Why are the lotus seeds I eat not sweet and delicious at all? I finally found the reason

July is the best time to enjoy lotus. Lotus has been loved by people since ancient times. As early as 3,000 years ago, the poem "There are Fusu in the mountains and lotus in the marshes" appeared in the "Book of Songs", which is also the earliest written record of lotus. The summer breeze gently blows across the lake, and a pool of lotus sways in the wind, exuding a faint fragrance. From ancient times to the present, lotus is not only a symbol of beauty, but also a symbol of the emotions of countless literati and poets.

Image source: Zhou Jiangtao

Today, there are still groups of scientists trying to decipher the secrets of lotus and contributing their wisdom and strength to the study of lotus. Facing this beautiful flower, let us learn more about the unknown secrets of lotus.

Classification and structure of lotus

Lotus, belonging to the genus Nelumbo of the family Nelumbo, is an important aquatic economic plant. It is one of the oldest dicotyledons and, along with Metasequoia, Ginkgo, and Chinese tulip tree, is a relict plant that survived the Quaternary glacial period and is known as a "living fossil."

At present, there are two existing species of Nelumbo nucifera in the world: Asian Nelumbo nucifera (N. nucifera Gaertn.) and American Nelumbo nucifera (N. lutea Pers.) (Zheng et al., 2022). Asian Nelumbo nucifera is mainly distributed in Asia and northern Australia, with a wide variety of species and abundant resources. American Nelumbo nucifera is mainly distributed in North America, the Caribbean, and the Atlantic Ocean, and is basically in a wild state.

Geographical distribution of Asian lotus and American golden lotus, image source: Reference 2.

There are some differences between the two in terms of plant shape, flower shape and color. The Asian lotus plant is tall, with single petals, double petals, and double-petaled flowers, and the flower color varies from white, pink to red; the American golden lotus plant is short, with yellow single petals. Although the Asian lotus and the American golden lotus are separated by the Pacific Ocean in terms of geographical distribution and have phenotypic differences, there is no reproductive isolation between the two, and both have eight pairs of chromosomes.

The lotus is mainly composed of the following parts: lotus flower, lotus leaf, lotus pod, lotus seed, lotus root and root.

Lotus structure diagram, Image source: Yang Hui

Lotus: Also known as lotus, lotus, lotus, water lotus, etc., it is one of the ten famous traditional flowers in China. Lotus is solitary and bisexual, and consists of calyx, stamen group, pistil group, receptacle and petals. The size, color, shape and number of petals vary, forming a gorgeous and colorful flower state.

Lotus leaf: The lotus leaf is shield-shaped, with an entire leaf margin and wavy edges. It is the main organ for photosynthesis in lotus. The surface of the lotus leaf has a thick wax layer and lipid papillae, which can prevent water from penetrating. Therefore, we can see that the lotus leaf is wet but not wet, which is also the main reason why it "emerges from the mud without being stained".

Lotus pod: Developed from the receptacle of the lotus, it is in the shape of an inverted cone with a flat top. After fertilization, the receptacle expands with the development of the fruit and seeds to form a lotus pod. It is yellow-green at first and turns brown when mature.

Lotus seeds: Lotus fruit is commonly known as "lotus seeds". After fertilization, the ovule develops into seeds, and the ovary wall develops into fruit skin. Lotus seed cotyledons are rich in starch and can be eaten. In addition, lotus seeds are particularly long-lived and can sleep for thousands of years before sprouting again.

Lotus root: Lotus root is the underground rhizome of lotus, which plays a role in storing nutrients and reproduction, and is an important food ingredient. When the lotus root is broken, it will form white filaments, which are very elastic, long, and not easy to break. This is what people often say "the lotus root is broken but the silk remains". Because the lotus root contains tannins, it is easy to oxidize and turn blue-black when it is cut with iron tools. Therefore, when cooking lotus root, you should reduce the use of iron cooking utensils.

Roots: There are two main types of lotus roots, namely seed roots and adventitious roots. Seed roots are taproots formed by the radicle when sowing, which are underdeveloped and do not play a major role. Adventitious roots grow on the nodes of the underground stems, and many lateral roots are attached to them, which play the role of fixing the plant and absorbing water and nutrients.

Lotus has a wide range of uses, and is used as a landscape ornamental, leisure food and vegetable. According to agronomic characteristics and cultivation purposes, lotus is generally divided into three categories, namely: flower lotus, seed lotus and lotus root .

The main classification of lotus, image source: Xin Jia

Why does lotus seed core taste bitter?

Lotus seeds are the mature seeds of lotus, slightly oval or spherical, 1.2 to 1.8 cm long, 0.8 to 1.4 cm in diameter, and consist of four parts: pericarp, seed coat, cotyledon and lotus seed core. The seed coat of mature lotus seeds is dark brown or brown, hard and not easy to peel off. The two yellow-white cotyledons are thick and have a green lotus seed core in the middle.

