Plants are the number one source of food for consumption to all herbivores in the environment. Herbivores naturally do not think twice before consuming certain plants that they believe adhere to their natural system, therefore they are generally unaware of the defenses plants exert to purge them. Research scientists H.M. Appel and R.B. Cocroft at the University of Missouri found out that plants can actually feel the vibrations of insects feeding on them. They also found out that the vibrations caused by insects chewing on plants versus birds caused plants to exert different chemical defenses than when plants could sense vibrations through the wind or insect song.
The main focus of Appel and Cocroft’s experiment was to really focus on the ecological significance of plant responses to vibrations rather than simply observing that plants react to vibrations, which has been an apparent conclusion that has been drawn from experiments on plants, for a long time. The goal of the experiment was to focus on acoustic energy, a unique observance of sound in an ecosystem. When one thinks of the term acoustic, word association of the term is often paired with music, however, one of the goals of the experiment was to communicate how acoustic energy should not just be put in a box with music, but that it is present in several other sources in the natural environment. As mentioned earlier, an early emphasis in the article was on vibrations, or rather, acoustic energy induced by insects. Appel and Cocroft explicitly state that, “We suggest that the vibrations produced by chewing herbivores are an important source of acoustic energy for plants. If plants can detect and use this conspicuous, reliable and rapidly transmitted source of information about herbivore feeding, tissues far from the site of attack could use feeding vibrations to respond quickly to the threat of herbivory.”
Appel and Cocroft went through a few experiments to validate this apparent fact of the ecosystem. In the first experiment, they studied leaves only after they were fed on to just solely focus on potential defenses they might have elicited after sensing vibration. In their second experiment however, they observed reaction of the plants that had never been fed on before in order to separate out “priming from direct effects of chewing vibrations on plant responses.”
The plant that Appel and Cocroft Chose to experiment with was the A. Thaliana plant, and they were grown in individual pots in order to be experimented with each individual goal of the study. The 4 caterpillars that were used for the experiment were of the P. rapae species and the vibrations of them chewing on the plant was recorded “at 24.5 ± 1 °C with laser Doppler vibrometry.”
After all the different variations that Appel and Cocroft chose to take with their experiments, they communicated their results which were that aliphatic glucosinolates were higher in the plants that had experienced herbivory previously than in the plants that had never felt a single vibration in their life.
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