Simplified, but: As nights lengthen, the primary trigger for onset of dormancy, photosynthesis decreases & the production of abscisic acid (ABA)increases. The reduction in auxin flow across the abscission zone of leaf petioles becomes inadequate to suppress the formation of of an abscission layer; it forms and leaves fall. High ABA levels, triggered by shorter photo-period and accompanied by dropping temperatures cause solutes (sugars, sugar alcohols, proteins, etc.) to concentrate in cell protoplasm. This is nature's anti-freeze and allows cells (in some plants) exposed to often very low temperatures to exist without freezing. Bound water (water contained within cell walls) is also drawn out of cells & into the inter-cellular spaces in living tissue. This, in effect, increases the concentration of solutes in remaining bound water & further enhances cellular cold-hardiness.

The major changes in cold-hardiness are in above-ground tissues. Roots also harden, especially the more lignified perennial roots, though to a lesser degree than above ground tissues.

So, "what's happening" is the plant responds to environmental stimuli and, as a defense mechanism, goes dormant. A varying length of time at (the generally agreed upon) temperatures between freezing and 45* F. is required to negate the effects of the physiological changes caused by the environmental triggers. Though I have never read a text that said definitively what occurs to release the trees from dormancy, many suggest that the high ABA levels are reduced to normal levels during the dormant period, which would then allow the plant to return to normal growth once cultural conditions are favorable.

After sufficient chilling occurs, the plant is technically released from dormancy, though in most cases it remains in a quiescent state until cultural conditions are favorable for growth (primarily temperature). Once the chilling requirement is met, soil temperatures need only rise above about 45* F, for as short a time as 3-4 days to initiate growth.

As you read the following, please consider that I am also including containerized plants:
Some temperate plants, if deprived of a dormant period, will still break bud in spring, usually very late. Signs of insufficient chilling are the late bud-break described, along with weak growth, primarily concentrated at/near the top of the plant and/or at branch tips. Weak, or no fruiting should be expected. Other temperate plants will simply not wake up in spring, or will continue to grow poorly through what would normally be their dormant period, only to go through leaf abscission and dormancy at a completely unexpected time - say in July. This is usually fatal to the plant, but in some cases can be reversed by providing a sufficient chilling period.

I've read extensively to try to discover the exact physiology involved in the actual release from dormancy at the molecular level & would be most interested if any can provide the info & cite a source.

Al

Mike - an afterthought: Since you frequently post on the fig forum, I thought I'd explain something additional that might clear things up a bit. Some trees, like Ficus carica or Ulmus parvifolia, can exist as a species either as temperate or subtropical trees. Within the species, and on a plant-by-plant basis, what determines cold-hardiness and the accompanying need for a chilling period is rooted (no pun) in provenance. If the plant is vegetatively cloned, as in cuttings, it will have the same genetically transmitted need, or lack of need, for a chilling period as the material from which the cutting was taken. Cold-hardiness potential is also transmitted through vegetatively cloned generations in the same way. That's why a fig that grows and fruits with vitality in FL will not show equal vitality in KY, e.g. Trees whose genetic material originated in a provenance where the trees saw no freezing temperatures will succumb to cold or freeze to the ground, while those that come from colder climes and are moved to warmer environs will grow poorly for lack of a chilling period.

Since provenance is largely determined by photo-period, roughly speaking, even north/south changes in latitude of 100 miles are enough to determine if a tree is able to grow to its full genetic potential vigor. The trees that will perform best in your neighborhood are those whose genetic material originated in your (or a very similar) provenance.

Al

Thanks for some very interesting info Al.
Let me start with your second reply. I have always been told that what I needed to focus on was ... find an open pollenated veggie that grows well for me then each year leave the very best growing / producing plant to go to seed. If I do this year after year I would wind up with a plant that would be adapted to my micro-climate and would be near impossible to find a better producer. Those plants would be adjusted to my particular site / climate so what you say about provenance (knowing the history of a thing ... in this case a plant) makes perfect sense and meshes with the idea I have come to believe is true.

Now for what I orginally was asking but not in a clear way. Why does a plant need a certain amount of chill hours? As you explain a plant goes dormant and the purpose is a form of protection against the ravages of winter. But that does not explain why a plant must have a certain amount of dormance (chill hours/rest period?) to fruit or even to function normally. What does a particular length of tempertures below ...again I think it is 45 degrees have to do with fruit set? Are chemical changes going on in the winter buds and that takes 'x' amount of time depending on the plant?

Thanks.
Mike

"Why does a plant need a certain amount of chill hours?"

The easy answer is to release the plant from dormancy, but I think you're looking for more than that as an answer.

I indicated above that as far as I know, there is no "one" accepted theory on exactly how chilling ends the true dormant stage. If I had to make a best guess, it would be that chilling negates the effects that a build-up of ABA has on the plant. There are several growth regulators (hormones, if you prefer) at work in the dormancy phase. Their individual effect and collective interactions determine, in part, how a plant reacts to its environment. If, once the plant undergoes a chilling period, higher levels of ABA are reduced to normal, the plant would then be able to grow normally as soon as cultural conditions (temp) allow. If, on the other hand, ABA levels remain high in the plant due to a lack of chilling, it is not unreasonable to expect that it would have a negative effect on the growth process, given that it was instrumental in the cessation of growth at the onset of dormancy.

