Physiology of Flowering

Lesson Prepared under MHRD project “National Mission on Education through ICT” Discipline: Botany Paper: Plant Physiology National Coordinator: Prof. S.C. Bhatla Lesson: Physiology of flowering Lesson Developer: Dr Geetika Kalra Department/College: Acharya Narendra Dev College Lesson Reviewer: Prof. S.C. Bhatla, Department of Botany, University of Delhi Language Editor: Namrata Dhaka Department/College: Department of Genetics, University of Delhi, South Campus Lesson Editor: Dr Rama Sisodia, Fellow in Botany ILLL

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Physiology of Flowering

Learning outcomes The student should be able to understand the following: 

What is photoperidism?



What are short day, long day and day neutral plants?



What role does dark period play in induction of flowering?



Different theories behind mechanism of photoperioidic induction of flowering.



Mechanism of vernalization.



Causes and mechanism of seed dormancy.

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Physiology of Flowering

Table of contents Chapter: Physiology of flowering 

Introduction 

Photoperiodism 

Short-day plants



Long-day plants



Day- neutral plants



Photoperiodic induction



The central role of dark period



Perception of photoperiodic signal



Mechanism behind photoperiodic Induction



Importance of photoperiodism



Vernalisation



Seed Dormancy



Summary



Glossary



Exercises



References



Web links



Causes of Dormancy



Mechanism behind Dormancy



Methods of Breaking Dormancy



Advantages of Dormancy

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Physiology of Flowering

Introduction Flowering is the most enigmatic phase in the life of a plant. Since long people have wondered how plants are able to flower only in a particular season? How do they know that now is the time to shed the leaves? And so on…...Plants must be able to anticipate seasons. They need to know when to produce flowers so that the fruit and seed development can be accomplished before the next season arrives. Flowers represent a specialised structure which differs from the vegetative plant body in form and cell types. Numerous biochemical and physiological changes take place within the shoot apex when it prepares itself for flowering. Many genes have so far been identified that play crucial role in the formation of flowers. These studies indicate that reproductive development in plants is genetically controlled. Also, events or the signals that bring about changes in apical meristem include both internal (like circadian rhythms, phase change and hormones) and external factors (like day length and temperature). The interaction of these external and internal factors enables the plant to synchronise their reproductive development with the environment. The transition from vegetative to reproductive development is generally marked by an increase in the frequency of cell divisions within the central zone of the shoot apical meristem.

Genetic regulation of flowering 

Floral meristems initiate four different types of floral organs - sepals, petals, stamens and carpels.



These organs are initiated in concentric rings, called whorls, around the flanks of the meristem.



Three types of homeotic genes control floral organ identity.



Mutations in these genes change floral organ identity without affecting the initiation of flowers.



These genes fall into three classes - A, B and C.



Type A – These genes (AP1 and AP2) control organ identity in the first and second whorls. Loss of type A activity results in the formation of carpels instead of sepals in the first whorl and stamens instead of petals in the second whorl.



Type B- These (AP3 and PI) control organ determination in the 2nd and 3rd whorl. Loss of type B activity results in the formation of sepals instead of petals in the second whorl and carpels instead of stamens in the third whorl.

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Physiology of Flowering 

Type C (AG) controls events in the third and fourth whorl. Loss of type C activity results in the formation of petals instead of stamens in the third whorl and replacement of fourth whorl by a new flower such that this whorl is occupied by sepals.



The ABC model explains determination of floral organ identity.



This model postulates that organ identity in each whorl is determined by a unique combination of the three organ identity gene activities.



Activity of type A alone specifies sepals.



Activities of both A and B are required for the formation of petals



Activities of B and C form stamens



Activity of C alone specifies carpels.

