The Incredible Adaptations For Water Conservation Of Cactus

Adaptations For Water Conservation Of Cactus

Cacti are a group of succulent plants that are well-adapted to survive in arid and semi-arid regions where water is scarce.

Adaptations for water conservation of cactus involves developing a range of features such as modified leaves, stems, roots, and even a metabolism system.

The adaptations allow different cacti plants to significantly reduce the loss of water while absorbing as much moisture as possible from their atmosphere. They have also developed their body features to store a large amount of water for long periods of time.

In this article, we will explore the adaptations for the water conservation of cacti in more detail. Let’s dive in.

Most cacti genera, including some of the largest, like cactoid and opuntia, have adapted in various ways to survive environments with a very small amount of water.

Some cacti species easily thrive in some of the aridest places on earth due to their adaptations.

And some exceptions to such adaptations are epiphytes and tree climbers like Rhipsalideae and Hylocereeae, which grow in tropical forests where the availability of water isn’t an issue.

The evolution history of water conservation of cacti goes way back, as the primary cacti’ ancestors were already adapted to survive frequent droughts.

And the first step to this evolution was developing succulent stems, which took place about 25 million years ago. More can be realized about the adaptations of cacti by studying their phylogeny and evolution history.

Different Adapted Features for Water Conservation in Cacti

Modified leaves, stems, roots and metabolism systems are some of the ways that cacti have developed to store water and reduce water loss. Let’s take a look at how these features contribute to water conservation.

Leaves

In order to save water, most cacti plants have either eliminated long-lasting leaves from their body features or have developed leaves that are thick and succulent.

One prominent example of cacti having long-lasting leaves is Pereskia which is a small genus of cacti consisting of only four species. Pereskia resembles most of the ancestral species that all cacti have descended from.

Plants from this genus have leaves that resemble the most with usual plants we notice around us every day. But with close inspection, you’ll notice that the leaves are comparatively thicker and fleshier than normal plant leaves.

Pereskiopsis, another genus from the opuntioid subfamily, also comes with thick and succulent leaves. A key deciding factor of water loss through leaves is the ratio of the surface area of a leaf and its volume.

More surface area means more loss of water through the surface. Contrarily, more volume indicates that the leaf can store more water.

That’s why thickened and succulent leaves of different cacti plants allow them to retain more water and minimize the loss of water through the surface of the leaves.

Spines

Although it may not be realized readily, spines, modified versions of leaves, play a significant role in conserving water for cacti plants. Spines have a high surface area to volume ratio, but that matters very little, as mature spines contain little to no water.

Spine of cactus

The reason for that is spines are made of fibers full of dead cells. Spines provide cacti plants protection from herbivore animals besides retaining water and moisture.

Here are the ways spines help to conserve water in cacti plants:

  1. Spines create a layer of moisture near the surface of the plant by trapping air. This layer reduces water loss through transpiration and evaporation.
  2. They can also keep the temperature of the plant surface low by providing shade. Low temperature reduces water loss through evaporation.
  3. Spines are able to condense moisture when moist air is present around the plant. This results in dripping water near the plant’s base, which is later absorbed by its roots.

Stems

For most cacti plants, their succulent stems are the prime water-conserving organ. Stems can store a huge amount of water, so much so that almost 90% of a cactus body mass can be pure water content.

Stems of cactus

Usually, there are 3 features by which succulent cacti stems can contribute to water conservation. They are –

1.    Reduced Surface Area to Volume Ratio

Naturally, cylindrical and spherical shapes have a low surface area-to-volume ratio compared to other shapes. Especially a spherical shape can take on the lowest surface area while occupying a certain volume.

Columnar and globose/globular cacti take advantage of their low surface area to volume shape by reducing water loss through transpiration and evaporation. These shapes also significantly reduce the heating effect from direct sunlight.

2.    Shrinkage and Swelling    

Manny cacti plants have fluted or ribbed stems that let them shrink when water is scarce and swell by absorbing a huge amount of water when it’s very much available.

Saguaro (Carnegiea gigantea) is a prime example of such a cacti. When available, a grown saguaro can absorb and store up to 200 gallons of water inside its stem.

3.    Waxy Cuticle Layer

Almost all cacti plants with succulent stems come with a thick, waxy layer of cuticle. This layer greatly prevents water loss through the surface of a plant. A grayish or bluish color tone seen often on the cacti stems is the result of this thick cuticle layer.

Roots

Cacti roots are developed differently to maximize water absorption. These following features are common for cacti roots that greatly help them survive in arid lands.

Roots of cactus

Wide but Shallow Roots for Efficient Water Capture

Most cacti roots spread very widely but don’t go much deep. This helps them swiftly capture a large amount of water during rainfall.

