Photoperiodism and Plant Movement

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 15. 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 flowering. 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. 

Phytochrome

Borthwick & Hendrics: Discovered a light sensitive pigment responsible for flowering

Butler: Give term "Phytochrome" for this pigment & isolate it.

Pigment phytochrome is a chromosphere billiprotein which is a open tetrapyrrolic related to phycobilin.

Phytochrome mainly localised on cell membrane of all type of plants.

Phytochrome: Exist in two different forms. Pr (phytochrome Red): Red light absorbing form, induce flowering in SDP.

Absorption Range: 630-670 nm. Absorption peak- 667nm.  

Pfr (phytochrome Far Red): This is far red light absorbing form, induce flowering in LDP.

Absorption Range: 720-740 nm, Absorption peak­-735 nm.

Both forms of phytochromes are photobiochemically inter-convertible into each other.

The Pfr (Yellowish) form, gradually changed in to Pr (Bluish) form in dark.

During the day the Pfr form is accumulated in the plants which is inhibitory to flowering in SDP but, stimulate in LDP.

Phytocrome - Pfr (P₇O) is active form which controls many photoPhysiological processes in plants.

Control of morphogenesis by light & phytochrome is called photomorphogenesis.

Now phytochrome is considered as universal distribution in plant kingdom. Photomorphogenests in higher plants appear to be under control of one of three photoreceptors.

Phytochrome - which absorbs red and far red region to light.

Cyptochrome - which absorbs blue and UV-A (380nm) light.

UV-B receptors - which absorbs UV-B (290 nm) light.
 

Vernalisation or Yarovization

Effect of low temperature on the initiation and development of flower, was first realised by Klippart 1857 (Exp. on winter & ~ Spring wheat) Detail study and term - Vernalisation by Lysenko (credit of discovery).

Chourad defined as 'Acceleration of ability to produce flower by chilling treatment is called Vernalisation'.

Mainly embryo tip, shoot apex & leaves percepts induction of low temperature on plants.

Concept of hormone, Vernalin in vernalisation was given by Melcher et. al. This is hypothetical plant hormone because has been isolated till today. Vernalisation of seeds or plant propagule in laboratory can be induced at 1°C to 10°C in presence of O₂ & H₂O.

If vernalized plant propagules are kept in high temperature just after the low temp treatment then effect of vernalisation is reverse, this effect is called as devernalization.

Requirement of vernalization:

(i)  Low temperature: Low temperature required for vernalization is usually 0-4°C is most of the cases. The chilling treatment should not be' immediately followed by high temperature (i.e., about 40°C), otherwise the effect of vernalization 15 lost. This phenomenon is called de-vernalization.

(II) Duration of low temperature treatment: It varies from species to species from a few hours to a few days.

(iii) 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.

(iv) Water: Proper hydration is must for perceiving the stimulus of vernalization.

(v) Oxygen: Aerobic respiration is also a requirements for vernalization. The stimulus has been named as vernalin (reported by Mechlers). Process of vernalization: Usually vernalization treatment is given to the germinating seeds. The seeds are moistened sufficiently to allow their germination. They are then exposed to a temperature of 0-4°C for a few weeks and sown to the fields. Lysenko developed the process of vernalization it is completed in two stages.

(i) Thermostage: Germinating seeds are treated with 0-5°C in presence of oxygen and slight moisture. The seed dormancy is broken.

(ii) 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 flowers.

Significance:

(i) Better & early flowering.

(ii) Vernalisation increases the resistance to fungal diseases.
 

Plant Senescence

Period from complete maturity to degredative changes during the death of an organ or plant is known as senescence.

During senescence a gradual destruction occurs in protoplasm, cell, tissue, organ or plant and functioning of the plant and plant parts.

During the senescence, higher rate of catabolism is starts, under the control of growth hormones like ABA, ethylene. Senescence occurs as a result of ageing and leads to death of plant parts or whole plant. (Senescence and ageing - studied in Phyto­gerontology)

Senescence may be of following types :

(a) Whole plant senescence: ex. Tomato, Wheat, Musturd, Rice, Beans.

(b) Organ senescence: When plant part above ground dies (shoot) each year and root & rhizone system alive.

Ex. Alfalfa,' Sugarcane, Banana, Ginger.

(c) Sequential or Progressive senescence :

Evergreen perennials show progressive or sequential senescence of older leaves, lateral organs anches, flowers, fruits and shoot.

