Are grapefruits or nectarines Kosher? Is it permitted to hybridize lemons and oranges?
ב"ה Shalom According to the Torah, one may not graft trees of different species. However, after the tree has been grafted the fruit it bears is not forbidden and may be eaten. (Rambam Kila'im 1:7, Shulchan Aruch YD 295: 9) Therefore, grapefruits which originally came from a graft may be eaten. Nectarines are not a grafted fruit, they were developed from a peach and by a natural mutation they became without fuzz, so they may also be eaten. As far as lemons and oranges, although they are both citrus fruits, according to the Torah and Land Institute (Machon HaTorah Ve'Haretz) based upon rulings of Rav Mordechai Eliyahu zt"l , and Rav Shaul Yisraeli zt"l, lemons and oranges are categorized as two different species and therefore they may not be grafted. All the best