A word appearing as ᴱQ. palo (palu-) in the Qenya Lexicon of the 1910s, glossed “plane surface, plain, the flat” and based on the early root ᴱ√PALA having to do with flat things (QL/71).
Neo-Quenya: Since √PAL continued to be used for wide and flat things in Tolkien’s later writing, I would retain ᴺQ. palo for purposes of Neo-Quenya. However, I would use it for abstractions like “plane (surface), the flat, ✱flatness” and not for a regional “plain” for which I would use later words like ᴹQ. landa or ᴹQ. palar. I would further assume that palo can be used for a flat plane in any direction, not just horizontal.
A verb attested in Qenya Word-lists of the 1920s in a last of rhymes in -alle, with a past for palle “shook” (PE16/143). It might be based on a later variant of the early root ᴱ√PAPA “✱tremble” (PE16/143).
Neo-Quenya: Helge Fauskanger use this as the verb for “to shake” in his NQNT (NQNT), but I prefer [ᴱQ.] quas- as a more distinctive verb for “shake”; see that entry for discussion.