Stomata
Adjustable pores on the leaf surface, each framed by a pair of guard cells, that let CO2 into the leaf and water vapor out. The plant opens and closes them constantly.
Every stoma is a compromise. Opening admits the carbon dioxide photosynthesis requires, and it simultaneously lets water escape. A plant cannot take up carbon without spending water, and the exchange rate between the two is set largely by how far the stomata are open.
Stomata respond to light, to internal CO2 concentration, to humidity and to water status. They open through the first part of the photoperiod rather than instantly at lights-on, which is why injecting CO2 before they open wastes gas.
Under elevated CO2 the plant can meet its carbon demand with the pores less far open, so stomata partly close. That single response drives most of the downstream irrigation and climate changes an enriched room needs.