Stress
physiology
Dormancy
is a special adaptation to overcome unfavorable condition. Dormancy is a
temporary suspension of physiological activity or growth, though conditions are
favourable for normal growth and developments. There are three types of
dormancy
- Imposed
dormancy due to the physical condition like deep sowing of seeds.
- Innate
Dormancy is by the nature of the material undergoing dormancy
- Secondary
or induced dormancy means artificial induction of dormancy
Bud dormancy
Induction of bud dormancy
Bud is unexpanded leaf primordial
covered by stipules or leaves. Mostly in woody plants, located in temperate
regions in late summer or autumn, the whole plant enters resting. In tropical
plants both dormant and active buds.
Reason for bud dormancy
- Due to
change in day length
- Role
of phytochrome in regulating the dormancy
- Exhaustion
of nutrients
- Presence
of chemical inhibitors like ABA
Removal of bud dormancy
- Treatment
of plant buds with winter chilling at 1-10oC for a period of 260 to 1000
hours
- Transferring
seeds from short day to long day condition
- Use of
chemicals like ethylene chlorhydrin, thiourea and dinitrophenol to break
the dormancy
- Providing
warm bath to dormant buds at 40oC
- Application
of gibberellins found to break the dormant buds
Seed Dormancy
Dormancy of seeds depend upon three
main factors
- Stage
and nature of development of seed embryo
- Seed
coat permeability to
- Imbibition
of water to activate enzymes and others so that stored food is utilized
and germination process initiated.
- Permeable
to oxygen to initiate respiration, so that more energy is supplied to
embryo
Mechanism of Seed dormancy
Two schools of thoughts
- Interference
with gaseous exchange by
seed coat and other structures even it may be a barrier for imbibition of
water.
- Seed
dormancy due to immature
embryo: Seeds are shed with immatured embryo which needs after
ripening period to complete development
- Hormonal
control:
Deficiency of some essential growth promoting substances such as
gibberellins result in seed dormancy. However, gibberellins found to
replace the chilling requirements thereby breaking the dormancy. Active
inhibitors like dormin or abscissin promote seed dormancy.
- Light: Red light found effective
against seed dormancy but infrared light induce dormancy
Overcoming seed dormancy
a). Hard seed coat:
- Seed
scarification- mechanical scarification with abrasive materials
- Soaking of seeds in water to soften the
seed coat
- Washing
seeds with sulphuric acid and then soaking in water to remove the acid
- Microbial
decomposition of seed coats
b). Immature embryo
- Storing
of seeds for some period of time until embryo completely develops
c). Chemical inhibitors
- Chilling
or heat treatment of seeds to inactivate/destroy the chemical inhibitors
responsible for seed dormancy
- Seed
stratification (placing seeds in different layers of soils) to wash out
the chemical inhibitors and also to stimulate synthesis of growth
promoters
d). Light sensitivity: use of red and infrared lights
according to the requirements
e). Chilling treatment done for temperate plants
Cold
tolerance
Low
temperature causes damages to plants in three ways
- Presences
of water in inter cellular space freezes at low temperature. As the
temperature goes down drastically, the water freezes into crystals. These
inter cellular crystals pulls the protoplasmic water of cells, leading to
dehydrations.
- Dehydration
of cells results in increased of salt concentration of the plant. Both the
dehydration and increase in salt concentration denature vital preoteins
and active enzymes.
- Ice
crystals enlarge in intercellular spaces and break the cellular membranes
Overcoming cold stress
- High
cell wall permeability
- Increased
free proteins and enzymes provide resistant to dehydration and low
temperature
Salt tolerance
Problems
- Most
important problem at high soil concentration is the water regulation.
Since the water concentration is high on outer environment, plant has to
pull water against negative water potential. So plants will suffer from
physiological dryness.
- Accumulation
of high concentration of Na+, SO4- and Cl- will be toxic to plants
- Absorption
of K+ becomes critical as Na+ competes better than it.
Overcoming salt tolerance
- Accumulation
of more solutes in cells further reduces the water potential, so that
water can be absorb ed from soil with ease.
- Synthesis
of large quantity of amino acid, Proline, other important amino acids and
organic compounds involved in osmoregulations.
- Salt
regulators/succulents tide over the high salt concentration by maintaining
low salt concentration
- Exudation
of excess salts to avoid accumulation of salts to toxic concentration
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