Question

In CAM cycle during the formation of malic acid stomata remains
A. Open
B. Closed
C. Semi open
D. Always closed

Answer 1

Xerophytic plants have adaptations that help plants minimize water loss while maintaining a clean uptake of $C{O_2}$ from the environment. In these plants or cuticles, there is sclerenchyma tissue which is very thick and abundant .This reduces transpiration but increases diffusion resistance to $C{O_2}$ absorption. As a result, xerophytes are less efficient than photosynthesis per unit area. Most succulents have Crassulacean Acid Metabolism (CAM).

Complete answer:
Crassulacean Acid Metabolism-In this type of metabolism, $C{O_2}$ is absorbed by the green stem leaves through the stomata which remain open at night. However, during the day, the stomata in these plants remain closed to retain moisture. As with C4 plants, the $C{O_2}$ absorbed by the succulents at night is fixed to malic acid, which is stored in vacuoles.
Therefore, the malic acid which is formed at night is used as a source of $C{O_2}$ for photosynthesis during the day to continue the C3 pathway.
CAM is a kind of adaptation present in some succulents such as pineapples to continue photosynthesis without losing much water, which is commonly found in plants with the C3 and C4 pathways.

Hence, the correct answer is option (A).

Additional information:
CAM has several advantages, e.g. Increase the efficiency of plant water use and, secondly, they adapt to very hot climates by the enzyme PEP carboxylase. In addition, the CAM system can also accept the zero point for $C{O_2}$ compensation at night and thus achieve a steeper gradient in $C{O_2}$ uptake.

Note: CAM plants are especially suitable for dry habitats like deserts. These plants have the extraordinary ability to achieve high biomass in conditions of high evaporation rates or low rainfall, which otherwise are not sufficient for plant growth. During the day when the stomata are closed to check for transpiration, photosynthesis can continue to use up $C{O_2}$ , which is released when malate is decarboxylated. The transpiration ratio (i.e. the ratio of the weight of water obtained to the weight of dry matter produced) was significantly lower in CAM than in C3 or C4 plants.