Lotus seed anatomy diagram, image source: Xin Jia

Lotus seeds begin to develop from day 0 after pollination, and their morphology gradually increases. The peel gradually changes from yellow to green, and then to dark brown. The complete development cycle of lotus seeds is about 30 days, which can be divided into four developmental stages, namely the organogenesis stage (1 to 3 days after pollination, 1-3 DAP), cell expansion stage (4-9 DAP), nutrient accumulation stage (10-25 DAP) and dehydration maturity stage (26-30 DAP) (Sun et al., 2020).

Dynamic changes in lotus seed development, Image source: Sun Heng

In the hot summer, we taste fresh lotus seeds and enjoy the sweet taste buds, but many friends will find that lotus seeds taste bitter when eaten. Why is this?

It turns out that the time of eating lotus seeds will greatly affect its fresh taste. Generally, we choose lotus seeds 15 days after pollination, when the lotus seeds have a higher water content and taste sweet and delicious. When the lotus seeds develop to 18 days, the cotyledons accumulate a higher content of starch and the water content decreases, so the taste is poor.

In addition, the lotus seeds taste bitter at this time, which is caused by not removing the clean green lotus seed core . The lotus seed core can synthesize a variety of alkaloids , mainly including liesinine, isoliensinine and methyl neferine. As the lotus seeds develop, the alkaloids in the lotus core continue to accumulate and the bitterness increases.

Chemical structure of lotus seed alkaloids, image source: Xin Jia

Does lotus seed core need light for photosynthesis?

The lotus seed core is located inside the lotus seed, and its outside is tightly wrapped by cotyledons, seed coat and pericarp in sequence. During the development of lotus seeds, the color of the lotus seed core undergoes a transformation from white, yellow-green to dark green, which is mainly caused by the accumulation of chlorophyll.

Generally speaking, the synthesis of chlorophyll in angiosperms requires light. As one of the earliest species in angiosperms, why can lotus seed core synthesize chlorophyll in a "dark" environment? This peculiar phenomenon has attracted the interest and attention of many scholars at home and abroad. During the development of lotus seeds, researchers gave the lotus pods a light-proof treatment and found that the color of the lotus seed core turned yellow, proving that the synthesis of chlorophyll in the lotus seed core requires light.

Lotus seed heart shading experiment picture, picture source: reference 4

By conducting whole-genome identification of the structural genes of the chlorophyll biosynthesis pathway in lotus, it was found that the chlorophyll biosynthesis pathway in lotus is consistent with that reported in other angiosperms, and during the development of lotus seed core, the expression of most chlorophyll biosynthesis structural genes is significantly upregulated, which is consistent with the accumulation pattern of chlorophyll.

In the chlorophyll biosynthesis pathway, the conversion of protochlorophyllide to chlorophyllide catalyzed by protochlorophyllide oxidoreductase (POR) is a key step.

For angiosperms (almost all flowering plants), light-dependent protochlorophyllide oxidoreductase (LPOR) helps them convert protochlorophyllide into chlorophyllide to synthesize chlorophyll.

Some gymnosperms (such as Ginkgo) and microorganisms use the light-independent protochlorophyllide oxidoreductase (DPOR) to complete this reaction.

Chlorophyll synthesis pathway, Image source: Xin Jia

Through the study of nuclear genome and chloroplast genome, researchers found that there are two LPOR coding genes in lotus, but no DPOR coding gene. This discovery provides important molecular evidence for the light dependence of chlorophyll synthesis in lotus.

Image source: Zhou Jiangtao

As early as in ancient my country, lotus has become a plant that we admire, eat and sing about. Nowadays, under the background of the development of characteristic agriculture in my country, lotus planting has become an important source of income for farmers in some areas. As an ancient species, lotus has gone through many changes, and its research value has far exceeded itself. In the accumulation of history, it exudes a unique charm.

However, there are still many unsolved mysteries in lotus, such as the germination of lotus seeds over a thousand years, the evolution of lotus, etc. The writing of these scientific chapters requires the joint efforts of generations of scientific researchers. We will unlock more unknown secrets of lotus and let this beautiful flower bloom forever.

References:

[1] Wang Qichao, Zhang Xingyan. Atlas of Chinese Lotus Varieties[M]. China Forestry Publishing House, 2005.

[2] Lin Z, Zhang C, Cao D, et al., 2019. The Latest Studies on Lotus (Nelumbo nucifera)-an Emerging Horticultural Model Plant. International Journal of Molecular Sciences, 20: 3680.

[3] Sun H, Li J, Song H, et al., 2020. Comprehensive analysis of AGPase genes uncovers their potential roles in starch biosynthesis in lotus seed. BMC Plant Biology, 20: 457.

[4] Sun H, Song H, Deng X, et al., 2022. Transcriptome-Wide Characterization of Alkaloids and Chlorophyll Biosynthesis in Lotus Plumule. Frontiers in Plant Science, 13: 885503.

[5] Zheng P, Sun H, Liu J, et al., 2022. Comparative analyzes of American and Asian lotus genomes reveal insights into petal color, carpel thermogenesis and domestication. The Plant Journal, 110: 1498-1515.

This article is produced by Science Popularization China, edited by Xin Jia and Sun Heng (Wuhan Botanical Garden, Chinese Academy of Sciences), and supervised by China Science Popularization Expo

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