They won't play at all, but it seems you can the other way round click here
"BluRay discs carry high definition television and therefore store data in a high density format. A standard definition DVD player will not recognise a BluRay disc and will simply report an error if one is loaded.
On the other hand, a BluRay player will not only play BluRay discs but will also handle standard DVDs. Most will also recognise and play CD format discs. Most Bluray players will also down convert the HD signal from a BluRay disc so that it can be viewed on a standard definition television. In this case, of course, the image quality will be the same as any other standard definition signal. To benefit from the high definition signal, an HD television is also required. "