Ex. Eucalyptus, Mango.

(d) Simultaneous or Synchronous leaf senescence :

In perermial deciduous plants, all the leaves undergo senescence and abscission Simultaneously. Ex. Dalbergia sisso, Azadirachta indica, Ficus religiosa.

Theories of Senescene

(1) Wear and tear: According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles.

(2) Toxicity: It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all.

(3)  Loss of metabolites: It is assumed that senescence leads to gradual depletion of essential metabolites in a cell.

(4) Genetic damage  

Importance of senescence: Biologically senescence and death have following advantages:

(1) It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and fruits, etc.

(2) 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 development of new parts.

(3) Shoot senescence is a mechanism to help the plants perennate during the unfavourable periods.

(4) Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in winter, when the soil in frozen and roots can not absorb water.

(5) Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the soil.
 

Abscission

Detachment of senescent or mature plant organs like leaves, fruits, flowers due to change in hormonal activity.

There is a separation layer (Abscission layer) within the region of attachment of these parts. Cell wall layers and middle lamella are dissolved by the activity of cellulase and pectinases (Polysaccharide hydrolysing enzymes) during the abscission. Hormone ABA is main controller of abscission process.
 

Plant Movement

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 show in 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.

(A)  Movement of locomotion: When the whole plant plant part or organs of plant moves from one place to another place.

(I)  Autonomous: (by internal stimulus)

(i)  Amoeboid: Ex. Gametes of Spirogyra

(ii) Ciliary movement: Ex: Chlamydomonas, Euglena Zoospores

(iii) Cyclosis:

(a) Rotation: Whole protoplasm moves around the one central vacuole, in one direction. Ex: Hydrilla Vallisnaria cells.

(b) Circulation: Protoplasm moves, around the different vacuoles in different directions.

Ex. Staminal hairs of Tradescantia.         

(II)   Induced/Paratonic Tactic movement: (Due to external stimulus)

(i) Phototactic: Due to stimulus of light.

Ex: Algae - by Eye spot/Stigma.

Three types of arrangement present in columular cells in chloroplast of dorsiventral leaves.

Parastrophe: In intense (maximum) light, chloroplasts of cells are arranged in longitudinal wall as a sequence manner.

Apostrophe: In minimum light, chloroplasts of cells are arranged in different manner.

Epistrophe: In dark, chloroplasts of cells are arranged in transverse wall as sequence manner. 

(ii) Chemotactic: Due to stimulus of chemicals.

Ex. Male gametes of lower plants (antherozoids).

By chemical gradient sensing mechanism.

(iii) Thermotactic: Due to stimulus of temperature.

        Ex: Chlamydomonas, Euglena.

(B) Movement of curvature: Movement of plant organs only.

(I) Autonomous: Movement of variation - Dance movement by Desmodium. (Indian telegraph plant). Due to turgour pressure change. Epinasty & hyponasty, the growth movements in flower & leaves.

(ii) Induced /Paratonic movement: By external stimulus but directional)

(i) Tropic movement:

Definite direction towards stimulus.

(a)Phototropism: Definite direction in relation to light.When a plant organ curves due to unilateral light stimulus it is called phototropism. 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 plagio­phototropic, 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. 

(b) Geotropism: Definite direction in relation to gravity (Root cap percept stimulus)Growth of movements induced by the stimulus of gravity are known as geotropism. 

Generally, the primary root grows towards the force of gravity and hence is positively geotropic. The stem coloptile and pneumatophores grows away from the force of gravity and is negatively geotropic. The secondary roots and stern branches arise at angle less than. 90°. They are thus plageotropic. Certain underground 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 Cyeas) possess little or no I 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.

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 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. 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 dinostat is stopped the rotation of the plant stops, the shoot apex moves upward (negative geotropism) and the root apex moves downwards (positive geotropism).

(c) Chemotropism: Ex. Pollen tubes & Fungal hyphae.Definite direction in relation to chemicals.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 peptones. 

(d) Thigmotropism (Haptotropism):

Ex: Tendrils, Haustoria of Cuscuia.

Definite direction in response to contact or support. The movement which are due to contact with a foreign body. It is most conspicuous in tendrils which coil around support and help the plant in climbing. e.g., Tendrils of cucurbitaceae, petiole of clematis, leaf apex of Gloriosa.

(e) Hydrotropism: Ex : Roots of seedlings.  

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 hydrotropic 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.

