In this case, the driver of the vehicle, you, will have to pay the fine. The owner will generally receive this by mail a while after the rental has ended and will get in contact with us or you directly.
If you refuse to go through with payment, the owner can send your contact details to the local authorities, with the rental contract enclosed. It is best to warn the owner that you have received a ticket, with all the necessary information, so that he can contest it upon reception.
*Be aware of low emission zones, and ask the owner of the vehicle, if you are not sure how to proceed.