Hello Inayat,
The plant code is not defaulted from the material master in the PR or in the PO.
To default organisational values in PR/PO, the system uses either parameters ID or user default settings.
See for instance thread for the PIDs : Procurement related Parameter ID and its Values
For the user default values, once in ME51n, click on personal setting, then go to the tab default values and choose more fields if the plant is not listed on the tab.
Cheers,
Alex
