macports which port owns a file

[Roger Pack]

Question:
is there a way to determine which port "owns" or "put down" a file,
for instance (relative to a comment at the bottom of
http://trac.macports.org/ticket/29198 )
I'm trying to add a dependency for this file, to mplayer:
/opt/rdp_project_local/lib/libXss.1.dylib
however, "port search xss" doesn't easily reveal an answer.
Thank you.
-roger-