I'll just mention this, regarding the question of fruit production impacting tuber yield, and this is not grafting related ... there was a study done at Colorado State University done one year with a potato variety. (I can't recall, but it was a Russet.) They did yield evaluation of two groups of the same variety, and measured yield from one set that they let flower and produce fruit (seed berries), and the other set they removed all the berries. This was done to see if berry production adversely affected tuber yield. There was no difference.
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