescence.
5. Ethene:
The most important role of ethene (a gas). is that it triggers ripening It affects the permeability of cell membrane, which allows enzymes responsible for destroying chloroplast with the result that red and yellow colours are unmasked and fruit assumes ripened colour. It contributes to leaf abscission and also breaks the dormancy of buds and seeds in some species. It also initiates flowering in some plants e.g. pineapple.
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References:
IAA
https://en.wikipedia.org/wiki/Indole-3-acetic_acid
cell elongation
https://www.quora.com/How-does-auxin-act-in-plants-for-it-to-cause-cell-elongation
root initiation
https://www.researchgate.net/figure/Auxin-Transport-Model-of-Lateral-Root-Initiation_fig4_23671770
Leaf Abscission
biologydiscussion.com/plants/senescence-and-abscission-of-leaves-botany/20618
flower to fruit
https://www.learnpick.in/questions/details/31217/which-part-of-flower-form-it-s-fruit
apical dominance
https://www.chegg.com/homework-help/questions-and-answers/select-experimental-conclusion-go-plant-3-auxin-iaa-capable-inducing-apical-dominance-abse-q17072807
seed section
https://courses.lumenlearning.com/wm-biology2/chapter/development-seeds-and-fruit/
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