In succulent plants (e.g., Opuntia), all the CO₂ produced during respiration is fixed internally for use in photosynthetic activity during the day, as a result O₂ is consumed but CO₂ is not evolved. So, R.Q. is zero.
2C6H12O6 + 3O₂ ----> 3C4H6O5 + 3H2O
R.Q.: = Zero CO2/3O2 = zero