Why are succulents drought-tolerant?

Produced by: Science Popularization China

Author: CPY (School of Life Sciences, Peking University)

Producer: China Science Expo

Succulents are no strangers to everyone in front of the screen. In recent years, various succulents have become the new favorites of green plants in many homes and offices with their cute postures, colorful colors and simple maintenance methods. Many people have gradually fallen in love with these cute things and become loyal "succulent fans". Among the many advantages of succulents, the most prominent one is undoubtedly their strong drought resistance. It is precisely because of their extraordinary drought resistance that they can survive in front of our desks and on the balcony even if we forget to water them for a week or even longer.

At present, there are many people who grow succulents, but few people have a systematic understanding of the mechanisms behind succulents and their extraordinary abilities. Let us explore this today.

A variety of succulent plants

(Photo source: Veer Gallery)

Do you really know your succulents?

If we want to truly understand the secrets behind succulents’ remarkable drought tolerance, we first have to know who they are and where they come from.

Succulent plants, also known as fleshy plants or succulent plants, are not a taxonomic concept, but a general term for many plants with the same characteristics. These plants all have at least one thick and juicy nutritional organ (root, stem, leaf).

For example, aloe plants have fleshy leaves, cacti have fleshy stems (the leaves degenerate into thorns), Hooker's bottle has fleshy roots... We call these plants succulents.

Therefore, there are actually many species of succulents, and they belong to Crassulaceae, Agavaceae, Aizoaceae, Euphorbiaceae and even Asteraceae.

Succulent plants from various sources: A. Aloe vera from the Aloe family; B. Lithops karasmontana from the Aizoaceae family; C. Agave americana from the Agavaceae family; D. Orostachys malacophylla var. iwarenge from the Crassulaceae family

(Image source: Wikipedia)

Judging from their distribution range, their origins are mainly in Africa (South Africa, Namibia, the Canary Islands and Madeira Islands, Madagascar Island, Somalia in East Africa, Ethiopia, etc.) and America (western Mexico and the California Peninsula). These regions either have distinct dry and wet seasons in a year, or have a dry and rainy climate all year round. The special climate has enabled local plants to evolve various drought-resistant abilities, and succulentization is one of the solutions to drought resistance.

So, how do succulents resist drought?

Hotspots for succulents

(Image source: References)

Fancy drought prevention strategies for succulents

At the cellular level, more than 95% of the water in plant cells is stored in structures called vacuoles.

If you happen to touch the "wound" of a succulent plant, you will definitely be impressed by the sticky liquid it secretes. This is mainly because a large amount of polysaccharides and soluble proteins are dissolved in the vacuole fluid, making the vacuole fluid sticky.

The viscous cell sap increases the water potential of the cells, which helps the cells of succulents absorb water from the surrounding environment, while also making their own water less likely to evaporate in a dry environment.

Most succulents have a sticky sap.

(Image source: Wikipedia)

Some friends who know or have grown succulents may know that the fleshy nutritional organs of succulents can store a lot of water to maintain the normal metabolism of the plant body during droughts. However, precipitation in the desert is usually short-lived, and the soil will dry quickly under strong sunlight. Therefore, in order to improve the efficiency of water storage, succulents need to absorb a lot of water in a short period of time.

In order to adapt to these conditions, almost all succulents have thick shallow root systems. For example, the roots of the giant column cactus (Carnegiea) can extend dozens of meters horizontally, but the roots with water absorption function are mostly distributed within a depth of ten centimeters or even shallower from the surface.

The giant cactus (Carnegiea gigantea) is a signature plant of the Central American deserts.

(Image source: Wikipedia)

Comparison between the shallow root system of a giant saguaro cactus and the deep root system of some trees

(Image source: https://www.vecteezy.com/vector-art/3468110-big-cactus-growing-next-to-the-tree)

While succulents improve their water storage efficiency, they must also maintain their water reserves in a dry environment to minimize water loss.

Normally, plants exchange gases through the stomatal structure on their surface, and evaporated water vapor also mainly escapes into the atmosphere through the stomata.

