There are many known ways in which plants communicate (if you want to call that communication):
- ethylene is a gas, but also a plant hormone that causes (among others) ripening of fruits. So if you got a field of apple-trees, when the first tree's fruits starts to ripen and release ethylene there's a chain reaction where other trees detect the gas and causes the other trees to ripen their own fruits (and release more ethylene). This is used in the industry so you can transport unripe fruits and ripen them later with ethylene.
- Trees of Heaven produce hormones that they use to tell other plants to stop growing, so to speak [1]
- When lima beans are attacked by herbivores or parasites, they release gaseous substances that warn other plants, who then in turn upregulate their defences [2]
There are many, many, many more examples, it's awesome :)
IMHO what's even cooler is a type of plant defense where they cooperate with other organisms to fend off herbivores. Some plants are known to emit organic volatiles which attracts predators of these feeding herbivores.
For example moth larvae (Manduca sexta) feeds on tobacco (Nicotiana attenuata) plants. The tobacco plant releases leaf volatiles which float through the air to attract a predator (a big eyed bug, Geocoris) to feed on the larvae. [1]
- ethylene is a gas, but also a plant hormone that causes (among others) ripening of fruits. So if you got a field of apple-trees, when the first tree's fruits starts to ripen and release ethylene there's a chain reaction where other trees detect the gas and causes the other trees to ripen their own fruits (and release more ethylene). This is used in the industry so you can transport unripe fruits and ripen them later with ethylene.
- Trees of Heaven produce hormones that they use to tell other plants to stop growing, so to speak [1]
- When lima beans are attacked by herbivores or parasites, they release gaseous substances that warn other plants, who then in turn upregulate their defences [2]
There are many, many, many more examples, it's awesome :)
[1] https://en.wikipedia.org/wiki/Ailanthus_altissima#Ecology
[2] http://www.mpg.de/942876/W001_Biology-Medicine_060_065.pdf