Some plants even want to drink the northwest wind with their roots! What is the secret of aerial roots "drinking water in the air"?

Some plants want to drink the northwest wind even with their roots?

Before we begin, let's enjoy the wonderful beauty of nature:

Jinping vine (Photo credit: Zhang Lijun)

Air plants (Photo credit: Zhang Lijun)

Banyan tree (Photo source: veer photo gallery)

When you are immersed in the beautiful scene of "A Curtain of Dreams" of the Brocade Wings, when you are attracted by the quirky air plants, when you marvel at the powerful ability of the banyan tree to form a forest, have you discovered that they have a common feature: their long roots are exposed to the air.

How are these beautiful and powerful natural wonders formed? What are their functions? Today, let’s talk about the big guys behind these natural phenomena: aerial roots.

Part 1

They are all roots, but why are some above ground and some underground?

Aerial roots are organs that plants develop during their long life to adapt to hot and humid environments. They are modified roots that grow from the stems or branches of plants above the ground and are exposed to the air. Aerial roots are commonly found in some plants of the Orchidaceae, Moraceae, Araceae, Liliaceae, and Amaryllis families. The beautiful red roots of the jingping vine mentioned at the beginning are a type of aerial roots.

As we all know, the roots of plants generally grow below the ground, which are called "ground roots". They play the role of fixing, absorbing, transporting and storing nutrients. However, due to environmental reasons, the roots of some plants have some special functions and their morphological structures change. This phenomenon is called root metamorphosis. "Aerial roots" are a kind of metamorphic roots that grow in the air.

Under certain conditions, the ground roots and aerial roots of plants can transform into each other. For example, when the aerial roots of a banyan tree penetrate into the ground, they become exactly the same as the ground roots, absorbing water from the ground and supporting the above-ground part of the banyan tree. When the ground roots are exposed to the air for a long time, their characteristics will also be exactly the same as the aerial roots.

The terrestrial roots and aerial roots of plants have similar internal structures, but from the electron microscope scanning of the aerial roots of Dendrobium, it can be seen that the top of the aerial roots has no root hairs, but undergoes several periclinal divisions to form a root integument. The root integument is a spongy tissue composed of multiple layers of dead cells. The cells are polygonal and arranged closely. The cell walls are partially corked and thickened into ribbons or reticular patterns. There are tiny pores, which are suitable for absorbing water and transporting it to the interior of the cells.

The orchid mycorrhizal roots of the aerial roots of Dendrobium were scanned by cell electron microscope

(Image source: Reference 2)

The appearance of aerial roots changes constantly from birth to aging. Take the small-leaved banyan as an example. Aerial roots grow in rainy or humid weather. The white or light yellow part at the tip 0-4cm is the new part, commonly known as tender roots. Sun exposure, dry air and self-aging will make the aerial roots slowly mature, and the appearance turns yellow, which is called mature state. After that, they continue to age. When the surface of the aerial roots turns brown, similar to the wood layer of bark, it is called aging state. The latter two forms are called old roots. The old root part has thicker roots, rough appearance, and micropores on the surface. As shown in the picture below, the tender root part of the brocade screen vine is red, and the old root part is green.

Old and new roots of Jinping vine

(Photo credit: Photo by Zhang Lijun)

Part 2

The secret of aerial roots "drinking water in the air"

Scientists have concluded through experiments that aerial roots in different states absorb water in different ways.

There are two ways for old aerial roots to absorb water: one is to convert water vapor in the humid air into liquid water through adsorption and transport it to the roots, which is called root adsorption. The other is to directly absorb water condensed on dust particles in the air through micropores, or to store and absorb water that falls on the root surface on rainy days and transport it to the roots, which is called root absorption.

The tender roots of aerial roots also absorb water in two ways: after the aerial roots grow in the flowing saturated air for a day, dense white hairs will grow on the surface, which will act as condensation nuclei, causing water vapor in the air to condense on its surface and then be transported to the roots. This is the same as the principle of water absorption of terrestrial roots, which mainly absorb moisture from the air through root hairs.

Another situation is that when the aerial roots grow in nearly saturated air and no white hairs grow on the surface, the condensation of water on the surface of the aerial roots is mainly achieved through the wetting phase transition. The wetting phase transition usually refers to the transformation of a liquid from a partially wetting state to a fully wetting state on a solid surface. When the wetting phase transition occurs, the air around the aerial roots is close to saturation, and an air-water interface can be formed on the surface of the aerial roots, and the surface temperature of the aerial roots is slightly lower than the ambient temperature. Therefore, water vapor in the air will condense on the surface of the aerial roots and then be transported into the plant body, which is called the condensation mechanism of the root integument.

Part 3

The growth of aerial roots is affected by these three factors

1. Environmental impact

The appearance of aerial roots is closely related to the growth environment. When the ambient temperature is high and the transpiration of the plant is large, relying solely on the ground roots is not enough to sustain the plant's own use. The plant can use the stems and leaves or aerial roots of the above-ground part to absorb water. For example, in order to adapt to the high rainfall, high temperature and high humidity environment in the south, the banyan tree grows many rope-like aerial roots on its trunk to assist breathing. Because this type of plant prefers a warm and humid environment, it is mostly distributed in tropical and subtropical regions, with the most species in tropical rainforests. In my country, they are mostly distributed in the area south of the Yangtze River.

