Question

The value of R.Q. of a succulent plant at night is
A. More than 1
B. Less than 1
C. Zero
D. Infinite

Answer 1

Succulent plants have special mechanisms to save water loss. At night in succulent plants during respiration, no $CO_2$ molecules are released and $O_2$ is used for oxidation of glucose. Glucose is partially oxidized this that’s why organic acid is made and $CO_2$ is not released.

Complete answer:
The respiration quotient is constant. This is equal to the ratio of the number of molecules of $CO_2$ released and the number of molecules of $O_2$ used during respiration.
During respiration, different types of the substrate are used for oxidation, and for every substrate the value of R.Q. is different. For example- for protein as a respiratory substrate, R.Q. is 0.7. for fat it equals 0.7. In succulent plants during the night glucose is used for respiration but it is not completely oxidized. Glucose is converted into organic acids like oxaloacetic acid and malic acid then stored in the vacuole. $CO_2$is not released. That’s why the value of R.Q. is zero. For anaerobic respiration R.Q. is infinite. For organic acids, it is more than one. Ex. for oxalo acetic acid, it is 4.

Note: In succulent plants stomata are open at night. So, $CO_2$ is absorbed and converted into acids like malic acid. $O_2$ is also absorbed and used for partial oxidation of glucose and organic acid is made. Acid is stored in the vacuole and during daytime used in photosynthesis.