NCERT Grounding
NCERT Class 11 Biology Chapter 13 (Plant Growth and Development) introduces vernalisation in its closing section on extrinsic factors that control flower initiation. The textbook states:
"The stimulation of flowering by cold treatment is called vernalisation."
NCERT Class 11 Biology, Chapter 13 — Plant Growth and Development
The chapter situates vernalisation alongside photoperiodism as the two principal environmental cues that govern the timing of reproduction. Both are extrinsic factors that act through intrinsic pathways — ultimately influencing plant growth regulators and gene expression — to trigger the transition from vegetative to reproductive growth. NIOS Chapter 20 similarly defines vernalisation as "the process of accelerating the process of flowering by subjecting or exposing the plant to low temperature," and notes its practical utility for converting biennials into annuals.
Vernalisation: The Core Concept
The word vernalisation is derived from the Latin vernus (of spring). It describes the process by which an extended cold period — typically temperatures between 0°C and 5°C lasting days to weeks — creates a physiological competence in the plant to flower when subsequent conditions (day length, warmth) are appropriate. Without the cold treatment, vernalisation-requiring plants either fail to flower entirely or flower very late.
The phenomenon was first studied systematically by the German botanist Gustav Gassner in 1918, who demonstrated that germinating seeds of winter rye exposed to low temperature before sowing flowered at the same time as spring varieties. This proved that the cold stimulus could be perceived at the seed stage, not just by the mature plant.
Effective Temperature Range
The window for effective vernalisation. Temperatures below 0°C are generally insufficient (frozen tissue cannot respond); temperatures above ~10°C are too warm to elicit the response. Duration of cold exposure (days to several weeks) matters as much as temperature.
How Vernalisation Works: A Step-by-Step View
Vernalisation pathway — from cold exposure to spring flowering
-
Step 1
Cold Perception
Shoot apex or embryo cells detect sustained low temperature (0–5°C) over days to weeks.
Site: apex / embryo -
Step 2
Epigenetic Change
Cold triggers chromatin remodelling — silencing of floral repressor genes (e.g., FLC in model plants). The plant becomes "competent" to flower.
Mitotically stable memory -
Step 3
Return to Warmth
Temperature rises in spring. The vernalised plant perceives the appropriate photoperiod and is now competent to produce the floral stimulus.
Photoperiod check -
Step 4
Floral Initiation
Shoot apex converts from vegetative to reproductive meristem. Flowering proceeds on schedule — in spring rather than the following year.
Outcome: timely flowering
Site of Cold Perception
The single most tested fact about vernalisation in NEET is the site of perception. Cold is perceived by the shoot apex (growing tip) and by the embryo of the seed. This has been demonstrated experimentally: if only the vegetative leaves of a plant are cooled while the shoot apex is kept warm, vernalisation does not occur. Conversely, cooling the shoot apex alone while keeping leaves at room temperature is sufficient to induce the response.
This contrasts sharply with photoperiodism. In photoperiodism, the leaf is the organ that perceives the light/dark cycle; the resulting floral stimulus (florigen) is then transported to the shoot apex. In vernalisation, there is no long-distance relay of a cold signal — the apex and embryo perceive the cold directly, and the memory of that cold is maintained in those meristematic cells through cell divisions.
Figure 1. Comparison of perception sites. In vernalisation (left), the cold stimulus (0–5°C) is perceived directly at the shoot apex and embryo — the leaf plays no role. In photoperiodism (right), the leaf perceives the light/dark cycle and generates florigen, which travels to the shoot apex. NEET frequently tests this distinction.
Plants That Require Vernalisation
Rule of thumb: Plants that naturally encounter winter in their life cycle — winter annuals, biennials, and some perennials — typically require vernalisation. Spring annuals and day-neutral tropicals do not.
Winter Cereals
Winter wheat (Triticum aestivum) — classic example; sown in autumn, overwinters, flowers in spring
Winter rye (Secale cereale) — Gassner's original experimental plant
Also: winter barley
NEET context: LDP + vernalisationBiennial Plants
Henbane (Hyoscyamus niger) — biennial variety; classic experimental plant for vernalisation research
Sugar beet (Beta vulgaris) — requires cold to bolt and flower in second year
Turnip, carrot, celery
NEET trap: biennial ≠ annualOther Cold-Requiring Plants
Annual meadow grass — low-temperature requirement
Many rosette plants that must bolt before flowering (e.g., cabbage under certain conditions)
Some varieties of cotton, rice (as noted by NIOS for low-temperature induction)
NIOS example: 1–10°C applied to wheat/riceDevernalisation: Reversing the Cold Effect
Vernalisation is not irreversible. If a plant that has received partial or complete cold treatment is subsequently exposed to a short period of high temperature (approximately 35–40°C), the promotive effect of the cold is cancelled. This reversal is called devernalisation.
