the idea of having a number of rootstocks is to promote competition between themselves, provided each rootstocks is not related to each other. So by having a competition between the rootstocks, everytime u apply fertilizer, each root will try to grab the most nutrients and faster than the rest. So who benefited?? Its the grafted scion!!!!! Here is a sample matured mutliple rootstocks Durian tree in Indonesia.. The very reason why i started this obsession..
Yes, I also think having all the rootstocks competing just makes the scion benefit.
My (limited) experience so far is showing that some seedling rootstocks are just better than others-- if multiple-grafted, some just grow thicker and others stay thinner. As conditions change over time, the "better" rootstocks may change-- the point is that each rootstock is a unique attempt to adapt to the conditions as best as possible. Some may be better in drought, some in wetness-- but the scion benefits regardless.
It's possible that extra stress hormones caused by root crowding may be what causes more compact growth (at least on all my experiments so far).
I ran an experiment with bell pepper plants this summer; for each of the 5 varieties I'm growing I planted single-rootstock controls and 2-5 different combinations of plants with extra rootstocks grafted on. The 25 multiple-rootstock plants all seem healthier than the single-rootstock plants and with 1 exception are all out-producing the single-rootstock plants, and with larger fruit. Some varieties proved to be better additional rootstocks than others; so far in my yard a 'Big Bertha' rootstock seems to make the plant twice as large and fruitful regardless of the variety of the top of the plant. Even a 'Big Bertha' top benefits from an additional (seedling) 'Big Bertha' rootstock. It could be that 'Big Bertha' is just the best-adapted to my soil or climate, or that it's just inherently vigorous; either way I want to experiment more with it next year.
Multiple-rootstock tomatoes seem more vigorous and fruitful as well-- growth is more dense but also larger overall than single-rootstock plants.
Kevin