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Unit VIII (Botany)
Class B.Sc. III Year
Growth and Development
Growth:- It is a quantitative and irreversible change. It reflects an increase in size and
volume of an organism, e.g. the change in the size of a leaf or increase in the length and
breadth of stem can be referred to as growth.
Characteristic features of growth
1. Growth in plants is restricted to certain zones called meristems.
2. Two principal meristematic zones are found near the root and shoot tips (apices)
formed during embryo development called primary meristems. Another is vascular cambium
and intercalary meristem of grasses is secondary meristem except intrafascicular cambium of
dicot stems.
3. Cell division alone does not cause increase in size but the cellular products received
after division do grow and cause growth.
4. Some plant parts are determinate i.e. structures grow to a certain size and then stop
growth undergo senescence and death e.g. leaves, flowers, fruits.
5. Some plant parts are indeterminate e.g. vegetative roots and the stem. A bristle
cone pine that has been growing for 4000 years probably could yield a cutting that would
form roots at the base, producing another tree that might live for another 4000 years.
6. Entire plants are in a sense either determinate or indeterminate. The terms used as
monocarpic species and polycarpic species.
7. Monocarpic species flower once and die.
8. Polycarpic species flower, return to vegetative mode of growth and flower at least
once more before dying.
9. Monocarpic species are annuals (live only one year), perpetuating themselves only
as seeds.
10. Biennials eg Beta vulgaris complete their life cycle in two seasons.
11. Perennials live for many years (or more than two growing seasons). One of the
monocarpic but perennial plant is Agave americana and another Bamboo.
12. Polycarpic plants did not convert all of their vegetative meristems to determinate
ones. Woody perennials (shrubs and trees) utilize only some of their axillary buds for the
formation of flowers, keeping the terminal buds vegetative, alternatively terminal buds may
flower while axillary buds remain vegetative. Herbaceous perennial dicots e.g Convolvolus
arvensus, oxalis etc die each year except for one or more perrenating buds close to the soil.
Some form bulbs, corms, tubers, rhizomes and other underground structures.
13. Seed contains a miniature plant. The plumule, radicle and some of the primordial
leaves form the embryo. Plantlet is produced after the differentiation of the embryo.
Normally meristematic cells of the root and shoot apices give rise to other cells that divide to
form branch roots, leaves, axillary buds and stem and root tissues including vascular
cambium. Many axillary and apical meristems form flowers. In woody perennials lateral
meristems (cambium) produced secondary xylem and phloem each year, and periderm for
secondary lateral meristem (phellogen cork cambium) resulting in the growth in diameter of
stems and roots.
Phases of growth: Plant growth is completed in three phases:
1. Cell division
2. Cell elongation
3.Cell maturation
1. Cell division or formative phase :- In this phase new cells are formed from preexisting cells by mitotic divisions. All the cells are genetically similar, because in mitosis, the
chromosomes are replicated and divided equally. In higher plants it occurs in meristems or
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growing points. The cells of meristems are rich in protoplasm, with large nucleus and thin
cellulose wall.
2. Cell enlargement :- Cell division is followed by cell enlargement. This phase is
mainly responsible for growth of plant parts. Cell enlargement may occur in all directions e.g.
in isodiametric cells. In many parts cells enlargement takes place permanently in the linear
direction, so called phase of cell elongation e.g. conducting tissues and fibers. The cell
increase in size due to vacuolization and water absorption. Pattern of cellulose microfibril
deposition is important in controlling final shape of the cell.
3. Cell maturation :- The cells now undergo structural and physiological
differentiation and takes up a particular function e.g. photosynthesis by mesophyll cells,
conduction by sieve tube cells tc.
The time interval from the formative phase to maturation phase is called grand period
of growth.
Growth curve ( Course of growth ) :- If
growth rate is plotted against time a “ S” shaped
curve is obtained which is called sigmoid curve
or grand period curve. It consists of 3 parts.
1. Lag phase:- During this phase the
growth rate is quite slow because it is initial
stage of growth.
2. Log phase:- In this phase growth rate
is maximum, at this stage the cell division and
physiological processes are quite fast. This
phase is also called exponential phase.
3. Stationary phase:- During this period the growth is almost complete and becomes
static. It stops completely during this phase.
Plant Growth Regulators
Thiman (1948) designated the plant hormones by the term phytohormones in order to
distinguish them from animal hormones. He defined it as an organic compound produced
naturally in higher plants controlling growth or other physiological functions at a site remote
from its place of production and active in minute amounts.
Plant development was thought to be regulated by only five types of hormones,
auxins, gibberellins, cytokinins, ethylene and abscisic acid. However there is now compelling
evidence for the existence of plant steroid hormone the brassinosteroids that have a wide
range of morphological effects on plant development.
A variety of other signaling molecules that play role in resistance to pathogens and
defense against herbivores have also been identified including jasmonic acid, salicylic acid
and polypeptide systemin.
Auxin ( Gk: Auxein = to grow ):- The hormone auxin was discovered first through some
experiments by Charles Darwin and his son Francis around 1880.