Figure: The mutations in the A, B and C types of genes cause alterations in the pattern of floral organogenesis. Source: ILLL inhouse

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Physiology of Flowering

Figure: The ABC model depicts the functional role of the three types of genes A, B and C in floral organogenesis Source:

http://www.adonline.id.au/flowers/floral-identity/;

(seek

permission

http://www.adonline.id.au/contact/)

Floral evocation thus depends on both internal and external factors.

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Physiology of Flowering

ABC model of flower development has now been extended to ABCDE model The ABC model depicts pattern of spatial gene expression in relation to phenotypes. The following figure shows the domains where the five gene classes A, B, C, D and E are expressed corresponding to the four types of floral organs: sepals, petals, stamens and carpels (including ovules). The figure shows that, for example, sepal identity is determined by expression of type A and type E genes. The petal identity is determined by the expression of type A, type B and type E genes. The formation of stamens is determined by the type B, C and E genes. The development of carpels is determined by type C and E genes and finally the development of ovules is determined by type C, D and E genes. In Arabidopsis, the five gene classes in the ABCDE model comprise several redundant genes. The A type genes are represented by APETALA1 (AP1) and

APETALA2 (AP2), the B type by AP3 and PI, the C type by AGAMOUS (AG), the D type by SHATTERPROOF1 and SHATTERPROOF2 (SHP1, SHP2) and SEEDSTICK

(STK), and the E type by SEPALLATA1-SEPALLATA4 (SEP1, SEP2, SEP3 and SEP4). SEP1-3 are expressed in whorl 2-4, and SEP4 is expressed in all whorls.

Figure: The ABCDE model of flower development. Source: http://www.biomedcentral.com/1752-0509/4/101 (cc)

Autonomous regulation of flowering The case in which flowering occurs strictly in response to internal development factors and does not depend on any particular environmental conditions is referred to as autonomous regulation. Some plants exhibit an absolute requirement for the proper

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Physiology of Flowering environmental conditions in order to flower. This is called obligate or qualitative response to environment. Photoperiodism and vernalization are two most important mechanisms underlying flowering response.

Photoperiodism Flowering is so predictable in plants that it is used as floral calendar. As we know that roses bloom in summer and chrysanthemums in winter. It is generally the length of day that gives the most reliable indication of advancing season and an organism’s capacity to measure day length is known as photoperiodism. The study of photoperiodism has focussed on the transition from vegetative growth to reproductive phase which is the most mysterious event in the life of a flowering plant. First controlled experiment was conducted by a French scientist J. Tournois in 1912. He observed that Humulus and Cannabis plants flowered precociously during winter in greenhouse. He concluded that shortening of day length or lengthening of night was responsible for early flowering.

Figure: Photoperiodism implies that length of day and night effect flowering induction. Source:http://hoopermuseum.earthsci.carleton.ca/vegetation/9a_photoperiodism.htm W.W.Garner and H.A. Allard (1920) demonstrated the impact of day length on flowering and coined the term ‘photoperiodism’ while working on tobacco (var. Maryland Mammoth). These plants flowered when the length of day was shorter than the length of the dark period. We can observe that when the critical dark period shortens the plants remain vegetative. Further, when the dark period is interrupted by a flash of light, the plants remained vegetative. These observations led to the discovery of photoperiodism. On the basis of photoperiod requirement, plants have been classified into: (1) Short-day plants

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Physiology of Flowering (2) Long-day plants (3) Day-neutral plants

Figure: Representation showing that plants are classified into short day, long day or day neutral plants depending upon requirement of day-length for flowering. Source: https://ib-bioplans.wikispaces.com/Plant+Science+%289.1-9.3%29 (cc) Short-day plants (SDP) For flowering, the day length must not exceed a critical value. These plants can also be called as long-night plants, as a certain minimum of uninterrupted dark period in 24hrs is necessary for their flowering. If dark period is less than a critical length, flowering will not occur. A flash of light provided during continuous dark period inhibits flowering. However, light interruption is not very effective if it is nearing the beginning of the end of the dark period. Hillman (1959) showed that SDP are capable of flowering even if kept continuously in dark but provided with adequate sucrose. This shows that the short day plants require light only for carrying on photosynthesis. Examples: Soyabean, Poinsettia, Potato, Sugarcane, Cosmos, Chrysanthemum.

