Photorespiration
A wasteful side reaction in which RuBisCO binds oxygen rather than CO2, costing energy and releasing carbon the plant had already fixed. It worsens as temperature rises and eases when CO2 is elevated.
When RuBisCO grabs oxygen instead of carbon dioxide, the product has to be recycled through a salvage pathway spanning three cell compartments. The plant spends ATP and reducing power and gives back a portion of the carbon it had already captured. Under ordinary air this can waste a substantial share of gross photosynthesis.
Two things make it worse. Higher temperature reduces RuBisCO's preference for CO2 and lowers CO2 solubility relative to oxygen, so the oxygenation reaction gains ground as a room warms. Low CO2 concentration does the same.
This is why CO2 enrichment and warmer canopy temperatures belong together. Suppressing photorespiration with elevated CO2 lifts the temperature at which net photosynthesis peaks, so an enriched room that is still held at conventional temperatures leaves much of the benefit unclaimed.