Control of Photosystem II in spinach leaves by continuous light and by light pulses given in the dark

The light-induced induction of components of non-photochemical quenching of chlorophyll fluorescence which are distinguished by different rates of dark relaxation (qNf, rapidly relaxing and qNs, slowly relaxing or not relaxing at all in the presence brief saturating light pulses which interrupt darkness at low frequencies) was studied in leaves of spinach.After dark adaptation of the leaves, a fast relaxing component developed in low light only after a lag phase. Quenching increased towards a maximum with increasing photon flux density. This fast component of quenching was identified as energy-dependent quenching qE. It required formation of an appreciable transthylakoid pH and was insignificant when darkened spinach leaves received 1 s pulses of light every 30 s even though zeaxanthin was formed from violaxanthin under these conditions.Another quenching component termed qNs developed in low light without a lag phase. It was not dependent on a transthylakoid pH gradient, decayed exponentially with a long half time of relaxation and was about 20% of total quenching irrespective of light intensity. When darkened leaves were flashed at frequencies higher than 0.004 Hz with 1 s light pulses, this quenching also appeared. Its extent was very considerable, and it did not require formation of zeaxanthin. Relaxation was accelerated by far-red light, and this acceleration was abolished by NaF.We suggest that qNs is the result of a so-called state transition, in which LHC II moves after its phosphorylation from fluorescent PS II to nonfluorescent PS I. This state transition was capable of decreasing in darkened leaves the potential maximum quantum efficiency of electron flow through Photosystem II by about 20%.Abbreviations PFD photon flux density - PS photosystem     

Photoinactivation of photosystem II by cumulative exposure to short light pulses during the induction period of photosynthesis

Photoinactivation of Photosystem (PS) II in vivo was investigated by cumulative exposure of pea, rice and spinach leaves to light pulses of variable duration from 2 to 100 s, separated by dark intervals of 30 min. During each light pulse, photosynthetic induction occurred to an extent depending on the time of illumination, but steady-state photosynthesis had not been achieved. During photosynthetic induction, it is clearly demonstrated that reciprocity of irradiance and duration of illumination did not hold: hence the same cumulative photon exposure (mol m^–2) does not necessarily give the same extent of photoinactivation of PS II. This contrasts with the situation of steady-state photosynthesis where the photoinactivation of PS II exhibited reciprocity of irradiance and duration of illumination (Park et al. (1995) Planta 196: 401–411). We suggest that, for reciprocity to hold between irradiance and duration of illumination, there must be a balance between photochemical (qP) and non-photochemical (NPQ) quenching at all irradiances. The index of susceptibility to light stress, which represents an intrinsic ability of PS II to balance photochemical and non-photochemical quenching, is defined by the quotient (1-qP)/NPQ. Although constant in steady-state photosynthesis under a wide range of irradiance (Park et al. (1995). Plant Cell Physiol 36: 1163–1169), this index of susceptibility for spinach leaves declined extremely rapidly during photosynthetic induction at a given irradiance, and, at a given cumulative photon exposure, was dependent on irradiance. During photosynthetic induction, only limited photoprotective strategies are developed: while the transthylakoid pH gradient conferred some degree of photoprotection, neither D1 protein turnover nor the xanthophyll cycle was operative. Thus, PS II is more easily photoinactivated during photosynthetic induction, a phenomenon that may have relevance for understorey leaves experiencing infrequent, short sunflecks.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - F_v, F_m, F_o variable, maximum, and initial (corresponding to open traps) chlorophyll fluorescence yield, respectively - NPQ non-photochemical quenching - PS Photosystem - Q_A primary quinone acceptor of PS II - qP photochemical quenching coefficient     

