Its good for the grade 12.

1. Seed germination :

2. It is the fundamental process by which the embryo in the viable dormant
seed becomes activated & begins to grow into a new seedling or baby plant
under favourable environmental conditions. During this process there is
emergence of radicle & plumule from the embryo of the seeds.
PROCESS OF SEED GERMINATION :
• During germination, the viable dormant seed take up water resulting
swelling & softening of the seed coat at an optimum temperature due to
the process of imbibition.
• It starts the growth process by activation of enzymes. Amylase, Lipase,
Proteases, GA activates some genes of embryonic cell to form these
enzymes.

3. • The seed activates its internal physiology & starts to respire &
produce proteins & metabolites the stored food. It is the lag phase of
seed germination.
• By rupturing the seed coat, radicle emerges to form a primary root.
• The seed starts absorbing underground water. After emerging of the
radicle & plumule, shoot starts growing upwards.
• In the final stage, the cells of the seed become metabolically active,
elongate & divide to give rise to the seedlings.

7. TYPES OF SEED GERMINATION : It is of two types :
a. Epigeal germination : The hypocotyl grows more rapidly so that
cotyledons are carried above the soil. It is common in dicot seeds &
absent in monocot seeds except onion & orchids.
b. Hypogeal germination : The epicotyl grows more rapidly so that
cotyledons remain buried into the soil. It is common in monocot
seeds & absent in dicot seeds except pea, mango, gram &
groundnut.
FACTORS AFFECTING SEED GERMINATION :

8. 1.External factors :
i. Water : The poor or additional supply of water affects the seed germination. It
activates the enzymes which digests the complex reserve food of the seeds. If
the water content of the seed goes below a critical level, the seeds fail to
germinate.
ii. Temperature : It affects the growth rate as well as the metabolism of the
seeds. Seed fails to germinate at very low & very high temperature. The
optimum temperature is 25 – 35 degree celcius for most tropic species.
iii. Oxygen : Germinating seeds respire vigorously & release energy required for
their growth. Therefore, deficiency of oxygen affects the seed germination.
iv. Light : Photoblastic seeds ie germination influenced by light required exposure
of light intensity in order to germinate.
v. Soil : Germination of seed is influenced by soil condition such as water holding
capacity, mineral composition & aeration of the soil.

9. 2.Internal factors :
i. Maturity of embryo : The seed can not germinate when it contains
immature embryo & germinates only after maturation of embryo.
ii. Viability : Usually seeds remain viable or living only for a particular
period. Viability of seed ranges from a few days (Oxalis) to more than
hundred years. Seeds germinate within the period of viability.
iii. Seed dormancy : This is the condition in which the seeds are prevented
from Germinating even under favourable condition.
Condition necessary for seed germination :
a. Water: It provides necessary hydration for all the vital activities of
protoplasm,, provides dissolved oxygen for the growing embryo, increases
seed permeability, softens the seed coat & facilitates the rupturing of seed
coat.

10. b. Oxygen : The seed can absorb oxygen from the pores of the soil, but
if the seed is buried too deep, it will be deprived of oxygen & seed can
not germinate.
c. Temperature : The average temperature for the seed germination is
25-30 degree celcius, at very low & high temperature seed fails to
germinate.
d. Light : It is one of the essential factor for seed germination. Many
seeds can not germinate in the absence of sunlight, while some seeds
can germinate even in the dark place.

11. Seed dormancy :
It can be defined as the state or condition in which seeds are prevented
from Germinating even under the favourable environmental condition
for germination. It is the resting period in the life of a plant during
which growth slows or appears to stop.
REASONS/CAUSES/FACTORS OF SEED GERMINATION :
• Light
• Temperature
• Hard seed coat
• Germination inhibitors

12. • Immaturity of the seed embryo
• Impermeability of seed coat to water
• Impermeability of seed coat to oxygen
• Mechanically resistant seed coat
• Presence of high concentrated solutes
• Light sensitivity of seeds
The seeds of tobacco tomato etc can germinate only when they
are exposed to light, such seeds are called positively photoblastic.
While in some other species, seeds are negatively photoblastic & thus
their seed germination is inhibited by light.

