Hi:

Variable xstage (real number) was devised to provide a continuos response to
changes in tissue N concentration with time.  In maize, wheat models xstage
did coincide with ISTAGE.  Therefore I would follow the above definition of
XSTAGE rather than Jones and Kiniry.  ISTAGE in CERES is also CERES-defined!

For more information consult Drs. Alagarswmy, Gangadhar Rao and Ritchie.


Regards,

Upendra