Can I apply in IPOs without having a demat Account?

You need to fill your DP ID and Client ID details in the IPO Application Form. If you fail to do so, your application will be rejected. So you definitely need a Demat Account to apply in IPO.

Also, the allocated shares through IPO bidding process can only be debited to your Demat Account, so you need Demat Account prior to investing in IPOs.