PO Number on Email Invoice

How can I get the PO number into the notification email sent to the orders department? I added {$payment_info.po_number} to /stores/2/skins/{my_skin}/mail/orders/invoice.tpl. That didn't work.

I added this to my invoice.tpl



{$lang.po_number}: {$order_info.payment_info.po_number} in mail/orders/invoice.tpl





Not sure this is what you looking for

Thanks