I posted elsewhere today that mizuna would cross with bok choi as they are both Brassica rapa. I got queried on this by someone who thought mizuna might actually be Brassica juncea. A quick google search does show that seed suppliers have 'mizuna' in their catalogues under both B. rapa and B. juncea depending on which one you look at.
Sources like wikipedia, pfaf.org and The American Phytopathological Society and some other research papers use B. rapa for mizuna.
I think the source of the confusion might be that the common name 'mizuna' is used for two different things.
There are chinese mustards, like Ruby Streaks, that sometimes get referred to as 'Red Mizuna'. I'm fairly sure these are B. juncea. So a green chinese mustard with finely cut leaves could easily be referred to as 'mizuna' and be B. juncea. While the plant I commonly refer to as mizuna is still B. rapa.
Anyone able to shed more light on this?
Anyone know the botanical differences so we can identify whether they are B. rapa or B. juncea in the garden without resorting to a lab?