Crassulacean Acid Metabolism (CAM) and Dark CO2 Fixation

In this article, we will discuss about the crassulacean acid metabolism (CAM) and dark CO₂ fixation.

Under natural conditions the acidity of green shoots of some non-halophytic succulents and semi-succulent plants increases at night and decreases during the following day. This diurnal change in acidity was first discovered in a Crassulacean plant Bryophyllum calycinum hence, it has been termed as Crassulacean acid metabolism (CAM). Crassulacean acid metabolism occurs only in green organs and the plants which exhibit it belong to a number of different families. It is especially noticeable in leaves of Bryophyl­lum, Kalanchoe, Sedum, Kleinia, Crassula and fleshy green stems of Opuntia.

In CAM plants, the stomata are open at night and are usually closed during most of the day.

The distinctive diurnal fluctuation in acidity in plants showing CAM is predominantly due to changes in amounts of vacuolar malic acid in the mesophyll cells. During night, malic acid is synthesized utilising CO₂ (dark CO₂ fixation) which then accumulates in the vacuole and may account for about 85% of the total titrable acid content.

During the following day, this malic acid is consumed resulting in decrease of acidity. Besides malic acid, some other acids like citric acid and iso-citric acid also contribute to the total titrable acidity but their amount is negligible and moreover these do not show consistent diurnal pattern of fluctuation as exhib­ited by malic acid.

Diurnal Changes in Gaseous Exchanges during CAM:

During the dark synthesis of malate in CAM (resulting in the acidification), oxygen is absorbed continuously but, in the early stages, little or no CO₂ is evolved. This results in a respi­ratory quotient (R.Q.) of very low or even zero and sometimes a negative value.

When the accumulation of malate is complete, the CO₂ is evolved rapidly and the R.Q. is unity.

During light de-acidification, O₂ is continuously evolved but initially the absorption of CO₂ is very slow. This results in high values of photosynthetic quotient (ml. O₂ evolved/ml. CO₂ uptake). However, as the de-acidification slows down, the uptake of CO₂ increases rapidly until de-acidification is complete. In successive light periods, the vol. of CO₂ uptake equals to the vol. of O₂ given out and hence, the photosynthetic quotient falls to unity.

Factors influencing diurnal fluctuations in acidity:

The amplitude of the diurnal fluctuations in acidity varies with growth conditions and age of the plants. It increases with unfolding of the leaves until they are fully expanded and de­creases when they enter senescence. Apart from these, the seasonal changes also have profound effect on it through changes in day and night temperatures, photosynthetic activity and at least in some plants in the length of the day.

Synthesis of malate during night or dark CO₂ fixation:

Large amounts of starch are consumed during acidification which indicates that carbohy­drates are the source of malate synthesis, the overall process being represented by the follow­ing equation:

C₆H₁₂O₆ + 2CO₂ → 2C₄H₆O₅ Malate

It is now generally believed that the malate is synthesized during night in reaction in which some product derived from carbohydrate reserves e.g., pyruvate or most likely phosphoenol pyruvate (PEP) is carboxylated to produce malate either directly or first forming oxaloacetic acid which is then reduced to malate according to the following reactions:

Consumption of malate in light deacidiflcation:

During the following day when acidified organs are exposed to light, rapid consump­tion of malate occurs resulting in de-acidification (due to CO₂ release from malate).

The malate may be decarboxylated in two ways:

(i) In some CAM plants, the malate is directly decarboxylated in the presence of NADP⁺— malic enzyme into CO₂ and pyruvate.

(ii) In other CAM plants, the malate is first oxidised to oxaloacetate by a malate dehydrogenase. The oxaloacetate is then converted into CO₂ and phosphoenolpyruvate with the utilization of ATP by PEP—Carboxykinase.

The CO₂ thus produced in either of the above two ways is then consumed in normal photosynthetic reaction sequence to yield carbohydrates. The pyruvate and phosphoenolpyruvate are probably also utilised for carbohydrates synthesis during the day. The pyruvate is first converted into phosphoenol pyruvate (PEP) in the presence of the enzyme pyruvate orthophosphate dikinase.

