Literary: This is a very clear reference to the Tree of Life in Lehi’s (and Nephi’s) dream (1 Nephi 8:11 for the description of the fruit, including its whiteness). We must suppose that even though we do not see much of that story in the Book of Mormon, it was nevertheless well known such that Alma could make this reference assuming that his audience would understand the reference to the white fruit. Even though the Zoramites were apostates from the Nephite gospel, they must have been participants in the general cultural worldview, and somewhere in that culture or the religion they abandoned, the Tree of Life as understood by Lehi must have been taught. Alma doesn’t explain it, he absolutely expects that the imagery will be understood.