Chapter 22 Plant Growth and Development Part 2 by Teaching Care online coaching classes
- Photoperiodism (Light period): The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W Garner and H.A.Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkning. It could be made to remain vegetative in winter by providing extra light. Later, it was found that most plants would flower only if they were exposed to light for less or more than a certain period, the critical photoperiod, each day. Subsequently, it was observed that in light dark cycle, dark period is crucial in initiating flowring and not the light period as thought earlier. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories.
- Short day plants (SDP): These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. The critical day length differs with different species. The short day plants remain vegetative, if the day length exceeds the critical periods. Most of winter flowering plants belong to this category e.g. cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry ,
- Long day plants (LDP): These plants begin flowering when the day length exceeds a critical This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. Some common examples of long day plants are spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc.
- Day neutral plants: These plants can flower in all possible photoperiods. The day neutral plants can blossom thorughout the year. Some common examples of this category of plants are cucumber, cotton, sunflower, tomato, some varieties of pea,
Short day plant
Day length less than 12 hrs for flowering
Long day plant
Day length less than 12 hrs for flowering
Neutral plant Day length immaterial
Fig : The day-length requirements for flowering in three catagories of plants
- Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. Examples of intermediate plants are Mikania scandens, Eupatorium hyssopi folium and Phaseolous polystacous.
- Ampiphotoperiodic plants: Such plants ramain vegetative on intermediate day length and flower only on shorter or longer day Example of such plant is Media elegans.
- Short long day plants: These plants require short photoperiods for initiation of flowering and long photoperiods for Examples of these plants are some varieties of Triticum vulgare, Secale cereale.
- Long short day plants: These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming some common examples of these plants are Bryophyllum, Cestrum.
Critical period: Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is
- 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is SDP as it flowers in photoperiods shorter than its critical value, whereas the latter is LDP requiring photoperiods longer than its critical value. Both Xanthium and Hyoscymus niger flower with 14 hours of light per day. Thus, day length in which a plant flowers is no indication of its response class in the absence of further information.
- Skotoperiodism (Dark period): When photoperiodism was discovered, the duration of the light period was thought to be critical for
Subsequently, it was found that when the long night period was interrupted by a brief exposure to light, the short day plants, failed to
flower. Thus, for flowering, these plants require a long night or critical dark period rather than a short day length. Similarly, long day plants respond to nights shorter than the critical dark period. Curiously, they do not need an uninterrupted dark period, Therefore, a short day plant is also called long night plant and a long day plant as a short night plant.
In the night interruption experiments, when the short day plants were exposed to a flash of light before achieving a critical dark period, flowering was prevented. It is called light break reaction. If this was followed by exposure to far-red light (740 nm), the effect was reversed. Red, far red exposures given in succession showed that plant response is determined by the last exposure. Thus, photoperiodic response (flowering) is a phytochrome mediated process. The phytochorme shows reversible change is red (660nm) and far-red (730nm) wavelength.
On absorbing red light Pr is converted into Pfr. The Pfr becomes Pr either rapidly by absorbing far-red light or slowly in darkness. Thus, darkness or far-red light promotes Pr
Red light (R)
Far red light (FR)
R FR R
Fig : Effect of night (Dark) interruption on flowering in a short-day plant
formation and stimulates flowering in short day plants, on the contrary, sunlight or red light promotes Pfr formation and stimulate flowering in long day plants.
(3) Mechanism of photoperiodism
- photoperiodic perception: Experiments have demonstrated that photoperiodic stimulus is perceived by the fully developed leaves. Very young or first few leaves are commonly insensitive. In Xanthium (a short day plant) single leaf or even one eight part of a leaf was sufficient for this Further, a single leaf exposed to short days was able to induce flowering, when it was grafted on to a plant kept under non- inductive conditions.
