If you want to go farther in this, here's a search page with lots of interesting leads --
http://www.goodsearch.com/search.aspx?keywords=buckwheat+related+to+rhubarb

...and related to rhubarb!

Buckwheat is actually not a grain at all, but a seed. It's completely unrelated to wheat, despite the name. More info here:
http://en.wikipedia.org/wiki/Buckwheat
It's quite safe for gluten-free diets, as well as delicious and nutritious!

Buckwheat is its own grain, and is gluten free. It is not wheat.