Plant hormones and photomorphogenesis

ISC 12>CONTENT>STRUCTURE AND FUNCTION OF PLANTS>7. PLANT GROWTH & DEVELOPMENT

Scope of syllabus

Plant growth: phases of growth, growth rate, measurement of growth, factors affecting growth.
Growth regulators: Brief idea of various theories leading to discovery of auxins by Went; role of growth regulators in development and growth of plants;
Synthetic growth regulators: Applications of synthetic growth regulators;
Seed dormancy: definition and explanation of dormancy and quiescence; causes and methods of breaking seed dormancy;
Seed germination: definition of hypogeal, epigeal and viviparous germination;
Photomorphogenesis : Brief idea of short day , long day and day neutral plants; critical day length, photoperiodic induction; experiment to prove that photoperiodic induction is perceived by the leaves; brief idea of phytochromes;
Idea of apical dominance, senescence, abscission,
Differences between photoperiodism and vernalisation

PLANT GROWTH

INTRODUCTION

Growth is one of the most fundamental and conspicuous characteristics of living organisms.
Growth may be defined as an irreversible increase in mass, weight and size of a living organisms.
In most cases, it results in increase in dry weight and the amount of protoplasm. Growth in higher plants includes cell division, enlargement and differentiation. Increase in the number and size of cells by itself cannot account for the development of an organized plant. For example, when a seed is sown, it does not become a larger seed but it grows as a seedling. Thus, growth is always accompanied by differentiation. Differentiation is the transformation of identical cells into different tissues.
Depending upon the various structural, functional and physiological needs of the plant the tissues are of different types.
Growth and differentiation results in development, which leads to gross form of the plant.
Meristematic cells present in the plant body viz., root, shoot apices, and the cambium are responsible for growth in plants.

PHASES OF GROWTH

The growth in length of the plant is due to the meristematic activity of the apical meristems that takes place in the root and shoot apices.
The period of growth is generally divided into three phases viz., formation, elongation and maturation.

Cell formation phase (Meristematic): The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.
Cell enlargement phase (Elongation): The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.
Cell differentiation phase (Maturation): Further away from the apex, i.e.,more proximal to the phase of elongation, undergoes to the phase of maturation. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

Stages of growth (Growth curve)

The rate of plant growth is slow in the initial stages and this phase is called lag phase.
It is followed by a rapid growth phase called log phase.
In the third and final phases, the growth slows down and the organism maintains the size it has already attained. This phase is known as stationary phase or steady state phase.
The growth in size or increase in number of cells if plotted against time the graph shows ‘S’ shaped curve known as sigmoid growth curve as shown in the figure.
In the annual plants the last phase i.e. steady state phase is followed by senescence i.e. arrest of growth and death. However, in the case of large trees each growing season exhibits a sigmoidal pattern of growth.

MEASUREMENT OF GROWTH

Growth in plants can be measured by

Increase in the number of cells produced.
Increase in the length or girth of a shoot or root.
Increase in dry weight of an organ.
Increase in the area of a leaf.
Increase in the volume of fruit

Methods of measurement

1. By direct method :

This is the simplest method, in which growth is measured directly by a scale at regular intervals.

2. By horizontal microscope

A horizontal microscope which slides on vertical scale is used for measurement of growth.
The microscope is focussed on the tip of the growing object.
After a time interval, the tip grows up and its focussed again.
The difference of initial and final focussed point is the the growth in length.

3. By Auxanometer :

It measures the rate of growth of plant in terms of short length.
The auxanometer consists of a movable pointer attached to a pulley and a graduated arc fixed to a stand. A thread passes around the pulley. One end of the thread is tied to the growing tip of the potted plant. The other end is tied to a small weight. As the plant grows in length the pulley rotates and needle attached to the pulley moves down the scale. From this, growth in length of the plant can be measured at a given interval of time.

FACTORS AFFECTING GROWTH

External factors

Temperature : Growth increases with increases in temperature until it reaches a maximum at optimum temperature.The rate of growth decrease with further increases in temperature.The growth of a species is typically adapted to its natural temperature environment.
Light: Light affects growth depending on its intensity, quality and periodicity.
Soil water treatment : Since growth depends on hydrostatic pressure, water deficiency will retard or completely stop.
Pollutants : Air pollutants like ethylene, sulphur dioxides, fluorides etc. Widespread use of pesticides, herbicides, fungicides etc. has a harmful effect on plant growth.

