Seed germination

Where it happens

Germination begins inside the seed once conditions are favourable, and needs three main conditions: water, oxygen, and a suitable temperature for enzyme activity.

The parts of the seed matter for the answer. Inside the seed coat (testa) lies the embryo, made of the radicle, the plumule and one or two cotyledons.

In a dicot such as the green bean, the cotyledons themselves hold the food store; in a monocot such as maize, the store is a separate endosperm and the single cotyledon acts as an absorbing organ. The micropyle, a tiny pore in the testa, is where water first enters.

Enzyme activity, and therefore germination, happens in the food store and in the growing tips of the radicle and plumule.

Inputs and outputs

• Input: water, entering through the micropyle, to swell the seed and activate enzymes.
• Input: oxygen, diffusing through the softened testa for aerobic respiration in the embryo.
• Input: a suitable temperature, so that the hydrolytic and respiratory enzymes work at a useful rate.
• Input: stored food, starch, protein and lipid, in the cotyledons or endosperm.
• Output: soluble products (glucose, amino acids, fatty acids and glycerol) used to build new cells and to release energy.
• Output: carbon dioxide, water and heat from respiration, and a seedling with a root and shoot ready to photosynthesise.

The steps

1. The seed absorbs water through the micropyle (imbibition); the testa softens and the seed swells.
2. Water activates the hydrolytic enzymes stored in the seed, and the embryo begins to release gibberellin, which stimulates the production of more enzymes such as amylase.
3. Amylase hydrolyses starch to maltose and then glucose; protease breaks protein into amino acids; lipase breaks lipid into fatty acids and glycerol.
4. The soluble products diffuse or are transported from the food store to the embryo.
5. The embryo respires the glucose aerobically, using the oxygen that entered the seed, to release energy as ATP.
6. ATP and amino acids drive cell division and elongation; the swelling embryo bursts the testa.
7. The radicle emerges first and grows downward (positive geotropism) to anchor the seedling and absorb water.
8. The plumule then emerges and grows upward; once its first leaves open and turn green, photosynthesis begins and the seedling becomes independent.

Why it matters and how it is controlled

Germination establishes a new, independent plant from the food reserves packed into the seed. Once the plumule reaches light and unfolds its first leaves, the seedling can begin photosynthesis and no longer depends entirely on the food stored in the seed or cotyledons.

Control comes from both the environment and the seed's own hormones. Until water is available, the seed stays dormant with very low metabolic activity, which lets it survive dry seasons.

Once water enters, gibberellin from the embryo switches on enzyme production while the dormancy hormone abscisic acid is broken down. Temperature sets the pace: too cold and the enzymes work slowly; too hot and they denature.

A seed buried too deep, or in waterlogged soil, runs short of oxygen and fails to germinate even when warm and wet, a common inference question.

How it is examined

You may be asked to list and explain the conditions needed for germination, to describe how stored starch is mobilised during germination, or to explain the order in which the radicle and plumule emerge and why.

The standard practical uses 4 test tubes of seeds on moist cotton wool: one with all conditions, one kept dry, one with boiled and cooled water under a layer of oil to exclude oxygen, and one placed in a refrigerator. Only the first tube germinates, and each of the others isolates one condition.

Questions ask you to name the control, identify the manipulated variable in each tube, and explain the result. A related graph question tracks the dry mass of a germinating seed: it falls at first, because stored food is respired, and rises again only after the leaves begin photosynthesis.

Common misconceptions

Worked exam-style question

Question. A student set up four test tubes, each containing 10 green bean seeds on cotton wool. Tube 1: moist cotton wool, room temperature.

Tube 2: dry cotton wool, room temperature. Tube 3: seeds under boiled-and-cooled water covered with a layer of oil, room temperature.

Tube 4: moist cotton wool, kept in a refrigerator. After 5 days, only the seeds in tube 1 had germinated.

(a) State the purpose of tube 1. (b) Explain why the seeds in tube 3 did not germinate.

(c) Explain why the seeds in tube 4 did not germinate. (d) Describe how the dry mass of a seed in tube 1 would change over the first 10 days, and explain why.

Model answer. (a) Tube 1 is the control: it supplies all three conditions so that the other tubes can be compared with it. (b) Boiling removes dissolved oxygen, and the oil layer stops oxygen re-entering.

Without oxygen the embryo cannot carry out aerobic respiration, so no energy is released for growth. (c) The low temperature means the enzymes such as amylase are inactive or work very slowly, so stored starch is not hydrolysed to glucose and the embryo has no food for respiration.

(d) Dry mass decreases at first because stored food is respired and released as carbon dioxide and water; after the plumule leaves open, photosynthesis makes new organic matter and dry mass increases.

Source:SRC-DSKP-EN

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