Before the text begins, let's enjoy a wave of wonderful beauty of nature:
Jinping Teng (Image source: Zhang Lijun photographed)
Air pineapple (Image source: Zhang Lijun photographed)
Banyan tree (Image source: veer gallery)
When you immerse yourself in the beautiful scene of Jinping vine", when you are attracted by the quaint air pineapple, when you marvel at the powerful ability of banyan trees to form a forest, do you also find that they have a common feature: the long roots are exposed to the air?
How did these beautiful and powerful natural wonders come about? What do they do? Today, I will talk to you about the big man behind these natural phenomena - qi rooting.
The same is the root, why are some on the ground and some underground?
Air rooting is an organ formed by plants in the long process of survival to adapt to the humid and hot environment and exercise special functions. It is a metamorphosed root that emanates from a stem or branch on the ground of a plant, exposed to air. Aerophysis is commonly found in some plants of the Orchidaceae, Mulberry Family, Tiannanxing family, Lily family, and Stone Garlic Family, and the beautiful red root of Jinping vine in the opening chapter is a kind of air rooting.
As we all know, the roots of plants generally grow below the ground, called "ground roots", which play a role in fixing, absorbing, transporting and storing nutrients. However, the roots of some plants will produce some special functions due to environmental and other reasons, and at the same time, the morphological structure will change, which is called root metamorphosis. "Qi rooting" is a kind of metamorphic root that grows in the air.
Ground roots and air roots of plants can be transformed into each other under certain conditions. For example, when the air roots of the banyan tree are rooted in the ground, they will become exactly the same as the ground roots, and they can draw water from the ground to support the aboveground parts of the banyan tree; and their ground roots will be exposed to the air for a long time, and the characteristics will be exactly the same as the air roots.
Ground and aerophytic roots of plants have similar internal structures, but it can be seen from the electron microscopic scans of Dendrobium aerophyllum rooting that there are no root hairs at the top of aerophyllum, but several peri-peri-ring splits are carried out to form a root indumentum. The root indument is a spongy tissue, which consists of multiple layers of dead cells, the cells are polyangular, tightly arranged, and the cell wall is locally wood-embolized and thickened into bands or reticulated patterns, with tiny small holes, suitable for absorbing water, and can be transported to the inside of the cell.
The orchid mycorrhizal roots of Dendrobium are scanned by cell electron microscopy
(Image source: Reference 2)
In the process of qi rooting from birth to aging, the appearance is also constantly changing. Taking fig fig as an example, in rainy or wet weather will grow aerophytic roots, its tip begins at 0-4cm, white or pale yellow for the new part, commonly known as tender roots; sun exposure, air drying and self-aging will make the gas roots slowly mature, the appearance becomes yellow, at this time called mature state; then continue to age, when the surface of the gas roots becomes brown, bark-like wood layer is called aging state, the latter two forms are called old roots. The old roots are partially thicker, rough on the outside, and have micropores on the surface. In the picture below, the Jinping vine, the young root part is red, and the old root part is green.
The old roots and new roots of Jinping vine
(Image source: Zhang Lijun photographed)
The mystery of "drinking water in the air" is rooted in qi
Scientists have concluded through experiments that different states of qi rooting and water absorption methods are also different.
The old roots of gas rooting have two ways to absorb water: one is to convert the water vapor in the wet air into liquid water through adsorption, which is transported to the root, which is called the adsorption of the root. The other is to directly absorb the moisture condensed on the air ash particles through micropores, or store and absorb the moisture that falls on the surface of the root on a rainy day, and transport it to the root, which is called the absorption of the root cover.
There are also two ways to absorb water in the young roots of aerophyll: when aerophytic roots grow in the flowing saturated air for a day, there will be dense white villi growing on the surface, and the villi play the role of condensing nuclei, so that the water vapor in the air condenses on its surface and is then transported to the root. This is the same principle as ground root water absorption, mainly through the root hairs to absorb moisture in the air.
Another case is that when aerophytic roots grow in near-saturated air and no white villi grow on the surface, the condensation of water on the surface of aerophytic roots is mainly achieved through infiltration phase transition. Infiltration phase transition usually refers to the transformation of a liquid on a solid surface from a partially infiltrated state to a fully infiltrated state. When the infiltration phase change occurs, the air around the aerophytic root is close to saturation, and a gas-moisture interface can be formed on the surface of the aerophytic root, and the surface temperature of the aerophytic root is slightly lower than the ambient temperature. Therefore, the water vapor in the air condenses on the surface of the aerophytic roots and is then transported to the plant, which is called the condensation mechanism of the root cover.
The growth of qi rooting is affected by these three factors
The emergence of air rooting is closely related to the growth environment. When the ambient temperature is high and the transpiration of plants is large, it is not enough to rely on ground rooting to maintain the use of the plant itself, and plants can use the stems and leaves of the aboveground part or gas to root to absorb water. For example, in order to adapt to the rainy, high temperature and high humidity environment in the south, many strands of air roots like ropes grow on the trunk of the tree to assist breathing. Because this type of plant prefers a warm and humid environment, it is mostly distributed in tropical and subtropical areas, with the largest variety in tropical rainforest areas. In China, it is mostly distributed in the area south of the Yangtze River.