Site of synthesis:-Auxins are mainly synthesized in the shoot apex, young leaves and
buds and are transported downwards in stems. The usual direction of auxin transport is
downward, but when added to the soil these are absorbed by the roots and carried upwards
along with transpiration stream to various plant organs. The down ward movement takes
place through the living phloem cells, where as the upward movement occurs through the
dead xylem elements.
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Skoog experimentally confirmed that tryptophan is the auxin precursor. Auxin name
has been given by Went. Went auxin is indole-3 acetic acid.
Auxins are chemically diverse. A common feature of all active auxins is a molecular
distance of about 0.5nm between a fractional positive charge on the aromatic ring and a
negatively charged carboxyl group.
The principal naturally occuring auxin is IAA ( Indole acetic acid ) present in all
plants. other natural auxins are 4 chloro Indole 3 – acetic acid found in pea, Indole 3 found in
mustard and corn.
Synthetic auxins include, Napthelene acetic acid, Phenyl acetic acid and 2, 4
dichlorophenoxy acetic acid, 2 methoxy 3, 6 dichloro benzoic acid.
Role:1. Growth :- Auxin promotes cell elongation in roots in its low concentration and in
stem under high concentration. Flower growth is also promoted under high concentration of
auxins. It also involves elongation of petioles, midribs. It induces cell enlargement by
increasing,
(i).Osmotic contents of cell.
(ii). Permeability of cell.
(iii).Decrease in wall pressure.
(iv). Increase in wall synthesis
Cambial activity is resumed in the favorable season.
Root formation is promoted by auxin in stem cuttings.
2. Apical dominance :- Apical bud exerts an inhibitory effect on the development of
lateral buds. If the apical bud is excised then lateral buds develop into new leafy stems.
3. Seed dormancy and germination :- Auxins do not effect the seed dormancy.
Seed germination is stimulated in presence of auxins.
4. Senescence :- Auxins delays leaf senescence.
5. Abscission :- Auxins inhibit abscission, prevents premature fruit drop in apples,
pears and citrus. Spray of 2,4,D, IAA, NAA has been used successfully to prevent the
defoliation of cabbage and cauliflower during harvesting.
6. Flowering :- In presence of auxins flowering is promoted as has been observed in
pine apple.
7. Auxin promotes fruit development.
8. Parthenocarpic fruits :- Auxins promote formation of seedless fruits. Fruit quality
is also improved by colouration, greatly increases red pigments.
9. Rate of respiration in increased in presence of auxins.
10. Increased resistance to frost damage :- Application of 2,4,5 trichlorophenoxy
acetic acid in apricot fruits before the onset of frost resulted in less damage than untreated
fruits.
Auxins were used as defoliants in Vietnam war by the code name Agent orange which
is 1:1mixture of 2,4,D and 2,4,5 T. This destroyed hundreds of square Kms. of Vietnam
forests.
Gibberellins:- In early part of 20th century Japanese farmers noted that some plants in rice
fields were taller, thinner and paler than the normal plants and were sometimes devoid of
fruits too. They named this disease as bakanee disease or foolish seedling disease, caused by
fungus Gibberella fujikuroi. Yabuta and Heyashi isolated this growth hormone promoting
substance in crystalline form and named it gibberellin. They are more than 100 reported from
both fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. However GA3
was one of the first gibberellins to be discovered and intensively studied. They are present in
all groups of plants including Bacteria.
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Site of synthesis:- Gibberellins are produced in young leaves around growing tips and
possibly in roots of some plants.
Gibberellins are tetracyclic diterpenoids.
Role:1. Growth :- Gibberellins induce elongation in the short internodes of the genetically
dwarf plant eg Pisum sativum. Involved in botting ( short elongation ) light inhibitory effect
on stem elongation can be reversed by the application of gibberellins.
2. Germination of dormant seeds and growth of dormant buds :- In Lettuce and
Tobacco seeds are light sensitive in these seeds light is compensated by the application of
gibberellins. It also substitutes for low temperatures long days and red light. Bud dormancy is
broken in potato tuber.
3. Senescence :- They delay aging in leaves and citrus fruits.
4. Abscission :- It has shown accelerated rate of abscission in explants of bean.
5. Sex expression :- They are capable of alternating the sex of flowers. Galun 1959
could induce maleness by foliar application of GA3 to the female flowers of cucumis.
6. Stimulation of enzymes :- GA stimulates the production of numerous enzymes,
notably - amylase in germinating cereal grains, which breaks down starch to sugar.
7. Fruit set :-Stimulation of fruit set by gibberellin has been observed in Malus
sylvestris.
Cytokinins:- Van overbeek et al 1941 found coconut milk as an active stimulant of cell
division. Later in 1955 Carlos Miller et al isolated a cell division stimulating factor from
yeast DNA. It was named as Kinetin. In subsequent years many other compounds promoting
cell division have been synthesized. Miller and his associates ( 1956 ) have grouped all such
compounds including kinetin under a generic name kinin. D.S. Leetham 1974 of Newzealand
proposed the term cytokinins for such substances. In 1964 the first natural cytokinin, zeatin
was described from maize seeds. Zeatin is the most natural occurring cytokinin.
