Plant Growth and Development-
INTRODUCTION
- All cells of a plant are descendants of the zygote. Development of a mature plant from a zygote follows a precise and highly ordered succession events.
- Development is the sum of two processes. GROWTH and DIFFERENTIATION.
- During this process a complex body organisation is formed that produce roots, leaves, branches, flowers, fruit, seeds and eventually they die.
- The first step in the process of plant growth is seed germination. The seed germinates when favorable conditions for growth exist in the environment, in absence of favorable conditions they go into a period of suspended growth or rest, and resume metabolic activities on return of favorable conditions and growth takes place.
Plant Growth and Development-
GROWTH
- Growth is irreversible permanent increase in size of an organ of its parts or even an individual cell.
- It is one of the most fundamental and conspicuous characteristic of a living being.
- Growth is accompanied by metabolic process and occur at the expense of energy.
- Plants retain the capacity of unlimited growth throughout their life due to presence of meristem at certain locations in their body.
- This form of growth wherein new cells are always being added to the plant body by the activity of meristem and shoot apical meristem are responsible for primary growth, i.e elongation along their axis.
- In dicotyledonous plants and gymnosperms, the lateral meristems like vascular cambium and cork-cambium, which appear later in life and cause increase in girth of the organs called secondary growth.
- Growth is measurable:- At cellular level, it is principally a consequence of increase in amount of protoplasm. It is measured by a variety of parameters like-increase in fresh weight, dry weight, length, area, volume and cell number.
Plant Growth and development-Phases of Growth
- The period of growth is generally divided into three phases MERISTEMATIC, ELONGATION & MATURATION.
- The constantly dividing cells at root apex and shoot apex represent meristematic phase of growth.
- Cells proximal to the tip, represent phase of elongation.
- More proximal to the phase of elongation is phase of maturation.
Plant Growth and Development-Growth Rates
Increased growth per unit time is termed as growth rate. It can be arithmetic or geometrical .
(a) Arithmetic growth:- Following mitotic cell division, only one daughter cell continues to divide e.g. root elongating at a constant rate.
Mathematically,expressed as Lt= Lo+rt
Lt= Length at time ‘t’
Lo= Length at time ‘zero’
r= growth rate/elongation per unit time
(b) Geometrical growth:- In most systems, initial growth is slow (lag phase), it increases rapidly thereafter at an exponential rate (Log or exponential phase), as both progeny cells of mitolic cell division retain ability to divide and continue to do so. However with limited nutrient supply, growth slows down leading to stationary phase, giving a typical sigmoid or S-curve
- A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.
The exponential growth can be expressed as:- W1= Woer
W1 = final size (weight, height, number etc)
Wo = initial size at the beginning of period.
r= growth rate; t= time of growth
e = base of natural logarithms.
Here, r = relative growth rate and measure of ability of plant to produce new material called efficiency index.
QUANTITATIVE COMPARISONS BETWEEN GROWTH OF LIVING SYSTEM CAN BE MADE BY
(I) Measurement and comparison of total growth per unit time called ABSOLUTE GROWTH RATE.
(II) The growth of given system per unit time expressed on a common basis, e.g. per unit initial parameter is called RELATIVE GROWTH RATE.
CONDITIONS FOR GROWTH
- Water:- For cell enlargement, i.e. extension growth by turgidity. Wter also provides medium for enzymatic activities.
- Oxygen:- For aerobic respiration to get metabolic energy.
- Macro and Micro nutrients:- For synthesis of protoplasm.
- Temperature:- Optimum range for best growth.
- Light and Gravity:- Also affect certain phases /stage of growth.
PLASTICITY
- Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called Plasticity.
- Heterophyll in cotton, coriander and larkspur-leaves of the juvenile plant are different in shape from those in mature plants.
- Environmental heterophyll in buttercup is also an example of plasticity, which shows difference in shape of leaves produced in air and water.
DIFFERENTIATION
- The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions, this act leading to maturation in termed differentiation.
DE-DIFFERENTIATION
- Living differentiated cells that have lost the capacity to divide can regain capacity of division under certain conditions, this phenomenon is de-differentiation, e.g. formation of interfascicular and cork-cambium from parenchyma cells.
RE-DIFFERENTIATION
- De-differentiated meristems are able to divide and produce cells that once again lose capacity to divide but mature to perform specific functions, i.e. get re-differentiated. e.g. secondary xylem, secondary cortex, cork etc..
DEVELOPMENT
- Development includes all changes that an organism goes through during its life cycle from germination of seed to senescence.
PLANT GROWTH REGULATORS
- Broadly divided into two groups based on their functions in a living body.
