3. Seed viability is the ability of the embryo to germinate
and is affected by a number of different conditions
(or)
Seed viability is the capability of plant structure (seed,
cuttings etc.) to show living properties like
germination and growth.
( or )
The degree to which the seed is alive (metabolically
active)
4. SEED VIABILITY
Viability is highest at the point of physiological maturity
and then gradually declines
Average life span of a seed is 10 to 15 years.
Some are very short-lived. e.g. willow (< 1 week)
Some are very long-lived. e.g. mimosa 221 years
Conditions are very important for longevity
Cold, dry, anaerobic conditions
These are the conditions which are maintained in seed
banks
8. Mechanical injury to seeds by threshing usually contributes to
immediate reduction in germination capacity and to an accelerated
loss of viability in storage.
Wahlen (1929) found that viability in clover seeds depend upon the
impermeability of the seed coat.
Alison et al (1990) studied poor seed emergence of pea, Vicia faba,
Phaseolus vulgaris, Chick pea, cow pea and long bean and found
associated with genotypes having white or unpigmented seed coat.
Genotypes with pigmented seed coats showed high levels of seed
viability .
9. Incomplete pollination results in a high proportion of
empty seeds.
They look like seeds, but there is no embryo, or one
which is only partly developed.
Most viable seeds are positioned around the outer
perimeter of the head.
As flowers and vegetables are further and further
hybridized to produce color and style which is more and
more remote from the natural parents, the viability of the
"germ" is reduced.
10. Mechanical damage during harvesting can severely
reduce the viability of some seeds, e.g.., certain large
seeded legumes.
Cereals are largely immune from mechanical injury,
presumably because of the protective outer structures,
the palea and lemma.
Small seeds tend to escape injury during harvest, and
seeds that are spherical tend to suffer less damage than
irregular or elongated shaped seeds.
11. Bacteria do not play a significant role in seed deterioration, since it
requires free water to grow.
If conditions were moist enough , this would encourage growth of
fungi which would suppress bacterial growth.
Two types of fungi invade seeds: 1) FIELD FUNGI
2) STORAGE FUNGI
FIELD FUNGI;
These invade seeds during their development on plants in field.
They need a high moisture content for growth.
A period of high rainfall at the time of harvest result in extensive
grain deterioration.
ex: Alternaria, Fusarium and Helminthosporium
12. STORAGE FUNGI:
They infest seeds only under storage conditions.
The major deleterious effects of storage fungi are to
Decrease viability
Cause discoloration
Produce mycotoxins
Cause heat production
13. Different seeds from the same seed pod will have
different degrees of viability in the embryo.
Some will germinate at once, or go into "deep dormancy",
or have insufficient viability to germinate at a later date.
Similarly, different seeds from the same seed pod will
have a "chemical" lock which will not degrade except for
the passage of time.
14. •Environmental variation during seed development
usually had a little effect on the viability of seeds unless
the ripening process is interrupted by premature
harvesting.
•Weathering of mature seeds in the field particularly in
conditions of excess moisture or freezing temperatures,
results in a product with inferior storage potential.
15.
16. The amount of moisture in the seeds is the most important factor
influencing seed viability during storage.
Generally if the seed moisture content increases storage life
decreases.
If seeds are kept at high moisture content the losses could be very
rapid due to mould growth
If they are kept at very low moisture content below 4% may also
damage seeds due to extreme desiccation or cause hard seededness in
some crops.
17. The life of a seed largely revolves around its moisture
content.
It is necessary to dry seeds to safe moisture contents.
The safe moisture content however depends upon
storage length, type of storage structure, kind / variety of
seed type of packing material used.
For cereals in ordinary storage conditions for 12-18
months, seed drying up to 10% moisture content appears
quite satisfactory.
However, for storage in sealed containers, drying upto
5-8 % moisture content depending upon particular kind
may be necessary.
18. Harringtons thumb rule on seed moisture content :
For every one per cent decrease in seed moisture content the life of seed
will be doubled. This is again hold good between 4- 12 °C.
