If a plant is stressed but no one hears it, does it still make a sound? While that's not quite the way the saying goes, researchers have discovered that the answer to the question is a resounding "yes." By monitoring tomato and tobacco plants, they discovered that plants which are under stress – from conditions such as being cut or from having a lack of water – emit a series of popping sounds. Even though these sounds are produced at human conversational levels, they are emitted at a frequency too high for our own ears to pick up, although that's likely not the case for insects and a range of other animals.
Researchers at Israel's Tel Aviv University (TAU) placed tobacco and tomato plants along with a series of specialized microphones first in soundproof acoustic chambers, and then in less sound-isolating greenhouses. They then recorded the sounds the plants made as they were cut and deprived of water. They discovered that the plants make popping noises when they were put through these trials, but the noises are in the ultrasonic frequency, so while the mics could pick them up, human ears could not. However, say the researchers, the noise could be heard by other species, which could provide them with valuable information.