rearrangement of string lists

I have a list:

lis = {{"abc", "def", "ghi"}, {"jkl", "mno"}}

and you want to obtain:

res = {"abc def ghi", "jkl mno"}

Is:

Table[StringJoin[lis[[i]]], {i, length[lis]}]

does not produce the desired "" among the original elements in lis. As always, thanks for the suggestions!