Chemical Coordination in Plants

Q: Name the fungus from which gibberellins were first obtained and the disease it causes.

Gibberella fujikuroi, which causes the "foolish seedling" (bakanae) disease of rice.

ICSE Class 10 · Biology · Chapter 7

What this chapter covers

Coordination without nerves
Auxins
Gibberellins
Cytokinins
Abscisic acid and ethylene
Tropic movements in plants
Phototropism
Geotropism
Hydrotropism
Thigmotropism
Chemotropism

Key definitions

Coordination is the working together of the different parts of an organism so that it responds to changes around it and functions as a single whole.

Chemical coordination is the control and linking of the activities of an organism by chemical substances that are produced in very small quantities and carried to the cells where they act.

Plant hormones are organic substances produced naturally in small amounts in one part of a plant that move to other parts and control growth, development or other responses there.

Plant growth regulators are natural or synthetic chemical substances that, in small quantities, promote, inhibit or modify the growth and development of plants.

Auxins are plant hormones, produced mainly at the growing tips of shoots, that promote the elongation of cells in the region just behind the tip.

Parthenocarpy is the development of a fruit from an ovary without fertilisation, which gives a seedless fruit.

Gibberellins are plant hormones that promote stem elongation, especially of the internodes, and also help to break seed and bud dormancy.

Bolting is the sudden elongation of the stem of a rosette plant, such as cabbage, just before it flowers.

Cytokinins are plant hormones that promote cell division and delay the ageing of leaves.

Senescence is the natural process of ageing in a plant or its parts, which ends in their death.

Abscisic acid (ABA) is a plant hormone that inhibits growth, induces dormancy in buds and seeds, and causes stomata to close when the plant is short of water.

Abscission is the natural shedding of a plant part, such as a leaf, flower or fruit, by the formation of a separation layer of cells at the base of its stalk.

Key points

Plants coordinate their activities only by chemicals, called plant hormones or phytohormones, which act in very small amounts.
Plant growth regulators include natural hormones and synthetic chemicals; auxins, gibberellins and cytokinins mainly promote growth, while abscisic acid and ethylene mainly inhibit it.
Auxin (IAA) is made at shoot tips; it elongates cells, causes apical dominance and tropic bending, roots cuttings, prevents early fruit drop and gives seedless fruits; 2,4-D is a synthetic auxin weedicide.
Gibberellins lengthen internodes, cause bolting, make dwarf plants tall, break seed dormancy and enlarge grapes; they were found from the fungus that causes foolish seedling disease of rice.
Cytokinins promote cell division, form shoots in tissue culture, delay senescence and oppose apical dominance; coconut water is a rich source.
Abscisic acid is the stress hormone; it closes stomata during drought, keeps seeds and buds dormant and opposes gibberellin.
Ethylene is a gas that ripens fruits, causes abscission and senescence, and makes pineapple plants flower together.
Hormones often work in opposing pairs: auxin and cytokinin, gibberellin and abscisic acid, auxin and ethylene.
A tropism is a directional growth movement in response to a one-sided stimulus; it is positive towards the stimulus and negative away from it.
Shoots are positively phototropic and negatively geotropic; roots are positively geotropic, positively hydrotropic and negatively phototropic.
Tendrils and twining stems show positive thigmotropism; the pollen tube shows positive chemotropism.
Phototropism and geotropism are explained by the uneven spread of auxin, with roots far more sensitive to auxin than shoots.

Questions answered

Define plant growth regulators. Name two growth promoters and two growth inhibitors.

Plant growth regulators are natural or synthetic chemicals that, in small amounts, promote, inhibit or modify plant growth and development. Promoters: auxin and gibberellin (or cytokinin). Inhibitors: abscisic acid and ethylene.

Name the plant hormone that is a gas. State two of its effects.

Ethylene. It ripens fruits and promotes the abscission (shedding) of leaves, flowers and fruits; it also hastens senescence.

Which hormone is called the stress hormone, and why?

Abscisic acid. Its level rises when a plant faces drought, cold or salinity, and it closes the stomata to reduce water loss and induces dormancy.

What is apical dominance? Which hormone causes it and which hormone opposes it?

Apical dominance is the suppression of lateral bud growth by the apical bud. It is caused by auxin from the shoot tip and opposed by cytokinin.

Give the scientific basis of the use of 2,4-D in wheat fields.

