Propagation - Sexual propagation and dormancy.pdf

Sexual propagation - Importance, Advantages and Disadvantages - Types of Dormancy - Methods of Enhancement of Seed Viability

1. Propagation
Sexual propagation -Importance, Advantages and
Disadvantages -Types of Dormancy -Methods of
Enhancement of Seed Viability
22CHOR11 - Fundamentals of Horticulture 2(1+1)
Dr. M. Kumaresan (Hort.)
Department of Horticulture
Vels Institute of Science, Technology & Advanced
Studies (VISTAS)
Pallavaram, Chennai, Tamil Nadu -600117

2. 1 = Sexual propagation 2 = Asexual propagation

Plant propagation
Propagation is a natural phenomenon in all plants
Plant propagation refers to the
Multiplication or perpetuation of individual or group of plants
Methods of propagation in various plant species can be
broadly grouped under two categories

3. Sexual Propagation
•Sexual propagation of
plants is by means of
seeds.
•Seeds are the fertilized
ovule - containing
embryos resulting from
the union of male and
female gametes during
fertilization.
•Embryo in the seed
gives rise to the new
plant during
germination.

4. Micro-sprogenesis
•Pollen produced in the anther, in the early stage of
development
•Anther consists of small group of meristematic cell.
•As it matures, four groups of microspore mother cells
develop with it.
•Each microscope mother cell divides meiotically to produce four microspores each of
which contains the haploid (n) number of chromosomes.
•Each microspore develops into a pollen grain alter undergoing the mitotic division
producing two nucleus.
•The two nuclei of the pollen grain are the ‘tube nucleus’ and the ‘generative nucleus’.
•The generative nucleus subsequently divides to produce two male gametes or sperm
cells before the pollen is shed.

5. Megasporogenesis
•Formation of a seed begins with the development of an ovule.
•In the young ovule, all the cells that compose the nucleus are identical.
•However, one of the nuclear cells, usually just below the epidermis near the top
differentiates from the surrounding cells, eventually forming the embryo sac
(megaspore) mother cells.
•Generally this is 2n or diploid tissue.
•Nucleus of this mother cell undergoes two successive meiotic divisions, forming a row
of four cells called ‘megaspores’.
•They are genetically ‘n’ or haploid cells.
•Generally, three of the four cells nearer the micropylar end disintegrate and the
remaining cell develops into the mature embryo sac.
•Nucleus of the remaining megaspore divides meiotically thrice to form the eight
nucleated mature embryo sac.
•Thus, each mature embryo sac contains eight nuclei (each n) consisting of one egg cell,
two synergid cells, three antipodal cells and one primary endosperm cells with two
nuclei.

6. •During pollination, pollen is transferred from the anther to the stigma
where it germinates and grows down the style until it reaches the embryo
sac.
•Pollen tube penetrates the tissue of the stigma, grows down the style and
enters the ovary usually through the micropyle.
•Tip of the pollen tube then ruptures and the sperm nuclei move towards
and fuse with the egg effecting fertilization and form the zygote (2n).
•Second sperm nucleus unites with the two polar nuclei in a double
fertilization forming the primary endosperm (3n).
•After fertilization of the ovule, the embryo and endosperm continue to
grow and differentiate, ultimately forming the seed.
Megasporogenesis

7. Apomixis
Embryos are produced not as a result of meiosis and fertilization
but by certain asexual processes. The occurrence of asexual
reproductive process in the place of the normal seed reproductive
process of reduction division and fertilization is known as
‘apomixis’
Plants which produce only apomictic embryos is known as
obligate apomicts
Plant produce both apomictic and sexual embryos are called
facultative apomicts

8. Types of apomixes
Recurrent apomixis
Non-Recurrent apomixis
Embryo develops directly from the
haploid egg cell or some other haploid
cells of the embryo cell
E.g: Solarium nigrum and Lilium sp.
Embryo develops from the diploid egg
cells or from the diploid cells of the
embryo sac without fertilization
E.g. Parthenium, Rubus, Malus, Allium.

9. Types of apomixes
Nuclear embryony or
adventitious embryony
Vegetative apomixis
Flowers in an inflorescence are replaced
by vegetative buds called bulbils which
sprout and produce new plants
Eg: Allium sativum, Agave, and
Dioscoria bulbifera
Embryo arises from a cell or a group of
cells either in the nucleus or in the
integuments and hence they are diploid
in nature having the genetic constitution
of its mother plant.
Eg: Citrus and certain varieties of mango

10. Significance of apomixes
Apomictic seedlings are identical with its mother plant, similar
to those propagated by vegetative means
Apomictic seedlings, when used as rootstocks, provide
uniformity to for scions when grafted
Apomictic seedlings are free from virus diseases

11. Polyembryony
Phenomenon in which more embryos are present within a single
seed is called polyembryony
It may result due to
(a) Nuclear embryony e.g. Citrus
(b)Development of more than on nucleus within the embryo sac leading
to multiple embryos e.g. Conifers.

