Chapter 21 Plant Growth and Development Part 1
Growth in plants, as in any organism, consists of an irreversible increase in size, which is commonly accompanied by increase in solid or dry weight and in the amount of protoplasm growth is essential character of life. In growth anabolic processes dominate over the catabolic processes and therefore growth is the final product of successful metabolism.
A correct definition of growth is difficult. In common parlance, the ‘growth’ may be applied to several things and situations. However, growth can be defined as a vital process which brings about permanent change in any plant or its part with respect to its size, form, weight and volume. Whole series of changes during life span of a plant or organism is termed as development. Growth is generally a quantitative matter and is concerned with increasing amount of organism. Development, on the other hand, is qualitative change referring to the changes in nature of growth made by the organism. Growth is measurable whereas the development is most commonly assessed by qualitative observation. During growth and development, there is formation of proteins and carbohydrates, thus increasing the protoplasm formation.
- Regions of growth : In unicellular plants there is overall growth and not confined to any specific region but in multicellular plants growth is restricted to specific regions having meristematic On the basis of their position in the plant body (higher plants) meristematic cells. On the basis of their position in the plant body (higher plants) meristems are divided into three main categories.
(i) Apical meristems, (ii) Intercalary meristems, (iii) Lateral meristems
- Apical meristems : These meristems are found at shoot and root As a result of activity of these meristems plant increases in length. In angiosperms and gymnosperms there is a group of meristematic cells but in bryophytes and pteridophytes there is a single tetrahedral cell found at the shoot apex.
- Intercalary meristems : These meristems are found above the As a result of the activity of these meristems increase in length takes place. e.g., Bambusa.
- Lateral meristems : These meristems are made up of cells which divide in radial direction They form laterally placed new cells towards the centre and
periphery. Cork cambium (phellogen) and vascular cambium are the examples of lateral meristems. Increase
in girth of shoots and roots take place because of the
Cell plate formation Nuclear division Meristematic cell
activity of this cambium.
- Phases of growth : Growth is not a very simple
Primary cell wall
Thin layer of
process. Before completion of this process a meristematic cell has to pass through three phases.
- Cell division
- Cell enlargement
- Cell maturation (differentiation)
Cell elongation Secondary wall patterns
- Cell division (Formative phase) : A cell is metabolically highly active at the time of cell Its cellular mass increases and replication of genetic material
Fig : Showing cell formation, cell elongation and cell differentiation
(nucleic acids) takes place. Growth as a result of divisions is based on mitotic cell division. In the stage of mitosis each chromosome is split lengthwise into two homologous chromatids which pass equally into daughter cells. As a result of division each cell is only half the size of parent cell. These cells then proceed to enlarge.
- Cell enlargement : Cell division is followed by cell enlargement. The cell increases in size due to vacuolation (by absorption of water). A big central vacuole appears which pushes the cytoplasm to be limited to a thin boundary layer against the cell The new cell wall materials is synthesized to cope with the enlargement. The cell enlargement has been explained in two different ways. According to the first view, the tergour of the cell increases. As a result, certain gaps or lacunae appear in the cell wall. The new wall material is deposited in the lacunae between particles of the old wall (intus-ucception) or below the lacunae (apposition). The other view considers that as a result of growth of the cell wall the volume of the cell increases.
- Cell maturation (Differentiation) : Cell differentiation following cell division and cell enlargement leads to the development of specialized mature tissue cell , g., some cells are differentiated into xylem tracheids and trachea and some others into sieve tubes and companion cells.
- Growth curve : The rate of growth varies in different species and different In certain species of plants such as Cacti, the rate of growth is exceedingly slow. In many plants, the growth rate is phenomally rapid e.g., the young leaf sheath of banana grows for a time at the rate of almost three
inches per hour. Growth begins slowly, then enters a period of rapid enlargement, following which it gradually decreases till no further enlargement occurs. The mathematical curve which represents this variation in growth rate is some what flattened S-shaped curve or sigmoid curve. Time in which growth takes place has been called grand period of growth. This term was coined by Sachs. The analysis of growth curve shows that it can be differentiated into three phases :
- Lag phase : It represents initial stages of The rate of growth is very slow in lag phase. More time is needed for little growth in this phase.
- Log phase (Exponential phase) : The growth rate becomes maximum and more rapid. Physiological activites of cells are at their The log phase is also referred to as grand period of growth.
- Final steady state (Stationary phase) or Adult phase : When the nutrients become limiting, growth slows down, so physiological activities of cells also slows This phase is indicated by the maturity of growth system. The rate of growth can be measured by an increase in size or area of an organ of plant like leaf, flower, fruit etc. The rate of growth is called efficiency index.
In many plants another phase is also evident in their growth curve. This is called linear phase or phase of maximum growth rate. Sachs called it as grand phase.
- Measurement of growth : Growth in plants can be measured in terms of (i) increase in length, g., stem, root (ii) increase
Fig : A typical S-shaped grand
period of growth curve
Fig : Horizontal microscope for measurement of growth
in volume, e.g., fruit, (iii) increase in area, e.g., leaves (iv) increase in diameter, e.g., tree trunk. (v) increase in fresh or dry weight. The following methods are designed to measure growth in length.
The following methods are designed to measure growth in length :
- Direct method : It is the simplest method of measuring growth and involves measurement of growth between two marked points directly by a scale at regular This is not much used as in this case, growth over short periods cannot be measured.
- Horizontal microscope (Travelling microscope) : In this method, tip of a growing plant is marked with the help of Indian Ink and horizontal microscope is focussed at the After a day or two, marked point is obserbed by microscope. It is little bit raised. Distance between the two readings shows the actual growth of a plant. It can be used for measuring growth of plants in the field.
- Auxanometer : Several kinds of auxanometers have been devised to measure the growth in length of a plant. Two of them are given below :
- Arch auxanometer : It conists of a vertical stand with a pulley. Attached to the pulley is a pointer which moves on an arc scale. A silken thread is passed over the pulley, one end of which is tied to the plant apex and the other carries a weight enough to keep the thread stretched. As growth occurs the pulley moves, causing the pointer to move on the Growth can be calculated on the basis of the distance moved by the pointer on the arc and the length of the pointer as follows.