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Physiology of Flowering

Figure: A certain minimum dark period is necessary for flowering of SDP. Source:http://plantphys.info/plant_physiology/photoperiodism.shtml Long-day plants (LDP) They require a photoperiod of more than a critical length which varies from 14 to 18 hrs. The best flowering usually occurs in continuous light. A flash of light during long dark periods can induce flowering even during short day conditions. Since darkness has inhibitory effect on flowering, these plants can also be called as short-night plants. Examples: Spinach, Lettuce, Radish, Alfa-alfa, Sugarbeet, Larkspur. Day-neutral plants Their flowering is not affected by the length of the day. Examples: Tomato, Cucumber, Cotton, Pea, Sunflower. 

Within the above categories we also recognise obligate or facultative requirements.



Plants that have absolute requirement for a particular photoperiod before they flower are called qualitative photoperiod types. Ex. Xanthium strumarium does not flower unless it receives an appropriate short photoperiod. It is a qualitative SDP.



Spring cereals like wheat and rye are quantitative LDPs. They do flower under short days but flowering is accelerated under long days.



Photoperiod requirement is often modified by external conditions like temperature.



There are also other response types that respond to long and short days in some combination like Bryophyllum is a long-short-day plant. It flowers when certain number of short days is preceded by a certain number of long days.

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Physiology of Flowering 

Trifolium repens exhibits a reverse condition of short-long-day plant.



Some plants, like winter cereals require a low temoerature treatment before they become responsive to photoperiod while others may have a qualitative photoperiodic requirement at one temperature but a quantitative requirement at another temperature.



Some plants are Intermediate-day length plants. They flower in response to day length of intermediate length but remain vegetative when the day is too long or too short.



Interestingly, in Madia elegans flowering is delayed under intermediate day length (12 to 14 hrs) but occurs under day length of 8hrs or 18 hrs.

Figure: Long-day crops (wheat, barley, pea, lentil and sugar beet) flower under long days rather than under short days. Short-day crops (rice, maize, sorghum and wild potato) flower more efficiently under short days than under long days. Source: http://pcp.oxfordjournals.org/content/early/2015/01/11/pcp.pcu181.full (cc) Photoperiodic Induction 

Based on experimental findings it has been observed that a continuous favourable photoperiod is not essential, rather a short and appropriate photoperiod is required for the production of flowers.



This influence persists even when a plant is preceded and followed by unfavourable photoperiods. This phenomenon is called as photoperiodic induction or photoinduction.

The central role of dark period Actually, plants measure neither relative length of day and night nor the length of photoperiod. They measure the length of dark period. This was demonstrated by Hammer and Bonner (1938). Under 24 hr cycle of light and dark periods Xanthium flowered with dark periods longer than 8.5 hrs but remained vegetative in 16hrs light and 8 hrs dark. In 4 hrs light, followed by 8 hrs dark it remained vegetative. In 16 hrs

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Physiology of Flowering light followed by 32 hrs dark, flowering occurred. This shows that the relative length of day and night is not the determining factor in photoperiodism. It is the length of dark period which is important. Xanthium flowers only when the dark period exceeds 8.5 hrs. Critical role of dark period was confirmed by interrupting the dark period with brief light exposures. Flowering effect of an 9hr dark period can be nullified by interrupting the dark period with a brief light break, but a dark interruption of long light period has no effect. Similarly long-day plants require a dark period shorter than some critical maximum. With LD plants a flash of light on the middle of an otherwise non-inductive long dark period will shorten the dark period to less than the maximum and permit flowering to occur. Thus, photoperiodism is a response to the duration and timing of light and dark periods.

