Plant Hormones and Growth

Why Plant Hormones Matter in CDS

A plant hormone (also called a phytohormone) is a chemical messenger made in tiny amounts in one part of a plant that controls growth, development and responses in another part. Unlike animals, plants have no nervous system and no glands; they coordinate their whole life — sprouting, bending, flowering, fruiting, shedding leaves — entirely through these chemicals.

In the CDS paper, hormone questions are almost always direct single-fact recall: which hormone ripens fruit, which causes a shoot to bend towards light, which hormone is gaseous, or which one promotes cell division. There is no maths involved, so a couple of focused hours here give a very high marks-per-minute return compared with calculation-heavy physics topics.

The smartest way to study this chapter is to remember that there are just five hormones. Three of them mainly promote growth (auxin, gibberellin, cytokinin) and two mainly inhibit or stress-control growth (abscisic acid and, in part, ethylene). Hold that simple split in your head and the whole topic becomes orderly.

The Five Major Plant Hormones

Keep this master list at your fingertips — almost every CDS question is built from it:

• Auxin — promotes cell elongation; controls bending towards light (phototropism) and apical dominance.
• Gibberellin (GA) — promotes stem elongation, seed germination and bolting; can make dwarf plants tall.
• Cytokinin — promotes cell division (cytokinesis) and delays the ageing of leaves.
• Ethylene — the only gaseous hormone; promotes fruit ripening and leaf and flower fall.
• Abscisic acid (ABA) — the growth inhibitor or stress hormone; closes stomata in drought and promotes dormancy.

Auxin: The Elongation Hormone

Auxin was the first plant hormone to be discovered. The pioneering work of Charles Darwin and his son Francis Darwin on grass seedlings (coleoptiles) bending towards light pointed to a substance moving down from the tip; it was later isolated and named auxin (from the Greek auxein, “to grow”). The most common natural auxin is indole-3-acetic acid (IAA).

Auxin is produced mainly at the growing tips (apical meristems) of shoots and roots and moves downward. Its chief job is to make cells elongate — stretch lengthwise — rather than divide. Key roles tested in CDS:

• Phototropism: auxin gathers on the shaded side of a shoot, that side's cells grow longer, and the shoot bends towards the light.
• Apical dominance: auxin from the main tip suppresses the growth of side buds, which is why a plant grows tall before bushy. Pruning the tip removes this suppression and the plant becomes bushy.
• Root initiation: synthetic auxins are sprayed on cuttings to promote rooting, and are used as weedkillers (e.g. 2,4-D).

Gibberellin: The Stem-Stretcher

Gibberellins (GA) were discovered through the fungus Gibberella fujikuroi, which caused rice plants to grow abnormally tall and spindly — the “foolish seedling” (bakanae) disease. The active growth substance from the fungus was named gibberellin.

Gibberellins are famous for causing dramatic stem elongation. Their main exam-relevant roles are:

• Bolting: rapid lengthening of the stem before flowering in rosette plants such as cabbage.
• Breaking seed dormancy and triggering germination by stimulating the production of enzymes that digest stored food in the seed.
• Making dwarf plants grow tall: applying GA to genetically dwarf varieties can restore normal height — a favourite CDS one-liner.
• Promoting flowering and producing larger, seedless fruit in some crops.

Cytokinin: The Cell-Division Hormone

Cytokinin takes its name from cytokinesis, the division of the cell. Its central job is to promote cell division, working alongside auxin. The first known cytokinin was kinetin; the most common natural one is zeatin, first found in maize (corn) kernels.

Cytokinins are present in greatest amounts in regions of active division, such as root tips, developing fruits and seeds. Roles to remember:

• Promote cell division and the formation of new shoots and roots in tissue culture.
• Delay ageing (senescence) of leaves by slowing the breakdown of chlorophyll — they help keep leaves green and fresh.
• Help break the dormancy of seeds and buds.
• Work in balance with auxin: a high cytokinin-to-auxin ratio favours shoot formation, while a high auxin-to-cytokinin ratio favours root formation.

Ethylene and Abscisic Acid: The Restrainers

These two hormones mostly slow, stress-control or end growth phases.

Ethylene — the gaseous hormone

Ethylene is the only plant hormone that is a gas at normal temperatures. It is the classic fruit-ripening hormone. A ripening apple releases ethylene, which is why "one rotten apple spoils the whole basket" — the gas spreads and ripens (then over-ripens) the rest. Its roles:

• Promotes ripening of fruits (bananas, mangoes, tomatoes are ripened commercially using ethylene).
• Causes abscission — the natural falling of leaves, flowers and fruits.
• Triggers ageing (senescence) of plant parts.

Abscisic acid (ABA) — the stress hormone

Abscisic acid is the main growth-inhibiting hormone and is called the stress hormone. Its roles:

• Closes stomata during water shortage to reduce water loss in drought — its most tested function.
• Promotes dormancy of seeds and buds, helping plants survive winter and dry spells.
• Generally counteracts the growth-promoting hormones, acting as a natural brake.

Tropic Movements: How Plants Respond

Plants cannot run, but they can grow towards or away from a stimulus. A tropism (tropic movement) is a directional growth movement of a plant part in response to an external stimulus; it is controlled by hormones, mainly auxin. The movement is positive if growth is towards the stimulus and negative if away from it. The main types:

• Phototropism — response to light. Shoots are positively phototropic (bend towards light); roots are negatively phototropic.
• Geotropism (gravitropism) — response to gravity. Roots are positively geotropic (grow downward); shoots are negatively geotropic (grow upward).
• Hydrotropism — response to water. Roots grow towards moisture (positively hydrotropic).
• Chemotropism — response to a chemical. The growth of a pollen tube down the style towards the ovule is the standard example.
• Thigmotropism — response to touch, as in the tendrils of climbing plants that coil around a support.

Growth, Nastic Movements and Quick Distinctions

A few related ideas are commonly mixed up in CDS, so pin them down clearly.

Growth in plants is mostly limited to special regions called meristems (at root and shoot tips, and in the cambium for thickening). This is why plants keep growing throughout their life — growth is said to be indeterminate — whereas animals stop growing after a point.

Tropic vs nastic movements: a tropic movement depends on the direction of the stimulus (the part bends towards or away from it). A nastic movement is non-directional — the response is the same regardless of where the stimulus comes from.

• The drooping of the leaves of the touch-me-not plant (Mimosa pudica) when touched is a nastic movement — it is not growth and not directional; it is caused by a sudden change in water pressure (turgor) in the cells, not by hormones.
• The opening and closing of flowers in response to light or temperature is also nastic.

Worked Example: Identifying the Hormone

Reasoning questions are easy once you map each clue to one hormone. Work through this typical CDS-style case.

Previous-Year-Style Question

Here is a question in the exact format CDS sets, with full reasoning.

Quick Revision

Run through this the night before the exam — it covers most of what CDS asks on this topic.

Learn the five-hormone table and the tropism codes by heart, practise a few matching questions, and this becomes one of the most reliable scoring areas in CDS General Science.