Growth of plant in length = Distance travelled by pointer ´ Radius of pulley
Length of pointer meausred from the centre of the pulley
Fig : Arc auxanometer (arc indicator) for measurement of growth
- Pfeffer’s auxanometer (Automatic auxanometer) : It is composed of two pulleys (large and small), revolving cylinder covered with a smoked paper, stand and 3 The two pulleys magnify the growth.
Magnification = Radius of larger pulley
Radius of smaller pulley
One end of thread is tied to the tip of stem and other end is tied to a small weight. The thread is passed over the
Fig : Pfeffer’s auxanometer
smaller pulley. One more thread having small weight on its both sides passes over the larger pulley. One side of this thread bears pointer towards the revolving cylinder, which is in close contact with the smoked paper.
Revolving of cylinder takes place by the start of clock work. If growth occurs, two pulleys move by the downward movement of weight which is attached to the tip of stem. There is ring like and stair case like marking developed on the smoked paper. However, if no growth occurs, a horizontal line is formed on the smoked paper.
- Bose’s crescograph : The crescograph invented by Sir Jagdish Chandra Bose is a more delicate instrument and gives magnification upto 10,000 times. The rate of growth of root can be measured by the use of a root auxanometer.
- Factors influencing rate of growth : Growth is affected by the factor which affect the activity of It is affected by a large number of factors both environmental and physiological. Physiologogical factors such as absorption of water, minerals, photosynthesis, respiration etc, and environmental factors including climatic and edaphic both. The effect on these factors on one region of plant are also transmitted to other region of the plant.
Since growth is a resultant of many metabolic processes, it is affected by many external and internal factors, which are as follows,
- External factors
- Light : Light affects variously g., light intensity, quality and periodicity.
- Intensity of light : In general, light retards growth in plants. High light intensities induce dwarfing of the Plants at hill tops are short whereas those of a valley are quite tall. Very weak light induces the rate of overall growth and also photosynthesis. Development of chlorophyll is dependent on light and in its absence etiolin compounds in formed which gives yellow colour to the plant. The phenomenon is called etiolation. Similarly high light intensity affecting indirectly increases the rate of water lose and reduces the rate of water growth.
- Quality of light : The different colours (different wavelengths) affect the growth of In blue-violet colour light internodal growth is pronounced while green colour light reduces the expansion of leaves as compared to complete spectrum of visible light. The red colour light favours elongation but they resemble etiolated plants. Infrared and ultraviolet are detrimental to growth. However, ultraviolet rays are necessary for the development of anthocyanin pigments in the flowers. Blue and violet colours increase size of lamina of leaf.
- Duration of light : There is remarkable effect on durtion of light on the growth of vegetative as well as reproductive structures. The induction and suppresion of flowering are dependent on duration. The phenomenon is termed
- Temperature : Temperature has pronounced effect on the growth of plant. The temperature cardinals for growth vary according to temperature zones. The minimum, optimum and maximum temperatures are usually 5oC (arctic), 20 – 30oC (temperate) and 35 – 40oC (tropical). The optimum temperature needed for the growth of a plant is much dependent on the stages of development. Low temperatures during nights reduces the rate of respiration and high temperature during days increases photosynthesis accumulated photosynthate also increases growth the tomato plants do not grow well under uniform temperatures condition of day and night but they grow well under low night temperature (nyctotemperature) and flucuating day temperature (phototemperature). This response of plant to temperature variation is called thermoperiodicity. When plants are exposed to extremes of temperature they get injured and the injuries are called descication, chilling and
Due to hot or cold spells of wind, when the transpiration exceeds absorption, the plant tissue gets injured and the injury is called desiccation. If a plant of hot climate is exposed to low temperature it gets injured and the injury is called chilling. During winter, in hill plants water is withdrawn from the cell into the intercellular space. As a
result, the dehydrated protoplasm coagulates. There is inter and intracellular ice formation due to further lowering of temperature and as a result the plant tissue is injured. This injury is called freezing. A plant develops high osmotic concentration of the cell sap and a thick bark to withstand these injuries. Besides, it also shows formation of seeds, spores, tubers etc. when the temperature goes down.
- Water : As water is an essential constituent of the living cell, a deficiency of water causes stunted growth. Moreover unless the cells are in a turgid condition, they cannot divide and unless new cells are added up by the activity of the meristems, growth cannot take water is also essential for photosynthesis not only as a raw material, but also for the photosynthetic activity of the cells. Water is also essential for the translocation of mineral salts and ready-made food to the growing regions of the plants. Without food supply growth cannot take place.
- Oxygen : In poorly aerated soil there is low concentration of oxygen and a high concentration of
Under such conditions plants usually show stunted growth. Normal growth of most plants occurs only when
abundant oxygen is present since
O2 is important for respiration. It has been reported that oxygen plays some
important role during GI stage of cell division.
- Mineral salt : Absence of essential mineral salts results in abnormal growth. For example, the absence of nitrogen prevents protein-synthesis, while the absence of iron prevents chlorophyll formation and thus leads to pale and sickly growth of plants, known as chlorotic
- Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticites etc have detrimental effect on plant Some plants are very sensitive to certain pollutants. Citrus and Gladiolus are very sensitive to fluorides. Poor growth of tobacco is observed in regions where ozone concentration is high. White pine cannot survive under high O3 concentration. Cotton plants are, similarly very sensitive to ethylene.
- Carbon dioxide : CO2 is essential for photosynthesis and hence nutrition. Due to change in photosynthetic rate, with the increase or decrease in CO2 concentration, the plant growth is also
- Internal factors : Amongst internal factor e., age, health, hereditory factors, growth regulator, nutritional relations, etc. growth regulators are very important. Some of the internal factors are :
- Nutrition : It provides raw material for growth and differentiation as well as source of energy. C/N (carbohydrate/protein) ratio determines the type of growth. High C/N ratio stimulates wall thickening. Less protoplasm is formed. Low C/N ratio favours more protoplasm producing thin walled soft cells. According to law of mass growth, the initial rate of growth depends upon the size of germinating structure (seed, tubes, rhizome, bulb, )
- Growth regulators : These are manufactured by living protoplasm and are important internal growth regulators which are essential for growth and development. These growth regulators include several phytohormones and some synthetic
Growth hormones and Growth regulators.
