PLANT GROWTH REGULATORSR

PLANT GROWTH REGULATORSR

POINTS TO REMEMBER :

Definition of Growth

Growth is defined as “an irreversible permanent increase in size of an organ or its part or even of an individual cell.”

In other words, Growth is the most fundamental and conspicuous characteristics of living beings and is accompanied by several metabolic processes that occurs at the expense of energy. These metabolic processes may be catabolic or anabolic. In case of plants, seed germinates, develops into seedling and later it takes the shape of an adult plant are different stages of growth. Plants displays indefinite growth.

On the other hand, animals show uniform and fixed growth.  

Characteristics of Growth


Plant Growth is generally Indeterminate –Plants possess the ability of growth throughout their life. This is due to the presence of meristems at certain locations in their body and these meristems have the ability to divide and self –perpetuate.


Growth is Measurable – At cellular level, Growth is the consequence of increase in protoplasm and this increase is difficult to measure. Growth, in plants, is measured via different methods like increase in dry weight, volume, cell number, volume or increase in fresh weight.



The following diagram represents the location of root apical meristem, shoot apical meristem and vascular cambium. The arrows display the direction of growth of cells and organs. 



The Growth of Plants has three phases:


Formative Phase – Cell division is the basic event in the growth of plant. All cells are the result of division of pre-existing cells. Mitosis is the type of cell division that happens during growth and includes both quantitative and qualitative division of cells. This division is carried out in two steps – Division of Nucleus, which is referred as Karyokinesis and division of cytoplasm referred as Cytokinesis. In case of higher plants, an increase of cells is carried out in meristematic region, whereby some daughter cells retain this meristematic activity while some enter in the next phase of growth, i.e. the phase of cell enlargement.


Cell Enlargement and Cell Differentiation – At this stage, the size of tissues and organs is increased and this enlargement occurs by forming Protoplasm, Hydration (absorbing water), developing vacuoles and then adding new cell wall to make it permanent and thicker.


Cell Maturation – At this stage, the enlarged cells acquire specific size and forms as per their location and role. Thus, several cells are differentiated from simple and complex tissues which perform different functions.

Phases of growth :

• The period of growth is generally divided into three phases
  • Meristematic.
    
  • Elongation.
  • Maturation.
• Root apex and shoot apex represent the meristematic phase of growth.
• The cells of this region are rich in protoplasm, possesses large conspicuous nuclei.
• Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connection.
• The cells proximal to that region are the phase of elongation.
• Increased vacuolation, cell enlargement and new cell wall deposition are the characteristic of the cells in this phase.
• Further away from the zone of elongation is the phase of maturation.
• The cells of this zone attain their maximal size in terms of wall thickening and protoplasmic modifications.

 Experiment to Study Phases of Growth

In order to study the phases of Growth, Germinate few seeds of peas in moist saw dust. Select the couple of seedlings with 2 – 3 cm of length, wash them and blot the surface water. Then, mark the radicles from tip to base with 10 – 15 point at interval of 2 mm via water proof ink. After drying of ink, place those seedlings on moist blotting paper and allow them to grow for 1 – 2 days. Finally measure the intervals between the marks and we can clearly observe the different phases of growth.

Following diagram shows the phases of growth in root. A is the marked radicle of seedling at the beginning of experiment and B is the condition of seedling after 48 hours. We can clearly identify zone of cell formation, cell elongation, cell differentiation and zone of matured cells. 



“The increased growth per unit time is termed as Growth Rate. Thus, the rate of growth is expressed mathematically.” An organism can produce cells in several ways and display Geometric as well as Arithmetic Growth.Following diagram shows both types of growth in plants:






The following diagram displays the various stages of embryo development showing both Geometric and Arithmetic Phases. Here dark blue blocks represent the cells capable of division while light blue blocks represents the cells that have lost the capacity to divide:



Thus, in Arithmetic Growth, only one daughter cell continues to divide while other differentiates and matures. The following graph represents the length of an organ against time, whereby a linear curve is obtained. We can clearly observe the constant linear growth against time t.



In Mathematical Terms, Growth Rate is expressed as:

Lt = Lo + rt

Where, Lt = length at time “t”

Lo = length at time “zero”

r = growth rate or elongation per unit time.

Now focusing on Geometrical Growth, In majority of systems, Initial Growth is slow and is referred as lag phase. Then, it increases rapidly at an exponential rate referred as log phase or exponential phase. The growth of plant slows down in cases of limited nutrient supply and results in stationery phase. When we plot the growth against time, it results in S-Curve or Sigmoid Curve. Following graph represents an idealized sigmoid growth curve typical of cells in culture and many higher plants and plant organs.



The above sigmoid curve is the characteristic of living organism growing in natural environment and is typical for all cells, tissues and organs. The exponential growth in expressed as:

W1 = Woert

Where,

W1 = final size (weight, height, number etc.)