12. Polyembryony
•Occurrence of polyembryony is widespread in all citrus species but the
number of embryos per seed varies from species to specie.
•On rough lemon, it varies from 3 to 5; in mango certain cultivars are reported
to be polyembryonic with the number of embryos ranging from 2 to 10, the
seedlings 1 to 7 and the germination per cent from 40 to 87.
• Polyembryonic seedlings can be identified from its true seedlings by their
uniformity and vigorous in growth.

14. Importance of Sexual Propagation
Seed propagation is necessary when the vegetative
propagation is unsuccessful or difficult or expensive
E.g.: Papaya
Seedlings are required in large number, seed propagation is
the only mean
E.g.: forest trees
The rootstocks used for grafting mango and sapota and for
budding peaches and plums are raised only by seed.

15. Advantages of Sexual Propagation
Plants raised by seeds are long - lived and hardy with deep roof system
Possibilities are there to obtain chance seedling the performance of which are better than their
parents; e.g., Mango variety (i) Chinna Suwarnarekha (ii) Mundappa
Seed propagation is necessary when vegetative propagation is unsuccessful o difficult or
expensive (Eg. Papaya)
Polyembryony produces true to type nucellar embryonic seedlings which could be used as
rootstock for uniform performance.
Eg. Mango- Vellaicolamban, Olour and Bappakai, Citrus all, Citrus spp. Except C.
grandis
Exploitation of hybrid vigour is possible only when the hybrids are multiplied in the first
instance through sexual propagation although subsequent fixing of heterosis is effected through
vegetative propagation, e.g., Co-1 sapota (Cricket Ball x Oval), PKM-1 mango
(Chinna suwarnarekha x Neelum) and PKM-2 mango (Neelum x mulgoa).

16. Disadvantages of Sexual Propagation
Progenies are not true to type and so they become inferior, because in the
commercial orchards it is necessary to have uniform growth and yielding
capacities besides, the quality attributes
Choice tree or any hybrid trees cannot be perpetuated true to type by seed
Seedlings have long juvenile period.
Oranges when raised by seedlings take 7 to 10 years for bearing while a budded
plant comes to bearing within 3-4 year
Seeds lose viability in a short period.
In crops like, citrus, cocoa and rubber the seeds must be sown afresh, i. e.,
immediately after extraction
It is not economical to handle larger trees, as less number of trees can be
accommodated per unit area and the cultural operations are difficult.

17. Dormancy in seeds
•If the seed can germinate immediately upon the absorption of water
without a barrier to germination, the embryo is said to be ‘quiescent or
‘non-dormant.
•But in certain cases the seeds do not germinate readily even when they are
provided with all conditions required for germination. Such seeds are
known as dormant seeds.
•Dormant seeds do not germinate because of any condition associated either
with the seeds itself or with existing environmental factors such as
temperature and moisture.

18. Types of Dormancy
Seed coat dormancy
In certain species of plants belonging to the families like
Leguminbceae, Malvaceae, Cannaceae, Convolvulaceae,
the seed coats or other tissues covering the embryo are hard and
are impermeable to soil and oxygen, thus preventing the
germination.
In certain species, the seed coats (e.g. pits of stone fruits or shells
of walnut or other nuts) are apparently permeable to water and
gases but they are so hard to resist the embryo expansion.
Hence, germination does not occur in such case

19. Types of Dormancy
Dormancy due to rudimentary embryos
Some plants shed their fruits before the embryo
within the seed has attained the maturity stage to
germinate
Such embryos require several weeks to several
months after harvest to attain its full maturity so
that it can germinate
E.g. Ilex sp, Pinus sp, Viburnum sp, Palms,
Orchids etc.,

20. Types of Dormancy
Dormancy due to chemical inhibitors
In certain species, specific chemical substances
that prevent germination occur in the seed coats,
endosperm or the embryo.
These are reduced or eliminated by leaching with
water or adsorption by soil

21. Types of Dormancy
Dormancy due to internal factors
It is due to physiologically dormant embryos.
In this case, the dormant embryos do not resume active growth
even though all environmental conditions are favorable, unless
the seeds are subject to moist, drilling. During this process, the
levels of endogenous growth promoting substances
(e.g. gibberellins and cytokinins) increase while the level of
growth inhibiting hormones (e.g. Abscissic acid decreases, thus
removing the block and permitting germination, e.g. freshly
harvested seeds of apple, pear, peaches, apricot, rose and grapes
do not germinate due to the above factor. They require ‘after
ripening’ during which the physiological changes occur in the
dormant seeds, permitting the germination to take place

22. Types of Dormancy
Double dormancy
Seeds of some species, E. g. Cercis
occidentalis- exhibit seed coat dormancy and
embryo dormancy

23. Pretreatment of seeds
It includes breaking or scratching the seed coats
mechanically to modify the hard or impervio0us seed
coat. This can be done easily by revolving the seeds in
drum lined with sand paper
Mechanical
scarification
Generally seeds will be soaked in hot water for a few
seconds and then soaked for 24 to 48 hours in cold
water which make the seed coat to get soften and wash
off the inhibitors. e. g. wattle seeds. In some case, the
seeds are soaked in running cold water for a period of
8-12 hours which help in removing the inhibitors,
e.g. beet foot
Soaking in
water