2,4-D is a synthetic auxin that, at high concentration, kills broad-leaved (dicot) weeds but does not harm narrow-leaved cereals like wheat, so it works as a selective weedicide.

Name the hormone used to (a) make grape berries larger, (b) keep cut lettuce fresh, (c) make cuttings root, (d) make pineapple plants flower together.

(a) Gibberellin; (b) cytokinin; (c) auxin; (d) ethylene.

Name the fungus from which gibberellins were first obtained and the disease it causes.

Gibberella fujikuroi, which causes the "foolish seedling" (bakanae) disease of rice.

Distinguish between a tropic movement and a nastic movement, with one example of each.

A tropic movement is a directional growth movement whose direction depends on the direction of the stimulus, for example a shoot bending towards light. A nastic movement does not depend on the direction of the stimulus, for example the folding of Mimosa pudica leaves when touched.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Assertion (A): Plants do not have sense organs but still respond to different stimuli. Reason (R): Plants have a well-developed hormonal system to respond to sunlight, gravity, water, etc.

(A) is true and (R) is false.
(A) is false and (R) is true.
Both (A) and (R) are true, and (R) is the correct explanation of (A).
Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Show answer

Both (A) and (R) are true, and (R) is the correct explanation of (A).

ICSE 2027 sample paper · 2 marks

Which hormone promotes the growth of plants towards sunlight? What is this tropic movement called?

Show answer

Auxin. Phototropism.

ICSE 2026 sample paper · 1 mark

A farmer notices that storing ripe mangoes with unripe ones speeds up the ripening process. The phytohormone primarily responsible for this phenomenon is:

Auxins
Cytokinins
Gibberellins
Ethylene

Show answer

Ethylene

ICSE 2026 board paper · 1 mark

Assertion (A): Abscisic acid promotes stomatal closure during a drought. Reason (R): Abscisic acid helps the plant to conserve water during stress.

(A) is true and (R) is false.
(A) is false and (R) is true.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true but (R) is not the correct explanation of (A).

Show answer

Both (A) and (R) are true and (R) is the correct explanation of (A).

ICSE 2026 board paper · 5 marks

Give the biological / technical terms for the following: (a) The tropic movement wherein the tendrils of a pea plant twine around a support. (b) A defect in our eye in which some parts of the object are in focus while the other parts are blurred. (c) The type of waste generated in hospitals and pathological laboratories. (d) The surgical technique for females that can be used to prevent pregnancy. (e) The evolutionary process by which new species arise from the existing ones.

Show answer

(a) Thigmotropism (haptotropism) (b) Astigmatism (c) Biomedical waste (hospital waste) (d) Tubectomy (e) Speciation

ICSE 2026 sample paper · 3 marks

Plant hormones are produced in extremely low concentrations in plants to regulate the physiological processes and tropic movements – The picture shows a leaf in three stages, joined by arrows: Healthy, green plant → Senescence, yellow plant → leaf fall. (a) Name one plant hormone responsible for the changes shown in the above picture. (b) Identify a hormone that has an opposite effect to the one depicted in the picture. (c) Give an example of a plant that shows Thigmotropism.

Show answer

(a) Abscisic acid (ABA) (or Ethylene) – causes senescence and leaf fall. (b) Cytokinin (delays senescence) / Auxin (prevents leaf abscission). (c) Pea / grapevine / ivy (any climber with tendrils).

ICSE 2026 sample paper · 3 marks

In a school experiment, a few students grow plants in pots placed horizontally (the pot lies on its side; the shoot curves upward and the roots grow downward). After a few days, the students observed that the roots bent downward, and the shoots curved upward. (a) Name the tropic movement seen in shoots. (b) Why do roots always grow downwards? (c) Which phytohormone regulates the above tropic movements?

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(a) Negative geotropism (shoots also show positive phototropism). (b) Roots are positively geotropic (and positively hydrotropic), so they grow towards gravity and water. (c) Auxin

ICSE 2025 board paper · 1 mark

Sunil tried to match the tropic movements in plants with the stimulus they respond to. The tabulation is given below: | | Tropism | Stimulus | |---|---|---| | P | Thigmotropism | Touch | | Q | Geotropism | Water | | R | Chemotropism | Chemicals | | S | Phototropism | Gravity | Identify the correct pair:

P and R
Q and S
R and S
P and S

Show answer

P and R