Hello vishvanatha_achary,

set RowSelectionChanged AutoPostBackFlag to true and handle RowSelectionChanged event:

            <Behaviors>
                <ig:Selection CellClickAction="Row" RowSelectType="Single">
                    <AutoPostBackFlags RowSelectionChanged="true" />
                </ig:Selection>
            </Behaviors>

 ...

    protected void WebHierarchicalDataGrid1_RowSelectionChanged(object sender, Infragistics.Web.UI.GridControls.SelectedRowEventArgs e)
    {
          Infragistics.Web.UI.GridControls.ContainerGridRecord parentRow = ((Infragistics.Web.UI.GridControls.ContainerGrid)(sender)).ParentRow;  
    }

Hope this helps