Phytochromes & Photoperiodicity | Additional Study Material for NEET PDF Download

PHOTOPERIODISM

• The relative length of day & night is called as photoperiod.

• The response of plants to the photoperiod, expressed in the form of flowering is called as photoperiodism. “Effect or requirement of relative length of day (photoperiod) & night (dark phase) on flowering of plants is called as photoperiodism”

• The phenomenon of photoperiodism was first discovered by Garner & Allard on Maryland mammoth (a mutant variety of tobacco) and biloxi  soyabean. 

• Garner & Allard classified the plants in following groups.

(1) SDP (Short Day Plants) :- “These plants give flowers on exposure to photoperiod equal or shorter than their critical day length”. . They need a continuous (uninterrupted) dark period for flowering. Thus SDP also called as LNP (Long Night Plants).

• Ex. of SDP :- Tobacco, Soyabean , Viola, Xanthium (Cocklebur), Chrysanthemum, Cannabis, Coleus, Chenopodium, Mustard, Dahlia, Sugarcane , Strawberry, Cosmos, Rice etc.

• In SDP the dark period is critical and must be continuous. If this dark period is breaks by a brief exposure to red light, then SDP will not flowers. 

• Maximum inhibition of flowering with red light occur at about the middle of critical dark period.

• Prolongation of the continuous dark period, initiates early & good flowering in SDP.

(2) LDP (Long Day Plants) :- These plants flowers only when they exposed to critical photoperiod or photoperiod longer than their critical day length”. . The light period is critical for LDP. Ex. :- Henbane (Hyoscyamus) Spinach, Sugarbeets, Radish, Carrot, Wheat, Larkspur, Barley, Avena, Potato.

•  A brief exposure in the dark period stimulates flowering in LDP

(3) DNP (Day Neutral Plants) or Intermediate plants :- These plants do not need specific light period for the flowering. Ex. Zea, Cotton, Tomato, Sunflower, Cucumber 

• L-SDP :- These are SDP but must be exposed to long days during early stage of their growth. Ex. Bryophyllum

• S-LDP :- These are LDP but must be exposed to short photoperiod during early stage of growth. Ex. Wheat & Rye sps.

• Callachan :- Reported that stimulation of critical photoperiod is percepted by leaves. 

• Chailakhyan :– Discovered “Florigen”, it has been not isolated, thus called as hypothetical hormone.

PHYTOCHROME

• Borthwick & Hendricks :- Discovered a light sensitive pigment responsible for flowering

• Butler :- Give term “phytochrome” for this pigment & isolated it.

• Pigment phytochrome is a chromophore biliprotein, which is an open tetrapyrrole related to phycobilin.

• Phytochrome mainly located on cell membrane of all type of plants.

• Phytochrome :- exists in two different forms Pr (Phytochrome red). – Red light absorbing form, induce flowering in SDP.

• Absorption range- 630-670 nm. absorption peak-667 nm.

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 and are stable.

•  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 inhibited to flowering in SDP but stimulates in LDP.

• Phytochrome - Pfr (P730) 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.

VERNALISATION OR YAROVIZATION

• Effect of low temperature on the initiation and development of flower, was first realised by Klippart 1857

• 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 perceps induction of low temperature on plants.

• Concept of hormone ‘vernalin’ in vernalisation was given by Melcher et. al. This is a hypothetical plant hormone,because not 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.

Significance :– 

(i) Better & early flowering.

(ii) Vernalisation increases the resistance to fungal diseases.

PLANT SENESCENCE

•  Period from complete maturity to degenerative 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 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 phyto-gerontology)

•  Senescence may be of following types :-

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

(b) Organ senescence : When plant part above ground dies (shoot) each year and root & rhizome system alive. Ex. Alfa-alfa, Sugarcane, Banana, Ginger.

(c) Sequential or progressive senescence : Evergreen perennials show progressive or sequential senescence of older leaves, lateral organs, branches, flowers, fruits and shoot.

Ex. Eucalyptus, Mango.

(d) Simultaneous or Synchronous leaf senescence : In perennial deciduous plants, all the leaves undergo senescence and abscission sp.

Ex. Dalbergia sissoo, Azadirachta indica, Ficus religiosa.

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) is formed within the region of attachment of these parts. Cell wall layers and middle lamella are dissolved by the activity of cellulase and pectinase (Polysaccharide hydrolysing enzymes) during the abscission.

•   Hormone ABA is main controller of abscission process.

PLANT   MOVEMENTS

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

(I) Autonomous :- (by internal stimulus )

(1) Amoeboid :- Ex : Gametes of Spirogyra

(2) Ciliary movement :- Ex : Chlamydomonas, Euglena, Zoospores

(3) Cyclosis :-

(i) Rotation – Whole protoplasm moves around the one central vacuole, in one direction. Ex : Hydrilla, Vallisneria cells.

(ii) 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).

(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 turgor 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.

Chemotropism  :- Ex. Pollen tubes & fungal hyphae

•  Definite direction in relation to chemicals.

(d) Thigmotropism (haptotropism) :- Ex : Tendrils, haustoria of Cuscuta.

•  Definite direction in response to contact or support.

(e) Hydrotropism :- Ex : Roots of seedlings

(ii) Nastic movement (External stimulus but diffused type or nondirectional) :-

(a) Nyctinasty :- Ex : Flowers, leaves, stomata, daily movement (Sleep movements)

•  Due to rhythemicity of day and night.

(b) Thigmonasty or haptonasty :- Tentacles of insectivorous plants

(c) Chemonasty :- Ex : Tentacles of insectivorous plants

(d) Seismonasty :- Ex : Mimosa (touch me not plant) turgor change in pulvinus leaf base K+ ion also involved in this movement.

SPECIAL POINTS

•  Many plant parts specially leaves exhibits nastic movement and involves differential growth, this type of movement is known as movement of growth.  This movement, 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 gravitational force) 

• Clinostat :- used for nullifying geotropism

• Traumatropism :- Injury induced plant movement

•  Plagiotropism :-  Shown by stem & root branches growing at an angle of 45° from axis of plant.

•  Climacteric fruits (Banana, Apple, Avocados):- Fruits, in which rate of respiration increases (climacteric respiration)during their ripening(ethylene). Citrus is non climacteric fruits.

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

•  Formation of nodule is a 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 the most widely used plant tool for the studies of developmental genetics and growth physiology of plants. 

•  Betacyanin or Betalain :- Pigment mainly found in vacuole of beet roots and flower of Bougainvillea are differ from anthocyanin due to the presence of nitrogen (N). These pigments do not show reversibility of colour change due to pH change.

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

Anthocyanin exhibits different colour like purple, pink, blue, scarlet etc.
The colour of anthocyanin is sensitive to pH change. For Example colour of anthocyanin changes from Red (acidic pH) to Violet (neutral pH) to blue (in alkaline pH).
Turgorin-a newly discovered hormone found in pulvinus leaf base regulates turgor pressure changing movements.

Growth rate : Increased growth per unit time.

Plants growth is of two types :

(a) 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).

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

(c) 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 common basis i.e. per unit initial parameter in plant parts. or Total growth occurs in unit time in comparison to initial growth in plant or plant parts. Relative growth rate is generally high in young developing plant parts.