Plant Growth and Development

KC MEENA

PRINCIPAL KV JAMALPUR

Characteristics of plant growth

• Indeterminate
• Meristem at certain regions causes growth
• Open form of growth
• Apical meristems cause primary growth, elongation
• Lateral meristems increase the girth of organs.

ZONES OF GROWTH

GROWTH RATES

• The increased growth per unit time is termed as growth rate
• GROWTH CAN BE OF TWO TYPES
• (a) Arithmetic (b) Geometric growth

Arithmetic

• A. In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The  simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate
• Mathematically, it is expressed as
• L_t = L₀ + rt�L_t = length at time ‘t’�L₀ = length at time ‘zero’�r = growth rate / elongation per unit time

Geometric growth

• In most systems, the initial growth is slow (lag phase), and it increases rapidly thereafter – at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so.
• The exponential growth can be expressed as
• W₁ = W₀ e^rt�W₁ = final size (weight, height, number etc.)�W₀ = initial size at the beginning of the period�r = growth rate�t = time of growth�e = base of natural logarithms
• Here, r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the final size of W₁depends on the initial size, W₀.

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GROWTH CURVE

• In arithmatic growth On plotting the length of the organ against time, a linear curve is obtained
• Sigmoid curve obtained in geomatric growth

Conditions for growth

• Nutrients
• Water
• Oxygen
• Temperature
• Light
• Mineral elements

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DIFFERENTIATION,

• 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 is termed as differentiation. During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm

DEDIFFERENTIATION AND REDIFFERENTIATION

• Plants show another interesting phenomenon. The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation. For example, formation of meristems – interfascicular cambium and cork cambium from fully differentiated parenchyma cells

DEVELOPMENT

• Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence.

Plasticity

• Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is alled plasticity, e.g., heterophylly in cotton, coriander and larkspur

Plant growth regulators (PGRs)

• Small and simple molecules
• Secreted minute quantities
• Influence physiological function of plants diverse chemical composition- (i) indol compound – IAA (ii) adenine derivative -kinetin (iii derivatives of carotenoids -ABA (iv) terpenes -GA (v) gases-ethylene
• plant growth promoters - auxins, gibberellins and cytokinins
• Plant growth Inhibitor – Absescic acid,Ethylene

Auxin

• DISCOVERY - Charles Darwin and his son Francis Darwin in  coleoptiles of canary grass (responded to unilateral  illumination by growing towards the light source (phototropism)).
• Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.

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Functions of Auxin

• Maintain Apical dominance
• Inhibits the growth of the lateral (axillary) buds
• Auxins also induce parthenocarpy, e.g., in tomatoes
• Control growth of plants
• Control cell division & differentiations
• Prevent formation of abscission layer, so prevent premature leaves, fruits drop.
• 2,4-D- weedicide kill dicot weeds

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Gibberellins (GA)

• DISCOVERY - The ‘bakane’ (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberella fujikuroi. E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filtrates of the fungus. The active substances were later identified as gibberellic acid.
• There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants

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Functions of Gibberellins

• Cause elongation of internodes
• Promote bolting in rosette plants
• Synthesise Hydrolases to digest and mobilise reserve food of seed to developing embryo.
• Delay senescence.
• Increase length of internode in sugarcane.

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Cytokinin

• F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Skoog and Miller, later identified and crystallised the cytokinesis promoting active substance that they termed kinetin.

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Functions of Cytokinin

• Growth in lateral buds into branches
• Overcome apical dominance
• promote adventitious shoot formation
• Produce new leaves and chloroplasts in leaves
• Promote nutrient metabolism
• Delay senescence

Abscisic acid

• During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical. It was named abscisic acid (ABA).
• Cytokinins have specific effects on cytokinesis, and were discovered as kinetin (a modified form of adenine, a purine) from the autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. Search for natural substances with cytokinin-like activities led to the isolation of zeatin from corn-kernels and coconut milk

Functions of Abscisic acid

• Induce seed dormancy due to which seed can withstand desiccation & unfavorable condition.
• Stimulate closure of stomata.
• Increase tolerance of plants to various stresses. (stress hormone)
• Stimulate formation of abscission layer & abscission of leaves, fruits etc..

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Ethylene

• Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR. 

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Functions of Ethylene

• Promote horizontal growth of seedlings & swelling of axis.
• Induce apical hook formation in dicot seedlings.
• Promote senescence and abscission of leaves, fruits and flowers.
• Promote root growth and root hair formation ,increase water absorption.
• Help in rapid growth of internode and petioles of deep water rice plants to help leaves above water level.

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PHOTOPERIODISM�

• defined as the developmental responses of plants to the relative lengths of light and dark periods.
• Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants
• In 1920, W. W. Garner and H. A. Allard published their discoveries on photoperiodism and felt it was the length of daylight that was critical,but it was later discovered that the length of the night was the controlling factor.
• ^{Based on photoperiodism plants are classified in to}
• ^{Long Day Plants}
• ^{Short Day Plants}
• ^{Day Neutral}

Long-day plants

• Long-day plants - Long-day plants flower when the night length falls below their critical photoperiod
• Some long-day obligate plants are:
• Carnation (Dianthus)
• Henbane (Hyoscyamus)
• Oat (Avena)
• Some long-day facultative plants are:
• Pea (Pisum sativum)
• Barley (Hordeum vulgare)
• Lettuce (Lactuca sativa

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Short-day plants

• Short-day plants flower when the night lengths exceed their critical photoperiod
• Some short-day facultative plants are:^[8]
• Hemp (Cannabis)
• Cotton (Gossypium)
• Rice

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Day-neutral plants�

• Day-neutral plants, do not initiate flowering based on photoperiodism.
• Instead, they may initiate flowering after attaining a certain overall developmental stage or age, or in response to alternative environmental stimuli, such as vernalisation (a period of low temperature)
•  cucumbers,
• roses, and
• tomatoes

VERNALISATION�

• Vernalization (from Latin vernus, "of the spring") is the acquisition of a plant's ability to flower in the spring by exposure to the prolonged cold of winter, or by an artificial equivalent.

• common in biennials and some perennial plants

eg. apple, cherry, pear, peaches

carrots, cabbage,

bulbs like tulip, onion

• a. cold storage is used to preserve (sets) bulbs during winter this causes the sets to flower and produce seed in spring
• b. to devernalize- sets are exposed to 27^oC for 2-3 weeks before planting

Temperature and time for vernalization of wheat

Types temperature range(℃ ) days

Spring wheat 5～15 5～8

Semi winter wheat 3 ～ 6 10～15

Winter wheat 0 ～3 40 ～45

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Seeds must imbibed water (50% of seed dry weight) and the germination process has been initiated and exposed to low temperature ( vernalized). The vernalized seeds are dried and stored.

- other plants in particular the biennial eg. cabbage, must reach a certain minimum size or age before they can be vernalized ie. whole plant

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• In general, the plant flowering need long day period and higher temperature after finishing vernalization, which induces flower differentiation

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Thanking you