What is a Succulent and What Makes Them Different From Other Plants?

By Jackie Warburton MAIH RH0164 (Photos: Jackie Warburton)

Succulent plants store water in their leaves, roots and stems and have much thicker leaves than other plants. The word succulent comes from the Latin word ‘sucus’, meaning juice, or sap. They are drought-resistant plants and have adapted to living in dry environments especially well.

Crassulacean Acid Metabolism (CAM)

Succulent plants store water in their leaves and stems and therefore can withstand long periods without water. This is described as Crassulacean Acid Metabolism (CAM).

Crassulacean Acid Metabolism is a carbon fixing pathway that has evolved in some plants as an adaptation to arid conditions. The Stomata (air holes) in the leaves are closed during the day to reduce evapotranspiration and open at night to collect carbon dioxide (CO2). Succulents have reduced stomata and photosynthesise from its stems rather than its leaves.

 

Figure 1

Figure 1:

CAM plants fix CO2 at night and convert it to carbohydrates during the day. This allows them to close their gas-exchanging pores during the day and minimising water loss.

This is a variant of the C4 pathway of photosynthesis and was first discovered in the Crassulaceae family of Succulents and there for named after the family. It has evolved in other plants such as Bromeliads, Orchids, terrestrial and ground dwelling plants such as Tillandsias, Euphorbia’s, Grapes, Lilium’s and some 25 other plant families.

Cacti are only CAM plants but one exception the Pereskia family. CAM plants can sit ‘idle’ and the internal recycling of the fixed carbon dioxide gets the plants through a dry spell.

 

 

Photosynthesis

There only three different ways plants can undergo Photosynthesis, taking carbon dioxide from the air, water, and roots as well as sunlight to transform into sugar and oxygen.

The first chemical made by the plant is a three or four chain molecule and is known as C3 and C4 and CAM.

 


Figure 2: C4 plants separate carbon fixation and the Calvin cycle by carrying out the pathways in different places. CAM plants separate them by carrying out the pathways at different times of day. These adaptations allow C4 and CAM plants to survive in environments where C3 plants cannot.

 

C3 Plants

C3 Plants are normal plants that don’t photosynthesise to reduce photorespiration and is the first step of the Calvin Cycle. About 85% of plants are C3 plants and like a cool wet environment to grow. Common C3 plants are all cereal grains such as wheat rice, barley, oats and include most trees and most lawn grasses such as rye and fescue.

C4 plants

C4 plants are created through the Calvin Cycle and made into an enzyme called RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). They have a two-step fixation mechanism. C4 pathway is used in about 3% of all vascular plants such as corn and sugar cane where common habitats are hot and dry conditions. C4 plants use Mesophyll cells then evolve the bundle sheath cells and have the highest Carbon Dioxide (CO2) output and the lowest oxygen (O2) and water loss through the leaves.

CAM Plants

CAM plants have a wonderful adaptability to fix the atmospheric CO2 and minimise photorespiration. The slow growing desert succulents exhibiting CAM cycles have the slowest photosynthetic rate, while the species possessing C4 pathway possess the highest rates.

Stomata Open                    Best Adapted Environment

C3 Day                                 Cool, Wet conditions

C4 Day                                 Hot, sunny environments, high water, high light

CAM                                     Very hot dry climate, Low water availability.

 

The Calvin Cycle

Calvin Cycle is a chemical reaction performed by plants to ‘fix’ carbon from CO2 into three carbon sugars which then can be used to build other sugars such as glucose, starch and cellulose and used by plants as a structural building material. The Calvin Cycle takes molecules of carbon straight out of the air and turns them into plant matter. The carbon created in the Calvin Cycle is also used by plants and animals to make proteins, nucleic acids, lipids, and all other building blocks of life.

The Calvin Cycle has four main steps: carbon fixation, reduction phase, carbohydrate formation, and regeneration phase. This is where the energy to fuel chemical reactions in this sugar-generating process starts and is provided by ATP (adenosine triphosphate) and NADPH, (molecules). They are chemical compounds which contain the energy plants have captured from sunlight.

 


Figure 3: The Calvin cycle is a part of photosynthesis, the process plants and other autotrophs use to create nutrients from sunlight and carbon dioxide. The process was first identified by American biochemist Dr. Melvin Calvin in 1957.

 

Step 1: Carbon Fixation – is the method plants use to attach carbon dioxide from the atmosphere to a chemical called RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) in order to start the process of photosynthesis.

Step 2: Reduction Phase – Second step in the Calvin Cycle of photosynthesis, where energy reacts with chemicals to create the simple sugar G3P.

Step 3: Carbohydrate Formation – Type of sugar that is an important nutrient for most organisms.

Step 4: Regeneration Phase – Fourth and final step in the Calvin Cycle of photosynthesis, where energy and sugar interact to form the molecule RuBP, allowing the cycle to start again.

 

So, what does all this mean to us Horticulturists? Do we try to grow more C4 and CAM plants for our changing climate? What will this mean for our everyday ‘popular’ C3 plants that we like to grow? If we adapt the plantings that we grow, will this benefit the Environment and the plantings will be more suited to our changing environment?

 

References:

Fig 1: https://evolution.berkeley.edu/evolibrary/search/imagedetail.php?id=369&topic_id=&keywords=
Fig 2: https://www.coursehero.com/sg/cell-biology/alternative-mechanisms-of-photosynthesis
Fig 3: https://www.nationalgeographic.org/media/calvincycle

 

  1. Anderson, E., Barthlott, W. and Brown, R., 2001. The Cactus Family. Portland: Timber Press.
  2. Bidlack, J. and Jansky, S., n.d. Introductory Plant Biology.
  3. Alternative Mechanisms of Photosynthesis – Course Hero. www.coursehero.com/sg/cell-biology/alternative-mechanisms-of-photosynthesis
  4. berkeley.edu. 2020. Stomata (1 Of 3) Function. evolution.berkeley.edu/evolibrary/search/imagedetail.
  5. Henry Shaw Cactus and Succulent Society. 2020. C3, C4, CAM Photosynthesis – Henry Shaw Cactus and Succulent Society.
  6. org/resources/digest/plant-info/c3-c4-cam/
  7. Ladiges, P., Evans, B., Saint, R. and Knox, B., n.d. Biology.
  8. Society, N., 2020. Calvin Cycle. National Geographic Society. www.nationalgeographic.org/media/calvincycle

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