Site of synthesis:- Fruits and endosperm are the richest source of kinins. Major site of
synthesis is he root. It is transported via xylem.
Cytokinins are Adenine of amino purine derivatives (6 furfuryl lamino purine)
Role :
1. Growth:- In wheat coleoptile cytokinins cause growth by cell elongation. These
promote cell division, Skoog while culturing the pith cells in the medium found that if high
cytokinin to auxin ratios only a shoot system first develops, then adventitious roots are
formed spontaneously from the stems while still in the callus, but if the cytokinin to auxin
ratio is lowered root formation is favoured. This formation of roots and shoots is called
organogenesis.
Cytokinins induce the formation of interfascicular cambium in plants.
These promote cell division and cell expansion in dicot cotyledons and in leaves.
2. Apical dominance: - These act antagonistically to auxins i.e promote lateral bud
growth.
3. Senescence:- Ctokinins delay the normal process of ageing in intact leaves also
delay fruit ripening. These delay the breakdown of chlorophyll in detached leaves. Cytokinins
promote chloroplast development and chlorophyll synthesis, by promoting grana formation.
4. Seed dormancy and germination:-Dormancy of many seeds, including light
sensitive lettuce, tobacco and barley is broken by the application of cytokinins and promote
germination of seeds.
5. Bud formation and bud number is increased in mosses and compensates light
treatment required for their development into new plants.
6. Cytokinins promote movement of nutrients.
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Ethylene : R.Gane 1934 proved that Ethylene a volatile gaseous compound is synthesized by
plants and is responsible for faster ripening
Site of synthesis:- It is formed in almost all plant parts roots, leaves, flowers, fruits and
seeds. In general meristematic and nodal regions are the most active in ethylene synthesis.
Excess of auxin also induces ethylene synthesis. Ethylene is also synthesized during leaf
abscission and flower senescence as well as during fruit ripening. Any type of wounding can
induce ethylene synthesis. Physiological stresses such as flooding, chilling, disease and
temperature and drought stress also induces synthesis of ethylene.
Ethylene is produced in plants from the amino acid Methionine.
Role:
1. Growth:- it inhibits the elongation of stem and root in longitudinal direction. This
inhibition of growth is associated with the radial enlargement of tissues but in submerged
plants it promotes stem and petiole elongation so that leaves can remain above water.
2. Seed dormancy and germination :- Ethylene is responsible for breaking the
dormancy in cereals thus promoting sed germination. It breaks bud dormancy. It promotes
bud sprouting in potato.
3. Apical dominance :- Ethylene inhibits the growth of lateral buds in pea seedlings
and thus causes apical dominance.
4. Senescence:- Ethylene promotes the senescence of leaves and flowers.
5. Abscission :- Abscisic acid formation in leaves under water stresses has been found
to be mediated through ethylene.
6. Root initiation :- In low concentrations ethylene helps in root initiation, growth of
lateral buds and root hairs.
7. Fruit ripening :- It hastens fruit ripening. It has been commercially exploited by
shipping Tomatoes, Bananas, Mangoes, Oranges and many fruits when green and then
gassed with ethylene before distributing to consumer.
8. Flowering :- It stimulates flowing in fruit apple.
9. Sex expression :- Genetically male plants of cannabis can be induced to promote
female flowers in presence of ethylene . The same happens in cucumber.
10. Epinasty :- It induces epinasty.
Abscisic acid or Abscissin II or Dormin:- Abscisic acid was first identified and
characterized chmically in 1963 by Fredrick T. Addicat and his coworkers in california. This
hormone is also called stress hormone because the production of hormone is stimulated by
draught, water logging and other adverse environmental conditions, it is also called dormin as
it induces dormancy in buds, underground stems and seeds.
Site of synthesis:- It is abundantly synthesized in chloroplasts from xanthophylls
(violaxanthin).. It is transported through xylem, phloem
Abscisic acid is 15 carbon terpenoid compound
Role:-.
1. Growth:- It stops mitosis in muscular cambium towards the approach of winter. It
inhibits the growth of stems and promotes root growth at low water potentials.
2. Dormancy:- In many species application of ABA is delays seed germination.
Similarly in many other plants the amount of ABA in seeds is decreased when seed
germinates. It also induces dormancy of buds towards the approach of winter. It promotes
dessication tolerance in embryo.
3. Senescence:- it accelerates senescence in leaves. It stops protein and RNA
synthesis in the leaves and hence stimulates their senescence.
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4. Abscission :- it promotes abscission of leaves and flowers.
5. Flowering:- In small quantities ABA is known to promote flowering in some short
day plants e.g. strawberry.
6. Parthenocarpy:- it has been seen to induce parthenocarpy in rose.
7. Rooting :- Rooting of stem cuttings is promoted in some cases by ABA. e.g., bean,
Ivy.
