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 rubisco is an enzyme that acts both as a carboxylae and oxygenase. Why do you think rubisco carries out more carboxylation in C4 plants? 

RuBisCO is found in abundance in the bundle sheath cells of C4 plants where it fixes CO2 to form 3-phosphoglycerate. RubisCO also has an oxygenase activity in the presence of low concentration of carbon dioxide and high concentration of oxygen which is responsible for photorespiration in plants. But due to high concentration of CO2 in bundle sheath cells, RuBisCO works only for Calvin cycle (i.e. works as carboxylase) and not for photorespiration. 

 

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 The enzyme RuBisCo is absent from the mesophyll cells of C4 plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In C4 plants, the Calvin cycle occurs in the bundle-sheath cells. The primary CO2 acceptor in the mesophyll cells is phosphoenol pyruvate – a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and CO2fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase

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 carboxylation of rubisco is triggered when the concentration of CO2 is more & has optimum temperature. in c4 plants the concentraton of CO2 is more because of the continuous break down of c4 acid like  malic acid ,aspartic acid into c3 acid & CO2 in bundle sheath.

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