- Photoperiodic induction: Conditions under which the effect of suitable cycle of light and dark periods can persist in a plant and leads to flowering is called photoperiodic induction. It generally occurs when the plant has achieved certain minimum vegetative e.g. 8 leaves in Xanthium strumarium. Minimum vegetative growth provides the plant with ripeness to flower. Some plants are however, exception to it and can be photo induced even in their cotyledonary stage.e.g. Chenopodium rubrum. The minimum number of appropriate photoperiods (inductive cycle) required for induction varies from species to species e.g. one for Xanthium.
- Photoreceptor: The chemical which perceives the photoperiodic stimulus in leaves is phytochrome. The wavelengths of light are absorbed by the leaves. This becomes evident by the fact that defoliated (leaves removed) plant does not flower. Presence of even a single leaf is sufficient to receive required amount of photoperiod. Partially mature leaves are more senstitive to light while very young or mature leaves are much less sensitive to photoperiodic
Garner and Allard’s early worked led to the discovery, isolation and much of the characterization of the pigment responsible for absorbing light involved in photoperiodic phenomenon of plants. Borthwick, Hendricks and their colleagues later termed this pigment phytochrome. Pigment was isolated by Butter et al. (1959). This pigment controls several light dependent developmental processes in plants besides flowering, phytochrome exist in two interconvertible forms. The red (660nm), absorbing form Pr and the far red (740 nm), absorbing form Pfr . Pr is converted to Pfr on absorbing far red light. Pfr is converted to Pr rapidly absorbing far red light or slowly in darkness. The slow conversion to red absorbing form is under thermal control. During the day when white light available, Pfr accumulates in the plant. This form of phytochrome is inhibitory to flowering in short day plants and stimulatory to flowering in long day plants. In evening, Pfr undergoes thermal and spontaneous decay to change into Pr. This pigment is stimulatory to flowering in short day plants and inhibitory to flowering in long day plants.
Fig : The phytocrome concept
Therefore, in SOP interruption of dark periods with a flash of red light converts Pr into Pfr and flowering is inhibited.
- Structure/Chemistry of phytochrome: The clarification of he chemical structure of phytochorme was due to isolation efforts and purification of phytochrome from several plant sources by Borthwick, Hendricks and their Phytochrome was initially isolated from cotyledons of etiolated turnip seedlings. Siegelman and Firer were responsible for a highly purified extract that led to further purifications and analysis of the phytochorme structure.
Phytochrome is a chromoprotein with a chromophore (Pigment coloured protein) prosthetic group (e.g. chromoprotein). The chromophore group is a linear tetrapyrrole that differs in the conformation and absorption spectrum of its Pr state clearly from its Pfr state. A similar group with comparable conformational changes occurs in the bilirubins of red algae, though they bear an ethyl group instead of the vinyl group at their D-ring. Further there is probably one chromatophore for each phytochrome molecule. The chromatophore is linked to the protein at ring
- Apparently the photo-conversions of the Pr and Pfr forms involve electronic changes in ring I, with either addition or loss of a proton. Conformational (structural) changes in the protein probably contribute to dark conversion and possibly
Fig : Structure of chromophore of phytochrome and its relation with protein element
The protein is a dimer of two identical subunits with molecular weights ranging from 120000 to 127000 in different plant species. It is an allosteric protein.
- Importance of phytochrome : Phytochrome is located in plasma membrane. Phytochorme far red Pfr form is considered to be biologically active form and is responsible to initiate a number of physiological process such
- Elongation of stem and
- Plastids morphology and differentiation of
- Photoperiodism and
- The florigen complex (Flowering hormone): When the proper amount of light is perceived by leaves, they produce a chemical (flowering hormone), which undergoes stabilisation in dark. Later on, this chemical passes to shoot apex and causes its differentiation into flowering
The various experiments discussed in the foregoing section provide strong evidence for the production of flowering hormone in plants under suitable photoperiods. Chailakhyan (1936) a Russian investigator on photoperiodism, proposed that it be called ‘florigen’. According to him (1958) the “Florigen complex” the true flowering hormone includes two groups of substances formed in leaves :
- Gibberellins : Which are necessary for formation and growth of
- Anthesins : Substances which are necessary for flower
Acting together Gibberellins and Anthesin produce the effect as ascribed to florigen.
