Seed propagation, Pollination and fertilization, Seed formation and development, Seed Germination and its process, factors affecting seed germination, Dormancy and its types, Methods to overcome it.

1. PRINCIPLES AND PRACTICES OF
SEED PROPAGATION
Rahul pandey
R-2016-HRT-01M
Department of Horticulture
IAAS, T.U
Kirtipur, Kathmandu

2. SEED PROPAGATION
 Seed propagation, from an ecological standpoint
is the beginning of the next sexual generation.
 Growth of plant begins with the germination of
its propagules i.e. seed (Sadhu, 1989).
 Botanically, seed is a ripened ovule which
consists stored food material which is encased in
a protective covering called seed coat.
 Seeds, essentially to germinate needs three
conditions to be fulfilled for germination;
1. viability
2. appropriate environmental conditions and
3. state of overcomed dormancy (Khan, 1977).

3. SEED DEVELOPMENT PROCESS
 Sexual propagation starts with seed formation.
 Seed development requires sexual reproduction which
involves the fusion of male and female gamete to form
a zygote (2n) which develops into embryo (Singh,
2015).
 The male and female gametes are produced in
specialized structures of flowers, the anthers and
pistil in which male and female gametophytes are
produced respectively.
 The functions of the gametophytes are the production
of the sperm cells and the female cells, and their
union in fertilization.
 In flowering plants, the pollen grain is the male
gametophyte and the embryo sac is the female
gametophyte (Mascarenhas, 1989).

4. MICROSPOROGENESIS AND
MICROGAMETOGENESIS

5. MICROSPOROGENESIS AND
MICROGAMETOGENESIS (CONTD….)
 After the maturation of pollen, the microspore
undergoes mitotic division to produce a generative
nucleus and a tube nucleus; a binucleate stage(Borg
and Twell, 2010).
 Pollen on landing on receptive stigma interacts and
introduces next phase called fertilization. (Faegri and
Van der Pijl, 2013).
 The pollen grain germinates on the stigma upon
water absorption and hydration and produces a tube
(pollen tube) that moves down the style into the
embryo sac (female gametophyte).
 The generative nucleus undergoes a mitotic division
to produce two male gametes or sperms.
 The pollen tube finally enters the ovule through a
small opening, micropyle, and discharges two sperms
in the embryo sac(Cheung, 1996)

6. CONTD…
• The tube nucleus controls the growth of pollen
tube while the generative nucleus divides
mitotically to give rise to two sperm cells i.e. male
gametes (Tanaka, 1997).
• The elongation of pollen tubes is directed to reach
the ovary, the guided entrance of each pollen tube
into an ovule to penetrate the embryo sac
(Cheung, 1996).

7. MEGASPOROGENESIS AND
MEGAGAMETOGENESIS
 This process occurs in ovules that are present
inside the ovary.
 A single cell in each ovule differentiates into a
megaspore mother cell which further undergoes
meiosis producing four haploid mother cells
leaving only one functional and degenerating
others.
 Megaspore mother cell undergoes three meiotic
divisions to produce eight nuclei.
i. Three antipodal cells.
ii. Two polar nuclei
iii. One egg cell
iv. Two synergid cells

9. POLLINATION AND FERTILIZATION
 Pollen grains land on a stigma and germinate.
 Pollen tubes containing sperm grow through the style.
 Pollen tube enters each ovule to deliver the sperm to
an awaiting egg.
 The female synergid cells are closely associated with
the egg cell and function to attract the male nuclei to
egg cell for fertilization (Higashiyama et al., 1998).
 Herrero (2000) and Higashiyama et al. (2001)
reported that carbohydrate rich molucules secreted
by the ovule, a high calcium ion concentration in the
synergid cells are responsible for the attraction of
pollen tube to the micropyle and proper release of
sperm cells to the ovule.
 The synergid cells degenerate soon after sperm
release and allowing the sperm cell access for
fertilization.

10. CONTD..
Fig: Pollen-pistil interaction
Adapted from: (Cheung, 1996)

11. FERTILIZATION
 The sexual union of a male and female gamete is
fertilization.
 The next step after fertilization is the development of ovule
(containing the zygote and 3n central cell) into a seed.
 The 3n cell divides repeatedly and develops into a
endosperm, a nutrient rich mass of cells that provides
nourishment to the embryo until it develops to a stage
where it becomes self-supporting (seedling stage) (Acquaah,
2008).
 The mature ovule is the seed.
 The outer tissue of the ovules (the inner and outer
integuments) fuses and loses water to become seed coat
 The endosperm contains large amount if starch that serves
as food reserves to be broken down into glucose for use
during seed germination (Lopes and Larkins, 1993).

