Could anyone explain the mechanism by which dwarfing rootstocks promote earlier fruiting?
I'm not sure how specific of a botanical description you are looking for, but dwarf rootsocks are trees that genetically stay small- are non vigorous (just plain weak and whimpy). This means the tree matures earlier- like a 3 year old boy sprouting a beard- and a mature tree invests most of it's energy in seed at the expense of vegetative growth.
If you allow an M26 rootstock to grow its own tree it will stop growing vigorously when still a bush sized tree. It will become sexually mature (produce fruit) probably 3 times as quickly as 111.
Like hman, I've always operated under the assumption that it's something of a physiologic stress situation - weak root system(hence the need to stake or trellis many) and perhaps, poorly-functioning vascular system that may serve to cause greater accumulation of carbs and auxins in the grafted scion portion, accelerating maturity and bearing. Or, perhaps, the weak support system causes the scion variety to 'think' that it's on the verge of starvation and death, so it makes a greater effort to procreate (yeah, I know, I'm anthropomorphising).
I've seen articles indicating that bark inversion - removing a 1" strip of bark all the way around the trunk, inverting it and replacing it, will interfere, for a number of years, with normal transport of sugars/auxins, etc. down the trunk, initiating early flowering/fruiting, due to accumulation of these compounds in the top of the tree; I've always imagined that perhaps dwarfing rootstocks might be creating a similar scenario.
Are we saying that stressing the scion causes it to try to push fruiting?
Or is there some chemical/hormonal/enzimatic agent in the dwarfing root stock that interacts with the scion to change it so that it fruits more quickly?
Personally, I also think it's stress related,..you see this allot in other trees, when in stress,...making fruits or seeds early to insure future generations,..smart trees!
I do not think it is stress related at all, except in that it is stressful to be putting your energy into producing the next generation as opposed to putting all of it into your own growth and survival. I believe the stress is entirely the result of early maturity.
Does an Arkansas Black apple tree bear fruit 3 times as quickly and grow much more slowly than a Northern Spy on the identical rootstock because it is stressed?
From what I read, the mechanism can be from a variety of factors, such as all those mentioned on this thread.
In the paper, "Understanding how Rootstocks Dwarf Fruit Trees", the authors claim the effect is purely hormonal.
Although this may be true for the rootstocks they tested, it is not the dwarfing mechanism for all rootstocks.
In the paper, "The Dwarfing Mechanism of Citrus Rootstocks..." (see link below) which is more recent research, the authors suggest the dwarfing mechanism for the citrus rootstocks tested was not hormonal.
They summarize the various mechanisms of dwarfing very well:
"The dwarfing influence of the rootstock over growth of the scion can have several characteristics.
First, there are rootstocks that induce intrinsic dwarfing effects over the scion (Cheng and Roose 1995). In this case, the self-rooted rootstocks also show a reduced size.
A second model implies an indirect effect that depends on the interaction between both rootstock and scion (McKenzie 1961, Lockard and Schneider 1981). Here, the dwarfing ability varies with the scion variety and disappears when self-rooted rootstocks are used.
Third, some rootstocks are postulated to modify canopy development as a result of a negative influence of reproductive development on vegetative growth (McKenzie 1961, Lenz 1967).
Other physiological aspects, such as photosynthetic capacity, nutrient metabolism and water relationships, may also be involved (Carlson 1974,Syvertsen and Graham 1985, Schechter et al. 1991).
Lastly, it is well known that citrus scion growth can be influenced by pathological agents such as the exocortis viroid on susceptible rootstocks (Rabe et al. 1992)."
Although this paper specifically applies to citrus, their sources used in the text I quoted above, are from research from other fruits as well as citrus. For example in their second mechanism postulated (i.e. some degree of graft incompatibility causes the dwarfing) they site research from apple testing (i.e. Lockard and Schneider 1981)
Additionally their last mechanism postulated doesn't apply to just citrus either. I remember reading once, one of the Malling roostocks had a dwarfing effect, but when researchers cleaned it from viruses (i.e. EMLA) the dwarfing effect was removed.
Here is a link that might be useful: The Dwarfing Mechanism of Citrus Rootstocks F7A 418 and #23...
This post was edited by olpea on Wed, Apr 30, 14 at 10:08
Thanx to all for the various responses.
This is why GW is such a great portal of information so easily accessable and understandable by the professional and non-professional grower alike.
I have access to an otherwise healthy 6-7 year old apple branch that needs to come off a neighbor's tree. I will air layer it over the summer and try an experiment. I will plant it and also graft off the same tree to a dwarfing stock and see what happens.
Earliest report next year
This post was edited by mes111 on Wed, Apr 30, 14 at 12:09
I was thinking of the history of the English apple dwarfing rootstocks, which has mostly to do with the nature of the rootstock itself, I believe, because they were originally selected based on the growth habit of each rootstock as an individual tree. Then they were tested on how they would interact on other scionwood- so they were all self dwarfing as a starting point.
I have no information on the starting point of subsequent breeding programs at Cornell and elsewhere but would be interested to know if they used the same approach.
Just because a virus plays into it it is still the genetics of the tree that makes it susceptible to the virus in a way that is dwarfing. They are not necessarily a mutually exclusive factors.
Maybe they should be experimenting with various viruses.