Cascading LOVS - default values

I have two cascading LOVs. After changing value of my first LOV in second LOV appropriate values are populated.

First LOV: name - P2_DEPTNO

select dname, deptno  
from dept  
order by 1;  

Second LOV: name - P2_EMPNO

select ename, empno  
from emp  
where deptno = :P2_DEPTNO;  

cascading LOV parent - P2_DEPTNO

What should I add to set default value of second LOV (first row from query) after changing first LOV value?