1. Photoperiodism
- Photoperiodism is the response of plants to periods of day/night.
- Some plants require specific light durations to induce flowering.
- Based on light duration, plants are classified into three groups:
- Long day plants: Require exposure to light exceeding a well-defined critical duration to flower.
- Short day plants: Require exposure to light less than the critical duration to initiate flowering.
- Day-neutral plants: Flowering is not correlated with light duration.
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While shoot apices modify into flowering apices, they cannot perceive photoperiods directly. The site of light/dark duration perception is the leaves. It is hypothesized that a flowering hormone(s) is produced in leaves under appropriate photoperiods and migrates to shoot apices to induce flowering.
2. Vernalisation
- It is the phenomenon where some plants require exposure to low temperatures, either quantitatively or qualitatively, to flower.
- It prevents precocious reproductive development late in the growing season, allowing sufficient time for plants to reach maturity.
Examples of Vernalisation:
1. Some food plants, such as wheat, barley, and rye, have two varieties:
- Spring varieties: Planted in spring, they flower and produce grain before the growing season ends.
- Winter varieties: Planted in autumn, they germinate, overwinter as small seedlings, resume growth in spring, and are harvested around mid-summer. If planted in spring, they typically fail to flower or produce mature grain within the season.
2. Vernalisation in biennial plants:
- Biennials are monocarpic plants that flower and die in their second season.
- E.g., sugar beet, cabbages, and carrots. Cold treatment stimulates a subsequent photoperiodic flowering response in these plants.
Seed Dormancy
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Certain seeds fail to germinate even under favorable external conditions due to dormancy. Dormancy is caused by endogenous conditions within the seed, such as:
- Hard seed coat.
- Chemical inhibitors (e.g., ABA, phenolic acids, para-ascorbic acid).
- Immature embryos.
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Dormancy can be overcome naturally or artificially, for example:
- Breaking seed coat barrier: Through mechanical abrasions (e.g., using knives, sandpaper, or vigorous shaking). In nature, abrasions occur via microbial action or passage through animals’ digestive tracts.
- Removing inhibitory substances: By chilling seeds or applying chemicals like gibberellic acid and nitrates.
- Changing environmental conditions: Adjusting light and temperature.