Induction of flowering
Fig : Photoperiodic induction and formation of florigen which is translocated to growing point for formation of flowers
The flowering stimulus moves readily not only through the plant but also from plant across the graft union between a flowering plant and a non-flowering or vegetative plant. Lang performed grafting experiments and demonstrated that every type of grafting is possible e.g., intervarietal, interspecific and intergeneric.
Short day (12 hrs) Long day (16 hrs)
- (B) (C)
Fig : Demonstration through grafting showing that flower inducing stimulus is a chemical
- Photomorphogenesis: When plants are grown in continuous darkness they become etiolated e. such plants are longer, weaker, having yellowish half opened leaves, while light grown plants do not show such conditions. When etiolated plants are kept in light they gradually develop green colour and become normal. The effect of light in reversing etiolation involves two kinds of action; one the biochemical level for the synthesis of the chorophyll and secondly at the level of morphogenesis light acts to promote expansion of the leaves and inhibits elongation of the internodes. This phenomenon is called photomorphogenesis and is independent of the direction of light.
The action spectrum of photomorphogenesis reveals that plants are most sensitive to red light, but blue light is ineffective.
- Vernalization: Many plants, especially biennials do not flower before they experience a low They grow vegetative during the warm season, receive low temperature during winter, grow further and then bear flowers and fruits. Russian agronomist Lysenko coined the term vernalization (1929-30). According to him vernalization may be defined as the method of inducing early flowering in plants by pretreatment of their seeds at low temperatures. Chourad (1960) has defined it as the acquisition or acceleration of the ability to flower by chilling treatment. The low temperature requirement for flowering was first noticed by Klipport (1857) while working with winter varieties of cereals such as wheat, barley, oat and rye. He observed that, these varieties when sown in spring failed to flower the same year but grow vegetatively. Such winter varieties, when
Chilled for two months at 5°C, planted at ordinary temperature
Slightly germinated seed
sown in the autumn, they flowered in spring of the same year.
- Site of vernalization: The stimulus of vernalization is perceived
No chilling planted at ordinary temperature
only by the meristematic cells such as shoot tip, embryo tips, root apex, developing leaves etc.
Fig : Experiment to show effect of vernalization on winter Rye.
- Requirement of vernalization : Vernalization treatment requires three conditions (a) Low temperature – Low temperature required for vernalization is usually 0-4oC is most of the cases. The chilling treatment should not be immediately followed by high temperature (e., about 40oC), otherwise the effect of vernalization is lost. This phenomenon is called de-vernalization. (b) Duration of low temperature treatment
– It varies from species to species from a few houses to a few days, (c) Actively dividing cells- Vernalization stimulus is perceived only by actively dividing cells. Therefore, vernalization treatment can be given to the germinating seeds or whole plant with meristematic tissues and other conditions are (d) Water- Proper hydration is must for perceiving the stimulus of vernalization (e) Oxygen – Aerobic respiration is also a requirements for vernalization.
- Process of vernalization: Usually vernalization treatment is given to the germinating The seeds are moistened sufficiently to allow their germination.They are then exposed to a temperature of 0-4oC for a few weeks and sown to the fields. Lysenko develops the process of vernalization it is completed in two stages.
- Thermostage: Germinating seeds are treated with 0-5oC in presence of oxygen and slight moisture. The seed dormancy is
- Photostage: The stage is very essential to initiate the reproductive phase. After vernalization plants must be subjected to a correct photoperiod in order that they may produce
- Mechanism of vernalization : The stimulus received by the actively dividing cells of shoot or embryo tip is translocated to all parts of the plant and prepare it to flower. The stimulus has been named as vernalin (reported by Mechlers). It can be passed from one plant to another through grafting in case of Henbane but not in However, vernalin has not been isolated and identified. In some plants cold treatment can be replaced by gibberellins. It was reported by Lang. It has also been observed that the endogenous level of gibberellins enhances in vernalized plants. Therefore, it is suggested that the stimulus of vernalization that induces flowering could be particular gibberellin or a mixture of gibberellins. However, the correct mechanism is still not known and needs through investigation.
