Syntax highlighting in R (1.6.1) files autoloaded in Emacs (21.2) from desktop

[M.Kondrin]

How to turn it on? If I run Emacs and it have R files stored in desktop 
file from previous session then syntax highlighting does not turned on 
(with C-x C-f loaded files it works OK). M-x ess-mode does not help and 
produce a message "Wrong type argument: syntax-table-p nil". Entering 
ess-mode after a child (inferior) R session is launched goes smoothly.
What is wrong?
Thanks in advance