It depends on the type of system you build. The roots need air, just as much as the leaves do. If you build a water culture system, the roots are submerged 24/7, but you need to aerate the water sufficiently. You can always leave a air gap between the water level and the growing media, but you still need to aerate the submerged roots. As well as make sure the above water roots get enough moisture and humidity so they don't dry out, and are able to wick up enough moisture to the growing media, so the main root-ball dosen't dry out.

In the other 5 types of systems the roots get air various ways. But how much and how well depends on how you build the system/s, the type of growing media used, how much of it is used, as well as how often you water, and water temperature. Water temp is important to water culture systems too.