Assertion : Dark reaction occurs only at night in the stroma of chloroplast. Reason : CO ₂ fixation occurs only during C ₃ cycle.

Step-by-step solution

Dark reaction is also known as lightindependent phase. Unlike, light reaction, it does not require light as an essential factor. Thus can takes place both in the presence or absence of light. The term dark reaction does not mean that it takes place only in dark period or at night. CO ₂ fixation occurs in both C ₃ and C ₄ cycle. In C ₃ cycle, CO ₂ is added by the enzyme, RuBisco to a 5 carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. In C ₄ cycle the first product of CO ₂ fixation (takes place