[R] add symbol above letter in R

[Zilefac Elvis]

Hi,

I would like to add a dash (—) on a letter in R.
How can I add a dash on letter P in the following:

mtext(text=expression(Winter(DJF)~mean~daily~precipitation~italic(P)), side=3, line=1, cex=1.3, col="black")

?plotmath could not provide an answer to my problem.

many thanks for your solutions.

Asong.