CAM Cycle (Crassulacean Acid Metabolism)

CAM Cycle

This metabolism was first of all reported in Bryophyllum, a member of family Crassulaceae and hence is called crassulacean acid metabolism.

This occurs in mostly succulents (xerophytes) like Opuntia, Agave, Aloe, Sedum, Kalanchoe, etc., But there occurs dark acidification, ,i.e., during night malic acid is formed. This malic acid breaks up into CO2 and pyruvic acid in day time and CO ₂ released is utilized in C ₃ cycle.

In these plants, stomata open during night and close during day time (Scotoactive stomata)

In these plants carbon fixation at night is by PEPCO and during day is by RUBISCO.

CAM plants lack Kranz anatomy.

They are not resistant to photorespiration but minimise transpiration as stomata is closed during day time.

Factors of Photosynthesis

Blackmann’s law of limiting factor : Rate of photosynthesis is affected by the pace of the slowest factor or limiting factor eg. light or CO2.

Sachs has determined three cardinal points for a factor; Minimum, Optimum and Maximum.

1. Light : The effect of light on photosynthesis is immense.  It is affected by the intensity and quality. No photosynthesis occurs in a very weak intensity of light.  As the intensity of light increases, the process begins and picks up the pace.  A very strong intensity of light causes solarization. During solarization, photo-oxidation occurs and if it continues for a few hours, the photosynthetic apparatus is destroyed.  The photosynthesis begins in the morning hour, reaches its peak in the noon/afternoon and then its rate declines.  The value of light at which further increase is not accompanied by an increase in CO2 uptake is called light saturation point. A plant continues to respire almost at a uniform rate.  During peak hour rate of photosynthesis is about twenty times faster than that the of respiration.  However, in the morning as well in the evening (twilight), photosynthesis equals the rate of respiration.  It is called light compensation point. During compensation point there is no evolution of any gas. Light compensation point for shade plants is between 2.5-100 ft. candles and for sun plants is 100-400 ft. candles.

2. Temperature : The plants can perform photosynthesis on a range of temperature.  While some cryophytes can do photosynthesis at - 35°C, some thermal algae can do this act even at 75°C. Usually the plants can perform photosynthesis between 10°C - 40°C.  The optimum temperature ranges between 25°C – 30°C.

3. CO ₂ : The concentration of CO ₂ in the atmosphere is 0.03% by volume.  If this concentration is increased by 15 to 20 times, the photosynthetic rate increases, if no other factor becomes limiting.  Still higher concentration of CO ₂ is toxic to plant.  However, the value of upper limit of CO ₂ concentration is variable.  A stage in CO ₂ concentration where there is no net absorption of CO2 by illuminated plant organ is called CO ₂ compensation point  or threshold value. Its less than 10 ppm for C ₄ plants and 50 - 100 ppm for C ₃ plants.

4. Water : It is a reactant in photosynthesis.  A plant utilizes less than 1% water from its total absorption and the rest is transpired.  A decrease in water contents cause loss of turgor thereby closing down the stomata.  The effect of water is more indirect than direct.

5. Oxygen : The concentration of oxygen in the atmosphere is about 21% volume and it seldom fluctuates.  An increase in oxygen concentration decreases photosynthesis and the phenomenon is called Warburg effect. The explanation to this problem lies in the phenomenon of photorespiration.  In C-3 plants the rate of day (light) respiration is faster than the dark respiration.  This enhanced rate of respiration is called photorespiration.  It operates in high light intensity, high oxygen concentration and high temperature.  Here oxygen competes with CO2 for the oxidation of RuBP to phosphoglycolic acid, thus reducing the fixation of CO ₂ .

6. Chemicals : The chemicals, which act as enzyme inhibitors like all other vital processes, inhibit photosynthesis also.  Such chemicals are cyanides, hydroxyl amine, H2S, CO, iodoacetates etc.  Besides, chloroform and ethers also inhibit photosynthesis.  The herbicides dichlorophenyl dimethyl urea (DCMU) and chlorophenyl dimethyl urea (CMU) are also photosynthetic inhibitors.  They inactivate PS II, thus inhibiting the Hill reaction.

7. Minerals : Mn ²⁺ , Cl ^– Ca ²⁺ ions help in photolysis of water.  Mg and Fe in synthesis of chlorophyll.  B and K help in translocation of solutes.

8. Hormones : Auxins, Gibberellin and Cytokinin increase the photosynthesis.  ABA decreases photosynthesis.