update panel inside an gridview

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ASP.NET

Gridview

UpdatePanel

Hello froends,
I am using updae panel inside an gridview.My motive is to enable a dropdown on the selectedindexchange event of an another dropdown.
But it always throw this error
Could not find an event named 'CheckedChanged' on associated control 'ddlreason2' for the trigger in UpdatePanel 'upd3'.

My code is :

<asp:TemplateField HeaderText="Reason" ItemStyle-Width = "150">
          <ItemTemplate>

             <asp:UpdatePanel ID="upd2" runat="server">
          <ContentTemplate>
           <asp:Label ID = "lblreason" runat="server" Text='<%# Eval("reason") %>'></asp:Label>
            <asp:DropDownList ID="ddlreason2"
              AutoPostBack="true"
              onselectedindexchanged="ddlreason2_SelectedIndexChanged"

               runat="server" Visible = "false">
              </asp:DropDownList>


                     <br />
              <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"  InitialValue="Select Reason"
              ControlToValidate="ddlreason2" Text="Select Reason" ForeColor="Red"
              ></asp:RequiredFieldValidator>
          </ContentTemplate>
            <%--  <Triggers>
          <asp:AsyncPostBackTrigger ControlID="chkAll" EventName="CheckedChanged" />
              <asp:AsyncPostBackTrigger ControlID="CheckBox1" EventName="CheckedChanged" />
          </Triggers>--%>
          </asp:UpdatePanel>
          </ItemTemplate>
      </asp:TemplateField>
      <asp:TemplateField HeaderText="Sub Reason" ItemStyle-Width = "150">
          <ItemTemplate>


             <asp:UpdatePanel ID="upd3" runat="server">
          <ContentTemplate>
            <asp:Label ID = "lblsubreason" runat="server" Text='<%# Eval("subreason") %>'></asp:Label>
           <asp:DropDownList ID="ddlsubreason" runat="server" Visible = "false">
              </asp:DropDownList>
                                     <br />
              <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server"
               InitialValue="Select Sub Reason"
              ControlToValidate="ddlsubreason" Text="Select Sub Reason" ForeColor="Red"
              ></asp:RequiredFieldValidator>
          </ContentTemplate>
           <Triggers>
          <asp:AsyncPostBackTrigger ControlID="ddlreason2" EventName="CheckedChanged" />
              <asp:AsyncPostBackTrigger ControlID="CheckBox1" EventName="CheckedChanged" />
          </Triggers>
          </asp:UpdatePanel>
          </ItemTemplate>
      </asp:TemplateField>

Please tell me can i do it indside an gridview and how.
Thnaks in advance friends.