of an organ or its parts. The growth is followed by development.
⚫ Development includes all the changes that an organism goes through
during its lifecycle from germination of the seed to the senescence.
⚫ The development of plants is controlled by intrinsic and extrinsic
factors.
⚫Intrinsic factors include both intracellular (genetic) or intercellular
factors (chemicals such as plant growth Hormones)
⚫ Extrinsic factors include light, temperature, water, oxygen, nutrition
etc.
Seed germination
The fundamental process by which different plant species grow from a
single seed into a complete plants is called germination.
⚫The germination brings about the sprouting of a seedling from a seed
of an angiosperm or gymnosperm.
Process of seed germination
It is completed in following steps:
1. Imbibition: In this step, the seeds take up water rapidly and
this results in swelling and softening of the seed coat at
favorable temperature. This stage is called imbibition.
2. Activation of enzymes: This is a lag phase of seed germination.
In this step, the absorbed water activates the enzymes present
inside the seed that starts the growth phase in the embryo.
3. Growth: As the rate of respiration increases, the seed ruptures
(breaks). Then the radicle emerges to form a primary root,
whereas then plumule develops into a shoot.
4. Morphogenesis: In the last step, the cell of the seeds become
metabolically active, elongates and divides to give rise to the seedling. Slowly the small leaves called foliage leaves sprout from shoot ends. After this stage the plant start to produce own food by photosynthesis.
Conditions required for seed germination:
1. Water: It provides dissolved oxygen for the growing embryo,
softens the seed coats and increase the seed permeability.
2. Oxygen: O2
found in the soil particles is utilized by
germinating seed for respiration and metabolic activities.
3. Temperature: An optimum temperature of 25-30° C is
required for a seed to germinate.
4. Light or darkness: Seed germinates well in darkness.
Factors affecting seed germination
a. Water: The poor supply of water affects the seed germination
negatively.
b. Temperature: A temperature too below optimum range slows
down the seed germination due to fungal growth and too high
temperature also affects the germination.
c. Oxygen: Deficiency of oxygen affects the respiration in
germinating seed and reduces the energy releasing during
respiration.
Dormancy in plants
• The condition in which seeds are prevented from germinating even under the favorable condition for germination is known as dormancy.
• In most of the plants, it is caused by had seed coat which is impermeable to water, gases or other nutrients.
• The major causes for seed dormancy are light, temperature, hard seed coat, germination inhibitors, immaturity of seed embryo.
• In dormancy the metabolic activity of plants either stops or reduced.
Types of seed dormancy
a. Innate dormancy: The dormancy in which seeds are incapable of
germinating even if suitable conditions are maintained may be due
to immaturity of embryo is known as innate dormancy.
b. Enforced dormancy: The dormancy in which seeds are incapable of
germinating due to the provided environmental control.e.g. enough
amount of moisture, oxygen, light and temperature.
c. Induced dormancy: The dormancy in which seeds are incapable of germinating even when the seed has imbibed water, but has been placed under the extremely unfavorable conditions for germination.
Methods of breaking seed dormancy
• Natural method: seed dormancy is broken naturally by:
i. Development of growth hormones to counter growth inhibitors,
ii Maturation and after-ripening of embryo,
iii. Weakening of impermeable and tough seed coats by microbial
action, abrasion, passage through digestive tract of animals, etc.
• Artificial method: seed dormancy is broken artificially by:
i. Rupturing of seed coat by chipping or threshing through machines
ii. Action with hot water
iii. Seed coats treated with conc. H2SO4
iv. Weakening the tough seed coats by applying hydraulic pressure
v. Exposure to heat, cold or light, depending upon the type of seed dormancy.
Photoperiodism
⚫ The response of plants to periods of relative length of light
and dark periods (day and night) is termed as photo-
periodism.
⚫It is used to determine flowering time of a plant.
⚫ The optimum exposure to light for getting response of plant (flowering) is known as critical duration or critical day length
⚫ The critical duration in plants varies from species to species
⚫ Critical duration of almost all plants tends to fall within a
range of 12—14 hours.
⚫ Based on the critical duration, the plants are grouped int
three categories:
1. Long day plants (LDP): The plants which require the exposure to light more than the critical duration are known as long day plants.e.g. spinach, radish, sugar beet, onion etc.
2. Short day plants (SDP): The plants which require the exposure to light
less than the critical duration before the flowering is started are known
as long day plants.e.g. tobacco, soyabean, rice, wheat, sunflower etc.
3. Day neutral plants (DNP): The plants in which there is no correlation
between exposure to light duration an induction of flowering response;
such plants are called day-neutral plants.e.g. tomato, pea, potato, beans
etc
Importance of Photo-periodism
⚫ It helps to determine the season for a particular plant to flower.
