Adventive embryony in citrus is due to : -

Adventive embryony is a type of apomixis in which embryos develop directly from diploid cells of the ovule other than the egg, without fertilisation.
In Citrus (and mango), some nucellar cells surrounding the embryo sac divide and grow into the embryo sac to form extra embryos (nucellar embryos). This leads to polyembryony.
These embryos are genetically identical to the mother plant.
So adventive embryony in citrus is due to the nucellus.