Carboxylation
The reaction in which RuBisCO attaches CO2 to a sugar backbone and commits that carbon to the plant. It is the productive half of the enzyme's behavior, competing directly with oxygenation.
RuBisCO can attach either of two gases to its substrate. Carboxylation attaches carbon dioxide and produces molecules the plant carries forward into sugar. Oxygenation attaches oxygen instead and starts photorespiration, which costs energy and gives back carbon the plant had already fixed.
Both reactions run on the same enzyme at the same time, so they compete for it. Which one wins is set by the ratio of CO2 to O2 at the active site and by temperature. Neither the plant nor the grower can switch one off.
Raising CO2 in a sealed room shifts that ratio toward carboxylation. This is the entire basis of enrichment: not a faster enzyme, just a larger share of its work going to the useful reaction.
Temperature pushes the other way, favoring oxygenation as it rises. That opposition is why an ambient room and an enriched room have different temperature optima, and why raising CO2 without raising temperature leaves most of the benefit unclaimed.