Lotus root: stained by mud, but delicious

Produced by: Science Popularization China

Author: Zhang Lijun (Wuhan Botanical Garden, Chinese Academy of Sciences)

Producer: China Science Expo

There is a traditional dish in Hubei called Pork Rib and Lotus Root Soup, which can be seen on almost every dining table, at banquets for entertaining guests, and at gatherings of relatives and friends. The most important ingredient of this "Pork Rib and Lotus Root Soup" with local characteristics is lotus root. Today, let's talk about lotus root.

Pork Ribs and Lotus Root Soup

(Photo source: veer photo gallery)

Is lotus root the root of lotus?

As we all know, lotus root is located under the mud where lotus is planted. Farmers have to spend a lot of effort to dig it out. In some places, lotus root is also called "lotus root". So is lotus root the root of lotus located underground?

Actually, it is not. Lotus root is a kind of rhizome of lotus, which is a type of plant organ metamorphosis. This metamorphosis is the same as other organ metamorphosis, which is caused by the change of organ function due to environmental influence, resulting in significant changes in morphological structure.

As the name implies, rhizomes are stems that have undergone metamorphosis and become root-like in shape, lying horizontally underground. Because they are stems, they have distinct nodes and internodes, with small and degenerate scale leaves on the nodes, and axillary buds or terminal buds in the axils of leaves can form upright above-ground stems, and adventitious roots will also be produced on the nodes.

Rhizomes store rich nutrients and have strong reproductive capacity. Many plants in the Gramineae family, bamboo, lotus root, ginger, etc. are rhizomes.

Rhizome

(Image source: Reference [1])

How is lotus root formed?

Since lotus root is a part of lotus, if you want to understand the formation process of lotus root, you need to first understand lotus.

Lotus is a familiar aquatic plant. Its round leaves stand upright on the water. In summer, it blooms holy and beautiful flowers. After the flowers bloom, lotus pods grow and produce lotus seeds. In winter, the underground part of the lotus produces lotus roots. Therefore, the lotus is mainly composed of lotus flowers, lotus leaves, lotus pods, lotus seeds, lotus roots and roots.

Lotus structure diagram

(Photo source: Lotus Germplasm Resources and Genetic Breeding Research Group, Wuhan Botanical Garden, Chinese Academy of Sciences)

As can be seen from the picture, there are two forms of lotus rhizomes. The non-swollen one is the lotus root whip, which is composed of multiple nodes and internodes, and is as slender as a whip; the other swollen one is the lotus root, which is the rhizome formed by the swelling of the top of the lotus root whip.

The entire growth period of lotus can be roughly divided into the early stage and the late stage. In the early stage, the plant mainly grows stems and leaves and blooms and bears fruit. When the highest rear leaves appear on the tail tip, the lotus enters the late stage of lotus root formation. This stage is mainly for nutrient storage. The assimilation products of photosynthesis are mainly supplied to the swollen rhizomes. The rhizomes quickly become thicker and shorter, and the lotus root formation period begins.

When the lotus root is swelling, the tip of the lotus root whip changes from horizontal growth to elongating obliquely downward, and the internodes shorten and thicken. After a lotus root section is formed, the top bud at the tip continues to grow forward to form the next section while the previous section thickens. Generally, a lotus root section swells every 5-7 days until the top bud curls up and takes shape. While the main lotus root is formed, the side buds of the main lotus root separate and swell to form sub-lots, which have fewer nodes than the main lotus root, and the sub-lots can also form grandson lotus roots.

At this time, someone may ask: What part of the lotus root do we eat in spring?

Lotus stems are rhizomes in their tender stage, which are also called lotus whips. They mainly grow in spring. If the lotus roots in the lotus pond grow too densely at this time, you can thin them out by picking the lotus stems. After the peak flowering period in summer, some of the lotus stems will swell into lotus roots, but even if they do not swell in the later stage, they have no edible value. Therefore, the seasonality of lotus stems is very strong and they can only be eaten in spring. People usually make them into delicacies by stir-frying or stir-frying with hot and sour sauces. If you want to preserve them for a long time, most people make them into kimchi lotus stems, which are also crisp and appetizing.

Fresh lotus root

(Photo source: veer photo gallery)

What determines the formation of lotus root?

Scientists have discovered that although the external environment and hormones both have an impact on the expansion of lotus roots, fundamentally, whether or not they can expand is controlled by genes.

In the process of studying lotus, scientists discovered an interesting phenomenon: the rhizomes of temperate lotus can swell into lotus roots in the later stages of development, while the rhizomes of tropical lotus cannot swell into lotus roots. When temperate lotus is planted in tropical areas, their rhizomes still maintain the swelling characteristics of their place of origin.

