Q.Respiratory quotient is zero in succulent plants. Why ?

Succulent (CAM) plants show RQ = 0 because the CO2 released internally by decarboxylation of stored organic acids is immediately used up in photosynthesis inside the same cell, so no CO2 actually escapes to the atmosphere even though O2 is still being consumed.

Respiratory Quotient (RQ) is defined as the ratio of the volume of CO2 released to the volume of O2 consumed during respiration (RQ = CO2 evolved / O2 consumed). Its value normally depends on the respiratory substrate - about 1 for carbohydrates, less than 1 for fats and proteins, and greater than 1 for organic acids used as substrate.

Succulent plants (such as cacti and other CAM - Crassulacean Acid Metabolism - plants) have a special adaptation to conserve water: they open their stomata at night to take in CO2 and fix it into organic acids (mainly malic acid), which accumulate and are stored in large vacuoles overnight. During the day, stomata close (to prevent water loss in their hot, dry habitat), and the stored organic acids are decarboxylated, releasing CO2 internally within the mesophyll cells. This internally released CO2 is immediately captured and used by the chloroplasts for photosynthesis (the Calvin cycle) in the very same cells, so it never leaves the cell as a …