yeah, I know. I'm still saying that it can't be done, that the sex of the seed is genetically programmed. Even if it could be done, what would be the point. A female that should have been a male will not turn out to be a very good plant.
I used to think this myself, but decided to do some research to better understand what factors were in play and this is what I found -
You're right in the sense that the plants sex is determined at fertilisation - it takes 10 of the chromosome pairs from the father and 10 from the mother and the seed can be either a genetic female, male or hermaphrodite. So the seeds sex is set at fertilsation and as soon as the seed is germinated that genetic program starts.
The seed starts out at its pre-determined sex, but environmental factors now come into play which will determine whether the seedling continues on its set genetic program or deviates from it depending on the amount of stress and unfavourable conditions the environment may impart. What actually happens is, the plant can 'reverse its sex' either fully or in part depending on the severity of the stress and environmental factors. A full sex reversal simply means the plant changes sex entirely either from female to male or male to female depending on the environmental factors. Partial sex reversal are basically hermaphrodites.
It's important to differentiate between genetic hermaphrodites, which receive the gene/chromosome responsible for hermaphroditism and partial sex reversal hermaphrodites that become that way through environmental factors.
This is why people tend to report a far higher percentage of hermaphrodites from feminised seeds than people growing from standard non-feminsed seeds, as the former will usually be genetic hermaphrodites and the latter environmentally influenced ones. Genetic hermaphrodites don't receive the male gene or chromosome - because the father is a hermaphrodite it either receives the female gene/chromosome or the hermaphrodite one.
Robert Connell Clarkes Marijuna Botany is an interesting read on this subject.