(v) Importance of vernalization
- Vernalization is believed to overcome some inhibitor and induce synthesis of growth hormones like
- It reduces the vegetative period of
- It prepares the plant for
- It increases yield, resistance to cold and
- Vernalization can remove kernel wrinkles in
- Vernalization is beneficial in reducing the period between germination and Thus more than one crop can be obtained during a year.
Senescence and Death .
Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology.
- Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous The last three are also called organ senescence.
- Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life The plants may be annual (e.g. rice, wheat, gram, mustard etc.), biennials (e.g. cabbage, henbane) or perennials (e.g. certain bamboos). The plant dies soon after ripening of seeds.
- Shoot senescence: This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g. banana, gladiolus, ginger etc.
Whole plant senescence (A)
Progressive or sequential leaf senescence
Shoot senescence (C)
Simultaneous leaf senescence (D)
Fig : Types of plants and senescence (shaded region is the part undergoing senescence)
- Sequential senescence: This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g. Eucalyptus, Pinus,
- Simultaneous or synchronous senescence: It is found is temperate deciduous trees such as elm and These plants shed all their leaves in autumn and develop new leaves in spring. Because of this shedding of leaves, autumn season is also called fall. e.g. Dalbergia, Elm, Mulberry, Poplar.
- Theories of senescence: Several theories have been put forth regarding Some important ones are given below.
- Wear and tear: According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of
- Theories of senescence: Several theories have been put forth regarding Some important ones are given below.
- Toxicity: It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in
- Loss of metabolites: It is assumed that senescence leads to gradual depletion of essential metabolites in a
(iv) Genetic damage
- Differences between senescence, ageing and death
|1. Definition||Senescence: It refers to all collective, progressive and deteriorative process which ultimately leads to complete loss of organization and function.||Ageing: It includes all the chemical and structural changes, which occur during the life span of a plant or its organ.||Death: It is the ultimate termination of functional life of plant part.|
|2. Changes||It includes only degenerative and deteriorative changes in a plant or its parts.||It is sum total of metabolic changes that occur in plant or its parts.||It is a regular feature of the annual cycle of plants which is usually preceded by senescence.|
|3. Occurrence||Senescence occurs as a result of ageing and leads to death.||Ageing is a permanent feature of all living organisms.||Death is a permanent feature of all living organisms.|
(3) Characteristics of ageing and senescence
- There is general decline in metabolic activities decline in ATP synthesis and also decreased potency of
- Decrease in RNA and DNA
- Decrease in semipermeability of cytoplasmic
- Decrease in the capacity to repair and replace wornout
- There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes
- Increased production of hydrolytic enzymes such as proteases and
- Deteriorative change in cell organelles and
- Decrease in the internal content of auxin and cytokinins and increases in the production of abscisic acid or
- Importance of senescence : Biologically senescence and death have following advantages :
- It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and etc.
- During senescence, the cellular breakdown results in release of many nutrients including amino acids, amides, nucleotides, simple sugars and minerals. The same are withdrawn from the senescing organs into the main trunk and later utilised in the growth and developed of new
- Shoot senescence is a mechanism to help the plants perennate during the unfavourable
- Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in survival in winter, when the soil in frozen and roots can not absorb
- Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the
The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The
middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.
Certain other degenerative changes also occur making the region soft and weak. The organ from the plant is then easily detached whenever there is heavy rainfall or wind, etc.
Vessels Cortex Epidermis
Cells of abscession layer
Fig : Leaf and fruit abscission due to the formation of abscission layer
The abscission occurs due to a change in the hormonal balance. It has been observed that the abscission layer formation occurs rapidly when the auxin gradient becomes less i.e., less auxin on distal side than the proximal side of the leaf, flower or fruit. The plant hormones like ethylene and abscissic acid promote the abscission. A high concentration of auxin prevents the formation of a abscission layer.
