NCERT Class 11-Biology: Chapter –15 Plant Growth and Development Part 6

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Question 6:

The role of ethylene and abscissic acid is both positive and negative. Justify the statement.

Answer:

Ethylene and abscisic acid are plant growth hormones which have both positive as well as negative effects on a plant.

Positive Roles of Ethylene:

(a) Ethylene acts as a ripening agent involved in ripening of fruits.

(b) When present in low concentration, it stimulates root meristem formation and promotes growth of lateral roots.

(c) It is responsible for breaking dormancy of seeds and hence has a role in seed germination.

(d) It increase the number of female flowers in the cucumber plant.

Negative Role of Ethylene:

(a) Ethylene is responsible for nullifying geotropism.

(b) Ethylene inhibits the growth of lateral buds thus stimulating growth of apical buds or leading to apical dominance.

(c) Ethylene promotes abscission of leaves, flowers and fruits thus leading to senescence.Positive Role of Abscisic acid:

(a) During stress conditions, abscisic acid concentration increases in the leaves of the plant due to which stomata closes preventing water loss via transpiration. Thus it increases the tolerance. Hence, it aids during stress conditions because of which it is called stress hormone.

(b) It increases the resistance of plants to cold climatic conditions.

(c) It leads to tuber formation in potato.

(d) It is also responsible for initiation of roots in stem cuttings of some selected plants. root initiation in stem cuttings of some plants.

Negative Role of Abscissic acid:

(a) It inhibits growth of a plant and in turn induces dormancy of buds.

(b) It leads to abscission of flowers and fruits.

(c) It promotes senescence of leaves by destroying the photosynthetic pigment called chlorophyll and inhibiting the synthesis of RNA and proteins.

Thus we see how these two hormones have both positive and negative effects on a plant.

Question 7:

While experimentation, why do you think it is difficult to assign any affect seen to any single hormone?

Answer:

Hormones have synergistic and antagonistic effect with respect to one another. Hence it becomes difficult to assign any affect seen to any single hormone because it can be a synergistic or an antagonistic effect as well.

Synergistic effect occurs when two hormones come together to have the same effect and when their effect is combined, it amplifies the needed objective.

Antagonistic effect on the other hand is when two hormones counter the effect of each other.

For eg: Auxin acts synergistically with Cytokinin to control the shoot stem‐cell meristem while both the hormones act antagonistically to maintain the root meristem.

Question 8:

What is the mechanism underlying the phenomenon by which the terminal/apical bud suppresses the growth of lateral buds? Suggest measures to overcome this phenomenon.

Answer:

The mechanism underlying the phenomenon by which the terminal/apical bud suppresses the growth of lateral buds is known as apical dominance where apical bud growth is dominant. This is mainly due to the presence of auxin hormones synthesized in the tips of the apex of bud.

There are measures to overcome this phenomenon like: Decapitation or removal of the apical bud (shoot cutting) that inhibits growth of the apical bud and promotes the growth of the lateral branches. Cytokinin application can also be used in order to promote lateral bud growth and inhibit apical dominance.

Question 9:

In animals there are special glands secreting hormones, whereas there are no glands in plants. Where are plant hormones formed? How are the hormones translocated to the site of activity?

Answer:

Plant hormones are found in the different tissues like the tip of the shoots, the tip of the roots, meristematic tissues, apical buds etc. Hormones like cytokinins, auxins which are usually plant growth promoters usually are synthesized in regions of active cell division. Hormones like ethylene which are plant growth inhibitors are found almost throughout the plant body but specially in the parts which are old and in regions of ripened fruits.

These hormones are translocated to the site of activity by the vascular bundle tissues like xylem and phloem.

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