Wo = Initial size at the beginning of the period

r = relative growth rate and the measure of the ability of plant to produce new plant material

t = time of growth

e = base of natural logarithms  

Types of Growth

There are five types of Growth:


Primary and Secondary Growth: “The mitotic divisions of meristematic cells present at the root and shoot apex increases the length of the plant body. This is referred as Primary Growth and the Secondary meristem that results in an increase in diameter of the body of plant is called as Secondary Growth.”


Unlimited Growth: This is the stage, when root and shoot of plant continuously grow from germination stage to death and throughout the entire lifespan.


Limited Growth: This is the stage, when fruits, leaves and flowers stop growing after attaining certain size. It is also called determinate type of Growth.


Vegetative Growth: The Growth of Plant before flowering in called Vegetative Growth. This Growth includes producing of stems, leaves and branches.


Reproductive Growth: At this stage, plants start flowering, which is the reproductive part of the plant.   



Factors Affecting Plant Growth


External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by Temperature and Light.


Temperature plays important role in the growth of plants. The minimum, optimum and maximum temperature varies and from species to species. As the temperature increases above minimum, growth is accelerated until the optimum temperature is attained, when the growth gets slower and is completely retarded. Effect of duration for which a plant is exposed to certain temperature also varies amongst different species. For Example: The plant shows good growth when it is exposed to 86°F for a short duration and the same temperature has negative impact if maintained for longer duration.


Light also affect the growth and development of plant. Several factors of light like light intensity, duration of light and quality of light influences several physiological processes like movement of stomata, chlorophyll synthesis, temperature of aerial organs, formation of anthocyanin, absorption of minerals streaming of protoplasm and rate of transpiration. Intensity of light also influences plant growth and the variation in intensity has significant impact on growth pattern. Most ornamental plants and crops, such as Peas, Corn, Tobacco and Peas makes stocky and vigorous growth will full sun and thus, is also called “Sun Plant.”


Difference in wave length of light also effects the growth of plant. Several experiments have proved that plants that has full spectrum of visible light shows proper development and increase in dry weight. Plants grown in violet and blue light tend to dwarf, while plants in red light are taller and spindly.


Duration of light also affects the plant growth as it affects the rate of photosynthesis. For instance, during winters when days are short, the growth is very slow, while, it increases during summers when the days are longer.


The plants with lesser availability of oxygen show retarded growth while it is vice versa in the presence of ample of oxygen. It is important to note that plants in flooded areas, results in deficiency of soil aeration which on the other hand, results in poor plant growth.


Water is very important for plants and inadequate water results in poor growth. Plants grow well only in the presence of optimum water. Plants respond to deficiency of moisture as well. For instance, peppers, spinach and radishes wilt and cease to grow when the percentage of water in soil is lower.


Soil nutrients, their quantity and nature also affect the growth of plant. For Luxuriant Growth, it is important to have adequate amount of nutrients.




External Factors: The Growth of Plant primarily depends on habitat in which it is growing. Along with this, external factors also play an integral role in the growth of plants. It includes availability of Oxygen, Water and Nutrients followed by temperature and light.These factors include growth regulators, C/N ratio and genotype and genetic factor.


There are several classes of growth regulators. Some promote the growth like Auxins, Florigen, Cytokinins, Gibberellins, etc., while some are growth inhibitors like ethylene, abscisic and chlorocholine chloride.


The ratio of carbohydrates and nitrogen also govern the growth of plants. Presence of more carbohydrates as compared to nitrogen facilitates vegetative growth, fruiting and flowering while presence of more nitrogenous compounds results in poor vegetative growth.


Following diagram shows the percentage contribution of various factors in the growth of plants. According to it, the percentage of mineral particles is 45% and air & water is 25%.


Genotypes are responsible for controlling all the metabolic activities, growth and development of plant. Expression of genes in the correct sequence is controlled by two things, i.e. environment and genes. These genes are located in chromosomes and transcribe information to m-RNA that translates in enzyme and structural protein.

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Condition of growth :

• Water, oxygen and nutrients as very essential element for growth.
• Turgidity of cells helps in extension growth.
• Water also provides the medium for enzymatic activities needed for growth.
• Oxygen helps in releasing metabolic energy essential for growth activities.
• Nutrients are required by plants for synthesis of protoplasm and act as source of energy.

Differentiation, dedifferentiation and redifferentiation :

• The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.
• This act of maturation is termed as differentiation.
• During differentiation major changes takes place in their cell wall and protoplasm.
• Differentiated tracheary element cells loose their protoplasm, develop a very strong, elastic lignocellulosic secondary cell walls.
• The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain condition is dedifferentiation.
• Development of interfascicular cambium and cork cambium from fully differentiated parenchymatous cells is the example of dedifferentiation.
• Cells produced by the dedifferentiated tissues again loose the capacity to divide and mature to perform specific function is called redifferentiation.
  