Dormancy and germination of seeds
(See detail in Embryology module -II)
- SDP’s contain anthesins and synthesize gibberellic acid for Whereas LDP’s contain GA and synthesize anthesins for flowering.
- Leaves show maximum expansion in violet
- Impaction is the treatment given to seeds when they are shaken
- The term negative growth is sometime used for
- Knott (1934) found that the locus of photoperiodic induction is the
- Wellensick (1964) found that the locus for perception of cold treatment is the meristmatic cells (at all places) especially the shoot apex .
- Reduced availability of auxin stimulates leaf fall while presence of auxin slows down leaf Cytokinin prevent senescence through stimulating anabolic activity. They are called artiageing hormones Florigen hormone synthesized in the leaves.
Plant movements .
Movement is a change in position or place of an organ or organism. The movements in plants is not as much apparent as in the case of animals. But plants also show movements though they are fixed. They show movements of their parts. Such movements are not apparent except when observed after a time interval. They can however, be seen with the help of time lapse cameras.
Usually higher plants exhibit growth movements. Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. The reaction of plant to a stimulus is known as response”. The power or ability of a plant to respond to a stimulus is called sensitivity or reactivity or irritability.
The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus. And if the movements are produced in response to external stimulus, they are known as paratonic or induced movements.
The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time.
Classification of plant movements
Plants movements are broadly classified into two types:
- Movements of locomotion
- Movements of curvature
- Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomation are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli).
- Autonomic movement of locomotion : These movement of locomotion are due to internal stimuli they are of following
- Ciliary movements: Certain motile algae (e.g. Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or
- Amoeboid movements: It is the movement of naked mass of protoplasm by means of producing pseudopodia like process g. members of Myxomycetes (slime fungi).
- Protoplasm (B) (C) (D)
Fig : Locomotory movements (A) Ciliary (B) Amoeboid (C) Rotation (D) Circulation
- Cyclosis: These are movements of cytoplasm with is a cell (also called protoplasmic streaming). These are of two
- Rotation: When the protoplasm moves around a single central vacuole in either clockwise or anticlockwise direction g. leaf cells of Hydrilla, Vallisneria.
- Circulation: When the movement of protoplasm accurs around different vaculoes in different directions within the cell g. staminal hair of Tradescantia, shoot hairs of gourds.
- Excretory movements : Apical part of oscillatoria like a It is considered that such movements are due to exerction of substances by the plants. (movements opposite to the side of exerction).
- Paratonic movement of locomotion (Tactic movement): These movements take place in whole small plants. g. chlamydomonas or small free ciliated organs e.g. gametes. These movements are due to external factors like light, temperature or chemicals and are of following types.
- Phototactic movements or phototaxisms: It is the movement of free living arganism towords or away from light. e.g. movement of Chlamydomones, Ulothrix, Cladophora, Volvox towards suitable light intensity. Three types of arrangement present in columular cells in chloroplast of dorsiventral leaves.
- Parastrophe : In intense (maximum) light chloroplast cells arranged in longitudinal wall as a sequence
- Apostrophe : In minimum light chloroplast cells arranged in different
- Epistrophe: In dark chloroplast cells are arranged in transverse wall as sequence
Parastrophe (In light)
Apostrophe (In darkness)
Fig : Photolactic movements on leaf of Lemna
Epistrophe (In shade)
- Chemotactic movements or chemotaxisms : It is the movement of plant or plant parts from one place to another towards or away from chemical e.g. Male gametes (antherozoids) of bryophyta move towards archegonia under the influence of sugars produced by neck canal cells and also in pteridophyta male gametes move towards archegonia due to the malic acid produced by disintegration of neck canal cells and ventral canal cells.
- Thermotactic movements or thermotaxism: It is the movement of free living organism in response to external stimuli of temperature. e.g. Chlamydomonas move from cold water to medium warm water and from very hot water to medium
- Movement of curvature : In these cases, plants are fixed, thus they fail to move from one place to Somehow, movement is noticed in the form of bend or curvature on any part of the plant. Movement of curvature can be classified into.