Internal factors

Nutrients
Genetic factor
Growth regulators

GROWTH REGULATORS

Unlike animals, plants can manufacture all the chemical substances they need for survival, given only light, water, carbon dioxide and trace elements from the soil. They manufacture all the amino acids, proteins, carbohydrates, nucleic acids and other compounds , and among these are hormones (from the Greek horman, meaning "to stimulate").
By definition (originally from animals), a hormone

is manufactured in a particular part of the body

is transported to another part of the body

induces a chemical responses that controls a specific physiological event

In plants, some hormones operate in the same tissues in which they are manufactured, and others are transported for use to different locations.

Plants don't have glands to produce hormones: various tissues throughout the plant body produce hormones.

A. AUXIN

Discovery :

Charles Darwin: Charles Darwin studies of auxin effects are published a book called, 'The Power of movement'.Darwin studied phototropism using the germinating stem of the canary grass (Phalaris canariensis).
The cylindrical shoot is enclosed in a sheath of cells called the coleoptile.
Darwin set out to determine which region of the coleoptile is sensitive to light.
Boysen- Jensen experiments : showed that the substance traveling down the coleoptile stem was of a chemical nature.
Paal: (1919) confirmed the work of Boysen-Jensen.
Went:

Chemical nature :

IAA (Indole acetic acid) is chemically similar to the amino acid tryptophan which is generally accepted to be the molecule from which IAA is derived.
Besides naturally occurring auxins, several other organic chemicals have been synthesized which behave like growth hormones.
They include

IBA : Indole-3 butyric acid
IPA : Indole-3 propionic acid
NAA: Naphthalene acetic acid
2,4-D: 2,4-dichlorophenoxyacetic acid
2,4,5-T : 2,4,5 trichlorophenoxyacetic acid

Occurrence in plants :

Auxins are universally present in plants and their highest concentration is in the growing tips of coleoptile, leaves and roots.
IAA is the only plant hormone known to be transported in a polar (i.e., unidirectional) fashion.

Functions: A) Physiological functions

1. Cell elongation :

The primary function is the elongation and growth of stem and roots enlargement of many fruits.
The cell elongation is promoted by a) increase in osmotic solutes b) decrease in wall pressure c) increase in permeability of cytoplasm to water d) increase in wall synthesis.

2. Apical dominance :

Growth of the shoot apex (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. This phenomenon is called apical dominance.
If the terminal shoot of a plant is removed, the inhibition is lifted, and lateral buds begin growth. Gardeners exploit this principle by pruning the terminal shoot of ornamental shrubs, etc. The release of apical dominance enables lateral branches to develop and the plant becomes bushier. The process usually must be repeated because one or two laterals will eventually outstrip the others and reimpose apical dominance.Apical dominance seems to result from the downward transport of auxin produced in the apical meristem. In fact, if the apical meristem is removed and IAA applied to the stump, inhibition of the lateral buds is maintained.

3. Abscission

Auxin also plays a role in the abscission of leaves and fruits.
Young leaves and fruits produce auxin and so long as they do so, they remain attached to the stem.
When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole or fruit stalk.
Soon the petiole or fruit stalk breaks free at this point and the leaf or fruit falls to the ground

Experimental evidence:

If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days.
The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time.
If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.
Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping.

Demonstration of the role of auxin in abscission.

4. Root initiation and development:

The localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots.
Auxin also stimulates the formation of adventitious roots in many species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant.

5. Stimulates formation of fruits

Pollen contains large amounts of auxin - pollen’s auxin is a chemical signal that pollination has happened and fruit formation can begin - synthetic auxins can cause fruit formation without pollination.

Agricultural uses of gibberellins:

Weedicides
Many synthetic auxins are used as selective weed killers and herbicides. 2, 4 - D (2, 4 - dichloro phenoxy acetic acid) is used to destroy broad leaved weeds. It does not affect mature monocotyledonous plants.

2. Rooting

Naphthalene acetic acid (NAA), indole butyric acid (IBA) and indole acetic acid (IAA) are used to induce rooting of cuttings of woody plants like guava.

3. Flowering

Foliar spray of NAA and 2, 4 - D induces flowering in litchi and pineapple. Percentage of ball setting in cotton plants increases when NAA or IBA are applied.

4. Parthenocarpy

NAA and IBA treatment induces parthenocarpy of fruits in tomatoes, ladies finger and brinjal.