The sensitivity to devernalisation diminishes as the cold treatment progresses. A plant that has received a full, extended vernalisation period becomes progressively more resistant to reversal by heat. Devernalisation does not occur simply from returning to normal growing temperatures; it requires specifically elevated heat. This distinguishes it from the mere interruption of cold treatment.
Devernalisation temperature
A brief exposure to high temperature after cold treatment cancels the vernalisation effect. Normal spring temperatures (15–20°C) do NOT devernalise. The heat must be applied soon after cold and before the memory becomes fully stable.
Vernalisation and Photoperiodism Working Together
Vernalisation rarely operates in isolation. In many species, a plant must satisfy both a cold requirement (vernalisation) and a specific photoperiod requirement before it will flower. Vernalisation makes the plant competent — it removes a developmental block — but the actual transition to reproductive growth still depends on an appropriate day length.
The clearest example is winter wheat: it is a vernalisation-requiring, long-day plant. Without cold treatment, it grows vegetatively indefinitely regardless of day length. After vernalisation, it responds to the long-day photoperiod of spring and flowers. Remove either cue (cold or long day), and flowering is delayed or absent.
Similarly, the biennial henbane (Hyoscyamus niger) shows two ecotypes: the annual ecotype flowers without vernalisation under long days; the biennial ecotype requires vernalisation first and then flowers under long days. This system illustrates how vernalisation and photoperiodism are sequentially gated — cold first, then light.
Vernalisation versus Photoperiodism: Full Comparison
Vernalisation
Cold
Stimulus type
- Stimulus: prolonged low temperature (0–5°C)
- Site of perception: shoot apex and embryo
- Signal transmissibility: NOT transmissible by grafting
- Hypothetical signal molecule: vernalin (proposed but not confirmed)
- Reversible by: brief high temperature (devernalisation)
- Works in: shoot tip and seeds; leaves not involved
- Plants: winter wheat, rye, biennial henbane, sugar beet, turnip
Photoperiodism
Light
Stimulus type
- Stimulus: duration of continuous dark period (critical night length)
- Site of perception: leaf (mesophyll cells via phytochrome)
- Signal transmissibility: transmissible by grafting (florigen hypothesis)
- Hypothetical signal molecule: florigen (later identified as FT protein)
- Reversible by: night interruption with red light
- Works in: leaf perceives, apex responds
- Plants: SDP (chrysanthemum, rice, soybean), LDP (wheat in spring, barley), DNP (tomato)
Figure 2. Seasonal timeline for winter wheat (Triticum aestivum). After autumn sowing, the plant overwinters (vernalisation: 0–5°C). In spring, the now-competent plant encounters long days and initiates flowering. Both cues are required; neither alone suffices.
Worked Examples
A winter variety of wheat is grown in a region where it never experiences temperatures below 8°C throughout the year. What would you expect to observe, and why?
Answer: The plant would grow vegetatively but fail to flower (or flower very late). Winter wheat requires vernalisation — exposure to 0–5°C for several weeks — before its shoot apex becomes competent to respond to the long-day photoperiod of spring. Without the cold treatment, the developmental block on the floral transition is never removed. The plant remains in a perpetual vegetative state.
A student grafts a vernalised shoot apex onto a non-vernalised plant. Will the non-vernalised partner now flower? Justify your answer, contrasting vernalisation with photoperiodism.
Answer: No. The vernalisation stimulus is not transmissible by grafting. The cold-induced competence is stored as an epigenetic change in the cells of the vernalised apex itself; it does not produce a systemic signal that can travel through the graft union to reprogram non-vernalised tissue. By contrast, in photoperiodism, the floral stimulus (florigen) produced in induced leaves can travel via phloem to a non-induced, grafted scion and trigger flowering. This distinction — graftable vs non-graftable — is a classic NEET-tested difference between the two phenomena.
A biennial sugar beet plant has been exposed to cold treatment for four weeks. It is then kept at 38°C for two days. What is the name of this process, and what is its expected outcome?
Answer: This is devernalisation. A brief exposure to high temperature (35–40°C) after cold treatment reverses the vernalisation effect, particularly when the cold treatment has been short or incomplete. After four weeks of cold followed by two days at 38°C, the sugar beet would likely lose its vernalisation-induced competence and would not flower on schedule in spring. For devernalisation to be effective, it must be applied shortly after the cold period and before the epigenetic memory is fully consolidated.