Crassulacean Acid Metabolism (CAM cycle)

 Under natural conditions, the acidity of green shoots of some non-halophytic succulents and semi-succulent plants increase at night and decrease during the daytime. This diurnal change in the acidity was first discovered in Bryophyllum belonging to the family Crassulaceae. Therefore, it is called Crassulacean Acid Metabolism (CAM). This metabolism occurs only in green organs and it is quite common in the plants belonging to the families like Crassulaceae, Cactaceae, Orchidaceae, Bromeliaceae, Liliaceae, Asclepiadaceae, Vitaceae, and Euphorbiaceae. All such plants are called Crassulacean Acid Metabolism plants(CAM plants). Most CAM plants possess the succulent habit. A typical example of a commercial crop possessing a CAM pathway is pineapple.

Mechanism of CAM cycle

The sequence of reactions taking place in the CAM pathway is shown in Fig.20.

1. During the night when stomata are open, CO2 is fixed through the action of PEP carboxylase to Malic acid. This is accomplised in two steps :

a. PEP fixes CO2 and is converted into oxaloacetic acid (OAA)

b. OAA is subsequently converted into malic acid by malic dehydrogenase enzyme.

2. During light (daytime), the malic acid releases one CO2 molecule with the formation of PEP. This PEP is ultimately converted into sugars or starch.

3. The leaves of CAM plants also contain enzymes of the Calvin cycle. Therefore, the released CO2 combines with PGA and completes the C3 cycle in the light. Thus, two cycles (CAM and Calvin cycle) occur in the mesophyll cells and there is no differentiation between the type of cells as found in C4 plants.

Chemosynthesis

There are certain aerobic bacteria, which do not have chlorophyll but can synthesise organic food from CO2 and H20. This process of manufacture of food materials by bacteria making use of the chemical energy is called Chemosynthesis. Some of the common examples of Chemosynthetic bacteria are given below :

1. Nitrifying Bacteria

Nitrosomonas and Nitrosococcus oxidize ammonia to nitrite and chemical energy is released.

2NH3 + 3O2 → 2HNO2 + 2H2O + Chemical energy + 2S

2. Sulphur Bacteria

Beggiatoa and Thiothrix oxidize hydrogen sulfide to sulfur and release a sufficient amount of chemical energy for subsequent food synthesis.

4FeSO4 + O2 + 6H2O → 4Fe(OH)3 + 4C2O + Chemical energy.

Factors Influencing Rate of Photosynthesis

The factors influencing the rate of photosynthesis can be classified into two categories, internal and external (environmental)

A. Internal Factors

1. Chlorophyll

The amount of chlorophyll present has a direct relationship with the rate of photosynthesis, since, it is the pigment, which is photoreceptive and is directly involved in trapping the light energy.

2. Photosynthetic Enzyme Systems

The amount and nature of enzymes play a direct role on the rate of photosynthesis. Greater enzyme activity at higher light intensity increases the capacity of the leaf to absorb more light and thus increases the photosynthetic rate.

3. Leaf Resistance

Photosynthesis shows close dependence upon leaf resistance. For C4 plants, leaf resistance (primarily controlled by stomatal aperture) appears to regulate photosynthesis, but in C3 plants, the internal resistances including carboxylation efficiency offer greater limitation to CO2 fixation than stomatal resistance. Environmental factors such as light intensity, photoperiod, CO2 concentration, humidity, and soil moisture also affect photosynthesis via stomatal resistance.

4. Demand for Photosyntate

Because of the greater demand, the rapidly growing plants show an increased rate of photosynthesis in comparison to the mature plants. However, if the demand for photosynthesis is lowered by the removal of the meristem, then the photosynthetic rate declines.

5. Leaf Age

The Photosynthetic rate is higher in the newly expanding leaves and reaches a maximum as the leaves achieve full size. The rate declines as the leaf ages due to reduced chloroplast functions and other anabolic reactions.

6. Role of Hormones

Photosynthesis may be regulated by some of the plant hormones. Gibberellic acid (GA) and Cytokinins (CK) increase both carboxylation activity and photosynthetic rate.

7. Genetic Control

Genetic control also plays a very important role in both CO2 fixing systems and the CO2 transport system of the leaf. The fixation of CO2 is the function of set of enzymes present in the chloroplast, which is in turn under the control of genes of the chloroplasts.

