Photosynthetic acclimation
The downward adjustment many C3 plants make under sustained high CO2, reducing RuBisCO content so the early gain in photosynthesis shrinks over time. Also called down-regulation.
RuBisCO is expensive to hold. It is the most abundant protein in a leaf and it carries a large share of the leaf's nitrogen, so a plant keeping more of it than conditions require is tying up resources it could spend elsewhere.
Given weeks of elevated CO2, many C3 species respond by making less of it, alongside related shifts in leaf nitrogen and in how much sugar the plant can actually use. Net photosynthesis stays above the ambient baseline, but the gap narrows compared with what the first days of enrichment suggested.
The effect is well documented across C3 crops and is one of the reasons the response to CO2 flattens rather than climbing indefinitely. It is also a practical argument against very high setpoints, since the gas costs money continuously while the plant adjusts part of the benefit away.
How strongly it applies to cannabis at commercial setpoints has not been measured directly.