Kautsky effect

When dark-adapted photosynthesising cells are illuminated with continuous light, chlorophyll fluorescence displays characteristic changes in intensity accompanying the induction of photosynthetic activity.

When a sample (leaf or algal suspension) is illuminated, the fluorescence intensity increases with a time constant in the microsecond or millisecond range.

The initial rise of the fluorescence intensity is attributed to the progressive saturation of the reaction centers of photosystem 2 (PSII).

The slow decrease of the fluorescence intensity at later times is caused, in addition to other processes, by non-photochemical quenching.

Non-photochemical quenching is a protection mechanism in photosynthetic organisms as they have to avoid the adverse effect of excess light.