Gujarat Board GSEB Textbook Solutions Class 11 Biology Chapter 15 Plant Growth and Development Textbook Questions and Answers.

Gujarat Board Textbook Solutions Class 11 Biology Chapter 15 Plant Growth and Development

GSEB Class 11 Biology Plant Growth and Development Text Book Questions and Answers

Question 1.
Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem, and growth rate.
Answer:

• Growth: It is an irreversible and permanent process, accomplished by an increase in the size of an organ or organ parts or even of an individual cell.
• Differentiation: It is a process in which the. cells derived from the apical meristem and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions.
• Development: It refers to the various changes occurring in an organism during its life cycle from the germination of seeds to senescence.
• De-differentiation: It is the process in which permanent plant cells regain the power to divide under certain conditions.
• Re-differentiation: It is the process in which differentiated cells become mature again and lose their capacity to divide.
• Determinate growth: It refers to limited growth. For example, animals and plant leaves stop growing after having reached maturity.
• Meristem: In plants, growth is restricted to specialized regions where active cell divisions take place. Such a region is called meristem.
• Growth rate: It can be defined as the increased growth in plants per unit of time.

Question 2.
Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer:
Growth at a cellular level is principally a consequence of the increase in the amount of protoplasm. Since the increase in protoplasm is difficult to measure directly, one generally measures some quantity that is more or less proportional to it. Growth is, therefore, measured by a variety of parameters some of which are: increase in fresh weight; dry weight; length; area; volume and cell number.

Question 3.
Describe briefly:

1. Arithmetic growth
2. Geometric growth
3. Sigmoid growth curve
4. Absolute and relative growth rates.

Answer:
1. Arithmetic growth: In arithmetic growth, from 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. Look at fig. 15.1. On plotting the length of the organ against time, a linear curve is obtained. Mathematically, it is expressed as
L_t = L_o + rt
L_t = length at time ’t’
L_o= length at time ’zero’
r= growth rate/ elongation per unit time.

2. Geometrical 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 a mitotic cell division retain the ability to divide and continue to do so. However, with a limited nutrient supply, the growth slows down leading to a stationary phase.

3. Sigmoid growth curve: If we plot the parameter of growth against time, we get a typical sigmoid or S-curve (Fig 15.2). An S-shaped curve is a characteristic of the 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 =W_oe^π
W₁= final size (weight, height, number, etc.)
W_o = 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 the efficiency index. Hence, the final size of W₁ depends on the initial size, W_o.

4. Absolute and relative growth rates: Quantitative comparisons
between the growth of living system can also be made in two ways:

• Measurement and the comparison of total growth per unit time is called the absolute growth rate.
• 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. In fig. 15.3 two leaves A and B are drawn that are of different sizes but show an exact absolute increase in area in the given time to give leaves, ‘A’ and ‘B’. However, one of them shows much higher relative growth.
  

Question 4.
List five main groups of natural plant growth regulators. Write a note on the discovery, physiological functions, and agricultural/horticultural applications of any one of them.
Answer:
Plant growth regulators are the chemical molecules secreted by plants affecting the physiological attributes of a plant. There are five main plant growth regulators.
They are:

• Auxins
• Gibberellic acid
• Cytokinins
• Ethylene
• Abscisic acid

Discovery: During the mid-1960s, inhibitor – B, abscission II and dormant were discovered by three independent researchers. These were later on found to be chemically similar and were thereafter called Abscisic acid (ABA).

Physiological Functions:

• It acts as an inhibitor to plant metabolism.
• It stimulates stomatal closure during water stress.
• It induces seed dormancy
• It induces the abscission of leaves, fruits and flowers.

Horticultural application:
It induces seed dormancy in stored seeds.

Question 5.
What do you understand by photoperiodism and vernalization? Describe their significance.
Answer:
Photoperiodism: Photoperiodism has been observed that some plants require periodic exposure to light to induce flowering. It is also seen that such plants are able to measure the duration of exposure to light. For example, some plants require exposure to light for a period exceeding a well-defined critical duration, while others must be exposed to light for a period less than this critical duration before the flowering is initiated in them.