The term hormone used by first Starling (1906). He called it stimulatory substance. The growth and development in plants is controlled by a special class of chemical substances called hormones. These chemicals are synthesized in one part of the plant body and translocated to another where they act in a specific manner. They regulate growth, differentiation and development by promoting or inhibiting the same. They are needed in small quantities at very low concentrations as compared to enzyme. They are rarely effective at the site of their synthesis.
Thus, growth hormones also called phytohormones term given by Thimann (1948), it can be defined as ‘the organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes’. Sometimes the term growth regulators is misled with phytohormones. The term phytohormones as the definition indicates, is implied to those chemical substances which are synthesized by plants and thus, they are naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even molecular structure. Thus the synthetic substances which resemble with hormones in their physiological action are termed as growth regulators.
Phytohormones can have a promoting or inhibiting effect on a process. A particular hormone may promote certain processes, inhibit some others and not effect many others. In general, developmental processes are controlled by more than one growth regulator. They may act synergistically i.e., in a cooperative and beneficial manner (e.g., morphogenesis by auxins and cytokinins) or antagonistically i.e., in opposite manner (e.g., seed germination is promoted by gibberallin and is inhibited by abscisic acid). A group of plant hormones including auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth.
- Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the Among the growth regulators, auxins were the first to be discovered.
- Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen- Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a Auxin was first collected by Went (1928) from coleoptile tip of Avena . Went also developed Avena curvature test for bioassay of auxin. Kogl and Haagen. Smit (1931) introduced the term auxin.
- Types of auxins : There are two major categories of auxins natural auxins and synthetic
- Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and
The first naturally occuring auxin was isolated by Kogl and Haagen-Smit (1913) from human urine. It was identified as auxin-a (auxentriolic acid, C18H32O5). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin
Indole acetic acid (IAA)
(C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3- acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid (4-chloro-IAA) etc., are some other natural auxins.
Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in
stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and not through the vascular tissues. Auxins help in the elongation of both roots and shoots. However, the optimum concentration for the two is quite different.
It is 10 ppm for stem and 0.0001 ppm for the root. Its translocation rate is 1–1.6 cm/hr. (In roots 0.1 to 0.2
cm/hr). Higher concentration of auxins show inhibitory effect on growth.
Natural auxins are of two types : free and bond auxins. The auxins which can easily be extracted are called free auxins, whereas auxins which are hard to extract and need the use of organic solvents are termed as bound auxins. The free form of auxin is active, while the bound auxin is inactive in growth. A dynamic equilibrium exists between these two forms.
- Synthetic auxins : These are synthetic compounds which cause various physiological responses common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the weedicide, 2, 4, 5-T (2, 4, 5-trichlorophenoxy acetic acid), IBA (indole 3-butyric acid), NAA (naphthalene acetic acid, PAA (Phenyl acetic acid), IPA (Indole 3-propionic acid). IBA is both natural and synthetic auxin. Certain compounds inhibit action of auxin and compete with auxins for active sites are called antiauxins. g., PCIB (p- chlorophenoxy isobutyric acid), TIBA (2, 3, 5-tri iodobenzoic acid). TIBA is used in picking cotton bolls.
Indole Butyric acid (IBA)
Naphthalene acetic acid (NAA)
acid (2, 4-D)
2, 4, 5-trichlorophenoxy-acetic
acid (2, 4, 5-T)
- Bioassay of Auxins : Testing of biological activity (growth) of a substance (auxin) by employing living material is called bioassay. Auxin bioassay is also quantitative test as it measures amount of effect in response to a particular concentration of
- Avena coleoptile curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant
growth by performing some experiments with the oat (Avena sativa) coleoptile.
- When the tips of the coleoptiles were removed, no growth took
- When the freshly cut coleoptiles were placed on agar
blocks for a few hours (during this period auxin diffused into the
(A) (B) (C) (D) (E)
Fig : Oat coleoptile experiment
agar block) and then the agar blocks were placed on the cut ends of the coleoptile, growth occurred.
- When the agar block with the diffused substance was placed laterally on the cut tip of the coleoptile, only that side of the coleoptile elongated resulting in a
- Split pea stem curvature test : This test was also discovered by Went, Dark germinated seeds of pea are decapitated. About half an inch part of stem between 2nd and 3rd node is removed and split longitudinally. It is then floated on the test solution contained in a beaker. At first, negative curvature occurs due to water uptake. Then positive curvature occurs which is proportional to the log of the concentration of auxin.
These experiments indicated that some substance is synthesised in the coleoptile tip is translocated downward.
He called this substance auxin.
- Root growth inhibition test (Cress root inhibition test) : Sterilized seeds of cress are germinated over moist filter Root lengths are measured. 50% of seedlings are placed in test solution while the rest are allowed to grow over the moist filter paper. Lengths of roots are measured after 48 hours. Seedlings placed in test solution show very little root growth while the roots of controlled seedlings show normal growth. The degree of root growth inhibition is proportional to auxin concentration.
- Functions of auxins : Auxins control several kinds of plant growth These are as follows :
- Cell elongation : Auxins promote elongations and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all
The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of microfibrils, synthesis of more wall materials, increased membrane permeability and respiration.
Sugars Starch Sugars
Loosening of cell wall microfibrils
More cell wall materials
Fig. Action of IAA in cell elongation.
- Apical dominance : In many plants, the apical bud grows and the lower axillary buds are Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is checked by applying synthetic auxin (NAA).
- Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced when the auxin content falls below a minimum. Addicot and Lynch (1951) put forward auxin gradient theory about abscission :
- No abscission if auxin content is high on the organ
- Abscission layer begins formation when auxin content becomes same on stem and organ
- Abscission is favoured when auxin content is low on the organ
Premature drop of fruits such as apple, pear and citrus can be prevented to a great extent by spraying the trees with a dilute solution of IAA, NAA or some other auxin.