Again, this is my best guess, based on my understanding of dormancy/chill requirements in temperate species. I have read this possible explanation in many texts, but always accompanied by the aside that it is the logical conclusion but not the proven one.

Al

Think I have it now.
The ABA levels are the determining factor (maybe ;o) and the amounts that a plant produces or reacts to is partly determined by dormance. A lack of dormany leaves the cell with too great of an ABA level and will not function properly. That is for temperate plants.
With tropical plants I then assume either the ABA has a lesser affect, is not produced in as great a levels or is reduced by another trigger.
Thanks again Al!
Mike

Al gave a very thorough explanation of what the plant is doing physiologically, one which goes way beyond my understanding. However to take an ecological view of the process, dormancy is a mechanism that plants have developed to protect them from growing at the wrong time of year. Without this mechanism, they might be encouraged to break bud and grow during a warm spell in the winter. It is not just fruit trees that do this but most, if not all, cold-climate trees. The number of chilling hours a plant requires to break dormancy depends on the climate it is adapted to.

Bob - probably a minor point, but the mechanism of dormancy is a protection against intra-cellular freezing, but doesn't technically stop the plant from growing at inappropriate times, though it is very valuable to the plant in the autumn transition period when above-ground growth is ending but soil temperatures are still conducive to growth.

Actually, by the time the January thaw comes around in your neighborhood, a high % of plants have seen sufficient chilling to end dormancy. Once dormancy is ended (for most Idaho plants 1,000 - 2,000 chilling units) the plant enters a quiescent period in which it is fully capable of normal growth as soon as soil temperatures warm sufficiently. For most of these plants, if a Jan or Feb thaw was warm enough to warm soils to 45 - 50* F. for several days, the tree/plant would break bud, begin growing & lose much of its cold-hardiness, only to have newly produced tissues freeze when temps return to normal ranges. Perennial organs would be able to regain substantial hardiness via re-exposure to cold, but opening buds, leaves, & blossoms would remain at the mercy of ambient temps. So, it's probably closer to accurate to say that soil temperatures are the limiting factor in onset of growth in post-dormant temperate plants, especially once we pass the mid-Jan - Feb mark.

Al

Thanks Al, that was very informative. Like Mike, I've often wondered the same thig.

Plant dormancy is not solely controlled by temperature. In some case both chill hours and photoperiod are at work.

Thanks Happy, but first line of my first post:

"Simplified, but: As nights lengthen, the primary trigger for onset of dormancy..."

You probably already know this, but technically, it is the lengthening of dark period & not the reduction of daylight or day length that is the primary stimulus telling plants to ready themselves for a winter rest.

Al

I have heard that about the dark period being the impotant aspect of photoperiod sensitive plants. That even a very short exposure of light during the dark phase will reset the plants 'clock'.
Thanks again for all of the great info guys.
Mike

Actually my photoperiod comment was directed at the statement.

Thanks for all the information everyone. Plant ecology has always been one of my favorite subjects, and your comments have helped me see more clearly an aspect of this.

The whole provenance issue is very interesting to me, especially with plants that have extensive natural ranges. As an example, in the US I've seen viable populations of mountain laurel growing from the Florida panhandle clear up to New England. The range in chill hours would be somewhere around 550-600 in Florida, and into the thousands (?) in New England. It is fascinating that some species are able to adapt to such wide ranges (especially a broad leaved evergreen) while others have very narrow ecological reqirements. It would be interesting to know why such plants show so much plasicity in their tolerances. No doubt the adaptive advantage is great in both the short and long term for such species. PF

Is anyone still paying attention on the idea of chill hours? If the question of 'why' is aimed at an ecological rationale that has been honed by natural selection, then a chilling requirement is clearly a mechanism to escape damage from late frosts.
How strong can such a requirement be? I have studied growth of Pinus elliottii in Florida that did not break dormancy in January (budbreak) though temps went up to 90F and rainfall was heavy. And that's a kinda subtropical beast.

Seems I'm stuck on fruit trees and infact it was partly the early flowering of the peach tree that got me thinking. I say early 'cause more often than not in Ms. we'll get a stretch of warm weather say Feb. / March and the peaches will begin to bloom. Then here comes winter again and the flowers all get taken out. No flowers no fruit.
It is a touchy subject when discussing fruiting plants grown as a food plant. Apples are an iffy plant down here. If I plant a low-chill apple and get one of those early warm ups the trees bloom and I lose those flowers when winter reasserts itself. PLant an apple with too high of a chill time requirement and I'll get no flowers at all. Again no flowers no fruit.

HA!!! Thats why I love those figs ... and I must also add blueberries. They have near failed to produce fruit for me. Them and any of the late blooming fruiting plants.

Thanks again all for the great info.

Mike

I think my apple trees need more chill, is there any commercial product for chilling deficiency?