Starting Arduino IDE as root fails on Archlinux with KDE

I need to run the IDE as root to get access to the serial port.

The command is ./arduino from inside the package folder.

On Fedora this works fine, but on Archlinux the pocess chrashes with the following output:

No protocol specified
Exception in thread "main" java.awt.AWTError: Can't connect to X11 window server using ':0' as the value of the DISPLAY variable.
        at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
        at sun.awt.X11GraphicsEnvironment.access$200(X11GraphicsEnvironment.java:65)
        at sun.awt.X11GraphicsEnvironment$1.run(X11GraphicsEnvironment.java:115)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.awt.X11GraphicsEnvironment.<clinit>(X11GraphicsEnvironment.java:74)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:260)
        at java.awt.GraphicsEnvironment.createGE(GraphicsEnvironment.java:102)
        at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:81)
        at sun.awt.X11.XToolkit.<clinit>(XToolkit.java:123)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:260)
        at java.awt.Toolkit$2.run(Toolkit.java:860)
        at java.awt.Toolkit$2.run(Toolkit.java:855)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.awt.Toolkit.getDefaultToolkit(Toolkit.java:854)
        at processing.core.PApplet.<clinit>(Unknown Source)
        at processing.app.Preferences.save(Preferences.java:747)
        at processing.app.Preferences.init(Preferences.java:250)
        at processing.app.Base.main(Base.java:117)