24. Pretreatment of seeds
Soaking the seeds for a few minutes (15-60seconds) in
concentrated hydrochloric acid or sulphuric acid modifies
the hard or impermeable seed covering. At the end of
treatment period, the seeds are washed to remove the
remnant acid. e.g. Mucana bracteata
Acid
treatment
During stratification, seeds are exposed to abundant
moisture, ample oxygen and a relatively cool
temperature. It consists of placing the seeds in a moist
medium of sand, peat or vermiculite and holding at a
temperature slightly above freezing. The time varies
between 1 to 4 months depending upon the type of
seeds. This permits the physiological changes within the
embryo to occur, e.g. Peaches
Cold
stratification

25. Pretreatment of seeds
It promotes the after ripening in certain seeds which are
dormant when freshly harvested. Freshly harvested seeds
of many annuals, and herbaceous plants fail to germinate
until after a period of dry storage. Such post-harvest
dormancy may last from few days to several months
Dry storage
Soaking in potassium nitrate (0.2%), gibberellic acid (200 to 500
ppm) or thiourea (0.2%) solution prior to sowing has been found
to stimulate germination of different kinds of seeds. For instance,
soaking of seeds for incidence, the soaking of seeds in gibberelic
acid stimulate the germination of many citrus species viz.,
Trifoliate orange, Rangpur lime, Sweet orange, Sour orange etc.
Cardamom seeds when presoaked for 10 minutes in 25% acetic
acid, 25% nitric acid or 50% hydrochloric acid show
improvement in the germination from 18 to more all the cases
Treatment
with
chemicals

26. Seed Viability and Longevity
•Seed viability means the presence of life in the embryo while longevity
refers to the length of time up to which the seeds will retain their
viability.
•Seeds have been broadly classified into two major groups’ viz., orthodox
and recalcitrant, based on longevity vis a vis seed moisture content and
response to drying.
•Orthodox seeds can be safely dried to low moisture content and the
storability of such seeds improves with the lowering of seed moisture.
•Seeds of most field crops, seasonal vegetables and flowers are orthodox in
nature on the other hand; many horticultural crops produce seeds which
lose viability; when dried to moisture contents below a critical level as
irreversible ultra-structural damages are caused to the seed. Such seeds are
known as recalcitrant seeds.

27. Seed Viability and Longevity
•Because of this factor, the longevity of horticultural seeds is relatively
shorter, ranging from few days to few months.
•Viability can be tested by germination test, excised embryo test and
tetrazolium test.
•Among them, the Tetrazolium test is more reliable and easy to do.
•One per cent aqueous solution of 2,3,5-triphenyl tetrazolium chloride
(pH 6-7) is taken in a petridish and water soaked seeds are placed in it
and kept in dark, warm place.
•A viable seed take red coloured stain while a non- viable seed remains
colourless

28. Seed Priming
•Seed priming is used to increase rate of germination and
uniformity
•Subsequently, the seeds are dried, distributed and planted in the
usual way
•Primed seed usually exhibit more rapid and uniform emergence
of seedlings from the soil compared to non-primed seed of the
same seed lot
•These differences are greatest under adverse environmental
conditions in the field
•E. g. Cold or hot soils

29. Types of Seed Priming
•Osmo priming: Soaking the seeds in osmotic solutions (mannitol or inorganic salts, or
polyethylene glycol).
•Halo priming: soaking the seeds in salt solutions
•Bio priming: soaking the seeds with biological agent’s solution like Bacteria,
Rhizobium etc.
•Solid priming: This consists of mixing seeds in an organic or inorganic carriers and
water for a particular period of time. The moisture content of the matric brought to a
level just below what is required for radicle protrusion. Seed water potential is
regulated by the matric potential of the seed and during priming water is held by
carrier of seeds can imbibe water form carrier till the equilibrium is reached.
•Matrix priming: mixing with moist solid particulate materials, such as exfoliated
vermiculite, diatomaceous earth or lignaceous shale.
•Hydro priming: controlled imbibition, i.e. the continuous or staged addition of a
limited amount of water, such as in ‘drum priming’, though hydro priming is also
used to mean imbibition in effectively unlimited water for a short period of time.

30. Seed Treatment
•Seed treatment refers to the application of fungicide, insecticide, or a
combination of both, to seeds so as to disinfect and disinfect them from seed-
borne or soil-borne pathogenic organisms and storage insects.
•It also refers to the subjecting of seeds to solar energy exposure, immersion in
conditioned water, etc.
•The seed treatment is done to achieve the following benefits.
Benefits of Seed Treatment:
1) Prevents spread of plant diseases
2) Protects seed from seed rot and seedling blights
3) Improves germination
4) Provides protection from storage insects
5) Controls soil insects.