very cool study
In (if I remember correctly) the North American Fruit Explorers publication North American Pomona more than 30 years ago there was some discussion of ways to improve hardiness of fruit trees, especially of peaches, by grafting onto hardy rootstocks and by grafting branches of a hardy variety (such as Siberian peach) onto the leader of a tree of a higher-quality but less hardy variety. The expectation was that the hardier variety would provide hormones or other growth control factors that would control the dormancy of the less-hardy variety, preventing early flowering, sending the tree into dormancy earlier in the fall so that it would suffer less from early freezes, and making the dormancy deeper so that flower buds would not be killed during the winter.
I did read some anecdotal support of the success of this tactic, but never saw the results of an objective, large-scale study of the success of this method. Maybe it would be successful on a small scale but not economically feasible on a large scale in a commercial orchard.
Griffith Buck developed hardy rootstocks such as the IT-9 and IT-18 so that grafted plants of his hardy hybrids would not be killed by the freezing of rootstocks less hardy than the scion cultivar. He was developing these rootstocks during about the same time that fruit tree enthusiasts were experimenting with grafting as a way to make it possible to grow good quality peaches and other fruits north of their normal ranges.