We can show the messages of purchase requisition in a new CHATTER BOX at Purchase Order/Quotation by adding a related field.

class purchase_order(orm.Model):
    _inherit='purchase.order' 
    _columns = {
                'requisition_message_ids': fields.related('requisition_id', 'message_ids', type="one2many"),
                } 

        <record model="ir.ui.view" id="purchase_order_form_inherit">
            <field name="name">purchase.order.sciencefirst</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form" />
            <field name="arch" type="xml">
                <xpath expr="//form[@string='Purchase Order']" position="inside">
                    <div class="oe_chatter">
                        <field name="requisition_message_ids" widget="mail_thread"/>
                    </div>
                </xpath>
            </field>
        </record>