- INVOLVED IN GROWTH PROMOTING ACTIVITIES :- (Like-cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation)
(i) Auxin (indole compounds) IAA
(ii) Gibberellins (GA3, terpenes)
(iii) Cytokinin (adenine derivatives) (N”- furfurylamino purine, kinetin)
2. INVOLVED IN GROWTH INHIBITING ACTIVITIES:- (Like-Response to wound and stresses of biotic and abiotic origin; dormancy and abscission). e.g. :- Abscisic acid (derivatives of carotenoids).
PLANT GROWTH REGULATORS
1. AUXIN:–
- CharlesDarwin and his son Francis Darwin studied photosynthesis in canary grass.
- F.W. Went isolated auxin from tips of coleoptiles of oat seedlings.
- First isolated from human urine.
- Produced by growing apices of stems and roots.
- IAA and IBA isolated from plants.
- NAA, 2,4-D are synthetic.
Physiological effects:–
- Initiate rooting in stem cultings.
- Promote flowering in pineapples.
- Help prevent fruit and leaf drop at early stages but promote abscission of older mature leaves and fruits.
- Apical dominance.
- Induce parthenocarpy in tomatoes.
- 2, 4-D kill dicot weeds. Used to prepare weed-free lawns.
- Auxin controls xylem differentiation and help in cells-division.
2. GIBBERELLINS:-
- Bakanae (foolish seedling) disease in rice was caused by fungal pathogen G. fujikuroi. Kurosawa helped understand it. Later the chemical Giberellic acid was identified.
- GA3 discovered first and remains intensively studied form.
- All GAs are acidic.
- Increase length of grapes stalk.
- Cause fruit like apples to elongate and improve shape.
- They delay SENESCENCE.
- Used to speed up malting process in brewing industry.
- Increases length of stem and yield by 20 tonnes per hectare in sugarcane.
- Spraying juvenile conifers with GAs hastens maturity period.
- Promotes BOLTING in beet, cabbages and many plants with rosette habit.
3. CYTOKININ:–
- Skoog and miller crystallized cytokinesis promoting active substance and termed it KINETIN a modified form of adenine from autoclaved herring sperm DNA. Kinetin does not occur naturally in plants.
- ZEATIN the naturally occurring cytokinin was isolated from corn-kernels and coconut milk.
- Sythesised in regions of rapid cell-division like root apices, developing shoot buds, young fruits etc.
- Helps produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation. Overcomes apical dominance.
- Promote nutrient mobilization.
- Helps delay LEAF SENESCENCE.
4. ETHYLENE:-
- COUSINS helped to identify Ethylene.
- Synthesised in large amounts by tissues undergoing sencescence and ripening fruits.
- Horizontal growth of seedlings, swelling of axis and apical hoop formation in dicot seedlings is influenced by ethylene.
- Promotes SENESCENCE and ABSCISSION in leaves and flowers.
- Effective in fruit ripening, by increasing rate of respiration called CLIMACTIC.
- Breaks seed and bud dormancy.
- Initiates germination in peanut seeds, sprouting of potatotubers.
- Promotes rapid intermode/petiole elongation in deep water rice plants.
- Promotes root growth and root hair formation.
- Initiates flowering and helps in synchronising fruit-set in pineapples.
- Induces flowering in MANGO.
- ETHEPHON is source of ethylene. It hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits, (thinning of cotton, cherry, walnut).
- Promotes female flowers in cucumbers, increasing yield.
5. ABSCISIC ACID:-
- Regulates abscission and dormancy.
- A general plant growth inhibitor and inhibitor of plant metabolism.
- Inhibits seed germination.
- Stimulates closure of stomata
- Plays important role in seed development, maturation and dormancy.
- By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
- In most situations, ABA acts as an antagonist to GAs.
PHOTOPERIODISM
Some plants require a periodic exposure to light to induce flowering. Such plants are able to measure duration of exposure to light.
(a) Long-day- plants:- Re light period exceeding well defined critical period.
(b) Short-day- plants:- Require light less than critical period.
(c) Day-neutral plants:- No such co- relation between exposure to light duration and induction of flowering response.
- Flowering in certain plants depends on combination of light and dark exposure and also their relative durations.
- This response of plants to periods of day/ night is termed PHOTOPERIODISM.
- The site of perception of light/dark duration are the leaves. A hypothesised hormonal substance is responsible for flowering.
SEED DORMANCY
- Controlled by ENDOGENOUS factors, i.e conditions within the seed itself.
- Impermeable and hard seed coat, presence of chemical inhibitors- ABA, phenolic acids,para- ascorbic acid and immature embryos cause seed dormancy.
- Man made measures like mechanical abrasions, using knives, sand paper or vigorous shaking can break dormancy.
- In nature microbial action, passage through digestive tract of animals can break dormancy.
- Chilling condition, use of gibberellic acid and nitrates can remove effect of inhibitory substances.
- Light and temperature can also break dormancy.
