How To Disable TAX On PO - R12.1.3

Hi

Recently we enable  TAX for  AR and AP .with 5%. 

After this  - today we created  new  PO -for example 1000

In PO screen it shows  PO value as 1000

But in PO workflow document and workflow EMail the amount shows  as 1050 ( which is  1000 + 50 ( 5% tax)

Actually we no need to show  TAX on PO  as the TAX is effected on AP Invocie.

Can someone help -how to DISABLE TAX ON PO 

Thanks