8. Closure of Stomata:- During drought conditions ABA induces stomata closure.
Thus reduces transpiration.
9. Controlled growth:- it counteracts the effect of all growth promoting hormones
and therefore keeps their activity under check.
Seed Dormancy
It is defined as the condition of the seed when it fails to germinate even though the
favourable environmental conditions are present. Wareing (1969) defined dormancy as any
phase in the life cycle of a plant in which the active growth is temporary suspended.
According to him that dormancy can be due to unfavorable environmental factors called
imposed dormancy or quiescence, while the dormancy due to conditions within the dormant
plant or organ is called innate dormancy or rest.
Causes of dormancy:- Dormancy can be due to single or combination of many different
factors. The various factors of the seed dormancy are.
1. Seed coat induced dormancy.
2. Embryo induced dormancy.
1. Seed coat induced dormancy :- Seed coat is formed from integuments of the ovule.
Chemically it is composed of complex mixture of polysaccharides, hemicelluloses, fats,
waxes and proteins. During seed ripening, the seed coat become dehydrated and forms a
tough and hard protective covering of the embryo. The seed coat induced dormancy may be
due to following causes
(i). Water impermeability:-The seed coat of many plant species are impermeable to
water e.g. Leguminosae, Malvaceae, Chenopodiaceae, Convolvolaceae, Solanaceae etc. In
the seeds permeability of the coat increases slowly in dry stage, due to action of
microorganisms like Bacteria and Fungi
(ii). Gas impermeability:- The seed coats of certain seeds are impermeable to gases
such as O2 and CO2 e.g. Xanthium. Since O2 is required for early respiratory activity in
germinating seeds, the seed fails to germinate. This is also found in many grasses. Such
dormancy of the seed is reduced by storing them for long time.
(iii). Mechanical resistance of seed coat to the growth of embryo:- The seeds of
some common weeds eg Allisma, Amaranthus and Capsella etc have such hard and tough
seed coats that it prevents any appreciable expansion of embryo. In these seeds water
penetrates the seed but enlargement of embryo is limited by mechanical strength of seed coat.
As long as the seed coat of Amaranthus are saturated with water, the dormancy in
Amaranthus may persist for about 30 years, however if seed coats become dry and then again
become saturated with water they are no longer able to resist the expansion of embryo. At
about 400C temperature these seeds germinate
2. Embryo induced dormancy:- It may be due to two reasons:
(i). Rudimentary and poorly developed embryo:- Rudimentary or immature
embryo are found in Fraxinus, Ginkgo, Orchidaceae, Orobanche etc at the time of shedding
of their seeds. So these seeds will need a time gap for the development of embryo, then only
they germinate.
(ii). Embryo fully developed but unable to resume growth :- In many species e.g.
seeds of Apple, Peach, Iris, Pears, Cherry, do not germinate even though their embryos are
fully developed and conditions for germination are favorable. Even if seed coats are removed
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in these seeds still these do not germinate. Dormancy of such seeds is due to physiological
conditions of embryo. In these seeds germination is induced if stored in moist well aerated
and low temperature conditions called stratification or after ripening.
3. Dormancy due to specific light requirement. Seeds of certain plants such as Nicotiana,
Lactuca need a specific light treatment for germination. In imbibed Lactuca sativa, seed
germination is stimulated by red light of 660nm wavelength while it is inhibited by far red
light of 730 nm wavelength.
In Rumex seeds germination promotes with increase in intensity and exposures to light
duration of more than 12 hours.These light sensitive seeds are called photoblastic seeds.
4. Dormancy due to germination inhibitors:- A numbe of chemicals such as Phenolics like
Alkaloids, Cyanogenic chemicals (Cyanide releasing substances),Coumarin (unsaturated
lactones) Ferulic acid and Abscisin II etc present in seed coat, endosperm, embryos, juice or
pulp of fruits (Tomatoes) inhibit seed germination. If these inhibitors are leached out of the
seed the germination of seeds takes place.
Advantages of seed dormancy
1. Dormancy allows the seed to remain in suspended animation without any harm
during drought, cold or high summer temperature.
2. The dormant seeds can remain alive in the soil for several years. They provide a
continuous source of new plants, even when all the mature plants of the area have died down
due to land slides, earth quake, floods or continued drought.
3. It helps the seed to get dispersed over long distances through unfavorable
environment.
4. The small seeds with impermeable seed coat belonging to edible fruits come out of
the alimentary canal of birds and other animals uninjured e.g. Guava.
5. Dormancy induced by the inhibitors present in seed coats is highly useful to desert
plants. The seeds germinate only after a good rainfall which dissolves the inhibitors. The
rainfall ensures the seed a proper supply of water during the germination.
6. It allows storage of seeds for latter use by animals and men.
Mthods of breaking seed dormancy:- The dormancy of seeds can be broken and dormant
seeds can be induced to germinate by one or combination of more than one methods
described below:
1. Scarification :- It involves softening or weakening of the seed coat which are
impermeable to H2O and O2. It is done by mechanical breaking of seed coat or by chemical
treatment.