Notes:-
- Development in plants can be under intrinsic and extrinsic control. Intrinsic can be intra- cellular (GENETIC) or inter cellular (PGR).
- In plants growth and tissues of same meristem have different structure at maturity.
- PGRs can be having complimentary or antagonistic role, which can be individualistic or synrgistic.
VERNALISATION
- Vernalisation is either QUALITATIVE or QUANTITATIVE exposure to low temperature for flowering in some plants.
- It prevents PRECOCIOUS reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity.
- Wheat, barley, rye have winter and spring varieties.
- Subjecting biennials like sugarbeet, cabbages carrots to cold treatment stimulates a subsequent photoperiodic flowering response.
FAQS
Plant growth refers to an increase in size or mass, while plant development encompasses changes in form, structure, and function over the plant’s life cycle, from seed germination to senescence.
lant growth and development are influenced by various factors, including genetics, environmental conditions (such as light, temperature, water, and nutrients), hormones, and interactions with other organisms.
he main stages of plant development include germination, seedling establishment, vegetative growth (including stem and leaf development), reproductive growth (including flower formation and fruit development), and senescence (aging and death).
Plant hormones, also known as phytohormones, are chemical messengers that regulate various aspects of plant growth and development, including cell division, elongation, differentiation, and responses to environmental stimuli. Examples of plant hormones include auxins, cytokinins, gibberellins, abscisic acid, and ethylene.
Plants can exhibit various responses to environmental cues, such as phototropism (bending toward light), gravitropism (responding to gravity), thigmotropism (responding to touch), and photoperiodism (responding to day length), which influence their growth and development.
Plants can exhibit determinate growth, where growth ceases after reaching a certain size or developmental stage, or indeterminate growth, where growth continues throughout the plant’s life cycle. Indeterminate growth is common in perennial plants and leads to continuous branching and elongation.
Plants exhibit various adaptive mechanisms to cope with environmental conditions, such as modifying leaf shape or size to optimize light capture, adjusting root architecture to access water and nutrients, and altering flowering time in response to seasonal changes.
Cell division, particularly at meristematic regions called apical and lateral meristems, is essential for plant growth. Apical meristems promote primary growth (lengthening), while lateral meristems promote secondary growth (increasing girth or diameter).
Plants reproduce sexually through the formation of flowers, pollination, fertilization, and seed development. They can also reproduce asexually through methods such as vegetative propagation, where new plants develop from stems, roots, or leaves of a parent plant.
Understanding plant growth and development is crucial for agricultural practices, horticulture, forestry, conservation efforts, and plant breeding. It allows us to optimize crop yields, manipulate plant traits, and conserve and restore natural ecosystems.
FUNFACTS:-
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Plant Intelligence: Plants are capable of “sensing” their environment and making decisions about growth and development. For example, they can detect changes in light direction and adjust their growth accordingly, a process known as phototropism.
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Fastest Growing Plant: The title for the fastest-growing plant goes to bamboo. Some species of bamboo can grow up to 35 inches (90 cm) in a single day under optimal conditions!
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Ageless Plants: Unlike animals, which have a fixed lifespan, some plants exhibit indeterminate growth, meaning they can continue growing indefinitely under favorable conditions. For example, the “Pando” quaking aspen colony in Utah is estimated to be over 80,000 years old!
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Root Communication: Plants can communicate with each other through their root systems via chemical signals. This communication allows them to share resources, such as nutrients and water, and even warn each other about impending threats, such as insect attacks.
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Incredible Regeneration: Many plants have remarkable regenerative abilities. For instance, certain species of ferns can regrow from just a single leaf fragment, while some succulents can generate new plants from individual leaves.
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Seasonal Clocks: Plants have internal clocks that allow them to sense changes in day length, temperature, and other environmental cues. This enables them to synchronize their growth and development with seasonal changes, ensuring optimal timing for activities like flowering and fruiting.
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Resilient Seeds: Seeds have evolved various adaptations to survive harsh conditions. Some seeds can remain dormant for years, waiting for favorable conditions to germinate. Others have tough seed coats or chemical defenses to protect them from predators or pathogens.
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Plant Memory: Plants can remember past experiences and adjust their growth and development accordingly. This phenomenon, known as “plant memory” or “priming,” allows plants to adapt to recurring environmental stresses, such as drought or herbivory.
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Creeping Plants: Certain plants, such as creeping vines and ground covers, have specialized structures called adventitious roots or nodes that allow them to spread horizontally along the ground or climb vertical surfaces, covering vast areas over time.
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Mimicking Plants: Some plants have evolved to mimic the appearance or scent of other organisms to attract pollinators or deter herbivores. For example, certain orchids resemble female insects to attract male pollinators, while others emit odors similar to decaying flesh to attract pollinating flies.
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