Figure: Effect of photoperiod and night-break interruption on flowering of SDPs and LDPs. The night interruption may be less than 5 min of very dim light, as in some SDPs, or may require prolonged (1-2 h) exposures, as in some LDPs. Source:

http://plantsinaction.science.uq.edu.au/edition1/?q=content/8-3-2-processes-

floral-induction-and-initiation

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Physiology of Flowering

Figure: Graph showing critical value where in LDPs and SDPs respond to flowering. Source: https://ib-bioplans.wikispaces.com/Plant+Science+%289.1-9.3%29 (cc) Perception of photoperiodic signal The photoperiodic signal is perceived by the leaves. A careful observation reveals that though photoperiodic influence is induced in leaves, flowers are formed in the shoot apex. Thus, stimulus needs to travel from leaves to flower forming region. Experimental findings support the view that stimulus moves through phloem but is independent of the transport of the carbohydrates. Transmission of stimulus indicates that it is in the form of some chemical; assumed to be hormonal in nature. Chailakhyan (1937), a Russian physiologist, named this flower inducing chemical as florigen. This has not been isolated from plants but still the evidences are indirect, based on physiological experiments only. The chemical nature of the stimulus has been supported by grafting experiment carried on Maryland Mammoth variety of tobacco. In this experiment a leaf of a plant which has received a proper photoperiod can induce flowering on grafting on a plant exposed to

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Physiology of Flowering unfavourable photoperiod. Using such techniques long-day plant could be made to flower under short days through grafting. The sensitivity of the leaf may vary with age, as the youngest fully expanded leaf was found to be most sensitive. Mechanism behind photoperiodic induction 1. Theory of endogenous rhythms (Bunning hypothesis) According to this hypothesis, flowering of plants depends on the length of photoperiods. These photoperiods affect the daily rhythms which consist of two phases: a) Photophilous phase - This occurs in light and is characterised by intensive photosynthesis and weak respiration (anabolic processes predominate). b) Skotophilous phase - This occurs in dark and is characterised by intensive respiration.

In

decomposition

this of

phase,

starch

into

hydrolytic sugars

activities

takes

place

are

increased

(catabolic

and

processes

predominate). According to Bunning, in this method, some kind of time regulating mechanism is present in the cell which is called oscillator. In SDPs, oscillator is present close to skotophilous phase while in LDPs it is close to photophilous phase. This theory was rejected since it appears to be hypothetical. 2. Phytochrome theory - This theory believes that this pigment is present in plants in two inter-convertible forms Pr ( P660) <------Pfr(P730) 

One form of phytochrome is P730nm which absorbs light at 730nm and other form of phytochrome is P660, which absorbs light at 660nm.



Under long day photoperiods, P730 is retained for a longer time and stimulates flowering of long day plants and suppresses flowering of short day plants.



Under short photoperiods. P660 is retained for a longer time and stimulates flowering of short day plants.

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Physiology of Flowering

Figure: Phytochrome theory of flowering Source: http://biology4isc.weebly.com/7-plant-hormones-and-photomorphogenesis.html (CC) 3. Nutrient diversion hypothesis - All attempts to isolate and identify florigen have proved unsuccessful and henceforth florigen model was replaced by nutrient diversion hypothesis. According to this, an inductive treatment stimulates an increase in the flow of nutrients into the apical meristem. This is based on the observation that there is a rapid increase in export of sucrose from the leaves to the shoot apex in sinapis alba before the induction of flowering. 4. Multifactorial hypothesis -This assumes that florigen is a complex of two substances- gibberellins and anthesins. It is already clear that there is a close relationship

between

the

nutrients

and

physiologically

active

compounds.

