Q.Explain vernalisation.

Vernalisation — The Cold That Makes Plants Flower

Imagine a wheat seed planted in spring. It grows leaves, stems, and then — nothing. No flowers, no grain. Now take the same seed, chill it for a few weeks before planting, and it flowers normally. That is vernalisation in action.

The word comes from Latin vernus meaning "spring". Many plants that flower in spring do not simply respond to warmer weather — they need a period of cold first. The cold is not a stress they endure; it is a signal they require. Without it, they stay vegetative indefinitely.

The Intuition

Think of it as a biological calendar. A plant that flowers in spring must know that winter has passed. If it flowered too early — say during a warm spell in autumn — frost would kill the flowers. So evolution gave these plants a safety lock: they will not commit to flowering until they have experienced sustained cold. The cold resets their internal clock, telling them "winter happened, spring is safe to bloom."

This is different from mere cold tolerance. Vernalisation is not about surviving winter; it is about using winter as information.

The Precise Statement

Key features:

• Quantitative or qualitative: In some plants (e.g., winter wheat), vernalisation is obligatory — no cold, no flowers. In others (e.g., many biennials), it merely accelerates flowering.
• Site of perception: The cold is sensed by the shoot apical meristem — the growing tip of the plant. The meristem itself is transformed, not just the leaves.
• Memory: Once vernalised, the plant "remembers" the cold even if moved to warm conditions. This memory is mitotically stable — all cells descended from the original meristem carry the floral-competent state.

The Molecular Mechanism (for context)

At the gene level, a key repressor of flowering — a gene called FLC (Flowering Locus C) in Arabidopsis — is actively expressed in non-vernalised plants. Cold exposure triggers a cascade of epigenetic silencing: histones around the FLC gene are modified (histone H3 lysine 27 trimethylation), compacting the chromatin and shutting the gene off. This silencing persists through cell divisions, so even after the plant warms up, FLC stays off. With the brake removed, floral pathway genes activate and flowering proceeds. …