- Mechanical movement
- Vital movement
(A) Mechanical movements: These movements depends upon the presence or absence of water and occurs in non-living parts of plants. It is of two types.
(i) Hydrochasy: This movement occurs due to the absorption of water.
Example: (a) Peristomial teeth of moss protrude out when the capsule is dry and curve when capsule is wet.
- Spores of the Equisetum coil and uncoil in the presence and absence of water
Fig : (A) Equisetum spore (B) Peristomial teeth of moss
(ii) Xerochasy : This movement occurs due to the loss of water.
Example: When water is lost from the annules of the sporangia of fern, it burst from stomium and spores are thus liberated out.
Fig : Movements in the annulus of fern sporangium
- Vital movement: These movement are of two types :
- Growth movements : These movements are due to unequal growth in different parts of an organ and are
They are further divided into two types-autonomic (occurs spontaneously) and paratonic (induced by external stimuli).
(a) Autonomic growth movements
- Nutation (Nutatory movements): These movements occur in the growing stem of twiners and tendrils. The stem exhibits a kind of nodding movements in two directions. This is because the stem apex shows more growth on one side at one time and a little later there is a greater growth on the opposite It is called nutation. In spirally growing stems the region of greater growth passes gradually around the growing point resulting in the spiral coiling of stem and tendrils. Such a movement is called circumnutation. Coiling of a tendril after coming in
contact with a support is a thigmotropic movement.
- Nastic movements: They are non-directional movements in which the response is determined by the structure of the responsive organ
and not the direction of the stimulus. The responsive organ has
Fig : Nutations :
- Nodding movement
an asymmetrical or dorsiventral structure. Greater growth on one side causes the organ to bend to the opposite side. Greater growth on the adaxial side is called hyponasty. e.g. circinate coiling and closed sepals and petals in a floral buds. Whereas
More growth on lower surface
More growth on
more growth on abaxial side is called epinasty. e.g. opening of fern leaf and spreading of sepals and petals during opening of the floral bud.
(b) Paratonic growth movement (Tropic movements
Fig : Nastie movements (A) Hyponasty, (B) Epinasty
or tropism): These are movements of curvature brought about by more growth on one side and less growth on the opposite side of plant organ induced by some external stimuli. Depending upon the nature of stimuli these movements are of the following type.
- Phototropism (Heliotropism): When a plant organ curves due to unilateral light stimulus it is called Some parts of the plant e.g., stem moves towards light. These organs are called positively phototropic. Some other organs e.g., roots move away from light and they are called negatively phototropic. If we keep a plant in a dark chamber (Heliotropic chamber) with an opening on one lateral side the stem tip moves towards light i.e., towards opening. Phototropism of stem and root are due to differential hormonal effect. Violet blue light is most effective. Photoreceptor seems to be a carotenoid.
Young stems are positively phototropic, leaves diaphototropic, shoots of Ivy plagio-phototropic, roots either non phototropic or negatively phototropic (e.g. white mustard, Sunflower). Mechanism is believed to be Cholodny-Went theory which states that unilateral light produces more auxin (IAA) and hence more growth on the shaded side resulting in bending.
- Geotropism (Gravitropism) : Growth of movements induced by the stimulus of gravity are known as
Fig : Heliotropic chamber showing positive photoropism in shoot
Generally, the primary root grows towards the force of gravity and hence is positively geotropic. The stem coloptile and phematophores grows away from the force of gravity and is negatively geotropic. The secondary roots and stem branches arise at angle less than 90o. They are thus plageotropic. Certain undergorund stems such as rhizomes, stolons of potato are oriented at right angle to the direction of force a gravity and are called diageotropic. Some of the lateral organs (e.g. corolloid roots of Cycas) possess little or no geotropic sensitivity, they are called ageotropic.
If some seedlings are kept in a dark chamber in different directions, root always move downwards and shoot away from the gravitational force.