(ii) Nastic movement (External stimulus but diffused type or non directional):

(a) Nyctinasty: 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 stimulus they may be photonastic (light stimulus) or thermonastic (temperature stimulus). Maranta (Prayer plant), an ornamental house plant provides most common examples of nyctinastic response.

(b) Thigmonasty or Haptonasty: Tentacles of insectivorous plants.

When marginal glandular hair of Drosera come in contact with some foreign body e.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 thigmonasry movements.

(c) Chemonasty: Ex : Tentacles of Insectivorous plants.

(d) Seismonasty: Ex: Mimosa (touch me not plant: turgour change in pulvinus leaf base

This type of movement is brought about in response to external stimulus of shock or touch. The best example of seisrnonastic movement is the leaves OJ sensitive plant Mimosa pudica (Touch me not). II shows both nyctinastic (Sleeping movement) and seismonastic movement (shock movements).

Note:

Many plant parts specially leaves exhibits nastic movement and involves differentiable growth, this type of movement is permanent or irreversible and is known as ephemeral type of movement. This is an example of growth movement which is caused due to unequal growth in plant organs.

Ex: Epinasty, Hyponasty, Nutation.

Epinasty & Hyponasty: Ex: Leaves, flower (petal) opening & closing respectively.

Epinasty - More growth on upper surface of plant parts. Hyponasty - More growth on lower surface of plant parts. Both Epinasty & Hyponasty are example of autonomic growth movements.

Nutation: Zig-zag growth of plant organ mainly shoot, is called as nutation.

Circumnutation: Spiral growth of plants in tendrils.

Portulaca is known as compass plant:

Rhizomes diageotropic (90^° to gravitation force),

Clinostat: Used for nullifying geotropism.

Ex. Ruellia fruit dehiscence

Plageotropism: Shown by stem .& root branches growing at an angle of 45^° from axis of plant.

Climactric fruit (Banana, Apple, Avocados):

Fruits, in which rate of respiration increase (climacteric respiration) during their ripenin (Ethylene).Citrus is non climactric fruits.

The flower stalk of the poppy is positively geotropic but after the    opening of flower, this stalk change as negative geotropic.

Formation of nodule is combined activity of Cytokinin (By bacteria) and Auxin.(By leguminous plant)

Blue light has more effects on most of physiological and growth processes in plants except photosynthesis and photoperiodism.

Arabidopsis thaliana (Brassicaceae) is most widely used plant tool for the studies of developmental genetics and growth physiology of plants.

Betacyanin or Betalains: Pigment mainly found in vacuole of Beet roots and flower of Bougainuilla are differ from anthocyanin due to the presence of nitrogen (N). These pigments do not show reverstbiltry of colour change due to pH change.

Anthocyanin is water soluble vacuolar pigment, which does not involve in photosynthesis.

Anthocyanins exhibits different colour like purple, pink, blue, scarlet etc.

The colour of anthocyanin in sensitive to pH change. For ex. colour of anthocyanin changes from red (acidic pH) to violet (neutral pH) to blue (in alkaline pH).

Turgonin-a newly discovered hormone found in pulvinous leaf base regulates turgor pressure changing movements.

Growth Rate: Increased growth per unit time. Plants growth is of two types:

Arithmetic growth: From dividing cell, two new cells are formed (by mitotic division), out of them one daughter cell continues to divide while other differentiate and mature (stop dividing) . Ex. Root & Shoot elongation at constant rate.  

It is mathematically expressed as

L_t = L₀ + rt

where L_t - length at time 't' L₀ - length at time 'zero'

r - growth rate/elongation per unit time. It's curve is linear.

Geometric I Exponential Growth: From dividing cell (by mitotic division). both daughter cells retains the ability to divide and continue to do so.

Ex: All cells, -Tissue, organs, developing seed, germinating seed, seasonal activities etc. It is mathematically represented as W₁ = W₀e^rt Where, W₁ = final size (Weight, height, number etc.)

W₀ = initial size at the beginning of period.

r = growth rate, e = base of natural logarithms. It's curve sigmoid

Absolute and Relative growth rates:

Absolute growth rate: Measurement and the comparison of total growth per unit time in plant or plant parts. Or

Total growth occurs in unit time in plant or plant­ parts.

Relative growth Rate: The growth of the given system per unit time expressed on a comment basis i.e. per unit initial parameter in plant parts. Or Total growth occurs in unit time in comparison to initial growth in plant an plant parts. Relative growth rate is generally high in young developing plant parts.

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