Mechanical breaking of seed coat is called mechanical scarification done by shaking
(impaction) the seeds with sand or by scratching or nicking the seed coat with knife.
Chemical scarification is usually done by dipping the seed into strong acid like H2
SO4 or into organic solvents like acetone or alcohol or by boiling seed in water.
2. Stratification or (after ripening):- This method is employed in the seeds in which
dormancy is due to conditions of emryo.
In this process the seeds are exposed to well aeriated moist conditions under low
temperature ( 0 to 10C ) for weeks to months.
During the stratification some changes occur in seed which are necessary for seed
germination.
(i). The concentration of nitrogen and phosphorous are shifted to the various
parts of the seed.
(ii). Various constituent aminoacids, organic acids and enzymes are also
shifted.
(iii). The concentration of various growth regulators is changed.
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3. Alternating temperature:- In some seeds Poa pratensis the dormancy is broken
by the treatment of an alternating low and high temperatures. The difference between the
alternating temperatures should be more than 10 – 20 C. This method is done in the seeds
where embryo is immature.
Alternating temperature of 15 C and 25C is useful in breaking the dormancy of
photoblastic seeds like Rumex crispus.
4. Light:- Light sensitive seeds are called photoblastic. The dormancy of positive
photoblastic seeds can be broken by exposing them to red light ( 660 nm ). For red light ( 730
nm ) inhibits the seed germination indicating the involvement photoreversible pigment
phytochrome in the proces of seed germination. This pigment occurs in two forms one red
absorbing and other far red absorbing. Both these forms are photochemically interconvertible.
The red absorbing form ( PR ) is converted into far red ( PFR ) after absorbing the red light.
The far red form absorbs the far red light and is converted back into red absorbing form of the
pigment.
It is supposed that in positive photoblastic seeds, the far red absorbing form of the
pigment is stimulating to seed germination while red absorbing form is inhibitory to seed
germination.
5. Pressure :- Seed germination in certain plants like Melilotus alba and Medicago
sativa can be greatly improved after being subjected to hydraulic pressure of about 2000
atmosphere at 10C for about 15 – 20 minutes. The pressure changes the permeability of seed
coat to water resulting in to seed germination.
6. Growth regulators:- Growth regualtors are most widely used to hasten the
development of roots in cuttings and to increase the number of roos.
Kinetins and gibberellins have been used to induce germination in positively
photoblastic seeds like lettuce and tobacco etc. Besides a no. of chemcials such as KNO3,
thiourea and ethylene etc have also the capacity to promote seed germination.
Seed germination
Seed germination is the sprouting of a seed and growth of the embryo present inside
the seed into a seedling or young plant capable of independent existance.
Factors affecting the process of seed germination:- The factors are characterized as
external and internal.
External factors:1. Water:- A seed is generally in a very dehydrated condition containing only 6 – 15
% water. The optimum hydration for active cells is between 75 – 95 %. Therefore presence of
water is the pre requisite for a seed to germinate. Water causes softening of seed coats,
increase permeability of seeds transports gases, causes hydrolysis of reserved good and its
transport and allows embryo cells to grow in size.
2. Temperature:- Commonly the temperature range for seed germination is 5 – 40 C.
while the optimum temperature for seed germination lies between 25 – 30 C. however it
varies from species to species. Below and above this range seed germination is inhibited.
3. Light:- Seeds are both non photoblastic ( are not affected by light ) and
photoblastic ( effected by light ). Pea seed germination is hastened in presence of light. While
as seed germination of Onion, Lily, Phlox seeds is inhibited called negatively photoblastic
seeds. Red light promotes seed germination while as far red light inhibits the seed
germination.
4. Oxygen:- With the exception of few plants like Rice and Typha the seeds usually
require oxygen for their germination. It is because of this reason that soils are ploughed when
seeds are sown.
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5. Other factors:- Seeds of some aquatic plants germinate only in the acidic medium
eg Potomogeton while others germinate in alkaline medium. In many obligate parasites eg
Orobanche seed germination occurs only near the host, as it is stimulated by the exudes of
the host.
Internal factors:
1. Maturity of embryo :- The seed germinates only when the embryo is mature.
2. After ripening:- or stratification:- There are seeds which are stored under high
aeriation, low temperature and moisture conditions, then only they germinate.
3. Viability:- It varies from seed to seed e.g. oxalis seed remains viable for few
weeks, seed of Trifolium remains viable for over one hundred years. Lotus seeds have
viability of 1000 years. The seeds germinate only within the period of their viability.
4. Dormancy :- Many seeds become dormant as soon as they shed from the plant
because of no. of reason. As soon as the dormancy period is over then only these seeds
germinate.
5. Other factors:- includes sufficient amount of food and minerals, enzymes,
vitamins etc. In Echinops the seeds vary in weight from 8 – 18 mg. Only those seeds having
weight of 15 mg or more only germinate.
