Material number mandatory (field selection) based on acc. ass. category

Hy Folks,

i am looking for a solution to define the Material Number as a mandatory field if the Account Assignment Category "Q" is used in a Purchase Order Item. Field Selection of Account ASsignment category Q is not sufficient as the material number is not included.

Any ideas for a solution in SAP standard customizing? (sure, user exit will be possible...)

THX!

Gregor