The light saturation point is the light level at which a leaf's photosynthetic rate plateaus: more photons no longer raise carbon gain. It is read from a light-response curve. It is not a fixed number. It rises with CO2 concentration and with temperature up to the optimum, and it differs between single leaves and whole canopies, where lower leaves are shaded.
In practice
Chandra and colleagues (2008) measured cannabis leaf photosynthesis rising to around 1,500 µmol/m²/s at 25–30 °C (77–86 °F) and ambient CO2. Whole-plant trials go further: Rodriguez-Morrison and colleagues (2021) saw flower yield keep rising up to about 1,800 µmol/m²/s, because a canopy saturates later than any one leaf. Established.
For growers, the useful idea is diminishing returns. Each extra 100 µmol/m²/s above about 1,000 buys less, costs the same in power and heat, and needs more CO2 and tighter VPD to pay off. Many rooms without CO2 enrichment stop at 900–1,100 µmol/m²/s for that reason.
Don't confuse with
- Light compensation point: the low end, where photosynthesis just balances respiration.
- Light stress: damage when absorbed light exceeds what the leaf can use or dissipate; it can happen below the saturation point if heat or VPD is wrong.