⚫It helps to keep some plants in vegetative growth for longer periods
to get high yield of tuber, rhizome eg. Potato
Importance of vernalization
(i) Vernalization can help in shortening the vegetative period of plants and bring about early flowering. It is not only applicable to temperate plants but also to some tropical plants, e.g., Wheat, Rice, Millets,Cotton,
(ii) It increases yield, resistance to cold and diseases.
(iii) Kernel wrinkles of Triticale can be removed by vernalization.
Senescence in plants
• The gradual destructing process that naturally terminate the
functional characteristics of plants with age is called as senescence and the plants or plant organs at this stage are called as senescent.
• It is also known as aging in plants.
• Senescence occurs due to the deposition of waste material.
• It is controlled by plant’s own genetic programme, so it is also called Programmed Cell death (PCD)
Types of Senescence
There are four types of senescence pattern:
1. Overall senescence : In some plants the whole plant is affected and dies after flowering and producing seeds. This is also called whole plant senescence. Example-annual plants like rice, wheat,
beans, and tomato
2. Top Senescence: This type of senescence occurs in perennial
herbs where senescence occurs only in the above ground parts
while the root system and underground system remains alive.
3. Deciduous Senescence: This type of senescence takes place in woody deciduous plants. Here senescence occurs in all the leaves simultaneously but the stem and root system remains alive.
4. Progressive Senescence: This is characterized by gradual progression of senescence and death of leaves from the base upwards as the plant grows.
Effects of senescence on plants
• It increases respiration in plants declining
photosynthesis and breaking the macromolecules.
• It also disintegrates chloroplast white is a site for photosynthesis along with other cell organelles.
• Concentration of growth promoting hormones declines due to senescence.
• Expression of senescence associated genes (SAGs) increases which encode hydrolytic enzymes that degrade proteins and other nutrient in plants.
• It degrades chlorophylls an brilliant (bright yellow) colors are developed in leaves of plants.
Plant movement
The capacity of plants to change their position with respect to the
external or internal stimuli is known as plant movement.
Plant growth movement can be grouped into two types:
1. Nastic movement: The movement of plant in response to an
external stimulus in which the direction of response is not determined by direction of stimulus is known as nastic movement.
⚫It is non directional movement of plants in response to a stimulus
(temperature, light, nutrient, gravity etc)
⚫Nastic movement can be further of following types:
i. Epinasty (Opening movement): It is the downward curvature of the leaves because more growth takes place on the upper. e.g. bending down of heavy flower, the old leaf of fern are erect due to epinastic movement.
ii. Hyponasty (Closing movement): It is an upward bending of the
leaves or other parts of plant caused by the increases growth of their lower surface.e. g. Young leaves of ferns are circinately coiled.
iii. Nyctinasty: The movement of plant at night or dark.
iv. Photonasty: It is a movement in response of light. . E. g. leaves of oxalis are opened during day and closed during night.
v. Chemonasty: It is a movement in response of chemicals.
Vi. Hydronasty: It is a response to water.
Vii. Thermonasty: It is a movement in response of temperature.e.g.Flowers of Crocus open in higher temperature and closed in lower temperature.
viii. Seismonasty: It is a movement in response of shock or touch.e.g. In Mimosa pudica (Touch me not plant) the leaflets are sensitive to touch and fold upward in very short time
ix. Geonasty: it is a response to gravity.
ix. Thigmonasty (Haptonastic): This type of movement is stimulated by the
touch E.g. touch of insect in Drosera (Sundew)
2. Trophic movement: The growth movement induced by due to external
stimuli are called trophic movements.
• The movement of stem, petiole, floral axis on the basis of external factors like
light, gravity, water, chemicals are examples of trophic movement.
• These are of following types:
1. Phototropism: The movement of parts of plant in response to light is called
phototropism. E. g. Roots are negative phototrophic as they grow away from light whereas stem are positive phototrophic as they bend towards light.
2. Geotropism: The movement of plant organ in response to gravity is called
Geotropism. E. g. Root shows positive geotropism and stem shows negative
geotropism.
3. Chemotropism: The movement of parts of plant in response to chemicals is
called chemotropism. E.g. Movement of pollen tube towards ovary.
4. Hydrotropism: Movement of parts of plant in response to water is called
hydrotropism. E.g. Roots grow towards the water in soil. So they show
positive hydrotropism
5. Thigmotropism: This type of movement occurs in response to touch or contact.
E.g. The tendrils of pea plant around support.
Homework
• Write differences between from book
1. trophic movement and nastic movement
2. Phototropism and photoperiodism
3. growth and development
The end
---------Happy learning