The upcoming discussion will update you about the Relationship between C4 and CAM.
A similarity between the carbon fixation mechanism in C4 and CAM has been noted. Laetsch (1974) has reviewed the relationship and suggested a model for spatial and temporal compartmentalization of carbon fixation in C4 and CAM (Fig. 8.29).
In CAM-type photosynthesis, C4 acid is produced by primary carboxylation in night when the stomata are open. Decarboxylation of the C4 acid occurs in the following daytime, when the stomata are closed and the released CO2 re-fixed by the C3 pathway. Thus, the primary carboxylation and the carbohydrate synthesis are separated with respect to time.
On the other hand, the anatomy of C4 plants results in the separation with respect to space in different compartments (mesophyll cells and bundle sheath cells) of essentially the same biochemical machinery found in the green cells of CAM.
CAM plants are adapted to environments of constant aridity while C4 plants are adapted to tropical areas where extreme drought conditions alternate with periods of abundant water regime. In land plants growing in dry and hot regions, the major problem of the aerial part is desiccation.
The aerial portions of CAM plants are succulent with water storage tissue in leaves and stem and have low surface area to volume ratios. This type of desiccation-avoiding structure is optimum for water retention but not efficient for gas exchange. Water seems to be the most important factor in CAM and the selective pressures would be for facilitating the efficiency of water use.
C4 plants must be able to survive drought conditions, at the same time they must be able to grow satisfactorily when water is not limiting.
In C4 leaves, there are rows of solid cylinders of vascular tissue and thick-walled, large bundle sheath cells which provide small surface area relative to volume as compared to the C3 leaves and this should minimize the evaporating surface.
The distinct advantage of C4 over CAM is that in C4 an efficient carbon trapping system comprising of the adjoining mesophyll cells with intercellular spaces is situated in the periphery of the cylinders. Thus Kranz anatomy is a structural compromise which restricts water loss and simultaneously permits efficient gas diffusion.
A striking similarity between carbon fixation mechanism in CAM and C4 points out the basic significance of β-carboxylation in both types. The basic theme is that CAM plants can fix CO2 in dark into C4 acids and that C4 plants can fix CO2 in light. Based on this common character shared by both types, CAM plants may be rightly named as ‘night C4’, alternatively, C4 plants are essentially ‘crypto-succulent’.
The Na+ requirement in both C4 and CAM may be interpreted as cation-induced anion synthesis in the form of malate.
Both C4 and CAM plants have been found to thrive well in saline environments and incidentally these plants absorb quite a large amount of cations. It is possible that β-carboxylation was originally an adaptation for the synthesis of anions (malate) to counter-balance the accumulation of cations occurring in saline regions.
If this is true, the transport of malate from mesophyll cells to bundle sheath cells may be looked upon as an ‘anion pump’ rather than a ‘carbon pump’.
The bundle sheath cells lie next to the vascular tissue and it is likely that accumulation of cations would take place in the large bundle sheath cells after absorption from the soil and the spatial separation of the carbon fixation reactions in C4 is a mechanism to maintain cation-anion balance.
Finally, it may be considered that C4 plants are the form and functional relatives of CAM plants. The relationship of C4 anatomy with carbon metabolism with the resultant advantage of cutting down photo respiratory CO2 loss are thought to be only secondary adaptations. Primary adaptations of both C4 and CAM plants are in response to selection pressures common in arid and saline environments.