Nissan is one of the biggest manufacturers in the UK and exports the vast majority of its vehicles to the EU. It has consistently committed itself to the NE community and Sunderland in particular because of the highly motivated workforce there.
Nissan will now have to renegotiate its import standards into the EU, a straightforward procedure as no manufacturing processes will change at this point, though a costly and time consuming procedure, a large company like Nissan will have the resources to deal with the situation, however the renegotiation of standards to meet EU imports will have to be carried out by every single company in the UK that does or wishes to export to the EU... the implications for smaller business are dire.
And that's without figuring in the drop in currency value... Nissan have invested heavily in the North East, but global free trade has seen many huge multinationals upsticks and move to more lucrative markets for small percentage advantages to their operating costs.