How to place leaves in a 12-well microtiter plate

Well, it depends on the size of your leaf. For Arabidopsis, rosette leaves are small enough to fit quite happily in 12-well plates (for most lines I've worked with). Ofcourse, the simplest solution is to switch to a 6-well plate if your leaves are bigger. This will not negatively influence the protocol. Another reasonable option is to make leaf discs or leaf cuts. I have tried with both, and the protocol works perfectly fine in both. The thing to consider here is, please float your leaf discs or cuts for at least 8 hours in water prior to starting the DAB staining experiment, because this will stabilize the levels of ROS that would be induced due to wounding when you make your leaf disc or cut. It is important to stabilize this. I have tried this many times and it works fine.