Based on the tolerance and susceptibility of seeds towards moisture loss,
seeds are classified into
Orthodox – the seeds able to tolerate moisture loss and less seed moisture
favors the storage. i.e. decreased moisture increased storage period.
Eg. Rice, sorghum , and most of the cultivated species.
Recalcitrant – just opposite to the orthodox. Seeds not able to tolerate
moisture loss. Required high moisture for viability maintenance.
19. Temperature also plays an important role in life of seed.
Insects and moulds increase as temperature increases.
The higher the moisture content of the seeds the more they are
adversely affected by temperature.
Decreasing temperature and seed moisture is an effective means
of maintaining seed quality in storage.
The thumb rules by Harrington are useful measures for assessing
the effect of moisture and temperature on seed storage.
20. These rules are as follows.
1.For every decrease of 1% seed moisture content the life of the
seed doubles. This rule is applicable between moisture content
of 5-14%.
2. For every decrease of 5°C in storage temperature the life of
the seed doubles. This rules applies between 0°C to 50°C.
3. Good seed storage is achieved when the % of relative
humidity in storage environment and the storage temperature
in degrees Fahrenheit add up to one hundred but the
contribution from temperature should not exceed 50 ° F.
21. ABSORPTION ISOTHERM
Phase 1
Phase2
Phase 3
10 20 30 40 50 60 70 80 90 100
Safe for seed
storage
Not safe for storage
Short term
storage
18
16
14
12
10
8
6
4
2
Relative humidity (%)
Moisturecontent(%)
22. Roberts (1973) developed formulae to describe the
relationship between temperature seed Moisture content
and period of viability.
From these relationships it was possible to construct a
seed viability nomograph.
These nomograph are helpful in predicting the retention
of seed viability indefined storage environment for a
particular period or to determine combinations of
temperature and moisture content which will ensure the
retention of a desired level of seed viability for specific
period.
24. Relative humidity is the amount of H2O present in the
air at a given temperature in proportion to its maximum
water holding capacity.
Relative Humidity and temperature are the most
important factors determining the storage life of seeds.
Seeds attain a specific and characteristic moisture
content when subjected to given levels of atmospheric
humidities.
This characteristic moisture content called equilibrium
moisture content.
25. Equilibrium moisture content for a particular kind of
seed at a given Relative Humidity tends to increase as
temperature decreases.
Thus the maintenance of seed moisture content
during seed germination and storage is a function of
relative humidity and to a lesser extent of
temperature.
At equilibrium moisture content there is no net gain
or loss in seed moisture content.
26. C. Jensen (1941) treated seeds of cauliflower by exposing
them to a quartz-lamp and a sollux-lamp together, with full
strength for one hour at a distance of 1 m from the lamps.
Following this treatment the seeds were stored in glass
containers plugged with an ordinary cork or a paraffin cork.
The light-treated seeds maintained a higher germination %
over a period of eight years than untreated seeds.
Light treatment not only extended the life span of fresh
seeds, but also increase the germination capacity and vigour
of seeds,
27. Clayton (1931) found that treatment of vegetable seeds
with mercuric chloride, liquid organic mercurials , or hot
water, greatly shortened the life of vegetables.
Cotton-seed may be treated with organic mercury dusts
at any time after harvest and thereafter stored for
periods of up to seventeen months without injurious
effects resulting from the treatment and without
decreasing the beneficial effects of the treatment in
increasing seed viability and yield.(Miles, 1939, 1941)
28. Increase in O2
pressure decrease the period of
viability.
N2
and CO2
atmosphere will increase the storage
life of seeds.
Gaseous exchange is directly related to moisture
content of the seed and temperature at which it is
stored.
Gas durinG
storaGe
29. The activity of all these organisms can lead to damage resulting in
loss of viability.
Treated seeds with fungicides can be stored for longer periods.
Fumigation to control insects will also help in longer period of
storage.
Fumigants - methyl bromide, hydrogen cyanide, ethylene
dichloride, carbon tetra chloride, carbon disulphide naphthalene
and aluminimum phosphine.
Weevils, flour beetles, or borers are rarely active below 8% moisture
content and 18- 20°C, but are increasingly destructive as the
moisture content rises to 15% and temperature to 30-35°C.
