Populate comboBox with actual relation name and rest records names

Question

Hi there i got some problem i got 2 tables:

plain_table
-------------------------
plain_id
plain_name
FK_temperature_id


temperature_table
-------------------------
temperature_id
temperature_name

it's 1:1 relation


I got a datagridview where i list values from plain_table and left join to temperature. I can choose some record from datagrid - event on click in datagridview some record from plain_table then i populate controls plain_id - textbox, plain_name - textbox and FK_temperature_id - by combobox

my problem is i want to populate this combo by set first value of this combo on actual relation name from temperature_name, and then rest populate it rest of temperature_name's

ok some of code i made:
this how's I go to other form and take plain_name value from datagrid:

Code:

myfrmPlainEdit.plain_name = GrdPlain.CurrentRow.Cells[0].Value.ToString()

Now i am on second form

Code:

public string plain_name;

now i trying to populate controls:

Code:

        public void frmSamolotyEdytuj_Load(object sender, EventArgs e)
        {
            try
            {
                DataSet dataSet = new DataSet();

                using (SqlConnection connection = new SqlConnection("Data Source=TOSHIBA;Initial Catalog=plain;User Id=sa;Password=qazwsx;"))
                {
                    connection.Open();

                    using (SqlCommand command = new SqlCommand())
                    {
                        command.CommandText = ("select * from temperature");
                        command.Connection = connection;

                                    using (SqlDataAdapter dataAdapter = new SqlDataAdapter(command))
                                    {
                                        dataAdapter.Fill(dataSet); //filles each query as result in seperate datatable

                                        DataTable dt1 = new DataTable();
                                        dataAdapter.Fill(dt1);

                                        CboNewTemperatura.DataSource = dt1;
                                        CboNewTemperatura.DisplayMember = "temperature_name";
                                        CboNewTemperatura.ValueMember = "temperature_id";
                                        //CboNewTemperatura.SelectedValue = "temperature_id";
                                    }

                                    using (SqlDataReader dr = command3.ExecuteReader(CommandBehavior.CloseConnection))
                                    {
                                        while (dr.Read())
                                        {
                                            TxtNewPlain_name.Text = Convert.ToString(dr["plain_name"]);
                                        }
                                        dr.Close();
                                    }
                                }
                            }
                        }
                    }
                }
            }
            catch
            {

               
            }
            finally
            {

            }
        }

i populate a combobox, but how to set first value to actual name in relation with record i pick from datagrid? I trying with selectedvalue but..
plz help