PR and SERP’s have little to nothing in common. A PR 2 site with good SEO will rank higher in the SERP’s than a PR 6 site with little to no SEO done. Not to mention if PR has any influence it would be google’s serps only as PR is a google thing.
It’s thought that if you have a gambling site and exchange links with other gambling sites your PR will increase faster than exchanging links with other non-gambling sites of equal caliber.