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1. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
Growth : Growth cane be defined as an irreversible permanent increase in size of an organ or ots parts or even of an individual cell. Generally, growth is accompanies by metabolic processes (both anabolic and catabolic), that occur at the expense of energy.
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. For example, to form a tracheary element, the cells would lose their protoplasm. They also develops a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tesnion.
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.
Dedifferentiation : The living differentiated cell, that buy 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 july differentiated parenchyma cells.
Redifferentiation : While undergoing defifferentiation plant cells once again lose their capacity to divide and become mature. This process is called redifferntiation.
Determinate Growth : Although growth in most of the plant parts is unlimited. Certain parts grow up to a certain level and then stop growing. This kind of growth is called determinate growth.
Growth Rate : The increased growth per unit time is termed as growth rate. Thus, rate of growth can expressed mathematically. An organism, or a part of the organism can produce more cells in a variety of ways. The growth rate shows an increase that may be arithmetic or geometrical.
Quantitative comparisons between the growth of living system can also be made in two ways :
(i) measurement and the comparison of total growth per unit time is called the absolute growth rate.
(ii) The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.
2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant ?
Like most of the organisms plants also go through various phases of growth. When a seed is germinating then the parameters of growth will be different compared to growth of a mature plant. Moreover, plants can be of a huge variety ; right from perennial plants to plants to plants living hundreds of years. Because of sheer diversity in size and life span any single parameter to define and measure the growth of a plant can never be sufficient.
3. Describe briefly :
(a) Arithmetic growth
(b) Geometric growth
(c) Sigmoid growth curve
(d) Absolute and relative growth rates
Arithmetic Growth : 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
Lt = Lt0 + rt
Lt = 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 mitiotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase. If we plot the parameter of growth against the time, we get 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 = W0 ert
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 W1 depends on the initial size, W0.
Quantitative comparisons between the growth of living system can also be made in two ways :
(i) measurement and the comparison of total growth per unit time is called the absolute growth rate.
(ii) The growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.
4. List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.
Five Plant Growth Regulators
(i) Auxins
(ii) Gibberellins
(iii) Cytokinins
(iv) Ethylene
(v) Abscisic Acid
Auxins :
Discovery of Auxins : Charles Darwin and his son Francis Darwin when they observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments , it was concluded that the tip of coleoptile was the site of transmittable influence that caused the bending of the entire coleoptile. Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings.
Characteristics of Auxins : The term 'auxin' is applied to the indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
Functions of Auxins : They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plantations, hedge-making.
Auxins also induce parthenocarpy, e.g., in tomatoes. They are widely used as herbicides. ,4-D, widely used to kill dicotyledonous seeds, does not affect mature monocotyledonous plants. It is used to prepare seed-free lawns by gardeners, Auxin also controls xylem differentiation and helps in cell division.
5. What do you undertsand by photoperiodism and vernalisation ? Describe their significance.
Photoperiodism : Flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations, This response of plants to periods of day/night is termed photoperiodism. It is also interesting to note that while shoot apices modify themselves into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves cannot perceive photoperiods. The site of perception of light / dark duration are the leaves.
It has been hypothesised that there is a hormonal substance (s) that is responsible for flowering. This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.
The significance of photoperiodism is in regulating flowering in plants. Flowering is an important step towards seed formation and seeds are responsible for continuing the generation of a plant. So, photoperiodism has an important role to play in evolution.
Vernalisation : There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed vernalisation. It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity. Vernalisation refers specially to the promotion of flowering by a period of low temperature.
Some important food plants, wheat, barley, rye have two kinds of varieties : winter and spring varieties. The 'spring' variety are normally planted in the spring and come to flower and produce grain before the end of the growing season. Winter varieties , however, if planted in spring would normally fail to flower or produce mature grain within a span of flowering season. Hence, they are planted in autumn. They germinate, and over winter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.
Another example of vernalisation is seen in biennial plants. Biennials are monocarpic plants that normally flower and die in the second season.Sugerbeet, cabbages, carrots are some of the common biennials. Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.
6. Why is abscisic acid also known as stress hormone ?
Abscisic Acid increases the tolerance of plants to various kinds of streams. Therefore, it is also called the stress hormone.
7. Would a defoliated plant respond to photoperiodic cycle ? Why ?
The hormone responsible for photoperiodism is found in leaves. So a defoliated plant will be devoid of such hormones and will not respond to photoperiodic cycle.
8. What would be expected to happen if :
(a) GA3 is applied to rice seedlings
(b) dividing cells stop differentiating
(c) a rotten fruit gets mixed with unripe fruits
(d) you forget to add cytokinin to the culture medium.
(a) GA3 increases the length of axis. This property will help increase the length of axis so that yield of rice can be increased.
(b) When dividing cells stop differentiating then it is the maturity stage of that part of the plant. Further growth of that particular region will be stopped.
(c) The ethylene present in rotten fruit will hasten the ripening process of unripe fruit and may lead to premature ripening.
(d) There wil be lesser cell division and culture will not grow as per the target.
9. Which one of the plant growth regulators would you use if you are asked to :
(a) induce rooting in a twig
(b) quickly ripen a fruit
(c) delay leaf senescence
(d) induce growth in axillary buds
(e) ' bolt ' a rosette plant.
(f) induce immediate stomatal closure in leaves.
(a) Auxins
(b) Ethylene
(c) Cytokinins
(d) Auxins
(e) Gibberellins
(f) Abscisic Acid
10. 'Both growth and differentiation in higher plants are open'. Comment.
Theoretically growth and differentiation in higher plants are open. This means that there is no limit to the extent a plant par can grow. But it is more correct to say that development and differentiation is open in higher plants. Once a cell loses its capacity to didvide then it differentiates. Differentiation is the process by which a particular plant starts doing the job it is meant to. For example the job of a leaf is to make food for plants. Sometimes environment or a particular phase of growth can dictate a particular part to behave differently. This is the phase when redifferentiation occurs and the plant part takes on a new role.Thus it can be said that development and differentiation are open to cahnge under the given environmental conditions and demands of those conditions.
11. ' Both a short day plant and a long day plant can produce can flower simultaneously in a given place'. Explain.
As the concept of photo periodism shows it is not only the duration of light but also that of darkness which governs the flowering in plants.
A long-day plant requires fewer than a certain number of hours of darkness in each 24-hour period to induce flowering . These plants typically flower in the late spring or early summer as days are getting longer.
Short-day plants flower when the night is longer than a critical length. They cannot flower under the long days of summer. In general, these plants flower in late summer or fall, as days are getting shorter. Short-days plants will not flower if a pulse of artificial light is shone on the plant for several minutes during the middle of the night ; they require a consolidated period of darkness before floral development can begin. Natural nighttime light, such as moonlight or lightening , is not of sufficient brightness or duration to interrupt flowering. Photoperiod affects the flowering, when shoot induces to produce floral instead of leaves and leteral buds.
