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