Allopolyploid infers a plant composed of the full genomes of two related species that usually have different chromosome numbers. A rutabaga is an allopolyploid of a turnip and a cabbage (we’ll argue details later, see triangle of U) that occurred long ago. The resulting plant contains 38 chromosomes which is the full complement of both parents. This differs from a tetraploid which usually results from chromosome duplication in a single species. Okra is an allopolyploid as another example. I have not seen anything published indicating an allopolyploid in apple other than the chromosome doubling event about 65 mya. Do you have documentation showing an example of an apples that is an allopolyploid? https://en.wikipedia.org/wiki/Triangle_of_U | http://www.jamesandthegiantcorn.com/2010/08/31/apple-genome-part-2/
I’m going to also add some information about the reason why a triploid can produce seed but not viable pollen. Meiosis in plants diverges significantly as compared to animals. The short version for pollen is that a pollen grain for most plants contains two duplicate cells each with a single set of chromosomes. When there are three sets of chromosomes, only two sets can pair up to initiate division leaving one set of chromosomes unpaired and therefore unable to duplicate and then split into paired pollen cells. Rare events can cause even a triploid to sometimes produce a small number of viable pollen grains. This is significant for example in potato breeding where triploids can be generated and sometimes be induced to produce just enough pollen to be used in breeding.
For the ovule, the process is dramatically different. I am going on memory with this so please check that it is correct. The primordial female cell initiates a duplication where it doubles the number of chromosomes, then doubles the number of cells, then repeats until there is a cluster of 16 cells in the proto-embryo. Eight of the cells are discarded leaving 8 cells to form the embryo. A single set of chromosomes winds up in the cell that can be fertilized by pollen, two sets wind up in the structure that becomes the endosperm, and 5 cells move into support functions. When the pollen grain moves through the style, it splits in half with one set of chromosomes merging with the single set of chromosomes in the ovule and the other merging with the 2 sets of chromosomes in the proto-endosperm. The result is that most of time the seed contains an embryo with 2 sets of chromosomes and triploid endosperm that has 3 full sets fo chromosomes. The endosperm is discarded when the seed germinates and depletes the stored reserves leaving only the growing plant which is normally disploid. Now ask the question, what happens when the pollen grain is from a tetraploid? The resulting seed has 3 sets of chromosomes in the embryo and 4 sets in the endosperm. Voila, now you have a new triploid apple!