seed dormancy, germination and establishment

Question 1

What are the key steps in seed germination?

Answer 1

1️⃣ Imbibition: Water uptake, three phases.
2️⃣ Metabolic reactivation: Gene transcription, protein translation, enzyme activation.
3️⃣ Reserve mobilization: Breakdown of starch, lipids, and proteins for energy.
4️⃣ Radicle emergence: Seed coat ruptures, radicle emerges (end of germination).
5️⃣ Seedling establishment: Shoot and root system development begins.

Question 2

What are the three phases of water uptake (imbibition) in seeds?

Answer 2

1️⃣ Phase I: Rapid water uptake (rehydration of dry seed).
2️⃣ Phase II: Water uptake slows; metabolic reactivation occurs (gene transcription, translation).
3️⃣ Phase III: Water uptake increases as cell division and expansion resume, leading to radicle emergence.

Question 3

What are the key processes and components of seed development?

Answer 3

• Formation of three tissues: embryo, endosperm, seed coat.
• Development of primary meristems (for future growth).
• Formation of embryonic organs (shoot, root, cotyledons).
• Accumulation of seed storage reserves (carbohydrates, lipids, proteins).

Question 4

What are the key characteristics of a mature seed?

Answer 4

• Seed is viable and can germinate under the right conditions.
• Seed becomes quiescent, with extremely slow metabolism.
• Embryo desiccates, reducing water content.
• Seed coat provides protection for the embryo and storage reserves.
• Growth resumes once the seed is rehydrated.

Question 5

What happens when a seed becomes quiescent?

Answer 5

• The embryo dehydrates, reducing water content.
• The seed acquires desiccation tolerance, protecting the embryo.
• Metabolic activity slows down.
• The seed enters a quiescent (inactive) state.
• Growth resumes after water uptake (imbibition) and germination begins.

Question 6

What defines seed germination, and when does it end?

Answer 6

• Germination begins with imbibition (water uptake) and metabolic reactivation.
• The seed must be viable and non-dormant to germinate.
• Radicle emergence (root tip) marks the end of germination.
• Germination is followed by seedling establishment as the shoot emerges and grows.

Question 7

What is seed dormancy, and how is it different from quiescence?

Answer 7

• Dormancy: Seed fails to germinate even under ideal conditions.
• Quiescence: Seed is inactive but will germinate once conditions are suitable.
• Dormancy involves a temporal block in germination.
• Dormancy is controlled by the balance of hormones (ABA vs. GA).

Question 8

How do ABA and GA regulate seed dormancy and germination?

Answer 8

• Abscisic Acid (ABA) promotes dormancy and prevents premature germination.
• Gibberellins (GA) promote germination by activating hydrolytic enzymes.
• The outcome depends on the ABA:GA ratio, which changes in response to environmental signals.

Question 9

What environmental factors can break seed dormancy?

Answer 9

1) Light: Red (R) light promotes germination, while Far Red (FR) light promotes dormancy.
2) Temperature:
Low temperatures (5°C, moist) synchronize germination (cold stratification).
Dry after-ripening at room temperature reduces ABA levels over weeks or years.
3) Light is sensed by the photoreceptor phytochrome, which controls the R/FR light response.

Question 10

How do hormones regulate the mobilization of stored reserves in seeds?

Answer 10

• Gibberellins (GA) from the embryo signal the aleurone layer.
• The aleurone layer produces hydrolytic enzymes (e.g., amylases) in response to GA.
• Enzymes are released into the endosperm to break down starch into sugars.
• Sugars fuel the growth of the embryo and radicle emergence.