- Weed control : Weeds are undesirable in a field with a crop. Weeds cause competition for water, mineral, light and space. This causes poor yield. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but 2, 4-D does not affect mature monocotyledonous
- Root differentiation : Many new plants are usually propogated by stem cutting g., Rose, Bougainvillea. If we dip the lower cut end of a cutting in dilute solution of auxins (specially IBA gives very good results) very soon large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
- Parthenocarpy : It is the process of formation of fruits without fertilization. Such fruits are called as parthenocarpic fruits and are without seeds. Parthenocarpy can be induced by application of IAA in a paste form to the stigma of a flower or by spraying the flowers with a dilute solution of IAA. Banana, oranges and grapes are now- a-days grown parthenocarpically on commercial
- Control of lodging : In some plants when the crop is ripe and there is heavy rain accompanied by strong winds, the plants bends as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute solution of any auxin is sprayed upon young plants the possibility of bending of plants is reduced as the stem becomes stronger by the application of
- Flowering : In pineapple, NAA promotes flowering. In lettuce, auxins help in delaying the flowering. In cotton plants, the use of auxins increases the cotton seeds
- Differentiation of vascular tissues : Auxins induce the differentiation of xylem and phloem in intact plants and also in callus produced in vitro during tissue culture
- Sex expression : The spray of auxins increases the number of female flowers in cucurbits. In maize application of NAA during the period of inflorescence differentiation can induce formation of hermaphrodite or female flowers in a male
Thus auxins cause femaleness in plants.
- Healing : Healing of injury is effected through auxin induced division in the cells around the injured area. The chemical was formerly named traumatic acid or
- Nodule formation : In legumes, IAA is known to stimulate nodule
- Respiration : According to French and Beevers (1953) the auxin may
increase the rate of respiration indirectly through increased supply of ADP by
rapidly utilizing the ATP in the expanding cells. CO
- Gibberellins : Gibberellins are weakly acidic hormones having gibbane HO
ring structure which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants (i.e., corn, pea) in
CH3 COOH CH2
- Discovery : Gibberellins were first isolated from the fungus Gibberella fujikuroi (Fusarium moniliforme) the causal organism of Bakanae disease or foolish seedling disease of rice plants in Japan by Kurosawa in 1926. The characteristic symptoms of this disease are abnormal growth of stem and leaves, thin plants with long internodes, early flowering or death before flowering and
In 1939, Yabuta and Sumiki and coworkers working in Tokyo isolated an active substance from the fungus and called it Gibberellin A. This gibberellin preparation was probably a mixture of several gibberellins. The first gibberellin to be obtained was Gibberellin A-3. Cross et al. (1961) explained the detailed structure of gibberellic acid. Now 60 gibberellins have been identified from different groups of plants (algae, fungi, mosses, ferns, gymnosperms and angiosperms).
Many of them occur naturally in plants. Gibberella Fujikuroi has as many as 15 gibberellins. A single plant also possesses a number of gibberellins. All the different types of gibberellins, known so far, have gibbane skeleton and
are acidic in nature. Therefore, these are termed as GA1
(C19 H24 O6 ), GA2
(C19 H 26 O6 ), GA3 (C19 H22O6 ) , GA4
(C19 H24 O5 )
and so on. Of these gibberellic acid or gibberellin
A3 (GA3 )
is the commonest. Gibberellins are
synthesised in plants in leaves of buds, developing embryos, root tips, young apical leaves, shoot tips and seeds. Gibberellins are transported readily in the plant, apparently moving passively in the stream either in xylem or phloem. Their transport in non-polar. Anti-gibberellins like malic hydrazide, phosphon D, Alar and chorocholine cheoride (CCC) or cycocel are also called antiretardants (stimulates flowering and inhibits the growth of nodes). Commercial production of GA is still carried out by culturing this fungus in large vats.
- Mechanism of action : Gibberellins are closely related with steroids. Gibberellins exhibit ecdysome like Ecdysome is a moulting hormone. The steroids have very specific effect in depressing genes and thus activating specific genes. Another significant gibberellin treatment is production of enzymes like amylase and protease. It is also considered that the effect of gibberellin is indirect.
Fig. Role of gibberellin in synthesis of auxin.
According to this view gibberellins show its physiological effects by altering the auxin status of the tissue.
- Bioassay of gibberellin : Gibberellin bioassay is performed through dwarf maize/pea test and cereal endosperm test.
- Dwarf pea bioassay : Seeds of dwarf pea are allowed to germinate till the just emergence of GA solution is applied to some seedlings others are kept as control. After 5 days, epicotyl length is measured. Increase in length of epicotyl over control seedlings is proportional to GA concentration.
- Barley endosperm bioassay : Endosperms are detached from embryos, sterilized and allow to remain in 1ml of test solution for 1-2 There is build up of reducing sugars which is proportional to GA concentrations. Reducing sugars do not occur in edoperms kept as control.
- Functions of gibberellin
- Stem elongation : The gibberellins induce elongation of the internodes. The cell growth is promoted by the increase in the hydrolysis of polysaccharides. It also increases the elasticity of cell wall. The elongation of stem results due to rapid cell division and cell elongation induced by gibberellins.
- Leaf expansion : In many plants leaves become broader and elongated when treated with gibberellic This leads to increase in photosynthetic area which finally increases the height of the plant. Interestingly, gibberellins show no effect on roots.
- Reversal of dwarfism : One of the most striking effects of gibberellins is the elongation of genetic dwarf (mutant) varieties of plants like corn and It is believed that dwarfism in the mutant variety of plant is due to blocking of the capacity for normal gibberellin production (deficiency of gibberellin). When gibberellin is applied to single gene dwarf mutants e.g., Pisum sativam, Vicia faba and Phaseolus multiflorus, they grow to their nomal heights. It is further interesting to note that application of gibberellins to normal plants fail to show any remarkable effects.