  
  
  

PLANT GROWTH REGULATORS :
Characteristics :

• The plant growth regulators are small, simple molecules of diverse chemical composition.
• They could be:
  • Indole compounds (indole-3-acetic acid, IAA);
  • adenine derivatives (N6-furfurylamino purine, kinetin)
  • derivatives of carotenoids (abscisic acid,ABA)
  • terpenes (gibberellic acid, GA₂)
  • Gases (ethylene, C₂H₄)
• One group of PGRs are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed germination. These are called plant growth promoters, e.g. auxin, gibberellins and cytokinin.


• Another group of PGRs play important role in plant responses towards to wounds and stresses of biotic and abiotic origin. They involved in inhibitory responses like dormancy and abscission, e.g. abscisic acid.

 Discovery of plant growth regulators :

• Auxin was isolated by F.W. Went from tips of oat seedlings.


• The ‘bakane’ (foolish seedling) a disease of rice seedlings, was caused by a fungal pathogen Gibberalla fujikuroi.


• E. Kurosawa reported the appearance of the symptom of the disease in uninfected rice seedlings when treated with sterile filtrate of the fungus. The active substance was later identified as Gibberellic acid.


• Skoog and Miller identified and crystallized the cytokinesis promoting active substance that they termed as kinetin.


• During mid 1960s three different kinds of inhibitors purified, i.e. inhibitor-B abscission II and dormin. Later all the three proved to be chemically identical and named as Abscisic acid (ABA).


• Cousinsdiscovered a gaseous PGR called ethylenefrom ripened orange.

Physiological effect of plant growth regulators :

Auxin :

• The term auxin is applied to indole-3-acetic acid
• Generally produced by growing apices of the stems and roots.
• IAA and IBA have been isolated from plants.
• NAA and 2, 4-D (2, 4-dichlorophenoxyacetic acid) are synthetic auxin.
• Promote rooting in stem cutting.
• Promote flowering.
• Inhibit fruit and leaf drop at early stages.
• Promote abscission of older mature leaves and fruits.
• The growing apical bud inhibit the growth of lateral bud, the phenomenon is called apical dominance.
• Auxin induces parthenocarpy.
• Used as herbicides.
• Controls xylem differentiation.
• Promote cell division.
  

Gibberellins :

• Ability to cause an increase in length of axis is used to increase the length of grapes stalks.
• Gibberellins cause fruits like apple to elongate and improve its shape.
• Delay senescence
• GA3 is used to speed up the malting process in brewing industry.
• Gibberellins promote to increase length of stem in sugar cane.
• Promote early seed production.
• Promote bolting (internodes elongation) in beet, cabbages.

Cytokinins :

• Cytokinins have specific effects on cytokinesis.
• Zeatin isolated from corn-kernels and coconut milk.
• Promote cell division.
• Help to produce new leaves, chloroplast in leaves, lateral shoot growth
• Promote formation of adventitious shoot.
• Cytokinins help to overcome apical dominance.
• Promote nutrient mobilization.
• Delay senescence.

Ethylene :

• Ethylene is a simple gaseous PGR.
• Synthesized in the tissue undergoing senescence and ripening fruits.
• Promote horizontal growth of seedling.
• Promote swelling of axis and apical hook formation in dicot seedlings.
• Promote senescence and abscission of plant organs like leaf and flower.
• Increase rate of respiration during ripening of fruits, called respiratory climactic.
• Breaks seed and bud dormancy.
• Initiate germination.
• Promote rapid internodes elongation.
• Promote root growth and root hair formation.
• Used to initiate flowering and for synchronizing fruit-set.
• Induce flowering in mango.
• The source of ethylene is ethephon.
• Promote female flower in cucumbers thereby increasing the yield.

Abscisic acid :

• Regulates abscission and dormancy.
• Acts as general plant growth inhibitor and an inhibitor of plant metabolism.
• Inhibit seed germination.
• Stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses, hence called as stress hormone.
• Important role in seed development, maturation and dormancy.
• Inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth.
• Acts as antagonist to Gas.

PHOTOPERIODISM :

• Some plants require a periodic exposure to light to induce flowering.
• Response of plants in terms of day/night in relation to flowering is called photoperiodism.
• Long day plant: plant requires the exposure to light for a period exceeding critical period.
• Short day plant: plant requires the exposure to light for a period less than critical period.
• Day neutral plant: there is no such correlation between exposure to light duration and induction of flowering response.
• The site of perception of light/dark duration is the leaves.
  
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VERNALISATION :

• Vernalisation: There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature.
• It prevents precocious reproductive development late in the growing season.
• Vernalisation refers to the promotion of flowering by a period of low temperature.