Banyan tree aerial roots

(Photo credit: Photo by Zhang Lijun)

Or when the underground root system is insufficiently nourished or the breathing state is not ideal, the plant also needs to use aerial roots to absorb moisture from the air. For example, during the period of dragon fruit cutting planting, aerial roots will emerge from the branches, temporarily playing the role of breathing air and absorbing water.

Aerial roots of dragon fruit

(Image source: China Plant Image Library)

2. Hormone influence

Plant hormones play a complex and important role in the formation of aerial roots. They can directly regulate the development of aerial roots by affecting cell division and growth, and indirectly regulate the development of aerial roots by interacting with other molecules. Studies have found that increasing the concentration of exogenous or endogenous auxins can make plants grow more aerial roots. For example, ethephon will promote the rate of aerial root formation, and as the concentration of ethephon increases, the number of layers, number of roots and total weight of aerial roots will also increase, but the roots will gradually become thinner.

3. Genetic influence

In addition to external factors, the formation of aerial roots is also controlled by genes. Through the study of the development process of corn aerial roots, scientists found that auxin-related genes account for an important proportion. For example, the first mutant rt1 discovered is a key gene affecting the development of aerial roots. The number of underground roots of plants with this mutant has basically not changed, and all aerial roots are missing. In addition, histological studies have found that mutations in the RTCS gene will also affect the development process of aerial roots, so that only the main root and lateral roots are retained in the mutant, and the homologous gene RTCL (RTCS-LIKE) of RTCS is also an auxin response gene.

Part 4

There are many kinds of aerial roots with different functions. Which ones have you seen?

According to the different shapes and functions of aerial roots, they can be divided into the following categories:

1. Support roots

Some herbaceous plants with shallow roots and tall plants, such as corn and sorghum, can grow several layers of aerial adventitious roots around several nodes near the ground after jointing and before heading, and grow geotropically. After entering the soil, lateral roots are generated in the soil, which have a special function of supporting the plant. They are called supporting roots. When the supporting roots are poorly developed, the plant is prone to lodging in strong winds. In addition, supporting roots also have the functions of absorption and conduction.

Corn support roots

(Image source: Botany)

2. Climbing roots

Some vines, such as creeper, trachelospermum, fig, ivy, etc., will produce many climbing roots with flat tips from one side of the stem, which are easy to climb on the surface of objects.

Aerial roots of Ficus pumila

(Photo credit: Photo by Zhang Lijun)

3. Breathing roots

Some plants that grow in coastal or swampy areas, such as mangroves and water dragons, can produce breathing roots that grow upward and extend out of the ground. They have breathing holes on the surface and well-developed aerenchyma inside, which is conducive to ventilation and air storage. For example, when the pond cypress grows in soil with high humidity, it will also produce aerial roots from the ground to help it breathe.

Aerial roots of pond cypress

(Photo credit: Photo by Zhang Lijun)

4. Buttress roots

Buttress roots are formed when the taproot is poorly developed under certain conditions, and the lateral roots grow upward, forming a well-developed wooden plate-like ridge at the junction with the base of the trunk. Buttress roots are common in tropical tree species, and plant species such as Fabaceae, Elaeocarpus, Rubiaceae, and Celastraceae are common buttress root plants. Some buttress roots can reach several meters, enhancing the support of the huge tree crown.

Elaeocarpus styracifolius

(Photo source: Official website of Xishuangbanna Botanical Garden, Chinese Academy of Sciences)

5. Epiphytic roots

Many epiphytes, that is, plants that do not grow on the ground but on the branches or trunks of trees, also produce aerial roots. Their roots have a thick rhizome that is suitable for absorbing and storing moisture from the air, such as epiphytic orchids in tropical rain forests.

Aerial roots of orchids

(Photo credit: Photo by Zhang Lijun)

Part.5

In addition to "self-use", aerial roots can also be used for "others"

Biological nitrogen fixation by aerial roots of maize

In 2018, scientists discovered that a local corn variety grown in the Sierra Mixe region of Mexico has a large number of aerial roots that secrete a large amount of mucus. The mucus is low in oxygen and rich in sugar, and is enriched with a large number of nitrogen-fixing bacteria, showing strong nitrogen-fixing activity.

Calculated by various methods, the contribution of this variety to nitrogen in the process of atmospheric nitrogen fixation is 29%-82%. The biological nitrogen fixation mechanism of this corn and the genetic mechanism that controls this process in plants are worth further study, in order to provide important theoretical guidance for reducing the use of chemical fertilizers and developing green agriculture.

The aqueous extract of aerial roots of Ficus microcarpa can be used as a natural culture medium

The aerial roots of Ficus microcarpa contain endogenous hormones and hormone precursors, and their water extract, as a natural culture medium component, can induce or increase the proliferation rate of orchid protocorms. Studies have shown that the temporary replacement of hormones with the water extract of the aerial roots of Ficus microcarpa has a significant contribution to the rooting of cymbidium orchids, opening up a new way for cymbidium orchid tissue culture and providing convenience for researchers and corporate producers.

In short, aerial roots not only provide the plant with a chance to survive, but are also an important research topic in plant developmental biology. The physiological and biochemical mechanisms of their formation and the research on their functions have gradually attracted people's attention and have broad research and application prospects.

Produced by: Science Popularization China

Produced by: Zhang Lijun

Author unit: Wuhan Botanical Garden, Chinese Academy of Sciences

Producer: Computer Network Information Center, Chinese Academy of Sciences

(The images with source indicated in this article have been authorized)

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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