It is impossible. A grafting expert I spoke with said “I heard about that but dismissed it out of hand. The flowem and xylem are going the wrong way. Not how grafts work.”
But it does work.
The following story was related by Noris Ledesma, currently the Tropical Fruit Curator at Fairchild Gardens near Miami. It was during a grafting class at the garden attended by the Melbourne/Brevard Rare Fruit Club. In the 1990s, Dr. Richard Campbell, then the T.F. Curator at Fairchild, was in Chile visiting a tropical fruit expert, and part of their discussion was said expert’s technique for grafting certain trees. He spoke no English, and Dr. Campbell spoke enough Spanish to get by. Sort of. Excited by the possibilities, he returned to Fairchild, described it to Noris, and said “make it so”. She worked on it for some time and was able to successfully graft in this new way. Several years later, Noris, from Columbia, visited the expert and because her native tongue is Spanish, she was able to get more details. Dr. Campbell misunderstood. The expert was not grafting a scion onto a taproot placed upside down and the original top removed. But in this case of sheer serendipity, the translation error worked in spite of going against accepted grafting rules. It had several advantageous effects. First, the inverted graft tree was dwarfed. Second, it was precocious (fruited unusually quickly after planting–often within 2 years), it was well branched, had a strong fibrous root system instead of a tap root, and was unusually prolific. The technique works best when the seed is large, which allows the graft to have the time and food resources to grow. Avocado, Mamey, Canistel, Sapodilla, and Jackfruit invert graft ok. Mango has a large seed but the seed tends to fall off relatively quickly. After experimenting, it was found that the effect was most clearly seen with Canistel and Mamey grafts, especially on very young sprouts with very little top growth. The results were published in a research paper by Noris Ledesma and Richard Campbell in Proceedings of the Florida State Horticultural Society 2007, page 13 – The Inverted Root Graft: Applications for the Home Garden in Florida.
When I first heard of this from Adam Shafran of Flying Fox Nursery near Orlando during one of his talks at our fruit club and a grafting class at Trees N’More, prior to the class with Noris, it sounded intriguing. I tried it with a Brogden avocado, and it was my first (then, only) successful graft. I have not noticed any particular precocity yet except that it grew well and quickly into a dense bushy plant. 2017 is 2.5 years in the ground. Time will tell.
After reading the research paper and gathering some Canistel seedlings from club buddy Eric, and scions from my Trompo Canistel, Eric and I went back to the tiny taproots for a marathon inverted grafting session. As of 7/17/2017, there is new growth on one graft as seen below. Note the use of a tomato grafting clip for additional support of the thin graft. This is one of four grafts I did, and is my first to show new growth. Eric had even better results, with more and larger flushes already. His photos follow.