Sexual Differentiation in Monoecious plants with unisexual flowers
I will be able to answer only a part of the question.

Sex determination in Zea Mays

Initially, all the flowers in the plant are perfect, ie they have both the male and the female
flowers. This is followed by abortion of stamen or pistil resulting in unisexual flowers.

Which part of the flower undergoes abortion is determined according to the location of the flower. The florets in the tassel undergo the suppression of the pistil primordia resulting in staminate flowers, while the florets in the ear undergo suppression of stamen primordia resulting in female flowers.



The hormone Gibberelic Acid seems to play a role in sex determination.

Source and other useful links :


Sex determination in Plants by CC Ainsworth.
Sex chromsomes in flowering plants
Sex determination in monoecious and dioecious plants
The sex determination process in maize