B. External Factors

1. Carbon dioxide

CO2 is one of the raw materials for photosynthesis; therefore, its concentration affects the rate of photosynthesis markedly. The rate of photosynthesis increases with the increase in the atmospheric CO2 concentration up to a certain extent. Because of its very low concentration in atmosphere (current level of 350-360ppm), it acts as a limiting factor in natural photosynthesis. The rate of photosynthesis increases with an increase in the atm. CO2 level of upto 1000ppm beyond which, there is a general decline in photosynthesis.

At this enhanced level of CO2, the increase in the photosynthetic rate may be 10 to 30 times more than the normal CO2 level.

2. Light

Light affects the rate of photosynthesis in several ways. In general, photosynthesis can occur under artificial lights of sufficient intensity. The role of light on photosynthesis can be discussed under the following sub-heads:

a. Intensity of light :

With the increase in light intensity, the rate of photosynthesis increases, i.e., the rate of photosynthesis is directly proportional to light intensity. However, at a stronger light intensity, an increase in the rate of photosynthesis is not proportional to light intensity. Except on cloudy days, light is never a limiting factor in nature.

At certain light intensity, the amount of CO2 used in photosynthesis and the amount of CO2 produced in respiration are volumetrically equal. This point of light intensity is known as Light Compensation Point. Light compensation point is frequently in the order of 100 to 200 f.c. for sunloving leaves; while, the value is 100f.c. for shade-loving leaves. Thus, in shade-loving plants, the compensation point lasts for a much shorter period than in sun-loving plants.

b. Wavelength of light :

For photosynthesis, the visible range of spectrum of light (PAR:400 to 700 nm) is essential. Maximum photosynthesis is known to occur in the red part of the spectrum with the next peak in the blue part and minimum in the green region (RED >BLUE > GREEN) The region between 575 and 750nm (yellow to red) is quite congenial for photosynthesis. Ultra violet light has a lethal effect on plants if exposure is for a prolonged period.

C. Duration of light :

Photosynthesis may be sustained for relatively long periods of time without any noticeable damaging effect on plants.

d. Photo-oxidation :

When the light intensity for photosynthesizing tissue is increased beyond a certain limit, the cells become vulnerable to chlorophyll photo-oxidation; due to this, many more chlorophyll molecules become excited than can possibly be utilized. This causes a damaging effect on the chloroplast membrane system.

In presence of O2, the damaging effect of photo-oxidation is severe. It results in the bleaching of chlorophyll and the inactivation of some important enzyme involved in photosynthesis.

3. Temperature

The effect of temperature on photosynthesis is little than on another process. Very high and very low temperatures affect the photosynthetic rate adversely. The rate of photosynthesis increases with rising in temperature from 5 to 350C; beyond which, there is a rapid fall in photosynthesis. In the optimum range of temperature, the Temperature Quotient (Q10) is found to be 2.0 for the rate of photosynthesis (Q10=2.0).

4. Water

Water is one of the raw materials in photosynthesis. It has an indirect effect on the rate of photosynthesis. Water availability affects the water relation of plants, thus affecting the rate of photosynthesis.

In scarcity of water, cells become flaccid. Depending upon the availability of water, the rate of photosynthesis may be decreased from 10 to 90%.

5. Oxygen

Oxygen is the by-product of photosynthesis. Accumulation of a greater amount of oxygen molecules causes substantial inhibition of photosynthesis. Oxygen is also known to have a direct and competitive inhibition for RuBP carboxylase. As a result, glycolate synthesis is enhanced which leads to photorespiration.

6. Warburg’s Effect

An increase in the concentration of O2 in many plants results in a decrease in the rate of photosynthesis. A German Scientist, Warburg first discovered this phenomenon of the inhibition of photosynthesis by the greater accumulation of O2 in 1920 in the green alga, Chlorella. It is now known to operate in soybean etc. (C3 plant). But, plants like maize, sugarcane, sorghum etc. (C4 plants) do not show the effect.

7. Mineral nutrients

Some nutrients like copper etc., which are components of photosynthetic enzymes, or magnesium as components of chlorophylls also affect the rate of photosynthesis indirectly by affecting the synthesis of photosynthetic enzymes and chlorophyll, respectively. Potassium also affects the rate through stomatal movement. Leaf N content plays a major role in increasing the photosynthetic rate of crops.

Inhibitors of Photosynthetic Process

1. Several urea derivatives such as motoneuron (or CMU) and diuron (or DCMU) block electron transport between Q and PQ.

2. Simazine, atrazine, bromacil and isocil block the same step.

3. Diquat and paraquat are common photosynthetic inhibitors. These compounds (commonly referred to as viologen dyes) accept electrons from PS I before ferredoxin and produce toxic forms of O2 (superoxide and hydroxy).