Thanks for your suggestions. Actually I tried with even lookup columns also. The problem with it is when I search for text items(values in the lookup column) search results is not returning anything.
By investigation I found out that if I search only by numbers i.e id of the lookup list items then it is returning the required items in the search result. But it is not intuitive in nature.
So as a workaround I created a Calculated field for every dropdownlist column. Then mapped Managed property to these column. It is working fine then.