Answer»
- CAM is an adaptation to extremely xerophytic environments. Rates of CARBON fixation in CAM, are, however, lower than those of C3and C4. Under conditions of severe water stress, C3 photosynthesis may cease altogether whereas CO2 uptake CONTINUES in CAM plants.
- Under severe drought condition, net carbon loss occurs in C3 and C4 leaves due to dark respiration, whereas CAM tissues may eliminate respiratory loss of CO2 since these are able to RETAIN and re-fix the respired CO2. This situation is clearly demonstrated in certain cacti which can survive in extremely hot regions.
- The stomata remain closed not only during the daytime but also may be closed during the night. Under this condition, evaporative water-loss as well as CO2 exchange is ALMOST nil. Thus there is no growth but the plant can survive by fixing CO2 made available internally by photorespiration and dark respiration.
- The CO2 and PEP resulting from the normal respiratory process may be converted back to malate which upon decarboxylation provides CO2. This recycling of CO2 through CAM pathway is referred to as ‘idling’ CAM which may undergo transition to productive CAM when water supply is not limiting. In this way, these cacti can maintain almost constant dry weight over a long period of drought condition.
- There may be two types of CAM plants. In one type represented by the cacti such as Opuntia basilaris and Zygocactus truncates which are obligate CAM, the plants behave like CAM even during periods of abundant water supply.
- By contrast, some CAM plants are facultative or inducible, such as Mesembryanthemum crystallinum, Kalanchoe tubiflora and a few others, which shift from CAM to C3 photosynthesis when adequate water is available. Such facultative CAM species require inorganic salts, particularly NaCl, in the soil for the development of CAM character which is similar to the sodium requirement of C4 plants.
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