12. CONTD….
 The relationship between flower tissue and
subsequent parts of the fruit and seed for a
typical angiosperm species is outlined as follows:
1. Ovary grows into fruit tissue.
2. Ovule becomes the mature seed.
3. Embryo sac is the inner part of the seed.
4. Polar nuclei plus a generative nucleus become
the endosperm.
5. Egg cell fuses with one generative nucleus to
form embryo.
6. Integuments form the layers of the seed coat
(also called testa).

13. STAGES OF SEED DEVELOPMENT
Source: Adapted from Hartmann et al. (1990)

14. STAGES OF SEED DEVELOPMENT
 Stage I‐ Histodifferentiation
1. embryo and endosperm differentiate due to cell
division
2. increase in weight
3. embryo begins cotyledon stage
 Stage II- Cell expansion
1. According to Bewley (1994), this stage is
characterized by rapid cell enlargement due to
accumulation of food reserves . The major food
reserves include carbohydrates (starch), storage
proteins, and lipids (oils or fats). These reserve food
materials provides essential energy to the
germinating seeds.
2. increase in DNA, RNA, and protein synthesis

15. STAGES OF SEED DEVELOPMENT
(CONTD…)
 Stage III: Maturation and drying
1. At the end of stage II, seeds have reached
physiological maturity.
2. the seed has reached maximum dry weight through
reserve accumulation.
3. Seeds at physiological maturity can be removed
from the fruit and show high germination potential
as measured by seed viability and vigor (Miles et al.,
1988).
4. Depending upon the tolerance of water loss, seeds
can be divided into two groups; orthodox seeds and
recalcitrant seeds.
 Orthodox seeds: seeds tolerate maturation drying less
than 10% moisture

16. STAGES OF SEED DEVELOPMENT
(CONTD…)
 At this point three things can happen:
1. Seeds will germinate prematurely on the plant
without drying; vivipary.
2. Following maturation drying, seeds will be in a
quiescent condition.
3. Following maturation drying, seeds will be in a
dormant condition.

17. SEED DORMANCY
 Baskin and Baskin (2004) defined seed dormancy
as the inability of seed to germinate in a specified
period of time under any combination of normal
physical environmental factors (temperature,
light/dark, etc.) that otherwise is favourable for
its germination .
 A non-dormant seed is one that that has the
capacity to germinate over the widest range of
normal physical environmental factors
(temperature, light/dark, etc.) possible for the
genotype.
 The non-dormant seeds may also not germinate
due to the absence of these factors known as
quiescent stage.

18. CLASSIFICATION OF SEED DORMANCY
 Harper (1957) has broadly classified the seed
dormancy into three groups;
1. innate,
2. induced (equivalent to secondary dormancy) and
3. enforced dormancy (also includes quiescent
stage) which is the frequently used one.
 Vleeshouwers et al. (1995) and Thompson et al.
(2003) felt that his classification of dormancy is
too restricted to accommodate to all diversity of
seed dormancy.
 Baskin and Baskin (2004) have developed a
system of classification of dormancy which
classifies seed dormancy into following groups:

19. PHYSIOLOGICAL DORMANCY
 Also known as embryo dormancy.
 Physiological dormancy occurs when the embryo
requires a special treatment of cold temperature
treatment (stratification) to induce it to start
active growth.
 According to Acquaah (2008), a cold temperature
application of about 1-7 o C is commonly required
to break dormancy.
 Stratification in fridge takes about 3-4 months to
complete.

20. Types of physiological dormancy Characteristics
Deep dormancy 1. Excised embryo produces abnormal seedling
2. GA does not promote germination
3. Seeds require 3–4 months of cold
stratification to germinate
Intermediate 1. Excised embryo produces normal seedling
2. GA promotes germination in some (but not
all) species
3. Seeds require 2–3 months of cold
stratification for dormancy break
4. Dry storage can shorten the cold
stratification period
Non-deep 1. Excised embryo produces normal seedling
2. GA promotes germination
3. Depending on species, cold (c. 0–10°C) or
warm (15°C) stratification breaks dormancy
4. Seeds may after-ripen in dry storage
5. Scarification may promote germination

21. MORPHOLOGICAL DORMANCY
 In seeds with morphological dormancy (MD), the
embryo is small (underdeveloped) and differentiated,
i.e. cotyledon(s) and hypocotyl–radicle can be
distinguished (Baskin et al., 1998).
 Embryos in seeds with morphological dormancy are
not physiologically dormant and do not require a
dormancy-breaking pretreatment itself in order to
germinate.
 They simply need time to grow to full size and then
germinate (radicle protrusion).
 Under appropriate conditions, embryos in freshly
matured seeds begin to grow (elongate) within a
period of a few days to 1–2 weeks, and seeds
germinate within about 30 days.