Process of seed germination
Physiologists speak of 4 stages in germination of the seed
1. Hydration or imbibition of seed.
2. The formation or activation of enzymes, leading to increased metabolic activity.
3. Emergence or growth of radical and plumule from the seed coat.
4. Subsequent growth of seedling.
1. Imbibition of water:- Seeds absorb water through micropyle and seed coat.
Imbibition causes hydration of embryo, proteins and other colloids, with this causes swelling
of seed and creates imbibition force which ruptures seed coat. Water also leads the growth of
plumule and radical due to turgor of their cells. It also increases rate of respiration.
Changes that occur in the seeds after the imbibition of water :- .
1. Degradation of stored food material :- Seeds store food in the form of fats,
proteins and starch. These can not be used directly by the embryo. So need to be degraded.
Fats are broken down by the enzyme lipase into fatty acids and glycerol. Fatty acids are
changed to acetyl CoA by B- oxidation. CoA through glyoxylate cycle produces succinate
which forms glucose. Starch is hydrolysed by amylase to maltose units which are converted
to glucose.
Different plants store proteins in different ways eg zein in maize, hordeins in barley
plants can not use those proteins for the growth of seeds. These are hydrolysed by proteases
into aminoacids, which are used for the synthesis of proteins of functional importance to the
embryo. Similarly in some cases nucleic acids are also degraded by nucleases to form
nucleotides which are nuetrilized as building blocks for the synthesis of DNA.
The degradation of food materials takes place in the endosperm or storage tissue
(cotyledons) and the energy and now materials formed are mobilised to the developing
embryo for its nourishment. Sugars are catabolised via respiration to yield ATP and NADH.
It has been found in some cases that initially that ATP is produced on breakdown of
phosphorous rich compounds phytin (Inositol 6 – phosphate) in seeds by an enzyme called
phytase.
2. Changes in hormonal level :- In the embryo of barley seeds, releases gibberellic
acid which acts on the aleurone layers and induces the synthesis of enzyme amylase protease
and lipase, which degrades starch lipids and proteins as discussed above.
In the seed some enzymes are present in the seed, while others are synthesized in
response to hormones. A no. of other cellular changes like changes in membrane organization
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e.g. ER, development of mitochondria are also triggered in the seed during germination. The
net result of these changes is to provide nutrition to the embryo in order to make it divide and
grow. The visible outcome of these events is the emergence of radical (roots) followed by the
plumule. (The shoot). The shoot grows and develops primary leaves and with chloroplasts the
plant becomes autotrophic and begins to live by establishing itself firmly by its roots on the
ground.
In some seeds, while germination cotyledons come out of the soil, turn green such a
type of seed germination is called epigeal germination e.g. Bean.
In some seeds while germination cotyledons remain underground. Such a type of seed
germination is known as hypogeal e.g. maize.
Senescence
The period from the seed germination to death is called the longevity or age or life
span, this varies from plant to plant. The period just before the death is called senescence.
This may be compared with old age in animal. Broadly the senescence can be of following
types.
1. Whole plant senescence :- This is found in monocorpic plants, which flower and
fruit only once in their life cycle. The plant may be annual (rice,) or biennial (henbane) or
perennial e.g. (Bamboo, Agave Americana)
2. Shoot senescence :- (Top senescence) The plants are perennial with underground
perennial structures like rhizomes, bulbs, corms etc. under favorable conditions the
underground parts give rise to aerial shoots, which manufacture food. The major part of
which is transferred to underground parts for storage. The aerial shoots now bear flowers and
fruits. After the maturity of fruits the aerial shoots undergo senescence and die e.g. Banana,
Ginger etc.
3. Sequential senescence:- This is found in most of the plants possessing growing points on
the tips. The older leaves and lateral organs like branches and reproductive organs show
senescence and die. Now growth (leaves, buds) continues near the tips ( Mango, Eucalyptus )
evergreen plants.
4. Simultaneous or synchronous Sensescence :- It is found in decidous plants
which shed all their leaves in autumn. Eg Apple, Dalbergia etc.
Regulation of Senescence:- Senescence is regulated by both internal and external
factors. External factors are plant hormones, length of the day temperature and nutrient
supply play an important role. The internal factors are size and age of plant, degree of
flowering and time of ripeness of fruits determine the onset of senescence. In one experiment,
where the normal time of senescence was 120 days, when mature fruits were removed from
the plant the time of senescence was delayed. It occurred after 140 days and when young
fruits and flowers were removed, the senescence was delayed to 160 to 180 days. This means
that senescence is initiated as soon as the process of reproduction is set in. The demand for
the nutrients increases, and are drained from the leaves, supply of these is lowered naturally
the system collapses.
Biotic factors also play a role in senescence e.g. due to an attack of mites, insects or
even parasitic fungi, the process is hastened.
Programmed cell death (specialized cell death):- The process whereby individual
cells activate an intrinsic senescence programmed is called programmed cell death (PCD).
