Dear Ed:
 
There is no species file in CERES-Wheat.  I don't think there is any way to
to change the specific leaf weight, short of getting into the FORTRAN code.
That restricts the ability to evaluate environmental effects on specific leaf
weight which as it works now, must come from the separate effects of leaf
area expansion and dry weight accumulation, each responding to their own
environmental signals.
 
But direct your question to Joe Ritchie.  He may have some help for you.
 
Best regards,
Ken Boote