First, I created the class 'product_template' inherit to add the field 'new_field'

class product_template(osv.osv):

  _name = 'product.template' 
  _inherit = 'product.template' 

 _columns = { 
 'new_field': fields.many2one('product.uom','Unité de Stockage',required=True, ondelete='cascade'), 
 } 
product_template() 

After, the xml

    <record id="purchase_order_form" model="ir.ui.view">

        <field name="name">purchase_order_form</field>

        <field name="model">purchase.order</field>

        <field name="inherit_id" ref="purchase.purchase_order_form" />

		<field eval="90" name="priority"/>

        <field name="arch" type="xml">

		<xpath expr="/form[@string='Purchase Order']/sheet/notebook/page[@string='Products']/field[@name='order_line']/tree[@string='Purchase Order Lines']/field[@name='price_unit']" position="before">

                <field name="new_field_po" string="U Stock"/>

			</xpath>

		</field>

    </record>	

And the new py

class purchase_order_line(osv.osv):

    _name = 'purchase.order.line'

    _inherit = 'purchase.order.line'

    _columns = {

        'new_field_po': fields.related('product','new_field',type='many2one',relation='product.product',string='Title',store=True),	

The creation of new_field is correct in the product_template.

But when I create a PO, this does not work, the line does not take the value of the field, it remains empty