Cactus & Succulent Society
of New Zealand (CSSNZ)

Crassulacae Acid Metabolism in C&S

Introduction

Romans of old noticed that certain fleshy plants like sedums tasted "sour" in the morning and less "sour" at night, what they were unknowingly observing was Crassulacean Acid Metabolism .

Crassulacean Acid Metabolism (CAM) was first researched in the 1800's in the crassula family, hence it's name.

CAM has evolved from normal photosynthesis many times (20 families of land plants have it) over the past 100 million years and is present in more than 20,000 species of plants including desert cacti and other succulents. The primitive leaf-bearing cactus, Pereskia, does not CAM. There is a strong connection between CAM and succulence, like one needs the other in other to facilitate survival of the plant.

Families with CAM include Agavacea (agaves), Cactaceae (cacti), Crassulaceae (crassula), Euphorbiaceae (euphorbia), Liliaceae (lillies), Orchidaceae (orchids) and Vitaceae (grapes).

Biochemistry of CAM

Plants breathe through stomata (little holes) in the stem/leaves taking in CO2 from the atmosphere , combining it with water (H2O) in the presence of chlorophyll (the green stuff!) to make sugars (CHO) using sunlight as the energy source for the manufacture of the sugars, releasing oxygen into the atmosphere as a by-product of the chemical process known as photosynthesis, (we humans do the reverse - take in oxygen to burn sugars and release energy, and release CO2 as a by-product or waste).

Because sunlight is needed to power this chemical reaction in most plants it occurs during the day, at night the stomata are closed. It is also called a C3 cycle as the first chemical made by the plant is a 3 chain Carbon molecule. The only cacti to use this process is the primitive Pereskia.

• 6 H2O + 6 CO2 + Sunlight ---> Sugars (C6H12O6) + 6 O2

Having stomata open during the day also means water can escape from the plant, especially where the air outside is hot and dry and of a low humidity ... like in the desert. This is not good if water is a scarcity in the first place.

CAM plants take in CO2 from the atmosphere during the night fixing it within the plant as an organic acid (malic acid). During the day the organic acid is broken down internally to release CO2, when sunlight is present to provide energy for the chemical reaction to make sugars.
This means the stomata can be open during the night when outside temperatures are low and humidity is (relativity) high and the moisture is not "sucked" out of the plant. Meanwhile during the day stomata remain closed using the internally stored and released CO2, thereby sealing the plant off from the outside world.

• Night: CO2 + H2O ----> Organic Acid ... Stomata open
  
• Day: Organic Acid Breaks down releasing ... CO2 + H2O + Sunlight ----> Sugars (CHO) + O2 ... Somata Closed

CAM photosynthesis is less energy efficient and results in slower growth.
There is only a limited amount of CO2 than be fixed the night before and this may be used up by midday the next day so there is no more photosynthesing the rest of the day.
Another reason it is less efficient is that the process of losing water through the stomata forms a "vaccuum" sucking up water through the plant, then roots and ultimately from the surrounding soil. But in CAM plants this does not occur to such a degree so the "sucking" power, to draw water out of the soil, is not present (some C&S; make up for this with their special root structure and ability to form new roots very quickly after rains).

However some species are able to use normal photosynthesis to maximize growth at times of sufficient water supply, but switch to CAM during periods of limited water supply.

Another method used by some arid region plants is C4, found in Euphorbias. Also plants growing in areas of high temperature, high light levels and high rainfull in summer use C4, like sugar cane and corn. In this process a 4 carbon sugar (malate) is formed (instead of the 3 carbon in "normal" C3 photosynthesis), CO2 is concentrated within the cell but the whole process is once again less efficient.