These keyboard shortcuts provide the fastest way to access content on your Google+ Notifications panel. They are usually more efficient because the shortcut takes you directly to the right spot of the new messages, instead of you rolling around the notifications panel using the middle mouse wheel.
Here are the shortcuts you can use to quickly navigate between Google+ notifications:
Directional Arrow Up and Down – These keys will allow you to scroll quickly through the open notification list. Active notifications are marked with a thin blue vertical bar next to each individual notification.
Enter and O – Both will expand/open the currently selected notification.
Scroll to the end using the Directional Arrow Down and then hit Enter – These actions will open the previously read notifications area.
Directional Arrow Key Left and Right – These will open the next or previous notification. You must have an open notification to use this shortcut.
D – Mark your notification selected as “read”.
U – This key will take you back to the notification list.
Esc – Closes the entire notification panel.
Unfortunately, Google has not yet developed a specific shortcut to open the notifications panel. That way, you’ll still have to use the mouse and click the bell icon on the top right corner of the Google search main screen or your Plus profile to access it.
See More At AndroidDump.