B. GIBBERELLINS

Discovery :

Gibberellins were discovered by a chance observation.
in 1890, a Japanese rice farmer noted that some seedlings growing in his field were much elongated with unusually long internodes.
in 1926, E Kurosawa observed that these elongated seedlings were infected by a fungus, Gibberella fujikuroi.
Subsequently, Kurosawa found that culture filtrates from dried rice seedlings caused marked elongation in rice and other sub-tropical grasses.
This compound was named gibberellin by Yabuta in 1935; the first use of the term "gibberellin" in the scientific literature.
In 1938, Yabuta and Sumiki finally succeeded in crystallizing a pale yellow solid to yield gibberellin A and gibberellin B.
So far more than 100 gibberellins have been discovered.
They are abbreviated as GA1, GA2, GA3 and so on.

Characteristic elongated rice seedling caused by bakanae

Occurrence in plants

There are currently 136 GAs identified from plants, fungi and bacteria.
The major sites of GA production in plants are embryos, roots and young leaves close to shoot tip.

Chemical nature

Gibberellins are chemically gibberellic acid. In general they are of isoprenoid nature and possess the same general carbon skeleton.
All gibberellins are derived via the ent-gibberellane skeleton.
The gibberellins are named GA1 through GAn in order of discovery.
Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA3.

Physiological functions :

Stimulate stem elongation by stimulating cell division and elongation.
Stimulates bolting/flowering in response to long days.
Breaks seed dormancy in some plants which require stratification or light to induce germination.
Stimulates enzyme production (alpha-amylase) in germinating cereal grains for mobilization of seed reserves.
Induces maleness in dioecious flowers (sex expression).
Can cause parthenocarpic (seedless) fruit development.
Can delay senescence in leaves and citrus fruits.

Agricultural uses of gibberellins:

They are used for uniform bolting and increase seed production in lettuce.
They improve the number and size of fruits in grapes.
They improve fruit set and the size, colour and quality of fruits in pears and apples.
They break dormancy and help in uniform crop emergence in potato.
They increase the amount of alpha amylase in germinating barley.
They are used to promote the elongation of internodes in sugarcane without decreasing sugar content.

C. CYTOKININS

Discovery

In 1913, Gottlieb Haberlandt discovered that a compound found in phloem had the ability to stimulate cell division.
In 1941, J. van Overbeek discovered that the milky endosperm from coconut also had this ability.
The first cytokinin was isolated from herring sperm in 1955 by Miller et al. (named kinetin)
The first naturally occurring cytokinin was isolated from corn in 1961 by Miller (Miller, 1961). It was later called zeatin.

Occurrence in plants

Cytokinins are more abundant in roots, young leaves and developing fruits.

Chemical nature

adenine derivatives (amino purines)
approximately 40 different structures known.
Zeatin (Z), which was first isolated from maize (Zea mays) is the most common cytokinin.
Other naturally occurring cytokinins include, dihydrozeatin (DHZ) and isopentenyladenosine (IPA)

Physiological functions :

Stimulates cell division. (Details)
Delay senescence (Richmond- Lang effect)
Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.
Stimulates the growth of lateral buds-release of apical dominance.
Breaking dormancy of seeds.
Increase femaleness in grapevine and causes sex reversal
Stimulates leaf expansion resulting from cell enlargement.
Accumulation and translocation of solutes.
Delay of senescence: Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

D. ABSCISIC ACID

Discovery

P.F. Wareing and associates (1963) isolated a dormancy inducing substance from the leaves of Acer and they called it dormin.
Cons and Addicott (1963) also isolated a substance abscisin from plant tissues which could accelerate leaf fall.
In 1966, it was found that both dormin and abscisin are chemically the same compound and they were named as abscisic acid.

Occurrence in plants

The plant hormone ABA is synthesized primarily in mature leaves and fruit.
The production of ABA is promoted by stresses such as water loss and freezing temperatures.

Chemical nature

ABA is a sesquiterpene consisting of 15-C atoms.

Physiological functions :

Induce bud dormancy: Delays or even stops the opening of both vegetative and flowering buds.The bud dormancy is regulated by a balance between gibberellins and ABA.
Induce and maintain seed dormancy : ABA interacts with GA and kinetin in controlling germination.
Accelerate senescence and abscission
Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis).
Flowering and sex expression- inhibits and reverses effect of Gibberellins
Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots.
Induces seeds to synthesize storage proteins.