13. TYPES OF SEED DORMANCY :
I. Innate dormancy : It is the condition of seed which is incapable of
germination even if conditions suitable for seedling growth are supplied.
This inability to germinate may be due to in certain species to the
embryo, being immature at the time of dispersal.
II. Enforced dormancy : It is the condition of seed which is incapable of
germination due to an environmental resistant which includes, an
adequate amount of moisture, oxygen, light & a suitable temperature.
III. Induced dormancy : This type of seed dormancy occurs when the seed
has imbibed water, but has been placed under extremely unfavourable
conditions for germination. Finally, seed fails to germinate even under
more favourable conditions.

14. Methods of breaking dormancy :
1.Natural breaking of seed dormancy :
• Completion of the over ripening period
• Leaching of the inhibitors present in the seed coat
• Inactivation of the inhibitors by the supply of cold, heat and light.
• Leaching of the excess & highly concentrated solutes from the seeds
• Production of growth hormones which can neutralize the effect of
inhibitors.
2. Artificial breaking of seed dormancy :

15. • Action with hot water for termination of waxes, surface inhibitors etc.
• Rupturing of seed coats by chipping or threshing through machines.
• Exposure to heat, cold & light depending up on the type of seed dormancy.
IMPORTANCE OF SEED DORMANCY :
• Dormancy follows the storage of seeds for later use by man.
• Due to the dormancy of seeds, the food grains can be stored for a certain period of time without
decay. If seeds were not having dormant period, all the seeds would germinate immediately after
harvest.
• Dormancy enables the seeds to survive during unfavourable conditions even for a long period of
time & thus provides a continuous source of new plants, even when all the mature plants of the
area have been destroyed by natural calamities.

16. NOTE :
• The process of breaking or rupturing of hard seed coat of seeds, is
called Scarification
• The process of breaking seed dormancy caused by physiological
conditions like aeration of seeds, moistening of seeds, cold treatment
of seeds, is called, stratification.
• In some seeds, water & oxygen are unable to penetrate pycropyle
due to blockage of Cork cells, these seeds are shaken vigorously to
remove the plug which is called impaction.

17. Photoperiodism:
The phenomenon of physiological changes occurring in plants in response to
relative length of day & night, is called photoperiodism. It was coined by
Garner & Allard in 1920.Some plants need to be exposed to sunlight for a
particular duration of time to induce flowering. This is the reason behind the
seasonal blooming of flowers. It has significant role in :
• Control of vegetative trait
• Bud dormancy
• Most importantly in flowering
• Bulb formation
• Dark carbon fixation in CAM plants.

18. Classification of plants according to their
photoperiodic response :
a. Long day plants (LDPs) :The plants which require longer duration of day
light periods usually 14-16 hours & short dark periods for subsequent
flowering, are called long day plants. They are also called short night
plants. Day or light period is the critical factor. Eg, Radish, Barley, Wheat
Oat etc.The show flowering in summer & spring seasons.
b. Short day plants (SDPs) : The plants which require a relatively short
duration of day light periods usually 8-10 hours & continuous or
uninterrupted long dark or night periods usually 14-16 hours for
subsequent flowering, are called short day plants. They are also called
long night plants & night or dark period is critical factor. They show
flowering in winter season. Eg, Rice, potato, sunflower, soybean,
sugarcane etc.

19. c. Day neutral plants (DNPs) : The plants in which the flowering occurs
irrespective of the relative length of day & night for subsequent
flowering, are called day neutral plants. They show flowering in all
seasons. Eg, Tomato, cotton, cucumber etc.
d. Long-short day plants (LSDPs) : These plants are short day plants but
they must be exposed to long days during early periods of growth for
subsequent flowering. Eg, Bryophyllum.
e. Short – long day plants (SLDPs) :These plants are long day plants but
they must be exposed to short days during early periods of growth for
subsequent flowering. Eg, certain species of wheat & rye(secale).