Examples:
1. During the differentiation of xylem treachery elements the nuclei and chromatin
degrade and the cytoplasm disappears. These changes result from the activation of genes that
encode nucleases and proteases.
2. When a pathogenic organism infects a plant, signals from the pathogens cause the
plant cells at the site of infection to quickly accumulate high concentration of toxic phenolic
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compounds and die. The dead cells form an area called necrotic lesion. These necrotic lesions
isolate and prevent the infection from spreading to surrounding healthy tissues by
surrounding the pathogen with toxic and nutritionally depleted environment. This rapid
localized death of cells due to pathogen attack is called hypersensitive response.
Biochemical changes associated with senescence
1. Level of the chlorophyll content is decreased and CO2 fixation is reduced.
2. Rate of respiration is reduced so supply of ATP is reduced.
3. No. of degradative enzymes like proteases, ribonucleases, hydrolases,
chlorophylases are produced. Genes signaling the synthesis of these genes are called
senescence associated genes.(SAGS)
4. Levels of most leaf MRNA decline. The genes involved in the synthesis of these
RNA are called senescence down regulated genes (SDGS)
Abscission
The abscission of leaves occurs when at the base of a leaf a layer of cells is laid
which is called abscission layer. The cells of this layer have high activity of cellulase
(degrading cell wall). Another enzyme that increases abscission is polygalacturonase which
hydrolyses pectin, a major component of the middle
lamella region of the wall. So the cells get separated and
the leaves fall. Below this layer the plant makes a
protective layer which has lot of lignified cells when leaf
falls, this layer protects the tissue from desiccation.
Plant movements
Movement is a change in position. If the whole organism moves form one place to
another it is called movement of locomotion, however most plants are fixed, they show
movements in their parts called curvature movements e.g. bending, twisting and elongation.
Movements are produced as a result of some internal changes called autonomic or
autogenic movements or by external changes called paratonic or induced movements.
Plant movements seem to fall into two natural categories the Nastic movements and
Tropic movements (Tropisms).
Nastic movements:- These are triggered by an external environmental stimulus or by
an internal timing mechanism and in which the direction of the stimulus does not determine
the direction of the movement.
Important nastic movements involve leaves or the leaflets of compound leaves. An
upward bending of a leaf (or any organ) is called hyponasty downward bending is called
epinasty.
Greater growth on one side causes the organ to bend to the opposite side, e.g. circinate
coiling and opening of fern leaf or opening and closing of flower.
These movements occur also because of water movement in and out of the special cells
at the base of the petiole, blade or leaflet called motor cells and the group of such cells forms
an organ called a pulvinus. The various nastic movements found in plants are.
1. Nyctinasty or sleeping movements:- The nyctinastic movements are
caused by relative changes in cell sizes on opposite sides of the base of leaflets in a shoot
zone called pulvinus. It is believed that during early mornings the accumulation of K+ occur
in the lower side of the pulvinus, which results into translocation of water in the cells. These
cells swell and compression of the cells on the upper side results in that the leaves become
erect during night, the reverse reaction takes place, and thus the leaves hang down.
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2. Thigmonasty or seismonasty:- Nastic movements resulting from touch e.g.
Mimosa pudica upon being touched, its leaflets and leaves rapidly fold up only one leaflet
might be stimulated. Same stimulus moves throughout the plant. Leaflet movement is caused
by water transport out of certain motor cells of pulvinus.
How a stimulus can be transported in Mimosa there are two distant evident
mechanisms, electrical and other chemical.
The electrical response was first studied by J.C.Bose. The electrical fluctuation is an
action potential, which is a change in voltage. Action potentials in Mimosa are similar to
those occurring in animal nerve cells but much slower. They apparently travel through
parenchyma cells of xylem and phloem from one pulvinus to another pulvinus of other
leaflet.
The action potential will not pass through a pulvinus from one leaflet to another
unless the chemical response is also elicited. Chemical response was first reported by Ubaldo
Ricca 1916 . It is caused by substance that moves through the xylem vessels along with
transpiration stream called Riccas factor, still unidentified..
Tropic movements (Tropism). The movements occuring in response to an
unidirectional stimulus are called tropic movements or tropisms. Here direction of the
stimulus does determine the direction of the movement these movements may be of following
types.
1. Phototropism :- The tropic movement in response to light stimulus is
called phototropism. Some parts of plants like stems, branches, leaves, pedicels in flowers
move towards light and are called positively phototropic. In contrast roots and rhizomes
move against light and are called negatively phototropic.
(i). Diagiophototropism:- When the plant parts move towards the
duration perpendicular to the incident light it is called diaphototropism.
B- carotene and riboflavin are the photoreceptors which are involved in phototropism.
These are located either at the tip of coleoptiles as grasses or in hypocotyls as in seedlings of
dicots.
The phototropic response is due to unequal growth rate of two sides of an organ caused
by a symmetrical distribution of auxins. This results into accumulation of more auxin on the
darkened side than an illuminated side.
(ii). Plagiophototropic :- When the plant part move towards the
direction root perpendicular to the incident light, it is called plagiophototropism. e.g. shoots
of Ivy are both diaphototropic and plagiophototropic.