The former group of plants is called short-day plants while the later ones are termed long-day plants. There are many plants, however, where there is no such correlation between exposure to the light duration and indication of flowering response. Such plants are called day-neutral plants. It is also known now that it is not only the duration of the light period but the duration of the dark period is also of equal importance. Hence, it can be said that flowering in certain plants depends not only on a combination of light and dark exposures but also on their relative durations.

This is termed photoperiodism or the response of plants in terms of flowering to the relative length of day and night is called photoperiodism. The site of perception of light/dark duration is the leaves. It has been hypothesized that there is a hormonal substance (s) called florigen that is responsible for flowering. Florigen migrates from leaves to shoot apices for inductive photoperiod.

Vernalization: There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed vernalization. It prevents precocious reproductive development late in the growing season, to enable the plant to have sufficient time to reach maturity. Vernalisation refers especially to the promotion of flowering by a period of low temperatures. Some important food plants, such as wheat, barley, rye have two kinds of varieties: winter and spring varieties.

The ‘spring’ variety is normally planted in the spring and comes to flower and produce grain before the end of the growing season. Winter varieties are planted in fall. They germinate and overwinter as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.

Another example of vernalization is seen in biennial plants. Biennials are monocarpic plants that normally flower and die in the second season. Sugarbeet, cabbages, carrots are some of the common biennials.

Question 6.
Why is abscisic acid also known as stress hormone?
Answer:
Abscisic acid is called stress hormone as it induces various responses in plants against stress conditions. It increases the tolerance of plants towards various stresses. It induces the closure of stomata during water stress. It promotes seed dormancy and ensures seed germination during favourable conditions. It helps seeds withstand desiccation. It also helps in inducing dormancy in plants at the end of the growing season and promotes the abscission of leaves, fruits, and flowers.

Question 7.
“Both growth and differentiation in higher plants are “open”. Comment.
Answer:
Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability of the plants just due to the presence of meristems at certain locations in their body. Tile cell (s) of such meristems have the capacity to divide and self -perpetuate. The product, however, soon loses the capacity to divide and such cells make up the plant body. This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth.

We have mentioned that the growth in plants is open i.e. it can be indeterminate or determinate. Now, we may say that even differentiation in plants is open because cells/tissues arising out of the same meristem have different structures at maturity. The final structure at maturity of a cell/tissue is also determined by the location of the cell within. For example, cells positioned away from root apical meristems differentiate as root-cap cells, while those pushed to the periphery mature as the epidermis.

Question 8.
“Both a short-day plant and a long-day plant produce flower simultaneously in a given place”. Explain.
Answer:
The flowering response in short-day plants and long-day plants is dependent on the duration for which these plants are exposed to light. The short-day plant and long-day plant can flower at the same place, provided they have been given an adequate photoperiod.

Question 9.
Which one of the plant growth regulators would you use if you are asked to:

1. Induce rooting in a twig
2. Quickly ripen a fruit
3. Delay leaf senescence
4. Induce growth in axillary buds
5. ‘bolt’ a rosette plant
6. Induce immediate stomatal closure in leaves

Answer:
4. Induce growth in axillary buds. 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.

Question 10.
Would a defoliated plant respond to the photoperiodic cycle? Why?
Answer:
the defoliated plant will not respond to the photoperiodic cycle.
It is hypothesized that the hormonal substance responsible for flowering is formed in the leaves, subsequently migrating to the shoot apices and modifying them into flowering apices. Therefore, in the absence of leaves, light perception would not occur i.e., the plant would not respond to light.

Question 11.
What would be expected to happen if:

1. GA₃ is applied to rice seedlings.
2. dividing cells stop differentiating,
3. a rotten fruit gets mixed with unripe fruits,
4. you forget to add cytokinin to the culture medium.

Answer:

1. Increase in the length of rice.
2. Maybe affected, major structural changes both in their cell walls and protoplasm. For example, to form a tracheary element, the cells would lose their protoplasm.
3. Senescence and abscission of plant organs especially of leaves and dowers may affect the rate of respiration.
4. Plant development and growth may be affected. These functions may stop like making new leaves, making chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.