Fig : Geotropism in maize seedings. Grains are placed in soil in different positions but in all cases the roots grow downwards and the shoot upwards
According to Cholodny-Went theory there is more auxin on the lower side of both stems and roots. In stem
higher auxin concentration increases growth while in roots it inhibits growth. Therefore, stem grow more on the lower side while roots grow more on the upper side causing the stem to
bend upwardly and roots to bend downwardly. Another theory
Fig : A Clinostat : If not rotated shoot curves upward.
If rotated the shoot grows straight.
is statolith theory which states that perceptive regions contain statoliths (microscopic particles). Change in their position causes irritation and hence differential growth. Clinostat / Klinostal is a instrument which can eliminate the effect of gravity and allow a plant to grow horizontly by slowly rotating it.
The main axis of which is attached to a rod. On the top of the rod is attached a flower pot. The clinostat is kept in a horizontal position as shown in fig. When the clock axis rotates the flower pot also rotates. As a result of this the plant grows horizontally as the effect of gravity is nullified by clinostat. If the clock of the clinostat is stopped the rotation of the plant stops, the shoot apex moves upward (negative geotropism) and the root apex moves downwards (positive geotropism).
- Hydrotropism: Growth movements in response to external stimulus of water are termed as hydrotropism Roots are positively hydrotropic (i.e. bend towards the source of water).
Stem are either indifferent or negatively hydrotropic. Positive hydroptropic movement of the roots is stronger than their geotropic response. In case of shortage of water, roots bend towards the sewage pipes and other sources of water in disregard to the stimulus of gravity.
Fig : Hydrotropism in young roots Fig : Hydrotropism in roots in the centre is a
porous pot filled with water
- Thigmotropism (Haptatropism): The movement which are due contact with a foreign body. It is most conspicuous in tendrils which coil around support and help the plant in climbing. The most sensitive regions are the ones which are actively growing. Tendrils also show mutations which help them to come in contact with the e.g. Tendrils of cucurbitaceae, petiole of clematis, leaf apex of Gloriosa.
Fig : Thigmotropic curvature in Cucumber
- Chemotropism: When a curvature takes place in response to a chemical stimulus. The growth of pollen tube through stigma and style towards the embryo sac occurs with the stimulus of chemical substances present in the carpel or movement of fungal hyphae towards sugars and
- Thermotropism : Curvature of plant parts towards normal temperatures from very high or very low E.g., peduncles of Tulip, Anemone.
- Variation movements (Turgor movements): These movements are caused by turgor changes especially due to efflux and influx of K+ (swelling or shrinkage of living cells due to change in osmotic potential) and are reversible. Variation movements are further divided into two types- Autonomic (not induced by external stimuli) and paratonic (induced by external stimuli).
- Autonomic variation movement: These movement of variation, which occurs without the external stimulus. Rhythmic autonomic turgor changes produce jerky rising and falling of two lateral leaflets in Indian Telegraph plant (Desmodium gyrans). Here, large thin walled motor cells found at the leaflet bases regularly lose and gain water bringing about changes in turgor
Motor (Bulliform) cells present in the epidermal cells of some grasses cause their folding and unfolding movements (hydronasty).
Fig : Autonomic variation movements in the leaves of Desmodium gyrans (telegraph plant)
- Paratonic variation movement (Nastic movements). These movements of variation are determined by some external stimuli such as light, temperature or contact but the direction of response is prefixed (not determined by the direction of stimuli). Nastic movements are of the following
- Nyctinastic (sleeping) movements: The diurnal (changes in day and night) movements of leaves and flowers of some species which take up sleeping position at night are called nyctinastic movement. Depending upon the stimilus they may be photonastic (light stimulus) or thermonastic (temperature stimulus). Maranta (Prayer plant), an ornamental house plant provides most common examples of nyctinastic
- Photonastic movements: Leaves of Oxalis take up horizontal position in sunlight and droop down during Many flowers open during the day and close during night or cloudy sky e.g., Oxalis.