22. MORPHOPHYSIOLOGICAL DORMANCY
 Seeds with this kind of dormancy have an
underdeveloped embryo with a physiological
component of dormancy.
 In order to germinate they require a dormancy-
breaking pretreatment.
 Embryo growth/radicle emergence requires a
considerably longer period of time than in seeds
with MD.
 There are eight known levels of MPD, based on
the protocol for seed dormancy break and
germination (Walck et al., 2000).

23. MORPHOPHYSIOLOGICAL DORMANCY
(BASKIN AND BASKIN, 1998; WALCK ET AL., 1999)
Temperature required
Types of MPD To break dormancy At the time of embryo
growth
GA3 overcomes dormancy
Non- deep simple warm or cold Warm Yes
Intermediate Warm + cold Warm Yes
Deep simple Warm + cold Warm yes/no
Deep simple epicotyl Warm + cold Warm yes/no
Deep simple double Cold+warm+cold Warm ??
Non-deep complex Cold Cold Yes
Intermediate complex Cold Cold Yes
Deep complex Cold Cold No

24. PHYSICAL DORMANCY
 Seeds with physical dormancyf fail to germinate
because seeds are impermeable to water.
 formation of water impermeable palisade cells.
 Physical dormancy is most often caused by a
modification of the seed coverings (seed coat or
pericarp) becoming hard, fibrous, or mucilaginous
during dehydration and ripening (Walck et al., 2000).
 For most seeds with physical dormancy, the outer
integument layer of the seed coat hardens and
becomes impervious to water.
 Deposition of water repellent materials: lignin,
suberin,cutin, and waxes (Young and Gallie, 2000).
 Seeds at such condition are termed as hard seeds.
 Mechanical or chemical scarification will help to
overcome this dormancy with non-deep physiological
dormancy.

25. COMBINATIONAL DORMANCY
 Seeds with PYD + PD.
 The seed coat is water impermeable and, in
addition, the embryo is physiologically dormant.
 The physiological component appears to be at the
non-deep level (Baskin and Baskin, 1998).
 Embryos of freshly matured seeds of some winter
annuals, e.g. Geranium (Geraniaceae) and
Trifolium (Fabaceae), have some conditional
dormancy and will come out of dormancy (after-
ripen) in dry storage.
 Require a few weeks of cold stratification, i.e.
after PY is broken and seeds imbibe water, before
they will germinate.

26. SEED GERMINATION AND EMERGENCE
 Seed germination is the resumption of active growth of
embryo that results in rupture of seed coat and emergence
of young plant.
 It is a complex process involving metabolic, respiratory,
and hormonal activities.
 Miles et al. (1988) states that a seed to germinate most
pose the following three conditions:
1. The seed must be viable i.e. the embryo must be alive and
capable of germination.
2. Seed should be subjected to appropriate conditions of
environment (moisture, oxygen, temperature and
sometimes light also).
3. Ozga et al. (1992) reports that a seed must overcome
primary dormancy. Primary dormancy is removed by the
interaction of seed with its environment. Watkins (1992)
states that if seeds are subjected to adverse environment
conditions, secondary dormancy may develop and delays
the germination.

27. PHASES OF SEED GERMINATION
 Phase I- Water imbibition
 Imbibition is characterized by an initial rapid
increase in water uptake by dry seeds ( less than 10%
moisture).
 Seed parts may wet differentially depending on their
contents.
 Starch is more hydrophobic than protein, and the
starchy endosperm will hydrate more slowly
compared to the protein-rich embryo.
 Upon imbibition the cells become turgid, the seed
increases in size and volume.
 There will be change in the sub cellular organization
of the embryo, endosperm and aleurone layer.
 Phytochrome becomes biologically active and triggers
the process of germination in some plant species.

