Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 μs to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O2-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplast

Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 mus to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD( P) H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD( P) H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD( P) H. The addition of mercury chloride to the chloroplast suspension decreased the NAD( P) H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD( P) H-PQ oxidoreductase and the reoxidation of reduced PQ by an O-2-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts

Fingerprints of climate changes on the photosynthetic apparatus' behaviour, monitored by the JIP-test : a case study on light and heat stress adaptation of the symbionts of temperate and coral reef foraminifers in hospite

We here present an outline of the JIP-test, a screening test based on the quantitative analysis of the chlorophyll α fluorescence rise 0-J-I-P transient exhibited by all photosynthetic organisms. The JIP-test leads to the calculation of several phenomenological and biophysical parameters quantifying the photosystem II (PSII) behaviour. The parameters express (a) the energy fluxes for absorption, trapping and electron transport, (b) the flux ratios or yields, (c) the concentration of reaction centres and (d) a performance index. The JlP-test was proved to be a very useful tool for the in vivo investigation of the adaptive behaviour of the photosynthetic apparatus to a wide variety and combination of stressors, e.g. high or low temperature, high light intensity, atmospheric C02 or ozone elevation etc. The successful "fingerprinting" of stressors on the behaviour/performance of the photosynthetic apparatus provides the basis for using this behaviour/performance as a bio-indicator of climate changes. As an example, a case study on light and heat stress adaptation of the symbionts of coral reef foraminifers in hospite is presented. This study tackles the question about the origin of the big environmental problem of massive bleaching of the reef ecosystem, which involves, besides corals, several other species among which large foraminifers, and corresponds to the loss of their photosynthetic symbionts and/or the symbionts' pigments. We used the JIP-test to investigate in three genera of large foraminifers the response of PSII of their symbionts in hospite upon light and heat stress (up to 32 °C). While low-light was found to offer a strong thermoprotection, heat stress in the dark was found to result in a wide decrease of the capacity for photosynthetic activity. Strong illumination induced even wider decreases of the photosynthetic activity, however highly reversible. The extent of the reversibility from the high-light stress was much bigger under low-light than in the dark. Since the symbionts' photosynthetic products are the major energy source for the symbiotic association, it is proposed that the reduction in their productivity and, concomitantly, of their delivery to the host, affects negatively the co-habitation and, hence, triggers bleaching. The results suggest that warm nights should be considered as a major factor provoking coral reef bleaching.</p

Non-photochemical quenching of chlorophyll a fluorescence by oxidised plastoquinone: new evidences based on modulation of the redox state of the endogenous plastoquinone pool in broken spinach chloroplasts

Twenty-five years ago, non-photochemical quenching of chlorophyll fluorescence by oxidised plastoquinone (PQ) was proposed to be responsible for the lowering of the maximum fluorescence yield reported to occur when leaves or chloroplasts were treated in the dark with 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of electron flow beyond the primary quinone electron acceptor (Q(A)) Of photosystem (PS) 11 C. Vernotte, A.L. Etienne, J.-M. Briantais, Quenching of the system 11 chlorophyll fluorescence by the plastoquinone pool, Biochim. Biophys. Acta 545 (1979) 519-527]. Since then, the notion of PQ-quenching has received support but has also been put in doubt, due to inconsistent experimental findings. In the present study, the possible role of the native PQ-pool as a non-photochemical quencher was reinivestigated, employing measurements of the fast chlorophyll a fluorescence kinetics (from 50 mus to 5 s). The about 20% lowering of the maximum fluorescence yield F-M, observed in osmotically broken spinach chloroplasts treated with DCMU, was eliminated when the oxidised PQ-pool was non-photochemically reduced to PQH(2) by dark incubation of the samples in the presence of NAD(P)H, both under anaerobic and aerobic conditions. Incubation under anaerobic conditions in the absence of NAD(P)H had comparatively minor effects. In DCMU-treated samples incubated in the presence of NAD(P)H fluorescence quenching started to develop again after 20-30 ms of illumination, i.e., the time when PQH(2) starts getting reoxidised by PS I activity. NAD(P)H-dependent restoration of F-M was largely, if not completely, eliminated when the samples were briefly (5 s) pre-illuminated with red or far-red light. Addition to the incubation medium of HgCl2 that inhibits dark reduction of PQ by NAD(P)H also abolished NAD(P)H-dependent restoration of F-M. Collectively, our results provide strong new evidence for the occurrence of PQ-quenching. The finding that DCMU alone did not affect the minimum fluorescence yield F-0 allowed us to calculate, for different redox states of the native PQ-pool, the fractional quenching at the F-0 level (Q(0)) and to compare it with the fractional quenching at the F-M level (Q(M)). The experimentally determined Q(0)/Q(M) ratios were found to be equal to the corresponding F-0/F-M ratios, demonstrating that PQ-quenching is solely exerted on the excited state of antenna chlorophylls. (C) 2004 Elsevier B.V. All rights reserved

Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 μs to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O2-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts