Previous year questions: Plant Growth and Development

These ten questions come from the NCERT Exemplar chapter on plant growth and development, the only official previous year source available for Class 11 Biology. The set covers the growth regulators, the growth rate, photoperiodism and vernalisation. Marks follow the book convention: 1 mark MCQ, 2 marks very short answer, 3 marks short answer and 5 marks long answer. The heading of each item quotes the Exemplar chapter and question number.

Q1 (NCERT Exemplar, 1 mark; Chapter 15, Multiple Choice 1)

Ethylene is used for which of these?

Answer: Ethylene hastens the ripening of fruits, so the answer is the option for hastening ripening. It is not used to retard ripening of tomatoes, and both retarding and slowing options are opposite to its action. Ethylene is also the gas that a ripe fruit gives out to ripen the fruits kept near it.

Q2 (NCERT Exemplar, 1 mark; Chapter 15, Multiple Choice 3)

The effect of apical dominance can be overcome by which hormone?

Answer: Cytokinin overcomes apical dominance, because it promotes the growth of lateral buds. Auxin maintains the dominance, gibberellin mainly lengthens the stem and ethylene is associated with ripening and abscission. Removing the apex is the physical way to do the same thing.

Q3 (NCERT Exemplar, 1 mark; Chapter 15, Multiple Choice 7)

The term synergistic action of hormones refers to what?

Answer: Synergistic action means that two or more hormones act together and contribute to the same function, so their combined effect is more than either alone. The option that describes opposite effects is antagonism, the option about one hormone doing many things is pleiotropy, and many hormones for one function is not the definition of synergy. The prefix syn in synergy is the clue that the hormones are working on the same side.

Q4 (NCERT Exemplar, 1 mark; Chapter 15, Multiple Choice 12)

The photoperiod in plants is perceived at which organ?

Answer: The photoperiod is perceived in the leaves, which sense the relative length of light and dark periods and produce the flowering signal that travels to the shoot apex. The meristem, the flower and the floral buds are the parts that respond, not the parts that sense the day length.

Q5 (NCERT Exemplar, 2 marks; Chapter 15, Very Short Answer 3)

A primary root grows from 5 cm to 19 cm in a week. Calculate the growth rate and the relative growth rate over the period.

Answer: The increase in length is 19 minus 5, which is 14 cm in one week, so the absolute growth rate is 14 cm per week, or 2 cm per day. The relative growth rate is the increase divided by the original length, which is 14 divided by 5, equal to 2.8. It is usually written as a rate per unit initial size, so the relative growth rate here is 2.8 per week, or 280 per cent of the starting length per week. Writing the units at every step keeps the absolute rate and the relative rate from being mixed up.

Q6 (NCERT Exemplar, 2 marks; Chapter 15, Very Short Answer 8)

Where are IAA, gibberellins and cytokinins synthesised in plants?

Answer: IAA is synthesised mainly in the growing apices of the shoot and root, especially in young leaves and the coleoptile tip. Gibberellins are synthesised in young leaves, shoot apices and developing seeds. Cytokinins are synthesised mainly in the roots and are transported to the shoot through the xylem, while developing seeds and fruits also produce them. Root tips are therefore the main source of the cytokinin that reaches the shoot, and this is why root growth and shoot branching are linked.

Q7 (NCERT Exemplar, 3 marks; Chapter 15, Short Answer 1)

Nicotiana tabacum, a short day plant, when exposed to more than the critical period of light fails to flower. Explain.

Answer: A short day plant needs a dark period longer than its critical night length before the flowering signal can be produced. When Nicotiana is exposed to a light period longer than its critical requirement, the darkness that follows is shorter than the critical night length and the plant fails to flower. The leaf senses the length of the dark period, and the signal that would normally pass to the shoot apex is not produced. This is why a short day plant can be kept vegetative by interrupting the dark period with a brief flash of light, even when the total day length seems unchanged. A long day plant in the same chamber behaves in the opposite way, which is why the two plants are used together in demonstrations of the light and dark requirement.

Q8 (NCERT Exemplar, 3 marks; Chapter 15, Short Answer 8)

What is the mechanism by which the terminal bud suppresses the growth of lateral buds, and what measures overcome it?

Answer: The terminal bud produces auxin, which moves down the stem and keeps the lateral buds suppressed. The lateral buds are not damaged, they are held inactive, so removing the terminal bud releases them and they grow into branches. The same effect can be produced by applying cytokinin directly to the lateral buds, which overcomes the auxin dominance, and by applying an inhibitor that blocks the action of auxin at the lateral buds. Gardeners use decapitation regularly to make a plant bushy, and the experiment in figure 13.11 shows the two plants side by side. Treating a lateral bud directly with cytokinin starts it growing even while the apex remains intact, which is why the balance between auxin and cytokinin is described as the control of bud growth.

Q9 (NCERT Exemplar, 3 marks; Chapter 15, Short Answer 13)

A rubber band stretches and returns to its original position, while bubble gum stretches and does not return. Discuss this difference with respect to plant growth.

Answer: The rubber band shows elasticity, a reversible change; the bubble gum shows plasticity, an irreversible change in shape. Plant growth has both sides. A cell wall can stretch as water enters, and this elastic expansion is reversible if the water leaves again, so a wilted leaf can recover. Growth itself is plastic, because the new size and shape produced by cell division and differentiation cannot be reversed. The same distinction explains why a mature organ keeps its new form while a stretched cell can shrink back. A plant can recover from a temporary wilt after watering, yet it cannot undo the new branches and leaves it has already built, and the same contrast separates the elastic stretch of a cell wall from the plastic growth of the plant.

Q10 (NCERT Exemplar, 5 marks; Chapter 15, Long Answer 1)

Some wheat varieties are spring wheat and some are winter wheat. Spring wheat is sown and harvested in the same season, but winter wheat if planted in spring fails to flower or produce mature grain. Explain.

Answer: Winter wheat needs a period of low temperature, called vernalisation, before it can flower. When it is sown in autumn the seedling receives the cold of winter, and this cold treatment prepares the shoot apex to flower in the following summer. Spring wheat does not have this requirement and can complete its life cycle in one spring season. If winter wheat is sown in spring, it does not get the cold treatment, so the floral signal is not produced and the plant stays vegetative or produces only immature grain within the season. The cold requirement is perceived by the growing point and its effect persists through later growth, which is why a short cold period early in life can decide flowering months later. Growing a winter variety without its cold period therefore wastes the season, while a spring variety planted at the same time flowers normally. In some plants the cold treatment can be replaced by applying gibberellin, which is why gibberellin can make rosette plants bolt, but the natural route for winter wheat is the winter cold. The practical point is that sowing time is part of the treatment: a winter variety sown late misses the cold period and the crop fails, which is why seed catalogues mark varieties as winter or spring types.