Agricultural uses of ABA

Induction of leaf senescence
Inhibition of shoot growth.
Increase the yield of tubers.
Stimulation flower drop and fruit ripening
Used as anti-transpirant.

E. ETHYLENE (The only gaseous plant growth regulator)

Discovery

Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening.
In 1901, Dimitry Neljubow showed that the active component was ethylene.
In 1935, Crocker proposed that ethylene was the plant hormone responsible for fruit ripening as well as inhibition of vegetative tissues

Occurrence in plants

All plants are capable of producing ethylene.
It occurs usually in high concentration in leaves, buds and flowers undergoing senescence.
Roots are the main sites of ethylene biosynthesis.
Unlike the other four classes of phytohormones, ethylene is a gas at room temperature. Ethylene gas diffuses easily through the air from one plant to another. The saying "one bad apple spoils the barrel" has its basis in the effects of ethylene gas. One rotting apple will produce ethylene gas, which stimulates nearby apples to ripen and eventually spoil because of over-ripening.

Chemical nature

Ethylene is an unsaturated hydrocarbon.
It is colourless gas which is lighter than air and sparingly soluble in water.

Physiological functions :

Inhibitory effect on growth
Suppression of bud growth
Abscission- accelerates abscission
Flowering- retards flowering
Sex modification- increases the number of female flowers
Fruit growth and ripening- stimulates ripening
Root initiation- stimulates
Dormancy- breaks dormancy
Ageing and senescence.- induces

Climacteric fruits :

The tripple response : Ethylene treated shoots show three characteristic growth response simultaneously.

Epinasty: Downward curvature of the leaves
Decreased elongation and lateral cell expansion
Loss of gravity response to give horizontal growth.

APPLICATIONS OF SYNTHETIC GROWTH REGULATORS

Vegetative propagation: IBA and NAA (initiation and increasing in number of roots on the cuttings)
Breaking of dormancy : GA3 helps in breaking of seed dormancy.
Inhibition of sprouting: Sprouting of potato tubers in storage can be prevented by the application of growth regulators. (NAA and Malaeic hydrazide)
Control of flowering : NAA and 2,4-D
Control of sex expression : Application of auxin increases the number of females flowers and decrease the male flowers in cuccurbits.

PHOTOMORPHOGENESIS

PHYTOCHROME

Phytochrome is a photo-receptor molecule which mediates several developmental and morphogenetic response of plants to light.
In 1952, Borthwick and Hendricks proposed the name phytochrome for a pigment that receives light.
Phytochrome exists in two inter-convertible forms.

1. Pr --- light absorption peak in 660nm
2. Pfr--- light absorption peak in 730nm

Pfr is biologically active, Pr is biologically inactive
Pr absorbs red light, converted quickly to Pfr , during day
Pfr absorbs far-red light and is converted slowly to Pr, at night or in dark.

Role of phytochrome:

Seed germination : Many seeds are stimulated to germinate by light in a phytochrome-mediated response.This may require only brief irradiation or prolonged illumination. In general, red light enhances seed germination and far-red light inhibits it.
Seedling growth : The growth of coleoptile, extension of mesocotyl, unrolling of grass leaves are some examples of phytochrome mediated processes.
Flowering: The photoperiodic flowering responses in most plants have been shown to be mediated via phytochrome.
Circadian rhythms: A number of plants processes follow a periodic cycle of 24 hours.The phytochrome response ensure synchronous of the rhythm with day-length.

SEED DORMANCY

Normally, every viable seed is capable to germinate and gives rise to anew plant. However , some times germination of the seed is suspended due to adverse environmental conditions.
Quiescence is the condition of the seed when its germination is suspended due to environmental conditions normally required for its germination are not available.
Dormancy is the condition of a seed when it fails to germinate even when the environmental conditions usually considered necessary for germination are available.Dormancy is a state of inhibition of growth of the seed due to some internal causes.
Dormancy is advantageous to the plant as it is an adaptation to ensure seed germination only under favorable conditions enabling successful establishment of the seedling.

Causes of seed dormancy :

Hard seed coat
Immature embryo
After ripening
Presence of germination inhibitor

Breaking of seed dormancy :

Scarification
Low temperature treatment
Exposure to light
Compounds stimulating germination
Pressure

SEED GERMINATION
A. STAGES OF SEED GERMINATION

Dicot: SEED

1st Stage
a)   imbibition - initial absorption of water to hydrate seed
b)   activation of metabolism - increased respiration and protein synthesis
2nd Stage
a)   digestion of stored food - for example, starch to sugars in cotyledon or endosperm
b)   translocation to embryo.
3rd Stage
a)   cell division and continued growth and development of seedling.

