﻿ Why is abscisic acid also known as stress hormone ? from Biology Plant Growth and Development Class 11 Assam Board

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Plant Growth and Development

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# Why is abscisic acid also known as stress hormone ?

Abscisic acid is known as the stress hormone because of the following reasons :

1. It helps plant to cope with adverse environmental conditions or stresses.

2. It causes temporary closure of stomata due to which there is reduction in the rate of transpiration. Therefore, it acts as antitranspirant and reduces water loss.

3. It acts as a general growth inhibtor and inhibitor of plant metabolism

4. It inhibits seed germination.

5. It induces dormancy to escape the unfavourable conditions.

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Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?

Different plant parts, tissues and cells grow at different rates in various stages of the life cycle of a plant. Moreover it is difficult to measure the increase in the protoplasm directly. Therefore, a number of parameters are taken into account to demonsstrate growth thrught out the life a flowering plants, so that a approximate growth rate.
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Describe briefly : (a) Arithmetic growth (b) Geometric growth (c) Sigmoid growth curve (d) Absolute and relative growth rates.

(a) Arithmetic growth : In this growth one daughter cell continues to divide after the mitosis while the other cell differentiates to perform specific functions and get matured.

Example : Elongation of root at a constant rate. When a graph is drawn of length against time ; linear curve is obtained.

Mathematically its expression is given below :

Lt = L0+rt

Here, Lt is length at time ‘t’, L0 is length at time 0 and r is the rate per unit time.

(b) Geometric growth : In this type of growth the initial growth is slow and is called the lag phase. After this, the growth is quite rapid and at an exponential rate. This phase is called the log or exponential phase. In this phase, both the daughter cells (formed after mitosis) continue to divide. The last phase marks a slowed down growth. This happens because of limited nutrient supply. This phase is called the stationary phase. The graph of the geometric growth gives a sigmoid curve.

The exponential growth can be mathematically represented as follows:

W1 = W0ert

Here, W1 = final size (weight, height, number etc.), W0 = initial size at the beginning of the period, r = growth rate, t = time of growth and e = base of natural logarithms

(c) Sigmoid growth curve : The S-shaped curve on graph; to show geometric growth is called the sigmoid growth curve. It is S shaped curve which is characteristic feature of living organism in natural environment. It consists of lag phase, log phase or exponential phase and stationary phase.

(d) Absolute and relative growth rates

Absolute growth rate : It is the measurement and comparison of total growth per unit time. When growth is measured in absolute terms, e.g. in terms of length or weight, it is called absolute growth.

Relative growth rate : It is the growth of given system per unit time expressed on common basis eg per unit initial parameter is relative growth rate. When growth is measure in terms of comparative terms; like percentage growth; it is called relative growth.

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Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.

Growth : Growth may be defined as permanent or irreversible change in the size of a cell, organ or whole organism usually accompanied by an increase in dry weight.

Differentiation : Differentiation is the process whereby cells undergo few to major structural changes that lead to the maturation. The cells mature to perform specialized functions.

Development: Development includes all the series of changes which an organism goes through during its life cycle.

Dedifferentiation : It is the process by which the living differentiated cells which had lost the capacity to divide regain  the capacity to divide under certain conditions.

Redifferentiation : Redifferentiation is defined as maturation or differentiation of dedifferentiated tissues.

Determination growth . The ability of a cell, tissue or the organism to grow for a limited period is called determinate growth.

Meristem : Meristem is a group of cells which have power of division and add new cells to the body.

Growth rate : It is the increased growth per unit time.

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List five main groups of natural plant growth regulators. Write a note on physiological functions and agricultural/horticultural/application of any one of them.

The main five groups of  plant growth regulators (PGRs) are :

1. Auxins
2. Cytokinins
3. Gibberellins
4. Ethylene
5. Abscisic acid.

Auxin -

Physiological functions of auxins are:

1. They  help to initiate and enhance rootings in the stem cuttings.

2. They promote flowering in certain plants like pineapple.

3. They prevent the leaf and fruit drop at early stages. They promote abscission of older or mature leaf and fruit.

4. They induce apical dominance.

5. They are responsible for tropic movements.

Horticultural/Agricultural importance:

1. They induce parthenocarpy in plants like tomatoes..

2. They are used as weedicides that kill dicotyledenous weeds.

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