- Bolting and Flowering : Gibberellins induce stem elongation in ‘rosette plants’ e.g., cabbage, henbane, Such plants show retarded internodal growth and profuse leaf development. In these plants just prior to the reproductive phase, the internodes elongate enormously causing a marked increase in stem height. This is called bolting.
Bolting needs long days or cold nights. It has been further noticed that if cabbage head is kept under warm nights, it remains vegetative. The exogenous application of gibberellins induced bolting in first year itself in plants like cabbage (normally bolting occurs next year due to effect of endogenous gibberellins).
- Enzyme formation : One of the most dramatic effects of GA is its induction of hydrolytic enzymes in the aleurone layer of endosperm of germinating barley seeds and cereal grains. GA stimulates the production of digestive enzymes like proteases, a-amylases, lipases which help to mobilise stored nutrients. GA treatment stimulates a substantial synthesis of new Thus GA acts to uncover or depress specific genes, which then cause the synthesis of these enzymes. It is assumed that GA acts on the DNA of the nucleus.
- Breaking of dormancy : Gibberellins overcome the natural dormancy of buds, tubers, seeds, and allow then to grow. In this function gibberellins act antagonistically to abscisic acid (ABA).
- Parthenocarpy : Gibberellins have been considered to be more effective than auxins for inducing parthenocarpy in fruits like apple, tomato and pear. GA application has also resulted in the production of large fruits and bunch length in seedless
- Sex expression : Gibberellins control sex expression in certain plants. In general, gibberellin promote the formation of male flowers either in place of female flowers in monoecious plants such as cucurbits or in genetically female plants like Cannabis, Cucumis.
- Substitution for vernalization : Vernalization is the low temperature requirement of certain plant (i.e., biennials) to induce flowering. The low temperature requirement of biennials for flowering can be replaced by
- Malt yield : There is increased malt production when gibberellins are provided to germinating barley grains (due to greater production of a-amylase).
- Delayed ripening : Ripening of citrus fruits can be delayed with the help of It is useful in safe and prolonged storage of fruits.
- Seed germination : Gibberellins induce germination of positively photo-blastic seeds of lettuce and tobacco in complete
- Cytokinins (Phytokinins) : Cytokinins are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives that promote cell division (cytokinesis) either alone or in conjugation with
- Discovery : The first cytokinin was discovered by Miller, Skoog and Strong (1955) during callus tissue culture of Nicotiana tobaccum (tobacco).
It was synthetic product formed by autoclaving Herring sperm (fish sperm) DNA. This synthetic product was identified as 6-furfuryl amino-purine and named as kinetin. He found that normal cell division induced by adding yeast extract.
Various terms such as kinetenoid (Burstran, 1961), phytokinin (Dendolph et al. 1963) phytocytomine (Pilet 1965) have been used for kinetin like substances but the term cytokinin proposed by Letham (1963) has been widely accepted. Letham et al. (1964) discovered first natural, cytokinin in unripe maize grain (Zea mays). It was named as zeatin (6 hydroxy 3 methyl trans 2-butenyl amino purine).
About 18 cytokinins have been discovered, e.g., dihydrozeatin, IPA (Isopentenyl adenine), benzyl adenine. The most widely occurring cytokinin in plant is IPA. It has been isolated from Pseudomonas tumefaciens. Many are found as constituents of tRNAs. Cytokinins are synthesized in roots as well as endosperm of seeds. Coconut millk and Apple fruit extract are rich in cytokinins. Cytokinins in coconut milk called coconut milk factor.
Kinetin (6 furfuryl amino purine) is a derivative of the nitrogen base adenine. Plant physiologists use the term cytokinins to designate group of substances that stimulate cell division in plants.
Cytokinins are prdouced in actively growing tissues such as embryos, developing fruits and roots. Kinetin is the derivative of purine base adenine, which bears furfuryl group at 9 position which migrated to 6 position of the adenine ring during autoclaving of DNA. According to Fox (1969) cytokinins are substances composed of one hydrophilic group of high specificity (adenine) and one lipophilic group without specificity.
Kinetin (6-furfuryl aminopurine)
Cytokinin is transported to different parts of the plant through xylem elements. According to Osborne and Black (1964), the movement of cytokinin is polar and basipetal.
- Mechanism of action : Most known cytokinins have an adenine nucleus with purine ring intact with N6 substituents of moderate Cytokinins never act alone. In conjugation with auxins, they stimulate cell division even in permanent cells. It was noticed by Skoog and Miller that callus cultures grew slowly on basal medium, but growth could be promoted by adding hormones like IAA and cytokinins. No response occurred with auxin or cytokinin alone. When both the hormones are present in equal amount, cells divide rapidly but fail to differentiate. However, when quantity of cytokinins is more than auxins, shoot bud appears from callus. With more concentration of auxins, roots develop fast. The similarity in structure of most cytokinins to adenine, a constituent of DNA and RNA suggests that basic effect of cytokinin might be at the level of protein synthesis.
- Bioassay of cytokinins : Bioassay is done through retention of chlorophyll by leaf discs, gains of weight of a tissue in culture, excised radish cotyledon expansion, ,
- Tobacco pith culture : Tobacco pith culture is divided into two weighted lots one supplied with cytokinin and the other without it. After 3-5 weeks, increase of fresh weight of treated tissue over control is noted. It is a measure of stimulation of cell division and hence cytokinin
- Retardation of leaf senescence : Leaves are cut into equal sized discs with the help of a cutter. They are devided into two lots. One lot is provided with cytokinin. After 48-72 hours, leaf discs are compared for chlorophyll Cytokinin retards chlorophyll degradation.
- Excised radish cotyledon expansion : Excised radish cotyledons are measured and placed in test solution as well as ordinary water (as control). Enlargement of cotyledons indicates cytokinin
- Root inhibition test : Kiraly and his coworkers (1966) used root inhibition test for cytokinin They found, that amount of root inhibition of actively growing seedlings is related to cytokinin activity.