20. Vernalization :
The low temperature treatment given to the seeds or plant for stimulating
flowering in them is called vernalization.A flowering hormone venalin is
formed after vernalization. It accelerates fo the ability of the flowering in
plants by exposing them to cold treatment. Cold treatment is given to
specially flower buds , seeds or seedlings.It shortens the vegetative phase &
hastens flowering in plants. It was given by T. D Lysenko I 1920. The stimulus
of vernalization is perceived only by meristematic cells, eg, shoot tip, embryo
tip, root apex, developing leaves etc. Low temperature required for the
vernalization is usually 0-5 degree celcius. Low temperature treatment
should not be immediately followed by very high temperature about 40
degree celcius, otherwise the effect of vernalization is lost, the phenomenon
is called de-vernalization.

21. Requirement of vernalization /factors
affecting vernalization :
a. Low temperature : Low temperature required for vernalization is 0-5
degree celcius which should not be followed by very high temperature
about 40 degree celcius, otherwise, the effect of vernalization is lost.
b. Time duration : It varies from few hours to few days.
c. Activity of dividing cells : Vernalization does not occur in dry seeds. The
seeds must be germinated so that they contain an active embryo. For
this, the seeds are moistened before exposing them to low temperature.
d. Water : Proper protoplasmic hydration is must for perceiving the
stimulus of vernalization.
e. Aerobic respiration & proper nourishment

22. Importance of vernalization :
• It increases yield in plants
• It provides resistance to cold & diseases
• It induces early flowering & reduces the vegetative phase of the
plants
• It also removes the wrinkle on the kernels of the triticale (wheat &
rye hybrid)
• It enables Biennial plants to behave like annual plants
• It is not only applicable to temperate plants but also to some tropical
plants. Eg, wheat, rice, millet, cotton etc.

23. Senescence :
Senescence or biological ageing is a process of gradual deterioration of
functional activities of cells, tissues or organs of plant body with age. It
involves an increase in death rate or decrease in fecundity with increasing
age. The death of plant or plant parts as a consequence of senescence, is
known as programmed cell death. The branch of botany which deals with
ageing, abscission & senescence, is called phytogerontology.
TYPES OF SENESCENCE :
• Monocarpic or whole body senescence
• Top senescence ie senescence of aerial shoots in perennial herbs
• Seasonal or deciduous leaf senescence

24. • Senescence of other organs ie branches, fruits, seeds.
• Senescence of cotyledonary leaves, floral organs.
• Cell senescence ie trichomes, tracheids, vessels, fibres etc.
• Sequential leaf senescence.
CONTROL OF SENESCENCE:
• Nutritional – input like fertilizers (mainly Nitrogen), water
• Environmental – stress condition
• Hormonal – Ethylene, cytokinin & ABA implicated in hormonal regulation
of senescence
• Ethylene & ABA enhance senescence while cytokinin delay senescence

25. Following changes occur in senescening
organs :
• Cells undergo reduction in their size
• The membrane bound sub-cellular inclusions disrupted
• Photosynthesis is reduced & starch content decreases in the cell
• Chromatin material changes its property & DNA molecule degenerate
• RNA content is decreased
• Protein synthesis is decreases & protein breakdown enhances
• Structural deterioration of membranes of cell organelles such as
chloroplast & mitochondria.
• Variation in growth hormone contents
• Breakdown of chlorophyll is accompanied by synthesis & accumulation of
anthocyanin pigments, thus leaf becomes yellowing.

26. Significance of senescence
• It maintains efficiency since the old & inefficient organs are replaced by
young efficient parts like leaves, buds, flowers & fruits etc.
• Short senescence is a mechanism to help the plants perennate during the
unfavourable periods.
• Top senescence helps the plant to survive during the unfavourable periods
• Deciduous senescence helps to reduce transpiration & thus enables the
plants to survive during unfavourable weather condition in winter.
• The senescent organs such as leaves, litter, twigs etc form the importance
source of humus & thus help to replenish nutrients in the soil.
• Assists in fruit dissemination (senescence makes them attractive to
animals).