Pyruvate + ATP + Pi → PEP + AMP + PPi

AMP + ATP → ADP + ADP

The PEP in both types of CAM plants is converted into 3 PGA (3-phosphoglyceric acid) by reverse reactions of glycolysis. Thereafter, the 3-PGA is utilised in the Calvin cycle.

The outline of CAM is given in Fig. 11.28.

Distribution of CAM Plants:

About 24 families of angiosperms (including, Crassulacee, Portulacaceae, Cactaceae, Euphorbiaceae, Bromeliaceae, Aizoaceae, Agavaceae & Orchidaceae) are known to contain plants that exhibit CAM. Most of these families with the exception of Crassulaceae and Cactaceae, are not exclusively CAM, because they have both CAM and C₃ representatives. Some families may even have representatives showing all three modes of photosynthesis viz., C₃, C₄, and CAM.

Examples of commercially important plants which show CAM are, pine apple (Ananas comosus), agave (century plant), Cacti, and orchids.

Besides angiosperms, CAM also occurs in some pteridophytes such as Isoetes (Family Isoetaceae), some lithophytic and epiphytic ferns is (Family Polypodiaceae), and Welwitschia mirabilis (a gymnosperm). CAM is also known to occur in some aquatic species of plants.

Comparison between CAM and C₄ Plants:

The CAM and C₄ plants show some close similarities and also significant differences, and some scientists even think that CAM might be a variant of the C₄ syndrome.

A brief account of similarities and differences between the two follows:

Similarities:

i. Both CAM and C4 cycles, use cytoplasmic phosphoenolpyruvate carboxylase (PEPcase) to form 4-C acids from phosphoenolpyruvate (PEP) and bicarbonate ions (HCO₃^–) in mesophyll cells.

ii. In both the cases, the 4-C acids thus formed are subsequently decarboxylated to yield CO₂ for use in Calvin cycle or PCR-cycle.

Differences:

i. C₄-plants are found only in angiosperms, while CAM plants occur also in some pteridophytes, and a gymnosperm.

ii. Succulence is prerequisite for CAM, but not for C₄-cycle.

iii. Stomata are closed during day and open at night in CAM plants. Reverse is true for C4- plants.

iv. Krantz anatomy occurs in C4-plants, but not in CAM plants.

v. In CAM plants, carboxylation of PEP and decarboxylation of 4-C acids, occur in the same mesophyll cells but are temporally separated (i.e., separated in time), the former taking place in night and the latter during the following day. Contrary to it, in C₄ plants, carboxylation of PEP occurs in mesophyll cells while decarboxylation of 4-C acids takes place in bundle sheath cells in quick succession, and therefore, both these reactions are spatially separated but not temporally.

vi. In C₄ pathway, there is closed cycle of carbon intermediates (Fig. 11.22) while in CAM, it is not.

vii. The transpiration ratio is substantially lower in CAM plants than in C₄-plants.

Ecological Significance of CAM:

CAM plants are especially suited to dry habitats such as deserts. These plants have remarkable capacity to attain high biomass under conditions of high evaporation rate or scanty rainfall, which are otherwise insufficient for growth of crop plants.

Nocturnal opening of stomata in CAM plants allows uptake of atmospheric CO₂ when conditions for transpiration are at a minimum. During day time, when stomata are closed to check transpiration, photosynthesis can proceed by using CO₂ released from decarboxylation of malate. The transpiration ratio (i.e., the ratio of the wt. of water transpired to the wt. of dry matter produced) for CAM plants is substantially lower than either with those of C₃ or C₄ plants.

Typically, a CAM plant loses about 50-100 gm of water for ev­ery gm of CO₂ gained, as compared with 250-300 grams for C₄-plants and 400-500 grams for C₃-plants. Therefore, CAM-plants have definite competitive advantage over C₃ and C₄ plants in dry habitats such as in deserts. However, rates for daily carbon assimilation in CAM plants are only about half those of C₃-plants and one third those of C₄-plants.

Daylight closure of stomata in CAM plants to conserve water in dry habitats, may not be the unique basis for evolution of CAM. It is because, paradoxically, some aquatic plant species are also known to exhibit CAM. According to some scientists, CAM probably also increases acquisition of inorganic carbon in the form of bicarbonate ions (HCO₃^–) in aquatic habitats, when availability of CO₂ is restricted due to high resistance to gas diffusion.