Seed Dormancy, Photoperiodism and Vernalisation: Quick Revision

A brief note on Seed Dormancy, Photoperiodism and Vernalisation. Read it before a test. Try each question at the end before you open its answer.

Seed dormancy

• A dormant seed is alive and does not germinate in good conditions.
• It helps a plant through a bad season, spreads germination, and stops sprouting on the parent plant.
• Causes: hard coat, immature embryo, inhibitors such as ABA, need for cold, need for light.
• Cures: scarification (hard coat), stratification (moist cold), gibberellic acid, nitrate, light, washing out an inhibitor.
• ABA keeps a seed dormant. GA wakes it. Ethylene can break the dormancy of buds and seeds.
• Germination needs water, oxygen and a suitable temperature.

Photoperiodism

• It is the response of a plant to the relative lengths of day and night. Garner and Allard found it in tobacco and soybean.
• Short-day plants flower when the unbroken dark period is longer than their critical dark period. Long-day plants flower when it is shorter. Day-neutral plants do not mind.
• The dark period is what counts. A flash in the night breaks it. A dark spell in the day does nothing.
• The leaf perceives it. A flowering signal, florigen, moves through the phloem to the shoot tip.

Vernalisation

• The promotion of flowering by a period of low temperature, sensed by the shoot apex.
• Winter varieties of wheat, barley and rye are sown in autumn. Spring varieties need no cold.
• Biennials (sugar beet, cabbage, carrot) flower and die in the second season.
• A warm spell soon after the cold undoes it (devernalisation). Gibberellic acid can replace the cold in some biennials.

A worked example

A short-day plant has a critical dark period of 11 h. The day has 15 h of light and no flash. Does it flower?

1. The dark period is 24 h minus 15 h, which is 9 h.
2. It is shorter than 11 h, so the plant does not flower.

Practice with new numbers

A program to try

The program lists the three ways of waiting: what is sensed, and where.

Mistakes to avoid

• Calling a dormant seed dead. It is alive.
• Mixing up scarification and stratification. One damages the coat. The other is moist cold.
• Saying that the light period decides flowering. The unbroken dark period decides.
• Saying that the shoot tip senses the day. The leaves do.
• Saying that vernalisation acts on the leaf. It acts on the shoot apex.

Check yourself

1. What is stratification?

2. A short-day plant gets a flash of light in the middle of its long night. Does it flower?

3. Which part of the plant perceives the cold in vernalisation?

4. Name a biennial.