B. FACTORS AFFECTING GERMINATION

The external factors that affect seed germination are as follows.

Water : Activate hormones and enzymes Swelling of the seeds = bursting of seed coat.
Transport of simple materials to the embryo – to be used for respiration and growth.
Metabolic and enzyme actions – occur in solution, therefore need water.
Conversion of storage compounds into simple components (i.e. starch to glucose)
Oxygen : Oxygen is needed for aerobic respiration. Without a supply of oxygen, seeds fail to germinate because of the lack of energy. Most seeds germinate well in air containing 20% oxygen.
Temperature : Varies according to the species. The optimum temperature is 25 -35 degree celsius for most tropical species.
Light : There are many seeds which respond to light for germination and these seeds are said to be photoblastic. The red and far-red sensitivity of seeds is due to the presence of a photo-receptive pigment known as phytochrome.
Soil conditions : Water holding capacity, mineral composition and aeration of the soil.High salinity inhibit the germination due to osmotic effects.

C. TYPES OF SEED GERMINATION

EPIGEAL GERMINATION :

Germination in which the cotyledons are forced above the ground.
The hypocotyl elongates.
Cotyledons become photosynthetic.
Seed coat emerges from soil.

Examples: Bean

HYPOGEAL GERMINATION

Germination in which the cotyledons remain in the seed below the ground.
The epicotyl elongates.
Cotyledons do not become photosynthetic.
Seed coat remains in soil

Examples: Maize, Pea

VIVIPARY (Special type of germination)

In this the seed germinates inside the fruits while it is still attached to the parent plant.
The embryo is thus nourished by the parent plant.
These plants shed dart-like seedling instead of seeds.
The seedlings after falling into the mud immediately develops a root system and establish itself.

Examples: Mangrove pants like Rhizophora and Sonneratia

PHOTOPERIODISM

Response of a plant to relative length of light and darkness within 24 hours is known as photoperiodism.
Garner and Allard established the concept of photoperiodism by working on Maryland mammoth (Nicotiana tabacum)
Photoperiodism was first studied in relation to flowering. Plants can be described in relation to their photo-period responses as short-day, long-day, day-neutral plants.

Short day plants (SDP) :

These plants flower in short days, i.e. with a long unbroken night.
However, day length is critical and varies from species to species. If these plants are kept in day length in excess of critical point, they continue to remain in vegetative state.
These plants normally flower in winter or early spring.
Examples: Nicotiana tabacum, Glycine max (soyabean), strawberry, oryza sativa, chenopodium, cocklebur and chrysanthemum.

2. Long day plants (LDP) :

These plants flower in long days, i.e. with a short unbroken night.
However, day length is critical and varies from species to species. If these plants are kept in day length in below of critical point, they continue to remain in vegetative state.
They normally flower in late spring or early summer.
Examples: Poppies, lettuce, spinach and potatoes, Tomato

3. Day neutral plants (DNP) :

These plants do not have any specific requirement of day length to flower, i.e. they are not affected by day lengths and flower around the year.
Examples: Lycopersicon esculentum (tomato), corn, cucumber, rice and some strawberries.

CRITICAL PHOTOPERIOD

The length of the daily period of light to which a plant is exposed is called photoperiod and the number of hours of light that determines its flowering response is said to be critical photoperiod.
For short day plants, the critical photoperiod is the photoperiod at or below which it flowers.
For long day plants, the critical photoperiod is the photoperiod at or above which it flowers.

PHOTOPERIODIC INDUCTION

IMPORTANCE OF DARK PERIOD

In 1938, K. C. Hammer and J. Bonner experimented with artificial lengths of dark and light periods.
Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
If dark period is interrupted by a flash, it does not flower; darkness amidst day cycle has no effect.
Long-day plants require a dark period shorter than a critical length regardless of length of light period.
Therefore, length of the dark period controls flowering, not length of the light period

EXPERIMENT TO PROVE THAT PHOTOPERIODIC INDUCTION IS PERCEIVED BY THE LEAVES.

