JOIN where you want null columns

I'm flummoxed on this one.  I have a class that's building a query which
selects data from 1-n tables based on a common indexed id, io_id.  These
tables may contain 1-n rows of data keyed on io_id.  What I want the
query to do is return nulls for replicated columns rather than just
replicating them.

Here's the (relevant) data:

opt_io_vegetables_id:
  id | io_id | opt_val
----+-------+---------
  27 |   274 |       1
  28 |   274 |       3
  29 |   274 |       5
  30 |   274 |       7

opt_io_fruits_id:

  id | io_id | opt_val
----+-------+---------
  12 |   274 |       9


opt_io_name_text:

  id | io_id |             opt_val
----+-------+---------------------------------
  12 |   274 | Text... text... text... text...

I have this query:

SELECT
     A.opt_val,
     B.opt_val,
     C.opt_val
FROM
     IO io
     INNER JOIN opt_io_vegetables_id A ON io.id = A.io_id
     INNER JOIN opt_io_fruits_id B ON io.id = B.io_id
     INNER JOIN opt_io_name_text C ON io.id = C.io_id
WHERE
     io.id = 274;

It returns:

  opt_val | opt_val |             opt_val
---------+---------+---------------------------------
        1 |       9 | Text... text... text... text...
        3 |       9 | Text... text... text... text...
        5 |       9 | Text... text... text... text...
        7 |       9 | Text... text... text... text...

What I'd *like* the query to do for the replicated columns in $col[1]
and $col[2] is return nulls.

Is there any way to do this?