Plants and insects actually do this at night...

Have you ever noticed this phenomenon: the tips or edges of the leaves of some plants are always covered with sparkling water droplets.

For example,

Image source: Wikipedia

Or like this,

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Many people may think that this is dew. However, this is not the case. Dew usually covers the leaf surface, forming a layer of water film on the leaf surface, and the arrangement of water drops is irregular. But these water drops are neatly arranged on the edge of the leaf. This wonderful phenomenon is called guttation, which often occurs during the growth period of plants.

Dewdrops on leaves, copyrighted image, unauthorized reproduction

01

It is not uncommon for plants to spit out water

The phenomenon of plants spitting water was observed more than 300 years ago, but people at that time just regarded it as an accidental phenomenon. As more and more plants (especially herbaceous plants) have been found to spit water, the academic community now recognizes that plant spitting water is a common phenomenon, and thus has an official name.

It is not uncommon for plants to spit water. Many common plants, such as rice, wheat, barley, and sugarcane, spit water during the growing season. Unlike plant wound bleeding or gum secretion, spitting water is not the secretion of liquid after the plant is injured, but a naturally formed measure to maintain water balance in the body. In summer, plants grow rapidly and have a high metabolism, so they spit water more frequently.

The water spitting phenomenon usually occurs at night, because at night, the roots of plants are still continuously absorbing water and inorganic salts from the soil, but the transpiration of plants becomes weaker, so they can only discharge excess water through the tiny water holes on the edges of the leaves, forming the phenomenon that the edges of the leaves are covered with sparkling water droplets.

Most of the spitted water will be arranged at the edge of the leaves. This is because the edge of the leaves has specialized drainage tissues, called hydathodes. The drainage holes formed by specialized stomata drain water out of the leaves. They are closely connected to a cavity to store water, and the water comes from the water-permeable tissue connected to the vascular tissue inside the leaves.

Plant leaf drain structure, Image source: Reference 1

02

The water that plants "spit" can feed a group of insects

Generally speaking, plants "spit water" to expel excess water absorbed by the roots at night, which plays an important role in maintaining the water balance between the plant leaves and the body.

Moreover, subsequent studies have found that the water drainers and vascular tissues of leaves are connected. Researchers speculate that the "spitted" water may also contain other substances transported by the vascular tissues, such as minerals, sugars, proteins, etc. Scientists have also long discovered that many insects will suck the small water droplets formed by the water spitted by plants, but initially they only thought that these insects were simply replenishing water.

The breakthrough in the conclusion that "water exuded by plants contains a lot of nutrients" comes from blueberries. Scientists have discovered that blueberries not only exude an astonishing amount of water, but also that the water they exude is very popular with insects.

The phenomenon of blueberries spitting out water, and various insects that eat water, Image source: Reference 2

This shows that the water spitted by plants may not only be a source of water for insects, but also an important source of food for insects.

To verify this conclusion, scientists collected the water spit out by blueberries and collected three insects that often collect the water spit out by blueberries: including a fruit fly (Drosophila suzukii), a herbivorous insect, a parasitic wasp (Aphidius ervi), a parasitic insect, and a lacewing (Chrysoperla rufilabris), a predatory insect, representing three different trophic levels.

Next, the scientists fed the three insects with water collected from blueberry plants, pure water, sugar water, pure protein, and a mixture of sugar water and protein.

The results showed that insects that drank only pure water could not survive, while insects that drank water "vomited" by blueberry plants, insects that drank sugar water, and insects that drank mixed liquids all survived well. The lifespan of insects that drank water "vomited" by blueberry plants was even comparable to that of insects fed with sugar water.

The lifespan of three different insects after feeding with different nutrient solutions. The green line represents the spitted liquid fed with blueberries. Image source: Reference 2

In addition, researchers also mated female insects fed with different nutrient solutions and allowed them to lay eggs after fertilization to see if their egg production would be affected. The results showed that insects that drank the plant's spit water were not affected, and the blueberry plant's spit water can be called a "natural nutrient solution."

Through measurements, scientists have also found that the sugar and protein content in the water expelled by blueberries is very high, reaching 1.5 grams per milliliter and 4.3 milligrams per milliliter respectively. The sugar content in nectar is about 0.4 grams per milliliter, and the sugar content of the water "expelled" by blueberry plants is three times higher than that of nectar. (It is so nutritious that the author has the urge to drink the water "expelled" by blueberries).

Therefore, it is a very good source of nutrition for insects that feed on blueberry spit. It may be the fourth largest type of nutrient produced by plants, in addition to pollen, nectar and honeydew. What makes plant spit better is that it produces more and lasts longer than the other three.

Plants exhale water that is rich in nutrients. Is it just to feed insects? According to the laws of nature, things are definitely not that simple.

03

Plants spit water: Raise natural enemies of natural enemies to protect themselves

Blueberries "spit out" so much water, and it is an open resource. It seems that as long as insects get there first, they can have a bite. So, will this water-spitting phenomenon attract a large number of herbivorous insects, which not only drink the "spitted" water, but also eat all the plant leaves?

Scientists conducted field experiments to statistically investigate the insect communities attracted by blueberry water. In the wild, a large number of insects feed on the water spit by blueberries. Compared with blueberry plants that have had their water removed, the water-spitting plants obviously attracted more parasitic wasps and predatory insects such as lacewings, but attracted fewer herbivorous insects. In other words, blueberries attract more natural enemies of their own natural enemies through the water they "spit," thus being able to better protect themselves.

Compared with the control group, the blueberry plants that spit water attracted more predatory insects and parasitic insects, and attracted fewer herbivorous insects. Image source: Reference 2

What's more interesting is that the number of mosquitoes on the plants that spit water has decreased significantly. It may be because of the deterrence of other predators that the mosquitoes have to give up this nutrient. This may become a new mosquito prevention method.

Water spitting is quite common in crops. If we can control the "water spitting formula" of crops, we may be able to achieve a new biological control method, allowing crops to attract more predatory insects to fight against natural enemies, "killing insects with a borrowed knife", and achieving insect control from top to bottom.

Plant exudation of water is not only an important means for plants to regulate water balance, it also has important ecological functions, regulating the "circle of friends" through the substances in the exuded water.

Maybe one day, we can achieve the enrichment of certain substances by artificially controlling the substances contained in the water expelled by plants. In this way, we can not only collect blueberry fruits, but also collect the water expelled by blueberries, and realize the production of natural blueberry "expelled water" juice, which is delicious and nutritious. I wonder if you are willing to try it at that time?

References:

[1] Mondal, FLS,FIAAT, Amal. (2014). Guttatin-A Natural Phenomenon: Useful medium for production of Recombinant Proteins.

[2] Urbaneja-Bernat P, Tena A, González-Cabrera J, et al. Plant guttation provides nutrient-rich food for insects[J]. Proceedings of the Royal Society B, 2020, 287(1935): 20201080.

This article is produced by Science Popularization China, edited by Liao Xinfeng, and supervised by China Science Popularization Expo

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