# The science behind chilling hours requirement for temperate fruit trees ? **URL:** \ **Category:** Factual Questions\ **Created:** [March 3, 2019, 3:30pm UTC](https://boards.straightdope.com/t/the-science-behind-chilling-hours-requirement-for-temperate-fruit-trees/830486 "2019-03-03T15:30:53Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [March 3, 2019, 3:30pm UTC](https://boards.straightdope.com/t/the-science-behind-chilling-hours-requirement-for-temperate-fruit-trees/830486/1 "2019-03-03T15:30:53Z")
A number of temperate climate fruit trees (apples, pears, …) and vegetables require chilling hours to flower / bear fruits. > **[Vernalization](https://en.m.wikipedia.org/wiki/Vernalization)** > > Vernalization (from Latin vernus, "of the spring") is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower. The term is sometimes used to refer to the need of herbal (non-woody) plants for a period of cold dormancy in order to produce new shoots and leaves, but this > **[Chilling requirement](https://en.m.wikipedia.org/wiki/Chilling_requirement)** > > The chilling requirement of a fruit is the minimum period of cold weather after which a fruit-bearing tree will blossom. It is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures. > Some bulbs have chilling requirements to bloom, and some seeds have chilling requirements to sprout. > Biologically, the chilling requirement is a way of ensuring that vernalization occur... So, what’s going on at the biochemistry level ? What does the chilling hours (1000, 500, …) requirement translate to at a cellular level ? ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [March 3, 2019, 4:30pm UTC](https://boards.straightdope.com/t/the-science-behind-chilling-hours-requirement-for-temperate-fruit-trees/830486/2 "2019-03-03T16:30:32Z")
It is right there in your first link: > [@](#): > > At the molecular level, flowering is repressed by the protein _Flowering Locus C_ (_FLC_), which binds to and represses genes that promote flowering, thus blocking flowering.[[3]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-sung-3)[[13]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-13) Winter annual ecotypes of Arabidopsis have an active copy of the gene _FRIGIDA_ (_FRI_), which promotes _FLC_ expression, thus repression of flowering.[[14]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-14) Prolonged exposure to cold (vernalization) induces expression of _VERNALIZATION INSENSTIVE3_, which interacts with the _VERNALIZATION2_ polycomb-like complex to reduce _FLC_ expression through chromatin remodeling.[[15]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-Sung_and_Amasino-15) The epigenetic silencing of _FLC_ by chromatin remodeling is also thought to involve the cold-induced expression of antisense _FLC COOLAIR_[[16]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-16)[[17]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-Csorba-17) or _COLDAIR_ transcripts.[[18]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-pmid21127216-18) Vernalization is registered by the plant by the stable silencing of individual _FLC_ [loci](https://en.m.wikipedia.org/wiki/Locus_(genetics)).[[19]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-Angel-19) The removal of silent chromatin marks at _FLC_ during embryogenesis prevents the inheritance of the vernalized state [[20]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-Crevillen-20) > Since vernalization also occurs in _flc_ mutants (lacking _FLC_), vernalization must also activate a non-_FLC_ pathway.[[21]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-pbvp-21) A day-length mechanism is also important.[[9]](https://en.m.wikipedia.org/wiki/Vernalization#cite_note-trevaskis-9) ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [March 3, 2019, 4:32pm UTC](https://boards.straightdope.com/t/the-science-behind-chilling-hours-requirement-for-temperate-fruit-trees/830486/3 "2019-03-03T16:32:16Z")
My bad - thank you