Some Experiences on this subject:
Denny’s experience of the smell of ethylene on lemons (ethylene is present in kerosene, in incense but also in ripe fruit). It only takes 1 per 100 million in the air for the lemon to react and accelerate its ripening. Because ethylene causes fruit to ripen quickly.
Experience of the Dodder, a parasitic tomato plant, filmed on video by Professor De Moraes at the University of Pennsylvania. The dodder feels the beta-myrcene released by the tomato to get close to it by convolution and attach itself to it.
Experience Rhoades and Orians and Baldwin and Schultz, of Dartmouth on the predatory caterpillar of the weeping willow. When attacked, weeping willows emit an odor that causes neighboring willows to react. The latter set up a chemical defense against the caterpillars. For 10 years, these communication and defense mechanisms between trees have been much better studied and have confirmed the first experiences.
Hueil experiment in Irapuato, Mexico on wild beans, (using the new gas chromatography-mass spectrography techniques) showing that these odors are indeed intended for other neighboring plants because they are useless for the attacked plant which reacts even without odors.
These odors are methyl jasmonate (for attacks by insects or by herbivores) and methyl salicylate (for bacterial attacks, the plant transforms its production of anti-bacterial salicylic acid into volatile compound, which can be again transformed into liquid from one plant to another, in order to heal itself).