- Functions of cytokinins
- Cell division : Cytokinins are essential for cytokinesis and thus promote cell division. In presence of auxin, cytokinins stimulate cell division even in non-meristematic In tissue cultures, cell division of callus (undifferentiated mass of parenchyma tissue) is enhanced when both auxin and cytokinin are present. But no response occurs with auxin or cytokinin alone.
- Cell enlargement and Differentiation : Under some conditions cytokinins enhance the expansion of leaf cells in leaf discs and cotyledons. These cells considered to be mature and under normal conditions do not Cytokinins play a vital role in morphogenesis and differentiation in plants. It is now known that kinetin- auxin interaction control the morphogenetic differentiation of shoot and root meristems.
- Delay in senescence : Cytokinin delay the senescence (ageing) of leaves and other organs by controlling protein synthesis and mobilization of resources (Disappearance of chlorophyll). It is called Richmond Lang effect. It was reported by Richmond and Lang (1957) while working on detached leaves of Xanthium.
- Counteraction of apical dominance : Auxins and cytokinins act antagonistically in the control of apical dominance. Auxins are responsible for stimulating growth of apical bud. On the other hand, cytokinins promote the growth of lateral buds. Thus exogenous application of cytokinin has been found to counteract the usual dominance of apical
- Breaking of dormancy : Cytokinins breaks seeds dormancy of various types and thus help in their They also induce germination of positively photoplastic seed like lettuce and tobacco even in darkness.
- Accumulation and Translocation of solutes : Cytokinins induce accumulation of salts inside the They also help solute translocation in phloem.
- Sex expression : Cytokinins promote formation of female flowers in some
- Enzyme activity : Cytokinins stimulate the activity of enzymes especially those concerned with
- Parthenocarpy : Development of parthenocarpic fruits through cytokinin treatment has been reported by Crane (1965).
- Pomalin : A combination of cytokinin (6-benzladenine) and gibberellin (GA4, GA7) called pomalin is particularly effective in increasing apple
- Initiation of interfasicular cambium : Cytokinins induce the formation of interfasicular cambium in plants g., Pinus radiata.
- Nucleic acid metabolism : Guttman (1957) found a quick increase in the amount of RNA in the nuclei of onion root after kinetin
- Protein synthesis : Osborne (1962) demonstrated the increased rate of protein synthesis on kinetin
- Flowering : Gibberellins also play an important role in the initiation of Lang (1960) demonstrated that added gibberellin could substitute for the proper environmental conditions in Hyoscyamus niger
which requires long day treatment for flowering. Such effects of gibberellin are common among vernalised and long day plants.
Gibberellin is also known to play essential role in germination of cereal seeds.
- Ethylene : Ethylene is a gaseous hormone which stimulates transverse growth but retards the longitudinal
- Discovery : The effect of ethylene had been known since Kerosene lamps and hay have been used
to fruit merchants to hasten colour development (ripening) in fruits. These effects are due to ethylene. Neljubow (1901) observed that ethylene gas alters the tropic responses of roots. Denny (1924) reported that ethylene induces ripening of fruits. Crocker et al. (1935) identified ethylene as natural plant hormone.
Ethylene is produced in plants from the amino acid methionine. It is synthesized in almost all plant parts-roots, leaves, flowers, fruits, seeds. It is more synthesized in nodal regions. Maximum synthesis of ethylene occurs during climacteric ripening of fruits. High concentration of auxin induce ethylene formation. When a fruit ripens its respiration rate gradually decreases but it is reversed by a sharp increase called climactric. Some of the inhibitory effects earlier attributed to auxin are known to be caused by ethylene.
The commercial product for providing ethylene is ethaphon (2-chloroethyl phosphoric acid). Ethaphon is a liquid from which ethylene gas is released, hence this substance is used for artificial ripening of fruits.
- Bioassay of ethylene : It is done on the principle of triple response which includes three characteristic effects of ehtylene on etiolated seedlings of pea-viz.
- Swelling of nodes,
- Inhibition of elongation of internodes of stem,
- Induction of horizontal growth of stem against
- Triple pea test : Pratt and Biale (1944) developed this method for bioassay of ethylene which base on the physiological effect of ethylene to cause
- Subapical thickening of stem,
- Reduction in the rate of elongation and
- Horizontal nutation (transverse geotropism) of stem in etiolated pea seedlings. In presence of ethylene, epicotyls show increase growth in thickness and reduced rate of longitudinal and horizontal
- Pea stem swelling test : Cherry (1973) used pea seedlings to measure ethylene concentration by marked increase of stem swelling expressed as a ratio of weight to length. In one ppm of ethylene the ratio is about 0.
- Functions of ethylene
- Fruit growth and Ripening : Ethylene promotes fruit growth and its The harmone is used in the artificial ripening of climacteric fruits (e.g., Apple, Banana, Mango).
- Transverse growth : Ethylene inhibits longitudinal growth but stimulates transverse growth so that stem looks
- Epinasty (leaf bending) : Epinasty represents more growth on upper surface of leaf than on lower Epinasty is said to be controlled by ethylene in many plants.
- Abscission : Ethylene stimulates formation of abscission zone in leaves, flowers and
- Apical dominance : Ethylene inhibits the growth of lateral buds and thus cause apical dominance (in pea). It is believed that auxin might be functioning partly through synthesis of ethylene in causing apical
- Root initiation : In low concentration, ethylene stimulates root initiation and growth of lateral roots and root
- Flowering : Ethylene stimulates flowering in pineapple and related plants though in other cases, the hormone causes fading of Fading flowers of Vanda are known to release ethylene. Sleep disease (inrolling of petals in blossomed flowers) in due to ethylene.
- Sex expression : Ethylene application increases the number of female flowers and fruits in cucumber
- Dormancy : It breaks dormancy of different plant organs but not of lateral
- Abscisic acid (ABA) : Abscisic acid is a mildly acidic growth hormone, which functions as a general growth inhibitor by counteracting other hormones (auxin, gibberellins, cytokinins) or reactions mediated by
- Discovery : The hormone was first isolated by Addicott et al. (1963) from cotton balls. They named it as abscisin II. Simultaneously, Wareing and Cornforth isolated a substance that can induce bud dormancy. They named the substance as dormin. Later, both these substances were found to be the same and were named as abscisic acid. It is produced in many parts of the plants but more abundantly inside the chloroplasts of green The synthesis of abscisic acid is stimulated by drought, water logging and other adverse environmental conditions. Therefore, it is also called stress hormone. The hormone is formed from mevalonic acid or xanthophylls. Chemically it is dextro-rotatory cis sesquiterpene. The hormone is transported to all parts of the plant through diffusion as well as through conductive channels.
