Seeds remain dormant and suppress germination under unsuitable conditions, then germinate all at once when suitable conditions are present. This precise control is essential to plants’ survival strategy.
Basic Knowledge
Seed structure and stored nutrients: Embryo (the future plant), endosperm (nutrient storage; grasses and others) or cotyledons (nutrient storage; legumes and others), and seed coat. Stored nutrients are starch, lipids, and proteins.
Conditions for germination: ①Water (imbibition = softening of the seed coat and activation of enzymes) ②Temperature (the optimum temperature differs among species) ③Air (O₂) (energy supply through respiration). Some are positively photoblastic seeds that require light (lettuce, tobacco, etc.; phytochrome Pfr promotes germination), while others are negatively photoblastic seeds whose germination is inhibited by light (pumpkin, tomato, etc.).
Role of gibberellin (rice-seed model): Imbibition → the embryo synthesizes and secretes gibberellin → aleurone-layer cells synthesize and secrete amylase and protease → starch and proteins in the endosperm are broken down to supply glucose and amino acids → the embryo grows and germinates.
Seed dormancy and awakening: ABA (abscisic acid) induces and maintains seed dormancy. Germination requires ABA breakdown or antagonism of ABA by GA. Methods of breaking dormancy: ① low-temperature treatment (after-ripening and stratification) ② light exposure (positively photoblastic seeds) ③ scarification (damage to the seed coat or passage through the digestive tract) ④ drying (desert plants) ⑤ fire (some species).
Coleoptile growth and auxin: Experiments by Darwin and his son → those by Cannon → Went’s experiments (1926), through which auxin was isolated and identified. Experiments on coleoptile phototropism initiated the discovery of plant hormones.
Explain step by step how gibberellin acts on the aleurone layer during rice-seed germination.
Answer: ① When the seed absorbs water, gibberellin is synthesized and secreted in the embryo. ② Gibberellin acts on the nuclei of adjacent aleurone-layer cells and induces transcription of genes for hydrolytic enzymes such as α-amylase and protease. ③ The synthesized enzymes are secreted into the endosperm and break down starch (→ glucose) and proteins (→ amino acids). ④ The breakdown products are supplied to the embryo, where cell division and elongation begin, leading to germination.
- Three conditions for germination = water, temperature, and O₂ (light depends on the species)
- Positively photoblastic seeds = phytochrome Pfr promotes germination (lettuce and tobacco)
- Gibberellin = induces amylase synthesis in the aleurone layer → starch breakdown → germination
- ABA = induces seed dormancy and inhibits germination
- Breaking dormancy = low-temperature treatment, light exposure, and scarification
- After-ripening = ABA breakdown and increased GA through low-temperature storage after harvest
- Darwin → Went’s experiments = history of auxin discovery
Points to Note
① Lettuce-seed germination is promoted by red light (660 nm) exposure (a state with abundant Pfr) and inhibited by far-red light (730 nm) exposure (a state with abundant Pr) → the wavelength of the last light exposure determines germination (a reversible reaction). ② ABA and GA act antagonistically. High ABA → maintenance of dormancy; high GA → antagonism of ABA and amylase induction → promotion of germination. ③ When a hard seed coat causes dormancy (physical dormancy), passage through the digestive tract or fire damages the seed coat and promotes germination.
Practice
- State which form of phytochrome (Pr or Pfr) promotes germination in positively photoblastic seeds, and explain why red-light exposure promotes germination.
- Name the hormone that induces seed dormancy and the hormone that promotes germination.
- In rice seeds, what enzymes does gibberellin induce the aleurone layer to synthesize, and what are the substrates of those enzymes?