In high PF ∆F (i.e. the difference between F′ and F m ′) is smaller compared to low PF. A similar discrepancy between both proxies for NPQ was noticed for phytoplankton in Lake Ijsselmeer (Kromkamp et al. 2008). We

are not aware of other studies making this comparison. Notice that whereas the maximum fluorescence was actually measured after 4 min, the maximum functional cross section was measured in the dark period preceding the high light exposure. We do not know how to explain these differences. It may be important to note that NPQ is based on changes in F m ′ whereas changes in σPSII′ AL3818 are based on fluorescence induction curves of open PSII only (i.e. the development of ∆F during the flashlet Selleckchem Temozolomide sequence). We noted a correlation between the connectivity parameter p and changes in F and F m ′ and NPQ. Connectivity of PSII centres might increase the quantum efficiency of PSII by use of excitons, which are transferred from a closed to an open PSII. If connectivity would be absent, as in the separate units model, an exciton hitting MNK inhibitor a closed PSII would be lost. Zhu et al. (2005) demonstrated that an increase in connectivity delayed the fluorescence induction from O to J, without affecting the level of O. This suggests that connectivity

might not influence the level of F 0. F′, however, is affected by connectivity as show in this study. We clearly show a strong correlation between connectivity and variations in F′ induced by exposure to (relatively low) irradiances (Fig. 9e, f). One explanation might be that the negative charges caused by reduced QB on the acceptor side of PSII repel other PSII centres, hence causing a positive relationship with NPQ (Fig. 9d). The decrease in connectivity with increasing irradiances could not be compared to other studies because this observation could not be found in the literature. However, if connectivity influences

fast fluorescence induction as shown by Zhu et al. (2005), σPSII′ and \( \textNPQ_\sigma_\textPSII \) depend on energy distribution amongst PSII centres. Because NPQ is calculated from F m and F m ′, while \( \textNPQ_1 \) is dependent on the fast fluorescence induction, connectivity is likely to affect both the parameters individually. The sum of the quantum efficiencies for photochemistry, heat dissipation and fluorescence should equal 1 (Schreiber et al. 1995a, b). In this case, the quantum efficiency of heat dissipation includes all processes affecting NPQ, thus including state-transitions, which is theoretically wrong because state-transitions change the (optical) cross sections of the photosystems without affecting loss of absorbed light as heat.