Kautsky effect is a phenomenon consisting on a typical variation on the behavior of a plant fluorescence when is exposed to light. It was discovered in 1931 by H. Kautsky and A. Hirsch.

When dark-adapted photosynthesising cells are illuminated with continuous light, chlorophyll fluorescence

Chlorophyll fluorescence

Chlorophyll fluorescence is light that has been re-emitted after being absorbed by chlorophyll molecules of plant leaves. By measuring the intensity and nature of this fluorescence, plant ecophysiology can be investigated....

 displays characteristic changes in intensity accompanying the induction of photosynthetic activity.

Explanation

When the plant is illuminated the fluorescence intensity increases with a time constant in the microsecond or millisecond range. After a few seconds the intensity falls again and finally reaches a steady-state level. The initial rise of the fluorescence intensity (A) is attributed to the progressive saturation of the reaction centers in the photosynthesis. Therefore the quenching of the fluorescence

Quenching (fluorescence)

Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on...

 by the photosynthesis decreases with the time of illumination, with a corresponding increase of the fluorescence intensity. The quenching by the photosynthesis pathway is called photochemical quenching. The slow decrease of the fluorescence intensity at later times (B) is termed non-photochemical quenching

Non-photochemical quenching

Non-photochemical quenching is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls via enhanced internal conversion to the ground state , thus harmlessly dissipating...

. Non-photochemical quenching is most likely due to a protection mechanism the plant has to avoid the adverse effect of an excess of light. Several processes cause non-photochemical quenching, e.g. photoinhibition

Photoinhibition

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII...

.