Do Stressed Plants Cry?

A recent post by the New York Times said “When some plants are dehydrated or under some other form of stress, they cry a mournful melody made of ultrasonic clicks” referring to some new research that collected sounds from stressed plants. CBS News reported that “Plants emit a “rather noisy” cry for help when under stress”. Can plants really cry?

Scientists have also tested moths to see if they can use these sounds to distinguish between healthy plants and those under stress.

Crying Sounds from Stressed Plants

Here is what a dehydrated tomato plant sounds like:

This real sound was recorded from stressed tomato plants using ultrasonic microphones. The sound frequency is too high for humans to hear but some insects and bats should be able to hear it. In this audio recording, the sounds have been adjusted so humans can hear them and the duration of silent sections between the click sounds has been reduced so you don’t have to wait forever to hear them. A stressed plant will make these sounds at most about every two minutes. An unstressed plant may make the sound once an hour.

The sounds were recorded for tomato and tobacco plants that were either drought-stressed (i. e. not watered) or had their stem cut.

Are these plants crying, as reported by the New York Times article?

The answer is no. Headlines from sources such as the New York Times and CBS News are completely misleading and harmful to the public’s knowledge and understanding of plants.

Crying implies emotion and feelings, things plants don’t have. Besides that, the plants are not actually making the sounds. The researchers who discovered these sounds admit that the most likely explanation is cavitation sounds produced by bubbles inside the plant.

A drought-stressed plant will form air bubbles inside the xylem and the compression and expansion of these causes the noise. When leaves or stems are cut they also absorb air, which forms internal bubbles and cavitation noises.

Similar noises are caused in our growling stomach and can result from air and liquids being compressed. You also hear such sounds in water pipes after a repair. The banging sounds stop once air is pushed out of the pipes. Nobody thinks your stomach or water pipes are crying!

The sounds are real but not caused by the plant for emotional reasons or to announce that it is stressed. The plant doesn’t even know they are happening and has no control over them.

Do Moths Hear the Sounds?

If sounds emanate from stressed plants more frequently than non-stressed plants, is this information used by other organisms? That is a question researchers asked. Specifically, can moths that hear ultrasonic sounds, distinguish between a normal plant and a stressed plant?

Why would this be important? Female moths are looking for egg-laying sites and laying them on healthy plants is a benefit to the future generation of the moth. So it is reasonable to think that moths would have evolved to make use of the sounds.

The research showed that moths can hear the sounds.

Female moths that were released into an empty space which contained no plants but did have some artificial egg-laying sites, tended to lay eggs near the sounds of a stressed plant. This is the opposite of what was expected.

Adding plants to the above experiment got the opposite result. In the presence of plants, moths lay eggs away from the sound of stressed plants.

The discrepancy in the two above-described experiments can be explained if other visual, texture, and olfactory cues of thriving plants are more important for finding egg-laying sites than sounds from stressed plants.

The Crying Mandrake Plant

The mandrake plant (Mandragora officinarum) is claimed to cry when it is pulled out of the ground. The root of this plant can look like a human and stories about it crying have existed since agent times. There are also claims for magical and pharmaceutical uses of the root, and that hearing the cry will kill you. The stories about crying mandrake plants are all myths.

This story was popularized by one of the Harry Potter movies, where the young magicians were trained to transplant it.

Is Guttation a Form of Crying?

I guess that depends on your definition of crying. It looks as if the plant is crying – see the top image in the post. Water droplets are oozing out of specific pores on the leaves.

Guttation is also a way of overcoming stress. When a plant has absorbed too much water, it builds up pressure inside the plant. To relieve this pressure the plant releases some of its fluids (xylem and phloem sap). The resulting “tears” tend to be sweet and contain lots of sugars, amino acids and many other plant-produced chemicals.

A classic example of guttation is the water droplets on your lawn in the morning. It could be dew but many times it is excess water being released by the leaves.

Why does this happen? During the day plants have their stomata open and they can easily expel excess water through them. At night, most plants close their stomata but roots are still actively absorbing water. In moist soil, they can absorb too much water which builds up in the plant.

Can Guttation Kill Bees?

Synthetic pesticides are absorbed by plants and move around the plant in its sap. Once there, they will also be expelled in the guttation fluid, where insects have access to them. Corn seeds that have been treated with neonicotinoids have been studied and pesticides have been found in the guttation fluid of seedlings. The concentrations were high enough to be deadly to bees who seem to be particularly sensitive to these chemicals.

The neonicotinoids from corn have also been found in the guttation fluid of weeds growing nearby.

Before we can declare this to be a problem, we have to know if bees use guttation fluids as a source of water. A review study dated 2018 concluded that, “The previous and current review suggests that exposure to pesticides via guttation water alone is unlikely to negatively affect honeybee colonies. ” Guttation water is a potential source of harm, but given the honeybee’s behaviors, it is an unlikely source of harm. The same study went on to say, “but water foraging preferences of honeybees are not well understood”.