1. Seed dormancy can occur due to hard seed coats, underdeveloped embryos, chemical inhibitors in seeds, or internal physiological factors in embryos. 2. Dormancy can be overcome through mechanical or chemical treatments like scarification, stratification, or gibberellic acid application. 3. Seeds have varying lifespans depending on moisture content - orthodox seeds can be dried while recalcitrant seeds lose viability when dried.

1. TYPES OF DORMANCY

2. 1.Seed coat dormancy
 In certain species of plants belonging
to the families like Leguminoceae,
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.

3.  In certain species, the seed coats (e.g.
piths 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.

4. 2.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
Eg. IIex sp, Pinus sp, Vibrunum sp,
palms and orchids.

5. 3.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 absorption by soil.

6. 4.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 favourable,
unless the seeds are subject to moist, chilling
treatments.
 During this process, the level of endogenous
growth promoting substances (e.g.,
gibberellins and cytokinins) increase while
the level of growth inhibiting hormones (e.g,
Abscissic acid) decreases.

7.  Thereby removing the block and
permitting germination.
 Eg. 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.

8. 5.Double dormancy
 Seeds of some species (e.g, Cercis
occidentals) exhibit seed coat
dormancy and embryo dormancy.

9. MEASURES TO OVERCOME
SEED DORMANCY

10. 1.Mechanical scarification
 It includes breaking or scratching the
seed coats mechanically to modify the
hard or impervious seed coats.
 This can be done easily by revolving
the seeds in a drum lined with sand
paper.

11. 2.Soaking in water
 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. Eg. 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, Beetroot.

12. 3.Acid treatment
 Soaking the seeds for a few minutes
(15-60 seconds) 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
remaining acid. Eg. Mucana bracteata.

13. 4.Cold stratification
 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. Eg. Peaches.

14. 5.Dry storage
 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.

15. 6.Treatment with chemicals
 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.

16.  For instance, soaking of seeds in
gibberellic acid stimulates 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 than 90% in all the
cases.

17. 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.

18.  The 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 damage
are caused to the seed.
 Such seeds are known as recalcitrant seeds.

19.  The longevity of Horticultural seeds is
relatively shorter, ranging from few days to
few months only as indicated below:
Species Longevity
Mango 80 days
Jackfruit 1 month
Mangoesteen 8 weeks
Cinnamon One week
Cocoa 4 months

20.  Viability can be tested by
 Germination tests
 Excised embryo test
 Tetrazolium test
 Among them, the tetrazolum test is
more reliable and easy to do.

21.  One per cent aqueous solution of 2,3,5-
triphenyl tetrazolium chloride (pH 6-7) is
taken in a petridish.
 Water soaked seeds are placed in it and
kept in dark, warm place.
 A viable seed takes red coloured stain
while a non viable seed remains
colourless

22. Techniques of seed propagation

23. Testing the purity
 It is done to find out whether seed is
true to name and is adulterated with
weed seeds and other inert matters.

24. Pretreatment of seeds
 Special treatments are necessary for
seeds of many plants to germinate
even when conditions appear to be
favourable.

25. Seed sowing
 The seeds may be sown in raised beds
in open or sown in containers like seed
pan or seed boxes.
 The soil in the bed must be porous and
light; may be made by incorporating
one part of loamy soil, one part of sand
and two parts of finely sifted cattle
manure.

26.  The seed pans may be of 10 cm in height
and 30 to 45 cm in diameter and the seed
boxes have a 45 cm length and 10 cm
depth.
 These may be filled with a good soil
mixture.
 Proper spacing and maintaining optimum
moisture level in the soil media are
essential to produce healthy seedlings.

27. APOMIXIS
 In some species, 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.
 Such seedlings plants produced in this
manner are known as apomicts.

28.  Plants which produce only apomictic
embryos are known as obligate apomicts.
 Those produce both apomictic and sexual
embryos are called facultative apomicts.
 There are four type of apomixes reported
in horticultural crops viz.

29. 1.Recurrent apomixis
 The embryo develops from the diploid
egg cells or from the diploid cells of the
embryo sac without fertilization.
 The egg has the normal diploid number
of chromosomes as that of the mother
plant Eg. Parthenium, Rubus, Malus,
Allium.

30. 2.Non-Recurrent apomixis
 The embryo develops directly from the
haploid egg cell or some other haploid
cells of the embryo sac and hence
haploid plants are produced.
 It occurs in Solanum nigrum and Lilium
sp.

31. 3.Nucellar embryony or adventitious
embryony
 The embryo arises from a cell or a group
of cells either in the nucleus or in the
integuments.
 Hence they are diploid in nature having
the genetic constitution of its mother
plant.
 It differs from recurrent apomixis in that
such embryos develop outside the
embryo sac besides a normal embryo
developing from the embryo sac Eg.
Citrus and certain varieties of Mango.

32. 4.Vegetative apomixis
 In some species, the flowers in an
inflorescence are replaced by
vegetative buds called bulbils.
 Which sprout and produce new plants
while still on mother plants Eg. Allium
sativum, Agave, Furcreae and
Dioscorea bulbifera.

33. Significance of Apomixis
 Apomictic seedlings are identical with
its mother plant, similar to those
propagated by vegetative means.
 Such seedlings, when used as
rootstocks, provide uniformity to the
scions when grafted.
 Apomictic seedlings are free from virus
diseases.

34. POLYEMBRYONY
 The phenomenon in which more embryos
are present within a single seed is called
polyembryony.
 It may result due to
 Nucellar embryony Eg., Citrus
 Development of more than one nucleus
within the embryo sac (in addition to
the egg embryo during the early stages
of development) leading to multiple
embryos Eg. Conifers.

35.  Occurrence of polyembryony is
widespread in all citrus species except
Citrus grandis (Pummelo).
 The number of embryos per seed varies
from species to species.
 In Rough Lemon it varies from 3 to 5.

36.  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.
 The greater vigour is probably due to the
elimination of viruses.

37. The End