2. Geotropism :- The tropic movement in response to gravitational force is
called geotropism. Stems are negatively geotropic. In contrast the roots and rhizoids grow
towards the gravitational force and are called positive geotropic. The term was first coined by
Frank also called gravitropism and barytropism. It may be of following types:
(i). Positively orthogeotropic:- When the organ grows in the centre of
each eg roots and rhizomes.
(ii). Negatively orthogeotropic:- When the organ grows vertically
away from the earth e.g. stems and pneumatophores.
(iii). Diageogeotropic:- When the axis of the organ tends to lie at
right angles to the direction of gravitational force e.g. rhizomes and runners.
(iv). Plageogeotropic:- When the axis of the organ makes an angle
other than a right angle with the vertical e.g. side root of first order.
Geotropism is due to the asymmetric distribution of auxin in the plants. It is said that
gibberellins and abscissic acids also play role in geotropism.
3. Hydrotropism:- The tropic movements in response to water, it was first
demonstrated by Johnson ( 1829 ). Roots are positively hydrotropic. Hydrotropism is
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stronger than geotropism e.g. in case of shortage of water roots bend towards the sewage
pipes and other sources of water in direction to the stimulus of gravity.
4. Thigmotropism:- The tropic movement in response to touch stimulus is called
thigmotropism e.g. stem tendrils. It is perceived by tip.
5. Chemotropism :- It occurs in response to chemical structures. Growth of pollen
tube on stigma.
6. Thermotropism :- Tropic movement in response to temperature change on one
side of a plant organ called thermotropism. E.g. Tulipa sylvestris curves peduncles towards
sun throughout the day. It is a special type of phototropism.
Both nastic and tropic movements are often the result of differential growth.
Photoperiodism
It is defined as the response of a plant to the relative length of light and dark period.
Critical day length:- The photoperiod required to induce flowering is called critical
day length. It is 12 hours for Maryland mammoth variety of tobacco an 15.5 hours for
Xanthium strumarium.
On the basis of photoperiodic response to flowering plants have
been divided into following categories.
1. Short day plants:- These are the plants which require a photoperiod of less than 12
hours for flowering are called short day plants (qualitative SDPS), or their flowering is
accelerated by short days (quantitaive SDPS) e.g. Xanthium, Crysanthemum, Strawberry.
2. Long day plants:- These are the plants which require photoperiod of more than 12
hours for flowering (qualitative LDPS), or their flowering is accelerated by long days
(qualitative LDPS), e.g. Hibiscus, Barley, Hyocymus,
3. Photoneutral plans:- Photoperiod has now effect on the flowering of such plants
e.g. Lycopersicum esculentum, Capsicum etc.
4. Long short day plants:- These plants require first long photoperiod and then short
photoperiod for flowering e.g. Bryophyllum.
5. Short long day plants:- These plants require first short photoperiod and then long
photoperiod for flowering e.g. Trifolium repens.
Photoperiodic induction or photoperiodic after effect:- The condition in which suitable
cycles of light and dark persist in the plant and lead ultimately to flowering under normal
suitable condition is called photoperiodic induction or photo induction or photo periodic after
effect e.g. Soya bean requires only 10 short days to bring about the flowering. After this plant
could be placed under long day’s condition. Hammer and Bonner (1938) demonstrated in
Xanthium and Soya bean that effectiveness of light treatment in flowering depends on the age
of plant. They suggested that young immature leaves play no part in photo induction and the
reproductive growth occurs only when the plant has achieved a certain amount of vegetative
growth or maturing.
Photo periodic Perception :-Photoperiodic stimulus is picked up by the fully developed
leaves ( Knott 1934 ). Even one leaf or a part of it is sufficient for this purpose.
Flowering hormone florigen:- The chemical which induces flower formation has been
named as florigen. It has not yet been identified. The name has been given by Mikhail
chailakhyen. By grafting experiments it has been found that the stimulus of flowering can
pass from the induced plant to the non induced plant even if the latter is growing under
unfavorable photoperiods.
Dark periods :- The importance of the dark period on flowering was first demonstrated by
Hammer and Bonner 1938 in short day plant Xanthium he demonstrated that in Xanthium the
dark period is critical , if the dark period is interrupted with a brief exposure of light the plant
does not flower.
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Photoreceptor: Phytochrome is the primary photoreceptor in photoperiodism. It has been
observed that a night break of red light promoted flowering an dthe subsequent exposure to
far red light prevented this respose
Importance
1. Photoperiodism determines the season in which a particular plant shall came to
flower.
2. A plant can be made to flower throughout the year under green house conditions if
a favorable photoperiod is being provided to it.
3. The phenomenon has helped the plant breeders in plants which normally develop
flowers in different seasons.
4. Different varieties of a particular plant can be raised by manipulating the gene
controlling photoperiodism.
5. A proper knowledge of photoperiodism in relation to flowering is also highly useful
in laying out gardens, orchards etc.