For example, in the early stage of a saguaro plant, when it’s around 12 cm tall, its root system covers almost a 2-meter area in diameter while going only 10 cm deep below.

Rapid Root Growth in Cacti as a Response to Drought and Rainfall

Cacti can quickly grow new roots whenever necessary, especially when rainfall occurs after a period of drought. The extensive growth of roots allows cacti plants to absorb a great amount of water.

Cacti Use Salts to Absorb Water Efficiently

The roots of cacti contain a lot of salts in the root cells. Increased salt density allows the roots to absorb a great amount of water within a very short amount of time. 

Some cacti plants store water in their roots in addition to their stem. These plants have large, fleshy taproots instead of having spread-out thin roots.

In some cases, the size of the root can get two to three times larger than the size of the body above ground. Copiapoa atacamensis, a cacti species from the Atacama Desert, northern Chile, has taproots larger than their body size.   

Metabolism

Plants produce their food through photosynthesis, where they require carbon dioxide (CO2) intake through their stomata. Here we’ll discuss the regular method of photosynthesis alongside the ones that cacti plants use.

  1. C3 Mechanism

The photosynthesis process typical plants follow is called the C3 mechanism, in which CO2 is continuously absorbed and converted into a 3-carbon compound called 3-phosphoglycerate.

This 3-carbon compound is later converted into carbohydrates. The process includes some steps which would be really detrimental for cacti plants. They are:

  • The process takes place during the day when heat in deserts are at peak. 
  • During this process of taking in CO2, plants lose a significant amount of water (almost 97%) they absorb through their roots via transpiration.
  • The CO2-capturing enzyme becomes less efficient due to high temperatures and starts capturing oxygen instead. This brings the efficiency of C3 photosynthesis down to 25% only.

As cacti plants can’t afford to lose 97% of their absorbed water for a 25% efficient procedure, they have evolved their photosynthesis mechanism.

  • Crassulacean Acid Metabolism (CAM)

Cacti plants follow the full crassulacean acid metabolism (CAM) method, where the stomata are open only during the night (when the temperature is the lowest and so is the water loss).

During this time, the CO2 is converted into a form of organic acid and stored. While in the daytime, their stomata remain closed, and the plants only utilize the stored organic acid for their photosynthesis.

  • Cycling Crassulacean Acid Metabolism (CAM)

There is a more flexible method called cycling CAM, where stomata open during the day and take in CO2. At certain times like during a drought, the plant will open its stomata only at night. This method is prevalent in the species of Pereskia.

More adaptive measures were required for cacti plants to do efficient photosynthesis in harsh arid environments. The primary ancestors of cacti plants had to develop stomata on the stems.

But that alone wasn’t enough. They also developed a specialized plant tissue that contains chloroplast at the stem core. This tissue is called chlorenchyma, and that is where the majority portion of photosynthesis occurs.

Short or Specific Season of Growth

A short or specific growing season is seen in cacti plants to survive in arid environments with limited water availability.

During the growing season, cacti will absorb water through their roots and store it in their thick stems or leaves. They will use this stored water to sustain themselves during long periods of drought when water is scarce.

The growing season of cacti varies depending on the species, but it is typically short and coincides with the rainy season in their native habitats.

During this time, cacti will rapidly grow and develop their morphology, such as features like flowers, fruits, and seeds.

After the growing season, cacti will enter a period of dormancy, during which they will conserve water by reducing their metabolic activities and shedding their leaves or stems.

This dormancy period may last for several months or even years, depending on the species and environmental conditions.

Frequently Asked Questions

Here are some common questions that people have asked to better understand the adaptations for the water conservation of cacti:

1. Do cacti carry out photosynthesis at night?

No, photosynthesis requires sunlight to occur. Via the CAM method, cacti only store the required CO2 during the night. But the photosynthesis process takes place during the day.

2. Can we drink cacti water?

Water stored in cacti is not drinkable in most cases as it may contain harmful chemicals and alkaloids. But there are some cacti in the opuntia subfamily which are comparatively safer as they contain very little amount of chemicals.

Conclusion

Cacti is an excellent example of how plants can adapt to their environment to survive, and they are a reminder of the importance of conserving water in our daily lives.

The adaptations for water conservation of cactus plants are nothing short of remarkable.

By studying cacti and other desert plants, we can better understand how plants adapt to their environment and how we can work to protect and conserve this precious natural resource of ours.

Reference

  • Andrea. (n.d.). How Does The Cactus Store Water? Unique Adaptations. Retrieved from Your Indoor Herbs and Garden: https://www.yourindoorherbs.com/how-cactus-store-water/
  • BYJU’S. (n.d.). How Are Cactus Adapted To Survive In A Desert? Retrieved from BYJU’S: https://byjus.com/biology/how-are-cactus-adapted-to-survive-in-a-desert/

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