project_task_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <record id="view_project_task_form_issue" model="ir.ui.view">
            <field name="name">project.task.form.issue</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_form2"/>
            <field name="arch" type="xml">

                <field name="date_deadline" position="before">
                    <field name="ref"/>
                </field>
                <field name="partner_id" position="after">
                    <field name="issue_id"/>
                </field>
                <field name="description" position="before">
                  <group>
                    <field name="reason_id"
                           attrs="{'invisible': [('issue_id','=',False)]}"/>
                  </group>
                </field>

            </field>
        </record>

    </data>
</openerp>