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (Y_O2) and photochemical yield of PS II (_{PS II}) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y_O2 and _{PS II} followed each other, suggesting a close coupling between the oxidation of water and Q_A reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between Q_A reduction and the fraction of PS II reaction centers capable of evolving O₂ became temporarily uncoupled. _{PS II} recovered to the preillumination levels within 5–10 s, while the Y_O2 required up to 60 s to recover under aerobic conditions. The recovery of Y_O2 was independent of the redox state of Q_A, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (_{PS II}). The hysteresis between Y_O2 and the reduction of Q_A during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting enzyme (agents) - Chl chlorophyll - cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_O minimum fluorescence yield in the dark-adapted state - F_I minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state - F_M maximum fluorescence yield in the dark-adapted state - F_M maximum fluorescence yield under ambient irradiance or during transition from light-adapted state - F_V, F_V variable fluorescence (F_V=F_M–F_O ; F_V=F_M–F_I) - FRR fast repetition rate (fluorometer) - _{PS II} quantum yield of Q_A reduction (_{PS II}=(F_M – F_O)/F_M or _{PS II})=(F_M= – F_I=)/F_M=) - LHCII Chl a/b light harvesting complexes of Photosystem II - OEC oxygen evolving complex of PS II - P680 reaction center chlorophyll of PS II - PQ plastoquinone - POH₂ plastoquinol - PS I Photosystem I - PS II Photosystem II - RC II reaction centers of Photosystem II - PS II the effective absorption cross-section of PHotosystem II - TL thermoluminescence - Y_O2 oxygen flash yield The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Species- and age-dependent sensitivity to ozone in young plants of pea,wheat and spinach: Effects on acyl lipid and pigment content and metabolism

Acyl lipids and pigments were analyzed in young plants of garden pea, spring wheat and spinach exposed to < 5 or 65 nl l^?1 ozone 12 h per day for 6 days. In one set of experiments, the plants were exposed to ¹⁴CO₂ for 2 h 3 days prior to ozone exposure. The plants responded differently to the moderately enhanced level of ozone used Spinach was not at all sensitive while in both pea and wheat, leaves of different ages differed in ozone sensitivity. In pea, ozone sensitivity increased with leaf age. In the second and third oldest leaves, the amounts of galactolipids per leaf area and the proportions of 18:3 of the total lipid extract and of phosphatidylglycerol decreased. In the second oldest leaf, ozone also caused a decreased proportion of 18:3 of monogalactosyldiacylglycerol. In the fourth oldest leaf, lipid composition and galactolipid unsaturation was unaffected, but ozone caused decreased leaf expansion resulting in increased acyl lipid content per leaf area. In both the first and second leaves of wheat, ozone fumigation caused a marked decrease in the content of monogalactosyldiacylglycerol and in the first leaf, the contents of phosphatidylcholine and phosphatidylethanolamine increased. The proportion of 18:3 in phosphatidylcholine was larger in ozone-fumigated than in control plants, while the reverse applied for phosphatidylglycerol. In the oldest sampled leaves of pea and wheat, ozone caused an increase in the radioactivity associated with β-carotene, indicating increased turnover. Thus, while spinach was unaffected, in both pea and wheat ozone caused a decrease in the proportion of chloroplast membrane lipids to non-chloroplast membrane lipids in older leaves while younger leaves were less sensitive.     

The role of sulphur in detoxication of cadmium in young sugar beet plants

In young sugar beet plants cadmium suppressed the activity of nitrate reductase, glutamine synthetase and glutamate dehydrogenase, whereas sulphur exhibited a protective role towards activity of these enzymes, except of glutamine synthetase. Protein synthesis was suppressed in the absence of S in nutrient medium; the lowest level was at 10^-3 M Cd²⁺. Chloroplast pigment contents were increased by S while Cd²⁺, even in the lowest concentration, (10⁻⁵ M) showed a repressive effect. The highest concentrations of Cd²⁺ (10−3 M) caused a decrease in dry mass, whereas S induced its increase. Nitrate content was increased in the presence of Cd²⁺ and decreased by increased concentration of S. Acknowledgement: The authors acknowledge financial support of the Ministry for Science and Technology of Serbia. The paper was presented at 9th Congress of the Federation of European Societies of Plant Physiology, Brno, Czech Republic, 3–8 July 1994.     

Photoprotective effect of kinetin on pigment content and photochemical activities of wheat chloroplasts agingin vitro

