I think you will find a lot of information - and good sources - in this article: Air Reconnaissance in World War One by Del Kostka. It seems that planes did have wireless transmitters (no receivers) and that they used an adaption of the Morse code system to give firing information to their designated batteries. A quote from the aforementioned article:

Wireless technology of the era had some obvious drawbacks. First, the equipment was too heavy and bulky for planes to carry both a transmitter and receiver, so planes flew with a transmitter only. Second, in order to omit a clear signal the aircraft would have to descend to an altitude below 5000 ft and unreel a wire antennae that dangled 150 feet behind the plane. And Morse code was hardly the fastest method of communication when relaying complex observations. A skilled observer could only send messages at a rate of up to 12 words per minute. Still, the benefits offered by this crude method of communication revolutionized air reconnaissance, and nowhere did aerial reconnaissance change the nature of warfare more than in the coordination and targeting of artillery.

'Bloody April' by Peter Hart is a good book to get hold of. He outlines how April 1917, seen as a massacre of the RFC by the then better-organised and -equipped Jastas of the German air arm, was in fact a hard attritional battle where the RFC paid the price for maintaining air superiority over the battlefield, and thus ensuring their artillery spotting and recce units could keep working.

From memory, the communications used were wireless (using morse code), flares, and dropped notes. The British certainly took air artillery spotting more seriously than the French or Germans, though the Germans usually held the high ground in 1917-18.