Q: 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) If GA3 is applied to rice seedlings, then the rice seedlings will exhibit internode elongation and increase in height.(b) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cell is called callus.(c) If a rotten fruit gets mixed with unripe fruits, then the ethylene produced from the rotten fruits will hasten the ripening of the unripe fruits.(d) If you forget to add cytokinin to the culture medium, then cell division, growth, and differentiation will not be observed.

Question 1

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

Accepted Answer

(a) 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.(b) Differentiation:It is a process in which the cells derived from the apical meristem (root and shoot apex) and the cambium undergo structural changes in the cell wall and the protoplasm, becoming mature to perform specific functions.(c) Development:It refers to the various changes occurring in an organism during its life cycle – from the germination of seeds to senescence.(d) De-differentiation:It is the process in which permanent plant cells regain the power to divide under certain conditions.(e) Re-differentiation: It is the process in which de-differentiated cells become mature again and lose their capacity to divide.(f) Determinate growth: It refers to limited growth. For example, animals and plant leaves stop growing after having reached maturity.(g) Meristem: In plants, growth is restricted to specialised regions where active cell divisions take place. Such a region is called meristem. There are three types of meristems – apical meristem, lateral meristem, and intercalary meristem.(h) Growth rate: It can be defined as the increased growth in plants per unit time.

Question 2

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

Accepted Answer

In plants, growth is said to have taken place when the amount of protoplasm increases. Measuring the growth of protoplasm involves many parameters such as the weight of the fresh tissue sample, the weight of the dry tissue sample, the differences in length, area, volume, and cell number measured during the growth period. Measuring the growth of plants using only one parameter does not provide enough information and hence, is insufficient for demonstrating growth.

Question 3

Describe briefly:(a) Arithmetic growth(b) Geometric growth(c) Sigmoid growth curve(d) Absolute and relative growth rates

Accepted Answer

Growth - It is a characteristic of living beings in which an irreversible permanent increase in the size of an organ or its parts occurs or an increase in the size of a cell.Types of Growth Rate :Growth rate can be defined as the increase in growth per unit time.Plants show two types of growth—Arithmetic and Geometric—according to the increase shown by the growth rate.Arithmetic growth - Only one daughter cell continues to divide while others differentiate or mature. For example, root elongating at a constant rate.Geometric growth - Initial growth is slow (lag phase), followed by a rapid increase in growth (log/exponential phase), and followed by a phase where growth slows down (stationary phase). For example, all cells, tissues and organs show this type of growth

Question 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.

Accepted 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. These are:(i) Auxins(ii) Gibberellic acid(iii) Cytokinins(iv) Ethylene(v) Abscisic acid(i) Auxins discovery: The first observations regarding the effects of auxins were made by Charles Darwin and Francis Darwin wherein they saw the coleoptiles of canary grass bending toward a unilateral source of light.It was concluded after a series of experiments that some substance produced at the tip of coleoptiles was responsible for the bending. Physiological functions:1. They control plant cell growth.2. They cause the phenomenon of apical dominance.3. They control division in the vascular cambium and xylem differentiation.4. They induce parthenocarpy and prevent abscission of leaves and fruits.Horticultural applications:1. They are used as the rooting hormones in stem cuttings.2. 2-4 D is used weedicide to kill broadleaf, dicotyledonous weeds.3. They induce parthenocarpy in tomatoes.4. They promote flowering in pineapple and litchi.(ii) Gibberellic acid discovery: Bakane or the “foolish rice seedling” disease was first observed by Japanese farmers. In this disease, rice seedlings appear to grow taller than natural plants and become slender and pale green. Later, after several experiments, it was found that this condition was caused by the infection from a certain fungus Gibberella fujikuroi. The active substance was isolated and identified as gibberellic acid.Physiological functions:1. It causes elongation of internodes.2. It promotes bolting in rosette plants.3. It helps in inducing seed germination by breaking seed dormancy.Horticultural applications:1. It helps in increasing the sugar content in sugarcane by increasing the length of the internodes.2. It increases the length of grape stalks.3. It improves the shape of an apple.4. It delays senescence.5. It hastens maturity and induces seed-production in juvenile conifers.(iii) Cytokinins discovery: Through their experimental observations, F. Skoog and his co-workers found that the tobacco callus differentiated when extracts of vascular tissues, yeast extract, coconut milk, or DNA were added to the culture medium.Physiological functions:1. They promote the growth of lateral branches by inhibiting apical dominance.2. They help in the production of new leaves, chloroplasts, and adventitious shoots.3. They help in delaying senescence by promoting nutrient mobilization.Horticultural applications:1. They are used for preventing apical dominance.2. They are used for delaying senescence in leaves.(iv) Ethylene discovery: It was observed that unripe bananas ripened faster when stored with ripe bananas. Later, the substance promoting the ripening was found to be ethylene.Physiological functions:1. It helps in breaking seed and bud dormancy.2. It promotes rapid internode elongation in deep-water rice plants.3. It promotes root growth and formation of root hairs.4. It promotes senescence and abscission of leaves and flowers.Horticultural applications :1. It is used to initiate flowering2. It promotes the number of female flowers in cucumbers.(v) Abscisic acid discovery: During the mid-1960s, inhibitor-B, abscission II, and dormin were discovered by three independent researchers. These were later on found to be chemically similar and were thereafter called ABA (Abscisic acid).Physiological functions:1. It acts as an inhibitor to plant metabolism.2. It stimulates stomatal closure during water stress.3. It induces seed dormancy.4. It induces abscission of leaves, fruits, and flowers.Horticultural application:It induces seed dormancy in stored seeds.

