North American Fruit Explorers mailing list at ibiblio

[Kevin Bradley <kbradley@ttlc.net>]

Dave, from MN wrote: 

I think the main benefit from having a hardy understock for a
tender
tree
> is induction of early dormancy of the cultivar by the rootstock.

I have never seen a
rootstock induce early growth in its grafted  cultivar and so
perhaps falsely assumed that it would not induce early dormancy. I will
check this this fall on my topgrafted peaches on p americana compared to
those on maheleb.
I have seen a ranetka sucker break bud under a dormant Sweet 16 that
broke bud exactly the same time as those grafted on antonovka that is,
about 10 days later on average. In other words, the rootstock had
absolutely no influence on the bud break of the cultivar.
I am nearly convinced that the main advantage to topworking is the
"tender crotch/trunk" theory, that is that the weak links in
deep winter cold endurance are the crotches and trunks and not the outer
branches. I don't think I buy the "inducement" theory (of
inducing hardiness from hardy frames to tender grafts.) 
There were some recent experements on this but very poorly conducted
planned and inconclusive in my opinion. 
I speak, BTW, not as a plant physiologist but as an amateur who reads a
lot on this subject and who makes observations in the nursery.