Therefore, in order to retain water, the number of stomata in many succulents is reduced, or the stomata sink to form stomatal pits. In addition, the stems and leaves of succulent species often have wax or cuticles, which can almost completely prevent water evaporation when the stomata are closed. In order to further reduce the surface area of ​​the plant exposed to the air, the leaves of some succulents may even be specialized (for example, the leaves of cacti are specialized into thorns), or fall off in the dry season (for example, the osmanthus tree).

Fouquieria splendens leaves fall off during drought

(Image source: Wikipedia)

In addition to these changes, succulents generally have a special ability that helps them better adapt to arid environments.

In the biology textbooks of middle school, we have learned that green plants can absorb carbon dioxide (CO2) from the air through photosynthesis to synthesize starch and release oxygen (O2). However, during this period, plants need to open their stomata to absorb CO2 from the air, so this process is accompanied by more water evaporation.

In order to deal with the problem of water loss during photosynthesis, many succulents choose to change their photosynthesis process. They open their stomata and absorb CO2 at night when the temperature is lower and water evaporates less, and then store CO2 in vacuoles. During the day, they close their stomata and release the CO2 stored the previous night to use light for photosynthesis.

This change in the process prevents the plant from opening its stomata during the hot day, so the whole process saves about 90% of water compared to normal photosynthesis. This special photosynthesis process was first discovered in Crassulaceae plants, so the metabolic process is also called "Crassulacean acid metabolism" (CAM) process.

Crassulacean acid metabolism splits photosynthesis into two phases during the day and at night

(Image source: References)

Want to grow a succulent plant? It's actually very simple!

After learning about the clever way succulents store water, I believe the biggest question for many people is: I understand the principle! So how can I grow a pot of lovely succulents? Don’t worry! Keep reading.

The cultivation history of succulents is longer than most of us expect. At least since the 17th century, they have been recorded as indoor plants in some European literary works. Due to their lovely appearance and easy-to-grow characteristics, gardeners and green plant merchants began to pay attention to and deliberately cultivate them. In recent years, with the increase in international trade and the rise of e-commerce trade, succulents have gradually come into the vision of more people and have come to the balconies and desks of every household.

As one of the plant groups with the most adverse natural living environment, succulents have strong environmental adaptability and can survive indoors without much care from us. When caring for succulents, we only need to pay attention to the following points, and there is almost no possibility of failure:

The first point is to avoid overwatering. Many beginners who are new to the "succulent pit" may have a preconceived notion that the more water a plant needs, the better. However, this is not the case for succulents. Overwatering and the subsequent fungal or bacterial infection are the main causes of death for succulents. Therefore, for most succulents, watering once a week is sufficient. In addition, you can wait until the soil is dry before watering again.

Second, choose the right soil matrix. Similar to overwatering, too dense a soil structure can easily lead to succulent root rot. It is best to use soil with high permeability and high humus and nutrient content for growing succulents.

The third point is to ensure sufficient sunlight. The importance of sunlight to plants is self-evident. In general, succulents prefer growing conditions with sufficient sunlight, but pay attention to proper shade in summer.

The last point is the right temperature. The most suitable temperature for succulents is about 15-30℃. Most succulents are not very resistant to low temperatures, so don't leave them outside to shiver in the winter. On the other hand, although succulents can tolerate high temperatures, they will enter a dormant state when the ambient temperature is too high and will not grow and develop normally. Therefore, it is best not to let the ambient temperature exceed 35℃.

Planting succulents has become a trend in recent years

(Photo source: Veer Gallery)

Conclusion

Interestingly, in addition to sexual reproduction, most succulents can reproduce asexually through their nutritional organs (roots, stems, and leaves). Cut off a cutting with stems and leaves and insert it into the soil. After the wound heals, callus tissue will be produced, and roots will grow out after about a week. Based on this characteristic of succulents, everyone can enjoy the fun of breeding succulents at home. Are you tempted by such a lively and interesting succulent?

Editor: Ying Yike

References:

【1】Yuan, Guoliang, et al. "Biosystems design to accelerate C3-to-CAM progression." BioDesign Research 2020 (2020).

【2】Fiennes, Celia. Through England on a Side Saddle: In the Time of William and Mary. Cambridge University Press, 1888. p. 91.

【3】Grace, Olwen M. "Succulent plant diversity as natural capital." Plants, People, Planet 1.4 (2019): 336-345.

(Note: Latin text should be italicized.)