Question 5

What do you understand by photoperiodism and vernalisation? Describe their significance.

Accepted Answer

The physiological mechanism for flower-ing is controlled by two factors: photoperiod or light period, i.e., photoperiodism and low temperature, i.e., vernalisation. Photoperiodism is defined as the flowering response of a plant to relative lengths of light/ dark period. Significance of photoperiodism is as follows:1. Photoperiodism determines the season in which a particular plant shall flower. For example, short day plants develop flowers in autumn-spring period (e.g., Dahlia, Xanthium) while long day plants produce flowers in summer (e.g., Amaranthus).2. Knowledge of photoperiodic effect is useful in keeping some plants in vegetative growth (many vegetables) to obtain higher yield of tubers, rhizomes etc. or keep the plant in reproductive stage to yield more flowers and fruits.3. A plant can be made to flower throughout the year by providing favourable photoperiod.4. Helps the plant breeders in effective cross-breeding in plants. Enable a plant to flower in different seasons.Vernalisation is promotion or induction of flowering by exposing a plant to low temperature for some time. Significance of vernalisation is as follows :(i) Crops can be grown earlier.(ii)Plants can be grown in such regions where normally they do not grow.(iii)Yield of the plant is increased.(iv)Resistance to cold and frost is increased.(v) Resistance to fungal diseases is increased.

Question 6

Why is abscisic acid also known as stress hormone?

Accepted Answer

Abscisic acid is called stress hormones as it induces various responses in plants against stress conditions.It increases the tolerance of plants toward various stresses. It induces the closure of the 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 abscission of leaves, fruits, and flowers.

Question 7

Both growth and differentiation in higher plants are open. Comment.

Accepted Answer

Growth and development in higher plants is referred to as being open. This is because various meristems, having the capacity for continuously dividing and producing new cells, are present at different locations in these plant bodies.

Question 8

Both a short day plant and a long day plant can produce flower simultaneously in a given place. Explain.

Accepted Answer

The flowering response in short-day plants and long-day plants is dependent on the durations 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:(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.

Accepted Answer

(a) Induce rooting in a twig – Auxins(b) Quickly ripen a fruit – Ethylene(c) Delay leaf senescence – Cytokinins(d) Induce growth in axillary buds – Cytokinins(e) Bolt a rosette plant – Gibberellic acid(f) Induce immediate stomatal closure in leaves – Abscisic acid

Question 10

Would a defoliated plant respond to photoperiodic cycle? Why?

Accepted Answer

A defoliated plant will not respond to the photoperiodic cycle.It is hypothesised 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:(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.

Accepted Answer

(a) If GA3​ is applied to rice seedlings, then the rice seedlings will exhibit internode elongation and increase in height.(b) If dividing cells stop differentiating, then the plant organs such as leaves and stem will not be formed. The mass of undifferentiated cell is called callus.(c) If a rotten fruit gets mixed with unripe fruits, then the ethylene produced from the rotten fruits will hasten the ripening of the unripe fruits.(d) If you forget to add cytokinin to the culture medium, then cell division, growth, and differentiation will not be observed.

Chapter 15 : Plant Growth and Development