Abscisic acid (ABA) Xanthoxine
In some plant tissues (especially in young shoots) occurs a related compound called xanthoxine.
Whether xanthoxine is an intermediate of the ABA-biosynthesis or whether it is an independent product remains unknown. The structure indicates that both ABA and xanthoxine are terpene derivatives. This was proven when it could be shown that radioactively labelled mevalonic acid is integrated into ABA though it does not elucidate which intermediates are produced. Two alternative biosyntheses have been discussed :
- ABA is a degradation product of xanthophyll (especially of violaxanthin).
- ABA is produced from a carotenoid/xanthophyll
precursor using a separate pathway and is thus independent from the
The first idea seemed initially more plausible since the structures of xanthophylls and ABA correspond to a large degree. In vitro occurs conversion only upon exposure to strong light and with an extremely low yield, though.
- Bioassay of abscisic acid
- Rice seedling growth inhibition test : Mohanty, Anjaneyulu and Sridhar (1979) used rice growth inhibition method to measure ABA like activity. The length of second leaf sheath after six days of growth is
- Inhibition of a-amylase synthesis in barley endosperm test : ABA inhibits the synthesis of a- amylase in the aleurone layers which is triggered by Goldschmidt and Monselise (1968) developed the
bioassay method to estimate ABA activity by determining the extent of inhibition of a-amylase synthesis induced by treating barley seed endosperm with GA.
- Functions of abscisic acid
- Control : It keeps growth under check by counter acting the effect of growth promoting hormones, i.e., auxins, cytokinins and gibberellins. As growth is primarily controlled by gibberellins, abscisic acid is popularly called antigibberellic hormone. It will inhibit seed germination, growth of excised embryos, growth of Duckweed and other
- Dormancy : Abscisic acid acts as growth inhibitor and induces dormancy of buds towards the approach of winter. Dormancy of seeds is mainly caused by abscisic Because of its action in inducing dormancy abscisic acid (ABA) is also called dormin. The buds as well as seeds sprout only when abscisic acid is overcome by gibberellins.
- Abscission : ABA promotes the abscission of leaves, flowers and fruits in
- Senescence : Abscisic acid stimulates senescence of leaves by causing destruction of chlorophyll (an effect opposite to that of cytokinins) and inhibition of protein and RNA synthesis. The effect, however, can be reversed by application of cytokinins in Lemna.
- Antitranspirant : Abscisic acid can be used as antitranspirant. Application of minute quantity of ABA to leaves reduces transpiration to a great extent through partial closure of stomata. It thus conserves water and reduces the requirement of
- Hardiness : Abscisic acid promotes cold hardiness and inhibits growth of
- Flowering : ABA delays flowering in long day plants. However, in some short day plants (e.g., strawberry, black current) it promotes
- Rooting : Abscisic acid can be used to promote rooting in many stem
- Wound hormone or Traumatic acid or Necrohormone : Haberlandt (1913) reported that injured plants cells release a chemical substance (wound hormone), which stimulate the adjacent cells to divide rapidly in order to heal up the wound. English et al. (1939) finally isolated and crystallized this wound hormone and named it as Traumatic acid. Although traumatic acid has been found to be very active in inducing meristematic activity in uninjured green bean pods, but it is not effective in most of the plant tissues including tobacco pith
- Morphactins : Morphactins are synthetic growth regulators which act in variety of ways on the natural regulation mechanisms of plants. The important ones are phenoxyalkancarboxylic acid (synthetic auxin), substituted benzoic acids, Malic acid hydrazide, Fluorene-9 carboxylic acids and their derivatives, Chlorflurenol, Chloroflurun, Flurenol, Methylbenzilate, Dichlorflurenol, Morphactins have fundamental action on morphogenesis of plants and this characteristic designation (morphactins) is derived from morphologically active substances.
O – CH3
O – CH2 – CH2 – CH2– OCH3
Morphactins IT 3456 and IT 3233
The actions of these substances are systematic and after their uptake they are transported and distributed not polarly (as seen by IAA) but basi- and acropetally. Generally these are growth inhibitors. These contain ‘fluorene ring’ in their structure.
- Seed germination : In general, morphactins inhibit germination seeds particularly the emergence of the radicle from the seed This property can be counteracted with GA3 and almost completely by cytokinins. The germination of fern spores is also delayed by morephactins.
- Growth seedling : Morphactins inhibit the growth of seedling affecting the shoot and often also With this property they show a similarity with cytokinin. The inhibitory effect of seedling shoot growth can be partly counteracted with GA3 but not the inhibition of root growth.
- Stem elongation : They have inhibitory effect on the stem elongation. Increased concentration produces dwarfing in the plants. The inhibitory effect of morphactins is not only observed in stem elongation but also on the new growing shoot
- Polarity of cell division : Denffer and others (1969) observed in the dividing cells of the root tips of Allium that treatment of morphactin (CFI) results in random orientation of the mitotic spindle and plane of cell division, e., they exercise depolarisation during cell division.
- Histogenesis and Morphogenesis : Morphactins induce anomalies espicially in new-formed organs in case of Begonia. Formation of cornets (fusion of leaf with stem) and ochria (fusion of calyx with other floral parts). The single flowers and the number and differentiation of floral organs were
- Apical dominance and branching : Morphactins treatment with grasses and cereals increased tillering and also increased number of lateral buds and stimulated extension growth of lateral
- Prolonged bud dormany : The emergence of buds from the storage organs of various perennial species is delayed when plants are treated with morphactins as reported in Solanum tuberosum and Malus sylvestris.