It can be seen that the expansion of lotus rhizomes is controlled by genetic material. Later, after continuous exploration by scientists, the gene NnBEL6 that promotes the expansion of underground stems was discovered. Other scientists have found that there are 4 genes closely related to the formation of lotus rhizomes starch during the expansion of lotus roots. The expression of these genes is synchronized with the increase of starch content, the increase of starch granules, and the enlargement of the rhizomes during the expansion of lotus roots.

What are the functions of lotus root?

Although lotus root, as the enlarged rhizome of lotus, has become a source of delicious food for people, it was not born to serve humans, but to serve the survival and reproduction of the plant itself.

1. Expand your territory

Lotus roots can randomly branch out from the mother plant and extend to new areas. It won't take long for them to become the dominant plant in the water area. Many lotus ponds need to be dug and cleaned every year or every other year. The main purpose is to control the area covered by lotus. Otherwise, the lotus will soon cover the entire pond.

Digging lotus root

(Photo source: Xinhua News Agency)

2. Store nutrients and maintain vitality

The thin-walled cells of the lotus root cortex store abundant sugars, starch grains and proteins. Scientists' research on the starch content during the swelling process of lotus roots shows that starch is rapidly synthesized, transported and accumulated in the middle and late stages of swelling, and the content increases sharply. The starch content of lotus roots accounts for more than 70% of the total mass of the dry matter of the rhizome. The lotus root also has a well-developed aeration tissue inside. These structures enable the lotus root buried in the soil to maintain vitality and survive for one to many years.

The morphology and distribution of starch granules in the late stage of rhizome expansion of 'Wuzhi No. 2' (lotus variety) at 1600 times magnification

(Image source: Reference [4])

3. Help plants survive the winter

The lotus root stems spreading in the soil wither before winter comes, but the swollen lotus root will not wither. The lotus root has a terminal bud protected by scales, which can sprout into a new individual when the next spring comes, ensuring the reproduction of offspring.

As mentioned above, the rhizomes of temperate lotuses swell, while those of tropical lotuses do not. Moreover, if the growth environments of the two are swapped, tropical lotuses will not be able to bear fruit and overwinter in subtropical and temperate regions. Therefore, it can be inferred that the swelling of lotus roots can ensure the successful overwintering of lotuses and the normal germination in the next spring.

Rhizome development status of temperate and tropical lotus

(Image source: Reference [8])

Lotus root soup is delicious, but not all lotus roots are suitable for making soup

Lotus root is rich in starch, but different types of lotus roots have different starch content, composition and properties, which directly affect the quality of lotus root and its processed products. The quality of lotus root can be roughly divided into two categories:

One type is the crisp type . This type of lotus root requires high water content, low starch content, high sugar content, and low crude fiber content. After processing, it is not easy to cause large amounts of starch precipitation and gelatinization. The product has low viscosity and a refreshing texture. It is suitable for cold salads, stir-frying, etc. It can also be processed into lotus root juice drinks.

The other type is the powdery type . This type of lotus root requires low water content, high starch content, and high content of amylopectin in the starch. The lotus root powder processed in this way has weak gelatinization and coagulation when eaten, low gel strength, and ideal stability. It has a soft, tough and smooth taste and high viscosity. This type of lotus root has good quality when stewed or boiled, and is suitable for processing honey lotus root, pork ribs and lotus root soup, etc.

In my country, there are many places that produce lotus roots, but it is rare to find lotus roots that can be stewed into lotus root soup. The lotus roots in Hubei are pink and tender, soft and sticky, and can be stewed into delicious soup. When you come to Hubei, don't forget to drink a bowl of authentic pork ribs and lotus root soup!

References:

1. Qiang Sheng. Botany. Higher Education Press. 2010: 160-161

2. Xin Jia, Sun Heng. Do you know how lotus heart develops? China Science Expo, July 2022

3. Cheng Libao, Li Shuyan, et al. Expression of starch synthesis-related genes during lotus root rhizome expansion. Chinese Agricultural Science, 2012, 45 (16): 3330-3336;

4. Zhang Xiaodong. Preliminary study on carbohydrate metabolism and starch characteristics of lotus rhizomes. Master's thesis. 2005. Yangzhou University;

5. Zhao Qinglei, Wang Yu, et al. Research progress on the mechanism and influencing factors of lotus rhizome expansion. Shandong Agricultural Science, 2013, 45 (5): 138-141

6. Li Feng. Preliminary study on the mechanism of lotus root rhizome expansion. Master's thesis. 2008. Huazhong Agricultural University

7.Yanling Liu, Hyun Song,et. Identification of QTLs and a putative candidate gene involved in rhizome enlargement of Asian lotus(Nelumbo nucifera). Plant Molecular Biology 110, 23-26(2022)

8.Mei Yang, Lingping Zhu,et. Transcriptomic Analysis of the Regulation of Rhizome Formation in Temperate and Tropical Lotus(Nelumbo nucifera). Scientific Reports 5, Article number: 13059(2015)