Photoperiodism: is it the day length or the night length that matters? And the flash of light in the dark confuses me
Mehak AroraDropper Sample
1 Sept 2026 · 1 min read
Writing this at 1 am because I can't sleep and apparently neither can my confusion 🙈
Photoperiodism, from NCERT: some plants need a certain light-dark cycle to flower. Short-day plants (SDP) flower when the day length is less than a critical length, long-day plants (LDP) when it's more than a critical length, and day-neutral plants (DNP) don't care. Fine.
But then the book says it's now known that it is actually the dark period that matters, not the day length, and that a brief flash of light in the middle of the dark period stops flowering in SDP. I don't understand this. If light is the signal, why would more light in the night stop an SDP from flowering? And what happens in LDP with the same flash?
Then vernalisation: promotion of flowering by exposure to low temperature, with biennials and winter varieties of wheat, barley, rye as examples. Why does a winter wheat sown in spring not give grains?
Finally, seed dormancy: what causes it and how is it broken? I have a list of 3 causes but I don't know how each connects to a way to break it.
Thank you, I know this is a long one 🥺
7 answers & replies
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[+][–]Ishita GoyalDropperSample· 1mo ago
Think of it as the plant measuring night with a stopwatch.
- SDP: flowers when the continuous dark period is longer than a critical length.
- LDP: flowers when the dark period is shorter than a critical length.
Now the flash: in an SDP, a brief light flash in the middle of the long night cuts that night into two shorter dark periods, and neither is long enough, so the plant does not flower. The leaves are where the plant senses it, and a flowering stimulus moves from leaf to shoot apex.
So day length is a stand-in. What the plant counts is how long the uninterrupted night is.
[+][–]Aarav Kulkarni11thSample· 1mo ago
maybe I'm wrong but I think a flash of light in the dark makes both SDP and LDP flower, because light is the flowering signal for all photoperiodic plants?
[+][–]Rahul DubeyDropperSample· 1mo ago
Aarav, opposite results for the two. In an SDP the flash prevents flowering (it breaks the long night). In an LDP the flash promotes flowering, because it shortens the effective dark period below the critical length. This difference is exactly why scientists concluded that the dark period is what is measured. If light were a universal flower-signal, both would respond the same way.
[+][–]Mehak AroraDropperSample· 1mo agoAuthor
the "stopwatch on the night" picture and the flash of light breaking the night into two halves finally makes it click. thank you Rahul bhaiya, and Aarav no worries, I thought the same at first 😅
[+][–]Ishita GoyalDropperSample· 1mo ago
you got it! now go to sleep Mehak, the plants will still be there tomorrow 🌱
[+][–]Nisha Verma12thSample· 1mo ago
Vernalisation: flowering is promoted by exposure to low temperature. Biennials (sugar beet, cabbage, carrot) grow vegetatively in the first season and flower and die in the second after the winter. Winter varieties of wheat, barley, rye are sown in autumn, germinate, pass the winter as seedlings, and flower in spring or early summer, so they can be harvested by mid-summer. If you sow them in spring they don't flower or produce grains before the growing season ends, because they never got the cold treatment. So vernalisation prevents precocious flowering late in the growing season and ensures the plant has enough time to reach maturity.
[+][–]Siddharth RaoDropperSample· 1mo ago
Seed dormancy: three causes, three ways to break:
Cause Way to break Hard, impermeable seed coat Scarification (mechanical abrasion with knives, sandpaper, or vigorous shaking) Chemical inhibitors in the seed (abscisic acid, phenolic acids, para-ascorbic acid) Leaching/removing them, chilling (stratification), gibberellic acid treatment Immature embryo Time/after-ripening, suitable conditions Not every method works for every seed, but pairing each cause with a method is a good way to remember it. Dormancy is useful for the plant: it ensures germination happens at a time favourable for growth.