Fig : Oxalis-showing photonastic movement of the leaf (A) Open leaf during day
(B) Closed leaf during night
- Thermonastic movements: Flowers of tulips and crocus open during high temperatures and close down during low
- Thigmonastic (Haptonastic) movements: When marginal glandular hair of Drosera come in contact with some foreign body g., body of insect, they show haptonastic movements. Due to this the insect comes in contact with the central glandular hair which after being stimulated bring the marginal glandular hair on the body of
insect. These later movements are chemotropic whereas the previous movements of marginal glandular hair is chemonastic movement Drosera shows both nyctinasty and thigmonasty movements.
- Seismonastic movements: This type of movement is bought about in response to external stimulus of shock or The best example of seismonastic movement
is the leaves of sensitive plant Mimosa pudica (Touch me not). It shows both nyctinastic (Sleeping movement) and seismonastic movement (shock movements) The leaves are compound with four pinna and each pinna bears numerous pinnules. Pulvini are present at the base of petiole, subpetiole
and still other tertiary petioles. Pulvini are swollen areas consisting of large number of loosely packed parenchymatous cells separated by large number of intercellular spaces. The central portion of the pulvinus is traversed by vascular strand. The cells of the upper half of pulvinus have thick cell walls and the cells of the lower half have thin walls.
If the flowerpot is moved or leaf or any other organ of
Fig : Mimosa pudica : Showing seismonasty
- Normal leaf B. Drooping leaf
Finger Drooping leaf
the plant is touched, the stimulus reaches the base of the leaf. Owing to this stimulus the turgor of lower half of pulvinus is lost and the leaf droops. After
Cells retaining turgor
Cells losing turgor
Fig : Mimosa pudica : showing seismonasty (A) Pulvinus in normal stage
(B) Pulvinus in normal stage in L.S. (C) Pulvinus L.S. of drooped leaf
some time the cells of the lower half of pulvinus becomes turgid again and the leaf attains its erect position. According to Snow (1924) and Bose (1926), chemical or hormone is produced at the place of touch and it travels through xylem, phloem and pith outwards and downwards finally to reach the pulvini. At pulvinus K+ are released into intercellular spaces. As a result, exosmosis takes place. Due to loss of turgor pressure typical drooping occurs. With a strong stimulus, pulvini gets folded in such a way that pinnules curve upwardly. The pinnae come close and finally main petiole droops down. When this response period is over (about 10 minutes), K+ ions are released back,
turgor is regained and the leaflets open out. It is assumed that from the point of stimulus the message to respond travels in waves through the plant at the speed of 1 cm per second. It is further interesting to note that electric impulses called action potentials like nervous message in animals have been observed in Mimosa pudica.
Movement of locomotion
Movement of curvature
Mechamical movement Vital movement
- Hydrochasy b. Xerochasy
Growth movement Variation movement
Photonastic Thermonastic Haptonastic
- Physical movements: Unstimulated movements caused by mechanical tensions (e.g. dehiscence of Balsam, clam and Squirting cucumber fruits) and hygroscopicity (Shrinkage/ xerochasy and swelling/ hydrochasy, e.g., dehiscence of fern sporangium,peristome teeth of moss).
- Vital movements: Movements due to internal change (autonomic) or in response to stimulus (paratonic/ induced).
- Autonomic movement: Spontaneous
- Paratonic movements: Induced movements
- Chemotactic movements Antherozoids of Marchantia move towards open archegonia in response to certain protiens, in other bryophytes by sucrose, in most pteridophytes by malic acid while citric acid causes movements in Zoospores of moulds swim towards acidic pH where decaying organic matter is present.
- Geotropism/Gravitropism (i) It is variable in the floral stalk of Poppy-positive in bud state and negative at maturity of flower. (ii) Haberlandt (1900) and Nemec (1900) put forward statolith theory of (iii) Root cap perceives the stimulus of gravity.
It is believed to produce an inhibitor like abscisic acid or IAA oxidase which diffusing basipetally reduces growth on the lower side and causes bending.
- Geotropic stimulus is perceived by root cap in case of root by stem apex in case of
- Movement due to air are called aerotropism g. positively aerotropic peneumatophores and movement induced by injury is called traumatropism