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

Respiratory Quotient (RQ) = volume of CO2 evolved / volume of O2 consumed. In succulents (which typically use CAM, Crassulacean Acid Metabolism), the CO2 released by respiration is immediately trapped and refixed into organic acids inside the plant rather than escaping, so the externally measurable CO2 output is essentially zero while oxygen uptake continues, driving RQ toward zero.

Respiratory Quotient (RQ) is defined as: RQ = (Volume of CO2 evolved) / (Volume of O2 consumed)

Its value normally depends on the type of respiratory substrate being oxidised (RQ = 1 for carbohydrates, less than 1 for fats/proteins, greater than 1 for organic acids). However, in succulent plants (many of which are CAM plants, adapted to arid conditions), a special situation arises:

• Succulents open their stomata mainly at night to minimise water loss, taking in CO2 and fixing it (via the enzyme PEP carboxylase) into 4-carbon organic acids (mainly malic acid), which accumulate in the vacuole overnight.
• During the day, stomata remain closed (again to conserve water), and the stored malic acid is decarboxylated to release CO2 internally, which is then fixed via the Calvin cycle using light energy. …