28. PHASES OF SEED GERMINATION
 Phase II: lag phase of germination
 This period is characterized by highly active
physiological period of reduced or no water uptake
and high metabolic activity.
 Phase II includes flowing cellular activites:
 Mitochondria maturation: After rehydration of
seeds, mitochondria becomes enzymatically active and
ATP synthesis and respiration increases gradually.
 Protein synthesis: Although mRNA is present
within the dry seed, protein synthesis does not occur
until polysomes form after seed hydration. New
proteins are formed within hours of the completion of
imbibition. New protein synthesis during the lag
period is required for germination.

29. PHASES OF SEED GERMINATION
 Storage Reserve Metabolism: This is the
enzymatic breakdown of storage macromolecules
to produce substrates for energy production and
amino acids for new protein synthesis. Reserve
metabolism also produces osmotically active
solutes (like sucrose) that can lead to a change
in water potential of cells within the embryo in
preparation for radicle protrusion.
 Specific enzymes, including those responsible for
cell wall loosening in the embryo or tissues
surrounding the embryo, can be produced. The
hormones GA, auxin and cytokinin necessary for
germination are synthesized and get activated.

30. PHASES OF SEED GERMINATION
 Radicle Protrusion (Phase III)
 The first visible evidence of germination is
protrusion of the radicle.
 This is initially the result of cell enlargement
rather than cell division (Barton, 1943).
 soon after radicle elongation begins, cell division
can be detected in the radicle tip.

31. Adapted from Hartmann
et al. (1990)

33. SEEDLING EMERGENCE
 Seedling emergence begins with elongation of the
root and shoot meristems in the embryo axis,
followed by expansion of the seedling structures.
 The embryo consists of a shoot axis bearing one
or more cotyledons and a root axis (radicle) and
the growth occurs in the expense of storage
reserves.
 The enlarging root-shoot axis exerts internal
pressure on seed coat and results in its rupture.
 In case of monocots, the pressure developed
between cotyledons helps in rupturing of seed
coat and emergence of growing point.

34. SEEDLING EMERGENCE (CONTD…)
 It is the primary root that emerges first and supports
the establishment of seedling.
 Once growth begins, fresh and dry weight of the new
seedling plant increases, as storage tissue weight
decreases.
 The respiration rate, as measured by oxygen uptake,
increases steadily with advance in growth.
 Seed storage tissues eventually cease to be involved
in metabolic activities except in plants where
persistent cotyledons become active in
photosynthesis.
 Finally, the seedling establishes and the plant
becomes autotrophic.

36. FACTORS AFFECTING SEED GERMINATION
 Moisture :
 Seed most imbibe water to a certain degree for
germination process to be initiated. Moisture is
needed to initiate the enzymatic breakdown of food
reserves.
 Temperature
 The rates of biochemical reactions are controlled by
temperature.
 In seeds that require cold temperature treatment to
break dormancy, abscisic acid or other inhibitors are
broken down under low temperatures.
 When warm spring temperatures arise, levels of
endogenous gibberellins increase, resulting in
germination.

37. FACTORS AFFECTING SEED GERMINATION
(CONTD…)
 Generally, a warm seedbed is desirable for seed
germination.
 Warm-season crops (such as bean and squash) do
better at warmer temperatures (15 to 25°C ).
 Cool-season crops (such as cole crops) do well at
cooler temperatures (less than 10°C )
 The minimum temperature requirement is 3-5 oC
, while the optimum for most of the seeds is 15oC
to 35oC .
 Light
 The greater promotion of light on germination
occurs in red region (660nm) followed by an
inhibition zone in the far red region (730nm).

38. FACTORS AFFECTING SEED GERMINATION
(CONTD…)
 Soil conditions
 Saline conditions, fertilizers placed in close
contact and low oxygen concentration of soil
conditions retard the germination rate.
 Disease free
 One of the most common diseases of seedlings is
damping-off, a fungal attack caused especially by
Pythium ultimum and Rhizoctonia solani. These
fungi are active at warm temperatures (20 to
30°C) and thus are less of a problem when
germination occurs under cooler conditions.

39. CONCLUSION
 Flowering plants produce seed from which they can be
propagated.
 Seeds are products of the meiotic process (except apomictic
seeds, or seeds produced without fertilization) and hence
produce offspring that are not clonal or identical to the
mother plant.
 A seed contains the miniature plant (the embryo), which is
protected by the cotyledon(s).
 Some seeds fail to germinate even under optimal
conditions because of either physiological or structural
barriers that induce dormancy.
 Under such conditions, seeds may be scarified or stratified
to break the dormancy. Other chemical treatments may be
applied to enhance seed germination.
 Seeds require moisture, air, temperature, light, and a
disease-free environment for germination.
 Species differ in their requirements for germination.