i haven’t tried that yet, maybe others can answer this question. Some taxonomists identify paks as Morus macroura, while others dub it M. alba. Regardless of nomenclature, it is supposedly closely-related to other albas(and considering the geographical proximity of pakistan to middle east), i can bet my bottom dollar they are compatible. Nigra’s are compatible with alba’s native to china, so pakistan being so much closer would indicate it would be at least as closely related to nigra’s as alba’s are.
i did graft a couple of pak onto a noir nigra in feb this year(when paks were already leafing out), and neither of them took. Failure was likely unsatisfactory technique, but also likely to our unusually long winter, resulting in nigra sap taking forever to start flowing(as a result of protracted cold weather). Since you brought it up, will try to graft nigra this year onto pak, and vice-versa, when am sure both interstems are still flowing with sap. Hopefully at least one for each permutation will take despite the extreme heat we’ve been having lately.
here, paks grow just as fast as the generic alba’s(russian/chinese rootstock types), so not sure if it will accelerate growth. Btw, nigra’s will grow fast in shady conditions and good irrigation, but unfortunately will not accelerate ‘seasoning’ of wood. Interstems will be longer, and buds will be tiny relative to girth of stems, which equates to poor berries and low berry output relative to unit length of stem. Unlike precocious fruit species (like jujubes), beheading an old alba tree with an enormous trunk caliper and bark-grafting with nigra will result in fast nigra growth(about 2 feet in one year, even in full-sun conditions here in las vegas), but this is more of a physiological response to the large rootball, relative to the few nigra buds grafted to it, so the early nigra growth tends to be more vegetative in function than fruit-bearing in function.
nigras have long gestation periods, and live for hundreds of years, so many of us yanks are novices, considering it was merely imported to the americas relatively recently, and virtually all as grafts to alba rootstock. Noirs and persians will grow 30 feet, you bet, but probably not the trees am growing—for what’s left of my projected lifespan! Black beauty nigra’s don’t grow any faster or slower than noirs or persians, so will probably attain 30 feet or taller as well. What some nurseries sell as ‘dwarf black beauty mulberries’ are nothing more than a black beauty cultivar grafted low to an alba rootstock. A noir or persian grafted low will have the same rate of growth as the black beauty, all things being equal. As for grafts overpowering others, it will depend on vigor of rootstock and location of the grafts and species of grafts. Safe to say that an alba graft will be ‘prioritized’ by an alba rootstock over a nigra graft. I don’t see a noir being any better or worse than a persian or black beauty grafted on the same rootstock, other than location of respective grafts(amount of sunlight the grafts will be getting).
incidentally, folks across the atlantic have centenarian nigra’s, and i personally defer to. Check out @Carld 's nigra posts here . Nigra’s are old world natives, and can grow from seed, so possible some of these 30 ft tall trees are on own roots and have uniquely individual growth habits