The effects of kinetin (Kn) on pigment content and electron transport activities (ETA) in wheat leavesin vivo and chloroplastsin vitro aging in light was investigated. Excised wheat leaves were infiltrated with Kn for 3 h under irradiation. The treatment increased zeaxanthin (Zx) content by 40% and also increased chlorophyll (Chia, Chib) and major carotenoid (Car) contents in the leaves (per fresh mass unit). Chloroplasts isolated from Kn treated leaves, when incubated in light for 4 h showed relatively lower pigment loss and slower loss of ETA compared to the chloroplasts of untreated leaves. These observations suggest photoprotective action of Kn. The photoprotection was more prominent when Kn was applied directly to the irradiated chloroplastsin vitro. Moreover, chloroplasts agingin vitro under irradiation without Kn treatment lost pigments and ETA. Within 3 h of irradiation, both whole chain (H₂O to methylviologen) electron transport as well as photosystem (PS) 2 activity were completely lost. However, in the chloroplasts treated with Kn, the loss of pigments was slow and even after 4 h of irradiation the chloroplasts retained 15 % of PS 2 and 9 % of whole chain ETA. In the untreated chloroplasts, the loss of Zx after 4 h of irradiation was 49 % whereas in Kn treated samples its level was 1.3 times higher than that of control. Since a higher level of Zx was maintained in Kn treated chloroplasts, photoprotective action of Kn is possibly mediated through Zx. One of us (NKC) thanks Sambalpur University for study leave and Department of Biological Sciences, Mankato State University, Mankato for labortory facilities.     

Phytoplankton pigments and adenosine triphosphate (ATP) in Lake Peipsi-Pihkva

The material for pigment analysis was collected 1–3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992–1995. Concentrations of chlorophyll a, b and c (Chla, Chlb, Chlc), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985–1986. The mean of all Chla values was 20.2 mg m^–1 (median 13.3 mg m^–1) indicating the eutrophic state of the lake. Average Chlb, Chlc, Pheo and carotenoid (Car) contents were 3.7 mg m^–3, 4.1 mg m^–3, 3.0 mg m^–3 and 4.8 mg m^–3, respectively. The average Chlb/Chla ratio was 22.9%, Chlc/Chla 23.4%, Pheo/Chla 38%, Car/Chla 37% and ATP/Chla 3%, the medians being 14.3, 13.6, 17.5, 39.4 and 1.9%, respectively. The proportion of Chla in phytoplankton biomass was 0.41%, median 0.32%. There were no significant differences in temperature, oxygen concentration, Chla, and ATP between the surface and bottom water; the lake was polymictic during the vegetation period. The Chla concentration had its first peak in May followed by a decrease in June and July. In late summer Chla increased again achieving its seasonal maximum in late autumn. The ATP concentration was the highest during spring and early summer, decreasing drastically in autumn together with the decline of primary production. ATP/Chla was the highest during the clear water period in June and early July, which coincided also with the high proportion of Chla in phytoplankton biomass. The highest Chla occurred in November (average 37.2 mg m^–3) when Secchi transparency was the lowest (1.05 m). Concentrations of Chlb, Chlc and carotenoids were the highest in August, that of Pheo in June. Concentrations of Chla and other pigments were the lowest in the northern part of Lake Peipsi (mean 14.7 mg m^–3, median 12.5 mg m^–3) and the highest in the southern part of Lake Pihkva (mean 47.9 mg m^–3, median 16.3 mg m^–3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite.     

Protein structure,electron transfer and evolution of prokaryotic photosynthetic reaction centers

Photosynthetic reaction centers from a variety of organisms have been isolated and characterized. The groups of prokaryotic photosynthetic organisms include the purple bacteria, the filamentous green bacteria, the green sulfur bacteria and the heliobacteria as anoxygenic representatives as well as the cyanobacteria and prochlorophytes as oxygenic representatives. This review focuses on structural and functional comparisons of the various groups of photosynthetic reaction centers and considers possible evolutionary scenarios to explain the diversity of existing photosynthetic organisms.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - Rb Rhodobacter - Rp Rhodopseudomonas     

Pigment interactions in chlorosomes of various green bacteria

Resonance Raman experiments were performed on different green bacteria. With blue excitation, i.e. under Soret resonance or preresonance conditions, resonance Raman contributions were essentially arising from the chlorosome pigments. By comparing these spectra and those of isolated chlorosomes, it is possible to evaluate how the latter retain their native structure during the isolation procedures. The structure of bacteriochlorophyll oligomers in chlorosomes was interspecifically compared, in bacteriochlorophyllc- and bacteriochlorophylle- synthesising bacteria. It appears that interactions assumed by the 9-keto carbonyl group are identical inChlorobium limicola, Chlorobium tepidum, andChlorobium phaeobacteroides. In the latter strain, the 3-formyl carbonyl group of bacteriochlorophylle is kept free from intermolecular interactions. By contrast, resonance Raman spectra unambiguously indicate that the structure of bacteriochlorophyll oligomers is slightly different in chlorosomes fromChloroflexus auranticus, either isolated or in the whole bacteria.