- Root growth and Root branching : Lateral buds are inhibited or retarde and primary roots are promoted at low concentration but inhibited at higher concentration of morphactins. In general, the action of morphactins on the longitudinal growth of the roots system may be considered as the reverse of their action on the shoot
- Realisation of flowering : Morphactins have also been found effective on flowering stimulation, sequence of flowering, position and number of flowers, formation of flowers, inflorescence and parthenocarpy,
- Depot effect : First morphactine are accumulated in plants and after sometime show their
- Jasmonic acid (Jasmonates) : According to Parthier (1991), jasmonic acid and its methyl esters are ubiquitous in They have hormone properties, help regulating plant growth, development and they seem to participate in leaf senescence and in the defense mechanism against fungi.
O 8 11 O
O O– Glucose
9 10 12
4 3 COOH
Just like all other plant hormones jasmonates have both activating and inhibiting effects. Synergistic and antagonistic effects on other hormones have also been observed. Jasmonate derivatives induces the accumulation of proteins so-called jasmonate-induced-proteins that were found in all plant species tested. Their accumulation can
also be caused by desiccation or ABA effects. Jasmonate-induced-proteins are of varying molecular weights, and molecules of different size classes have immunologically been shown to be related. The major portion of these proteins is not glycosylated, has no proteolytic activity and is metabolically stable. Labelling with immunogold and electron microscopy showed that some of them are located within the nucleus, while others were detected in the vacuole. None have ever been found in mitochondria. Their synthesis can be inhibited by cycloheximid, but not by chloramphenicol. Chloramphenicol affects mitochondrial proteins. Jasmonate-induced-proteins are lacking in roots, in bleached leaves and in leaves of chlorophyll-deficient Hordeum vulgare mutants. They exist in etiolated leaves, though. Jasmonates do not only regulate the transcription of these proteins, they do also influence the rate of translation of different groups of mRNA. They do, for example, decrease the production rate of several essential housekeeping proteins.
Just like ABA jasmonates inhibit a premature germination of the oil-containing seeds of Brassica and Linum. After germination they induce the synthesis of the seed storage proteins Napin and Cruciferin as well as that of several more elaiosome-associated proteins.
- Calines (Formative hormones) : Certain other natural growth hormones in plants called as calines or formative hormone which are throught to be essential for the effect of auxin an root, stem and leaf growth they are :
- Rhizocaline or Root forming hormone : It is produced by the leaves and translocated in a polar manner down the
- Caulocaline or Stem forming hormone : It is produced by the roots and is transported upward in the
- Phyllocaline or Self forming hormone : It is produced probably by the It stimulates
mesophyll development in the leaves and is synthesized only in the presence of light.
None of these caline has yet been isolated.
- The term vitamin has been derived from vital They are heterogenous group of organic compounds which are needed in very small quantities (as accessory food factors) for different metabolic processes.
- They are essential for normal growth and development and maintenance of health as well as vigour.
- Vitamins are synthesised by plants and microbes, though they are required by all types of
- Most of the vitamins functions as coenzymes and prosthetic groups of various enzymes connected with protein, fat and carbohydrate
- Vitamin K is component of electron transport Some vitamins are essential in maintaining cell membranes and acting as antioxidants. Therefore, vitamins are important growth regulators.
- Vitamins are of two types : Fat soluble- it includes vitamins A, D, E and K, water soluble – vitamin B complex and vitamin
- The double sigmoid growth curve occurs in some fruits g., Grapes, plum.
- Measurement of growth in young root by making it at 1mm intervals with Indian Ink was first done by Strasburger.
- Inflexion point is the point at which growth begins to decline (beginning of decelerating phase) after the exponential
- 20000 tons of Avena tips give 1g of
- The development of shoot and root is determined by cytokinin and auxin ratio.
- IBA is the most potent root
- Mixture of 2, 4-D and 2, 4, 5-T (dioxin) is given the name ‘Agent orange’ which was used by USA in Vietnam war for defoliation of forests (i.e., in chemical warfare).
- At 260l, it is dextrorotatory, whereas at 280l, it is leavorotatory and this phenomenon is known as cotton
- In glass houses when plants are kept on artificial light and temperature, then this method is called phytotron and is applicable in agriculture, horticulture and tissue
- When each meristem influences other meristems then this phenomenon is called growth correction.
- Dinitrophenol when comes in contact with any plant which destroy Such herbicide is called contact herbicide.
- Ethylene like special chemicals are called sterenes and a natural
- ABA is used in
- Malic hydrazide is a growth retardant which checks cell So during seed storage this is applied for checking sprouting of potato tubers so that the importance of potato may be lowered down.
- In lower plants or thalophyta diffused growth is
- When AMO 1618 sprayed on aerial parts of the plants, it inhibits the It is used during wave for destroying plants.
- Dalapon (2-2 dichloropropionic acid) is a
- Auxin and Cytokinin in combined form shows synergistic effect (affects development of physical structure).
- Growing zone present 1 cm at root tip, 2-5 cm at shoot
Physiology of flowering.
Flower is a modified shoot specialized to carry out sexual reproduction of the plant. Flower initiation takes place by the transformation of vegetative apex into reproductive structure. Hence, it signifies a transition from vegetative to reproductive phase. The pattern and timing of flower initiation varies from species to species. A flower must attain a stage of ripening before it flowers. The period of maturity after which plant can produce flowers if exposed to inductive conditions is called ripeness of flower. The external conditions necessary for the initiation of flowers in a plant are called inductive conditions. The duration for which inductive conditions are required for inducing the flowers in a plant is called the inductive period. The external conditions under which plant continues to grow vegetatively for unlimited period are called non-inductive conditions. Daily and season fluctuations in a particular location. Daily and season fluctuations in a particular location are directly related with latitude. At equator, day length is of 12 hours throughout the year, temperature also remains constant at equator. Long warm days of summer are distinct from short cold days of winter. Such variations in environment can be observed as one moves away from equator. Flowering in a plant occurs at a particular time of the year and controlled by many morphological and environmental conditions. Two important controlling factors are photoperiod or light period, i.e. photoperiodism, low temperature i.e. vernalization