TurkeyCreekTrees,
It does seem to be something not discussed a lot so little is known about it. Not just rootstocks but interstems. I made a post specifically in regards to apples and interstems just last week Interstem questions - #6 by clarkinks. The article https://www.researchgate.net/publication/239804625_Interstem_and_Its_Relationship_to_‘Fuji’_Apple_Quality says the following
“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.”
The part that discusses in the article “more acidity” imparts flavor. Typically you hear experts discuss acid / sugar balance in regards to taste. Beyond knowing size of fruit and mineral content varied based on rootstock i’m not sure why I overlooked acidity and starches previously. Even more interesting the article notes “interstem effects on fruit quality were significant”. You might like this article on plums as well http://www.sciencedirect.com/science/article/pii/S030442380800229X or this one http://irrec.ifas.ufl.edu/flcitrus/pdfs/short_course_and_workshop/factors_fruit_quality/Wutscher-Rootstock_Effects.pdf