The USDA is a great source of information on rootstocks and you will find their research and knowledge on the subject is extensive http://portal.nifa.usda.gov/web/crisprojectpages/0073725-rootstock-and-interstem-effects-of-pome-and-stone-fruit-trees.html. The original question about interstocks aka interstems has been around awhile and you might be interested in a historical document http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3233&context=rtd&sei-redir=1&referer=http%3A%2F%2Fwww.bing.com%2Fsearch%3Fq%3Dinterstem%2Bcommerical%2Bapple%26src%3DIE-SearchBox%26FORM%3DIENTSR%26pc%3DEUPP_UE02#search=“interstem%20commerical%20apple” the research continued http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3469&context=rtd&sei-redir=1&referer=http%3A%2F%2Fwww.bing.com%2Fsearch%3Fq%3Dinterstem%2Bcommerical%2Bapple%26src%3DIE-SearchBox%26FORM%3DIENTSR%26pc%3DEUPP_UE02#search=“interstem%20commerical%20apple”
At this link http://www.boyernurseries.com/rootstocks.html you will again see confirmation of the widely known “EMLA 9 / EMLA 111-This interstem combination uses the dwarfing growth habit of the EMLA 9 with the benefits of the EMLA 111 root system.
BUD 9 / EMLA 111-This interstem combination uses the dwarfing growth habit of the BUD 9 with the benefits of the EMLA 111 root system.” Cornell and others mention similar information http://www.gardening.cornell.edu/factsheets/ecogardening/appleroot.html "M.9/MM.111 Interstem – An interstem tree consists of three parts: the scion variety, the interstem piece, and the understock. M.9/M.M.111 has all the advantages of M.9, but a stronger root system and somewhat better drought tolerance. Should be planted deep, with only the interstem piece showing. "
Ok so now onto the interesting stuff. This link https://www.researchgate.net/publication/239804625_Interstem_and_Its_Relationship_to_‘Fuji’_Apple_Quality clearly states the interstem has a signifigant impact on fruit size. It’s also widely known rootstocks have an influence on fruit taste, texture, mineral content etc. The article I just referenced states “In 1989, an 8-yr-old commercial block of ‘Granny Smith’ and Oregon Spur I Delicious’ apple trees on M.26 rootstock were grafted over to ‘Fuji’ (Nagafu 6). Before grafting, the original trees were cut back to 90 cm, leaving 20–25 cm of the original scion to serve as an interstem. In 1991 through 1993, 1 to 2 days before commercial harvest, 20 apples were harvested from each of 20 trees of each interstem treatment selected randomly throughout the block. Fruit characteristics (firmness, color, soluble solids concentration, acids, fruit weight, physiological disorders, and mineral content) were assessed on half the apples at harvest and on the other half after 120 days at 1 °C. At harvest, apples had similar starch levels regardless of interstem, but interstem effects on fruit quality were significant. ‘Fuji’ apples from trees with ‘Delicious’ interstems were larger, and had more acidity and more bitter pit than apples from trees with ‘Granny Smith’ interstems. ‘Fuji’ apples from trees with ‘Granny Smith’ interstems were firmer with more red color development and higher soluble solids concentration than apples from trees with a ‘Delicious’ interstem. After storage, ‘Fuji’ apples from trees with ‘Delicious’ interstem developed more scald than apples from ‘Granny Smith’ interstem. Fruit-flesh Mg, P, N and K; fruit-peel Mg, K, P, N and B; and leaf P, K, B, and N were greater in ‘Fuji’ apples with ‘Delicious’ interstems compared to ‘Fuji’ apples from trees with ‘Granny Smith’ interstems.” . Rootstocks typically used can be referenced here http://extension.psu.edu/plants/tree-fruit/commercial-tree-fruit-production/cultivars-and-rootstocks/apple-rootstocks . This is a summary of what influence the interstocks have Characteristics of Apple Rootstocks and Interstem Combinations
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