Judging by the derivative chain lecture appears to be the root itself (comp. lecture, lectur-er, lecture-ship, un-lectur-ed etc.). But I'm not sure if it can be divided by analogy with failure (to fail), creature (to create), departure (to depart) etc. The question is whether "lecture" functions as a monomorphemic or bimorphemic word in Modern English.
I think one can safely deem "lecture" as monomorphemic, as its etymology, by Etymonline.com, shows:
As you can see, it comes from the Latin word lectura, which means that "lecture" is itself the root word, as its Latin root isn't something like lec or tur. This can be compared to other monomorphemic words, such as "wait" as well as "man"