[R] do calculation within expression(), text in plot

[Tonja Krueger]

   Dear Helpers,
   I would like to add some text to my plot, containing a variable and the
   calculated value for the variable.
   As  I  would  like  to  produce  many  plots,  I hope this can be done
   automatically.
   Unfortunately I can't get R to do the calculation for cor(nou,dak) when I
   use expression()..

   text(0,2.2,expression(paste(r[Pearson],"=", cor(nou,dak) , sep= " ")),pos=4,
   cex=1.5)

   Is there a command that "stops" expression() so the result is calculated and
   printed in the plot?
   Thank you in advance,
   Tonja