Chailakhyan (1968) assumes that reverse kind of metabolism involved in flowering is an adaptation which occurred during the course of evolution. The long day plants arose from neutral plants with the loss of their ability to form flowers during autumn short days. Short day plants arose with the loss of the ability to form flowers during long days of spring. Flowering plant passes through two phases of life. a) Floral stem formation stage- This phase requires metabolic changes as observed under long day conditions i.e. increased carbohydrate metabolism and respiration and increased gibberellic acid content. b) Flower formation phase- This phase requires metabolic changes as observed under short day conditions .i.e. intensive nitrogen metabolism, higher content of anthesins in leaves and nucleic acid metabolites in stem buds. LD

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Physiology of Flowering conditions favours first phase while SD conditions favours second phase. Gibberellins are critical in LDPs and anthesins are critical in SDPs. Importance of photoperiodism 1. The yield of tubers, corms, bulbs and rhizomes can e increased by knowing favourable photoperiod for their formation. 2. Vegetative crops like spinach, radish, carrot, sugarcane can be made to remain vegetative for longer periods. 3. Annuals may be grown twice or thrice a year. 4. Winter dormancy and autumnal leaf fall can be prevented by increasing light hours.

Vernalization Light and temperature both interact with each other and affect flowering behaviour of plants. Vernalization is stimulation of flowering by cold treatment given to fully hydrated seeds. The concept was introduced by T. D. Lysenko (1920) who observed the ability of cold treatment to make the winter cereal behave as spring cereal. This could be of immense practical utility like: 1. Crops can be harvested much earlier than the control crop. 2. Crops can be grown in regions where they do not naturally reproduce. 3. Plant breeding experiments can thus be accelerated. For vernalization seeds are allowed to germinate for some time and then given cold treatment by keeping them at 0 – 50 C. Period of chilling varies from few days to weeks and from plant to plant. Response of vernalization decreases if the period of vernalisation is interrupted by periods of heat treatment. Generally stem apex is the region which perceives the effect of vernalization. Transmission of vernalization effect across graft union also occurs. In contrast to photoperiodic effect which leads to flower initiation, vernalization prepares the plant for flowering. Gregory and Purvis in 1930s through their experiments in rye, suggested that vernalization renders the seedling sensitive to photoperiod. The following image shows the examples of vernalization in various plants, as they remain as rosette plants when they do not receive low temperature treatment

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Physiology of Flowering

Figure: Pictures of few plants requiring vernalization. (A) A biennial cabbage (Brassica oleracea) variety with an obligate vernalization requirement that had been growing for five years without cold exposure. The small plant in the girl’s hands is a summer-annual variety of B. oleracea that flowers rapidly without vernalization. (B) and (C) Summer annual and vernalization-requiring types of henbane and Arabidopsis. Source: http://www.plantcell.org/content/16/10/2553.full Experiments have shown that vernalization treatments are effective only when the excised embryo is supplied with carbohydrate and oxygen, this indicates that it is an energy dependent metabolic process. All these experiments led G. Melchers to propose the existence of a transmissible vernalization stimulus called vernalin. Lysenko gave a two phasic theory, according to which growth and development are two different phases of plant life and an exposure to low temperature is necessary for the change over from growth phase to development phase. Lang and Melcher postulated that hormone called vernalin is produced on the meristematic shoot apex of the embryo due to vernalization treatment which is responsible for inducing flowering. Lang stated a direct connection between vernalin and florigen. Low Temperature---- Vernalin----- Florigen

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Physiology of Flowering Chailakhyan further stated that vernalin hormone may be a precursor of Gibberellin. Under long day conditions it is converted to Gibberellin. Another hormone called Anthesin is present in long day plants, which along with vernalin causes flowering in long day plants. In short day conditions vernalin is not converted to Gibberellin hence flowering does not occur. Addition of Gibberellin to long day unvernalised plants in long day conditions leads to flower formation as these plants contain anthesin. Gibberellin is ineffective in producing flower in short day plants as they lack anthesin.

