The Picosecond Kinetics of Non-Photochemical Quenching in Leaves in the Presence of Open and Closed Reaction Centers

Abstract

When the photosynthetic apparatus of plants becomes saturated in high light, a protective mechanism, called non-photochemical quenching (NPQ), is switched on. This is particularly important when the reaction centers (RCs) of photosystem II (PSII) are closed because in that case enhanced chlorophyll triplet formation might occur, leading to the production of singlet oxygen, which can lead to severe damage or even death of the plant. NPQ provides an additional decay channel for chlorophyll excited states, thereby shortening the excited-state lifetime and lowering the probability that excitations end up in a closed RC. However, the exact mechanism of NPQ is still unknown and also the rate of NPQ has never been determined. It is therefore not known whether this rate depends on the state of the RCs, i.e. whether they are open or closed. We have designed a new setup on which it is now possible to measure spectrally-resolved picosecond fluorescence on leaves with either open or closed RCs in the presence and absence of NPQ. We have measured remarkable differences for leaves with open and closed RCs, which are in disagreement with the current views on NPQ and the results will be presented.