I suppose you want an aswer with a bit more foundation than this wikipedia entry:

http://de.wikipedia.org/wiki/Hiroshimana

"It is named after the Hiroshima prefecture in Japan, where it is abbundantly grown."

Henk-jan, thank you, you are right. I used to travel across Hiroshima-ken and learned a story about local monk who visited Kyoto at the begining of Edo bakufu, and brought some seeds of pak-choi (???) that was crossed to local Brassica rapa (???) and gave plants most suitable for planting at rice fields before winter. They have high tolerance to short day and low light (highest among the species). Local cultivars are mostly self-compartible and very uniform in morphology and physiological reaction. My colleaguse believed that it was some introgression of B. japonica (mizuna/mibuna/Asian turnip), but, there is no solid confirmation for such idea.

I thought you knew the easy bits already. When it is something with extreme flowering times you may want to ask Judith Irwin @ JIC, she is quite a specialist on heading in brassica.