Light/Dark

Leaves/Cotyledons

Low Temperature

Physiologically active form of Pfr

Changes in growth hormones and metabolites

Synthesis of florigen

Transport of florigen to shoot apex

Initiation of floral primordial Figure: Events leading to initiation of flowering Source: Author

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Physiology of Flowering Flowering of plants depend on duration and timing of light and dark periods, termed as photoperiodism. Plants are classified into three classes according to their requirement of photoperiod, like long day plants, short day plants and day neutral plants. It is observed that leafless plants do not produce flowers. Leaf is the site of perception of photoperiodic signals.This means that some agent (a flowering hormone) is synthesised in leaf and passed to the flowering apex. This hormone was named as florigen. Vernalisation is defined as the method of inducing early flowering in plants by pretreatment of their seeds or young seedlings at very low temperature.Apical buds are the sites of vernalisation. Melchers (1939) proposed the role of a hormone called vernalin which can be transmitted from a vernalised plant to an unvernalised plant through a graft union. Seeds of some plants do not germinate even in all suitable conditions. These seeds are called dormant and the phenomenon is called dormancy.

Glossary ABC model - According to this model three type of homeotic genes namely, type ABC, are responsible for organ identity in each whorl of a flower. A unique combination of these three types of genes determines the organ formed in each whorl. Antiflorigen - A hypothetical hormone proposed to inhibit the formation of flower in certain long day plants under non inductive photoperiod. Critical day length - The minimum length of the day required for flowering of a long day plant. Florigen - The hypothetical hormone which is synthesised by the leaf and translocated to the shoot apex to induce flowering under inductive conditions. Long day plant - A plant which flowers only in long days when critical day length exceeds the minimum. Photoperiodism - A biological response to length and timing of day and night. Vernalization - cold temperature required for flowering as a pre-treatment.

Exercises 1.

What is photoperiodism?

2.

Distinguish between short-day plants, long-day plants and day-neutral plants.

3.

How does temperature influence flowering?

4.

Discuss

various

theories

of

flowering

in

relation

to

photoperiodism

and

vernalization. 5.

Enumerate methods to induce flowering in biennials at will.

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Physiology of Flowering Multiple choice questions 1.

When the dark period of short day plant is interrupted by a brief exposure of light then the plant:

I)

Flowers immediately

II)

Turns into a long day plant

III)

Will not flower at all

IV)

Gives more flowers

2.

In many plants the change over from vegetative to reproductive phase takes place in response to:

I)

The severity of temperature

II)

The length of the day

III)

The oxygen present in the air

IV)

Mainly the food material available in the soil

3.

Hormone responsible for vernalization is

I)

Abciscin

II)

Vernalin

III)

Florigen

IV)

Colchicine

4.

The short day winter annual ‘Pansy’ belongs to the genus

I)

Viola

II)

Papaver

III)

Iberis

IV)

Malva

5.

Vernalization is:

I)

Flowering induction by low temperature treatment of moist seeds

II)

Growth curvature in response to light

III)

Cycles of day and night

IV)

Effect of day length on plant growth

6.

Which of the following discovered and studied photoperiodism?

I)

Garner and Allard

II)

Dutrochet and Overbeak

III)

Went and Paal

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Physiology of Flowering IV)

Bose and Blackman

References Buchanan, Bob B., and Russell L. Jones. Biochemistry & molecular biology of plants. Vol. 40. Rockville: American Society of Plant Physiologists, 2000.

Hopkins, William G., and Norman PA Hüner. Introduction to plant physiology. Vol. 355. New York: Wiley, 1995.

Taiz, Lincoln, and Eduardo Zeiger. "Plant physiology." Sinauer Asso. Inc., Pub., Sunderland, Massachusetts, USA (2006).

Web links http://plantphys.info/plant_physiology/photoperiodism.shtml http://millar.bio.ed.ac.uk/andrewM/Jo%20Selwood%20site/photoperiodism.htm http://www.plantcell.org/content/16/10/2553.full.pdf+html

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