Phytochrome and Flowering

it has been indicated that the pigment phytochrome is involved in photoperiodic perception and flowering.
Phytochrome is a blue-green leaf pigment that exists in two forms:
Pr (phytochrome red) absorbs red light (wavelength of 660 ;nm); it is converted to Pfr.
Pfr is phytochrome far-red and absorbs far-red light (wavelength of 730 ;nm); it is converted to Pr.
During a 24-hour period, there is a shift in ratio of these two pigments.

Direct sunlight contains more red than far-red light; Pfr is present in plant leaves during the day.
Shade and sunsets have more far-red than red light; Pfr is converted to Pr as night approaches.
There is a slow metabolic replacement of Pfr by Pr during night.

Phytochrome conversion may be a first step in reception-transduction-response pathway resulting in flowering.

VERNALIZATION (A method of inducing early flowering in plants by pretreatment of their seeds at very low temperature. )

In many plants, temperature has a profound effect on flowering.
Some plants do not flower under the inductive photoperiod conditions but flower only when a cold temperature treatment is given to them.
The acquisition or acceleration of the ability to flower by a chilling treatment is said to be vernalization.
The technique was first applied a Russian scientist, Lysenko (1928) and he called it Jarovization.
In nature, vernalization can be demonstrated in biennials, which normally require a season of wintering before flowering. An artificial cold treatment followed by correct photoperiod and temperature will result flowering in the first growing season itself. Thus a biennial can be made to flower in the same period of time as required by an annual.
Vernalization stimulus is perceived by shoot apical meristem.
Grafting experiment have demonstrated that the shoot tip receives vernalization stimulus, and then stimulus is translocated to the other parts of the plant.
The most effective temperature for vernalization is is between 0-5 degree celsius for a duration of four days to eight weeks.
On the basis of grafting experiment, Melchers (1939) suggested that a transmissible flowering stimulus is formed as a result of chilling. He called this stimulus as vernalin. Unfortunately, vernalin or any other such chemical substance has not been isolated so far.

SENESCENCE

Senescence is the stage in the life history of an individual when the rate metabolic activities decline that leads to ageing and then eventually death.
Senescence is thus the period between reproductive maturity and death of a plant or plant part.
Senescence is marked by

general decline in metabolic activities (ATP synthesis and potency of chloroplasts) and
decrease in nucleic acid, semi-permeability of cytoplasmic membrane and in the capacity to repair and replace wornout cells.

The study of plant senescence is called phytogerontology.
Depending upon the part of the plant involved in senescence , the following types of senescence have been recognized.

Whole plant senescence: In this, there is senescence and death of the entire plant, which ususally takes place at the end of the reproductive phase. Most annuals shows this type of senescence.
Shoot senescence : In this, the above ground part of the shoot dies each year after flowering and fruiting, but the underground part survives which puts out new shoots again next year. Eg. Perennials like banana and gladiolus.
Sequential progressive senescence : In this, the oldest leaves senesce and die first. The senescence then moves from leaves to the stem to underground parts. Most perennials show this type of senescence.
Simultaneous or synchronous senescence: In this, all leaves senesce and die, leaving the stem and roots alive. In many trees, flowers and fruits also senesce. Deciduous trees shows this kind of senescence.

Significance of senescence:

Old and inefficient organs are replaced by young and developing leaves, buds, flowers and fruits.
Nutrients from older parts can be withdrawn to the younger plant parts.
Leaf fall in deciduous trees reduces transpiration loss which is essential for survival in winter when soil is frozen and roots cannot absorb water.
Leaf litter releases mineral nutrients available for cycling process.

ABSCISSION

Abscission is the shedding of leaves, fruits or flowers by a plant, usually due to change in hormonal balance.
In this process, a separation layer is formed within the region of attachment.
The middle lamella between certain cells in layer is often digested by polysaccharide- hydrolyzing enzymes, such as cellulase and pectinase. often, the entire cells disintegrates.
As a result, the abscission zone becomes the weakest point of attachment. Due to its weight, the leaf detaches itself from the plant and falls.
The exposed leaf scar on the stem becomes healed by the deposition of suberin.
Auxin and cytokinin retard while the abscisic acid and ethylene accelerate the process of abscission.
Types:

Normal abscission caused by senescence or ripening .
Abnormal abscission caused by stresses:higher or lower temperature, drought or flood, insects or diseases.
Physiological abscission caused by disorders in physiology itself:such as nutritional competition between vegetation and regeneration, sink and source.