hi Rafael,
You need to pass the material master data into line item of bapi_po_create1.
for example in line item of bapi_po_create1 short text, you need to pass the description,
like wise for material group you need to pass to MATL_GROUP,
kindly check the structure BAPIMEPOITEM.
This functional module wont retreive data from master, you need to maually fetch data from tables and need to populate in item table of the bapi.
With regards,
sampath kumar.