Vernalization

Why must some plants sit through weeks of cold before they will flower, when the warmth of spring is what they are waiting for?

A winter wheat sown in spring grows handsomely all season and refuses to make an ear. Sow the same seed in autumn, let it sit through the frosts, and it flowers the following May. The cold is not what makes it flower — the warm lengthening days of spring do that. So the plant insists on an experience that does nothing for it directly, and declines to act on the signal that marks the right moment.

That looks like bad design until you ask the question the plant is facing. It is not "is it warm?" It is "has winter happened?" Why should those differ?

Ask what a plant can measure. Temperature now. Day length now. Nothing else about the calendar reaches it. Does either, alone, distinguish spring from autumn?

Temperature plainly does not. A mild fortnight in October is indistinguishable from a mild fortnight in April, and a plant flowering on warmth alone would be tricked by every autumn warm spell into throwing its flowers into the coming frost — a terminal error for an annual that gets one attempt.

Day length seems better until you look at it properly. It is a symmetric signal: twelve hours of light happens twice a year, once going up and once coming down, and so does thirteen, and fourteen. Photoperiod tells a plant where it is on the curve, not which way it is travelling. Alone it is exactly as ambiguous as temperature.

So the plant needs something one-way — a marker obtainable only by having gone through the trough. A long sustained cold is what is available, and autumn cannot counterfeit it, because a proxy requiring weeks of accumulation is not forged by a few good days. That reframes the whole arrangement: the cold is not a trigger. It is a licence. Winter removes a block, and the block's absence is what lets the ordinary spring cues do their work.

Which raises the mechanical question. If the cold acts months before the flowering it permits, something has to remember it. What would such a memory look like?

In Arabidopsis, the block is a protein called FLC, a repressor sitting on the genes that would otherwise commit the plant to flowering. While FLC is being made, spring cues arrive and achieve nothing. Prolonged cold shuts the FLC gene down — not by degrading the protein but by silencing the gene itself, with Polycomb protein complexes laying a repressive mark across the locus, assisted by cold-induced RNAs transcribed from within the gene. The part that answers "how is it remembered" is that a chromatin state of this kind is copied through cell division. The silencing survives return to warmth: a record of last winter carried forward in the plant's own cells.

A further detail reveals how the dose is counted, and it is not what most people guess. Work on Arabidopsis published around 2011 — recalled, not cited — indicated that in any single nucleus the switch is essentially all-or-none, and that longer cold flips a larger fraction of nuclei. The graded response to a graded winter is a tally across cells, not a dimmer inside each one. The requirement runs to weeks near a few degrees above freezing, though that figure varies so widely by species and cultivar that I would not put a number on it.

Is this an adaptation, or am I telling a story? Here the answer is genuinely selection, and it is worth naming what selects on whom: individuals flowering into a frost lose their entire reproductive output while those that hold back leave descendants, so the selecting agent is the frost, acting on individual timing. The strongest evidence against a just-so telling is that the requirement is polymorphic within a single species — many Arabidopsis populations are rapid-cycling summer annuals carrying loss-of-function alleles at the gene that keeps FLC switched on, and skip vernalization altogether. A trait that appears and disappears with local climate is under current selection, not a fixed inheritance rationalised after the fact.

Three refinements. First, the reset is not an oversight — it is the point. Vernalization is erased in the embryo of the next generation, so every plant must experience its own winter. An inherited memory would be a record of someone else's winter, and would misfire the first season the climate shifted.

Second, an incompletely vernalized plant can lose ground: warmth following a short cold spell undoes part of the effect, which is how a tally still adding up behaves, and not how a switch already thrown behaves.

Third, as a check on over-generalising: the FLC-and-Polycomb account is Arabidopsis. Cereals reach the same functional result through a different set of genes — the VRN loci of wheat and barley — with no FLC involved. The mechanism is not one thing that evolved once; the logic is what recurs, as you would expect when an environment poses the same problem repeatedly to unrelated lineages.

Is it falsifiable? Directly. The model predicts the cold effect is dose-dependent and saturating, persists in continued warmth with no further cold, and is absent in progeny. Any one failing breaks it: a plant flowering as readily after one frosty night as after two months would kill the accumulation account; one that lapsed the moment it was warmed would mean no stored state at all, only an ongoing one; and offspring born competent would refute the resetting claim outright, turning the whole thing into inheritance rather than memory.

• Why the same cold that carries this information is, a few degrees lower, simple damage — the chilling injury that ruins a refrigerated banana.
• How perennials that flower every year re-establish the block, given that they never pass through the embryonic reset.

Every term the collection defines is gathered in the glossary.