How closely do the plants have to be related for them to be grafted? What are the possible constraints which can prevent two plant species from being grafted together? I guess there is no prominent immune system fr plants
there have been rare instances of lemons grafted onto apple trees . But this is the exception rather than the rule .
In general all stone fruit can be grafted in the same tree apriocots , plums, prunes, peaches, cherries, necterines.
all core fruits : apples, pears,MOUNTAIN ASH, quince , crabapples ,Asain pears and quince can be grafted and crossbred as well.tomato tops have been grafted onto potato roots but I don't recomend it. If you are thinking of something just try it the worst that could happpen is you will fail . AND YOU MIGHT SUCCEED WHERE NO OTHER PERSON HAS YET. GOOD LUCK!