The air roots of the banyan tree
Or when the local root system is not well nourished or the breathing state is not ideal, the plant also needs to absorb the moisture in the air with the help of air rooting. For example, during the planting of dragon fruit, there will be branches that will emerge and take root, temporarily playing a role in breathing air and absorbing water.
The qi of dragon fruit takes root
(Source: China Plant Image Library)
Plant hormones play a complex and important role in the formation of qi rooting, which can directly regulate the development of qi root by influencing the division and growth of cells, and indirectly regulate the development of gas rooting by interacting with other molecules. Studies have found that increasing the concentration of exogenous or endogenous auxin can make plants grow more aerograin. For example, ethylation promotes the rate of gas rooting, and the concentration of ethylene increases, and the number of layers, strips and total weight of gas rooting will also increase, but the roots gradually thin.
In addition to external factors, the formation of gas roots is also controlled by genes. Through the study of the development process of maize gas rooting, scientists found that auxin-related genes account for an important proportion. For example, the first mutant rt1 found is a key gene that affects the development of aerogenesis, and the number of underground roots in plants with this mutant has remained basically unchanged, while all aerophytic roots are missing. In addition, histological studies have found that mutations in the RTCS gene also affect the development of aerogranated roots, so that only the main root and lateral roots are retained in the mutants, and the homologous gene RTCL (RTCS-LIKE) of RTCS is also a growth hormone response gene.
A variety of qi roots with different functions, which have you seen?
According to the different forms and functions of qi rooting, it can be divided into the following categories:
Some herbaceous plants that belong to the shallow root system and have taller plants, such as corn and sorghum, can grow several layers of aerophytic adventitious roots on several nodes near the ground before the joint is pulled, and do local growth. After entering the soil, lateral roots are produced in the soil, which has a special role in supporting the plant, called supporting roots, and when the supporting roots are stunted, the plant is prone to lodging when it encounters high winds. In addition, the supporting root also has the functions of absorption and transfusion.
Corn supports the root
(Image source: Botany)
Some vines, such as climbing tigers, rocks, coixes, ivy, etc., produce many climbing roots with flat apexes from the side of the stem that are easy to climb on the surface of the object.
The qi of the coix takes root
Some plants that grow along the coast or in swampy areas, such as mangroves, water dragons, etc., can produce breathing roots that grow upwards and protrude from the ground. Its surface has breathing holes and has a well-developed ventilation tissue inside, which is conducive to ventilation and storage of air. For example, pond fir grows in moist soil, and it also rises from the ground to take root to help breathe.
The qi of the pond fir takes root
In a specific environment, the main root is underdeveloped, the lateral root grows upwards, and the junction with the base of the trunk forms a well-developed woody plate ridge. Plate root is commonly found in tropical tree species, and plant species such as Butterfly Flower Family, Duin family, Madder family, and Venom family are common plate root plants. Some of the roots can reach several meters, enhancing the support of the huge canopy.
Mountain Duying with plate roots
(Source: Official website of Xishuangbanna Botanical Garden, Chinese Academy of Sciences)
Many epiphytes, i.e. a class of plants that do not grow on the ground, but on the branches or trunks of trees, also produce gas roots. Its roots have a thick root indument, which is suitable for absorbing and storing moisture from the air, such as epiphytic orchids in tropical rainforests.
The angry roots of orchids
In addition to "self-use", qi rooting can also "other use"
In 2018, scientists discovered a local maize variety grown in Sierra Mixe, Mexico, with a large number of aerophytic roots that secrete a lot of mucus, mucus with low-oxygen and sugar-rich properties, and enriched with a large number of nitrogen-fixing bacteria, showing strong nitrogen fixation activity.
After a variety of methods, the contribution rate of this variety to nitrogen in the process of atmospheric nitrogen fixation has reached 29%-82%. The biological nitrogen fixation mechanism of the maize and the genetic mechanisms in plants that control the process also deserve further study in order to provide important theoretical guidance for reducing fertilizer use and developing green agriculture.
Ficus microphylla aerophyll contains endogenous hormones and hormone precursors, and its aqueous extract as a natural medium component can induce or improve the proliferation rate of orchid protobulbs. Studies have shown that the method of temporary replacement of hormones by the rooting water of Fig microphylla has made great contributions to the rooting of the large flower orchid, which has opened up a new way for the tissue culture of the large flower orchid, and also provided convenience for researchers and enterprise producers.
In short, plant air rooting not only provides the possibility of survival for the plant itself, it is also an important research topic in plant developmental biology. The physiological and biochemical mechanism of its formation, as well as the study of its function, have gradually attracted people's attention, and have broad research and application prospects.
Producer: Popular Science China
Producer: Zhang Lijun
Author Affilications:Wuhan Botanical Garden, Chinese Academy of Sciences
Producer: Computer Network Information Center, Chinese Academy of Sciences
(The images indicated in this article are authorized)
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: China Science Expo
Edit: fiufiu