Modulation of photosynthetic electron transport in the absence of terminal electron acceptors: Characterization of the rbcL deletion mutant of tobacco

Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The ΔrbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q_A and Q_B, whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of ΔrbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F₀ ‘dark rise’ in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O₂ in ΔrbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the ΔrbcL mutant under growth conditions. This protective capacity was rapidly exceeded in ΔrbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.     

CO2 response of cyclic electron flow around PSI (CEF-PSI) in tobacco leaves--relative electron fluxes through PSI and PSII determine the magnitude of non-photochemical quenching (NPQ) of Chl fluorescence

We hypothesized that cyclic electron flow around photosystem I (CEF-PSI) participates in the induction of non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence when the rate of photosynthetic linear electron flow (LEF) is electron-acceptor limited. To test this hypothesis, the relationships among photosynthesis rate, electron fluxes through both PSI and PSII [Je(PSI) and Je(PSII)] and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants at several light intensities and partial pressures of ambient CO2 (Ca). At low light intensities, decreasing Ca lowered the photosynthesis rate, but Je(PSI) and Je(PSII) remained constant. Je(PSI) was larger than Je(PSII), indicating the existence of CEF-PSI. Increasing the light intensity enhanced photosynthesis and both Je(PSI) and Je (PSII). Je(PSI)/Je(PSII) also increased at high light and at high light and low Ca combined, showing a strong, positive relationship with NPQ of Chl fluorescence. These results indicated that CEF-PSI contributed to the dissipation of photon energy in excess of that consumed by photosynthesis by driving NPQ of Chl fluorescence. The main physiological function of CEF-PSI in photosynthesis of higher plants is discussed.     

Changes in cyclic and respiratory electron transport by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

Phycobilisomes (PBS) are the major accessory light-harvesting complexes in cyanobacteria and their mobility affects the light energy distribution between the two photosystems. We investigated the effect of PBS mobility on state transitions, photosynthetic and respiratory electron transport, and various fluorescence parameters in Synechocystis sp. strain PCC 6803, using glycinebetaine to immobilize and couple PBS to photosystem II (PSII) or photosystem I (PSI) by applying under far-red or green light, respectively. The immobilization of PBS at PSII inhibited the increase in cyclic electron flow, photochemical and non-photochemical quenching, and decrease in respiration that occurred during the movement of PBS from PSII to PSI. In contrast, the immobilization of PBS at PSI inhibited the increase in respiration and photochemical quenching and decrease in cyclic electron flow and non-photochemical quenching that occurred when PBS moved from PSI to PSII. Linear electron transport did not change during PBS movement but increased or decreased significantly during longer illumination with far-red or green light, respectively. This implies that PBS movement is completed in a short time but it takes longer for the overall photosynthetic reactions to be tuned to a new state.     

Deletion of PsbM in tobacco alters the QB site properties and the electron flow within photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.     

Changes in cyclic and respiratory electron transport by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

Phycobilisomes (PBS) are the major accessory light-harvesting complexes in cyanobacteria and their mobility affects the light energy distribution between the two photosystems. We investigated the effect of PBS mobility on state transitions, photosynthetic and respiratory electron transport, and various fluorescence parameters in Synechocystis sp. strain PCC 6803, using glycinebetaine to immobilize and couple PBS to photosystem II (PSII) or photosystem I (PSI) by applying under far-red or green light, respectively. The immobilization of PBS at PSII inhibited the increase in cyclic electron flow, photochemical and non-photochemical quenching, and decrease in respiration that occurred during the movement of PBS from PSII to PSI. In contrast, the immobilization of PBS at PSI inhibited the increase in respiration and photochemical quenching and decrease in cyclic electron flow and non-photochemical quenching that occurred when PBS moved from PSI to PSII. Linear electron transport did not change during PBS movement but increased or decreased significantly during longer illumination with far-red or green light, respectively. This implies that PBS movement is completed in a short time but it takes longer for the overall photosynthetic reactions to be tuned to a new state.     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue–green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.     

High root temperature blocks both linear and cyclic electron transport in the dark during chilling of the leaves of rice seedlings

The most photosynthetically active leaves of rice seedlings were severely damaged when shoots but not roots were chilled (10°C/25°C, respectively), but no such injury was observed when the whole seedling was chilled (10°C/10°C). To elucidate the mechanisms, we compared the photosynthetic characteristics of the seedlings during the dark chilling treatments. Simultaneous analyses of Chl fluorescence and the change in absorbance of P700 showed that electron transport almost disappeared in both PSII and PSI in the 10°C/25°C leaves, whereas the electron transport rate in PSI in the 10°C/10°C leaves was similar to or higher than that in non-chilled control leaves. Light-induced non-photochemical quenching in PSII was inhibited in the 10°C/25°C leaves, occurring at only half the level in the 10°C/10°C leaves, whereas non-light-induced non-photochemical quenching remained high in the 10°C/25°C leaves. The light induction of Chl a fluorescence (OJIP curves) in the 10°C/25°C leaves was similar to that in leaves treated with DCMU. The fluorescence decay after a single turnover saturating flash in the 10°C/25°C leaves was much slower than in the 10°C/10°C leaves. In vivo analyses of the 550-515 nm difference signal indicated decreased formation of a proton gradient across the thylakoid membrane and decreased zeaxanthin formation in the 10°C/25°C leaves. Our results suggest that electron transport was blocked between Q(A) and Q(B) in the dark 10°C/25°C leaves, but without irreversible damage to the components of this system. The consequent light-dependent losses of electron transport, proton gradient formation across the thylakoids and thermal dissipation may therefore be responsible for the visible injury.     

Quenching of excited states of chlorophyll molecules in submembrane fractions of Photosystem I by exogenous quinones

The ability of three substituted quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duriquinone) to quench the excited states of chlorophyll (Chl) molecules in Photosystem I (PSI) was studied. Chl fluorescence emission measured with isolated PSI submembrane fractions was reduced following the addition of exogenous quinones. This quenching progressively increased with rising concentrations of the exogenous quinones according to the Stern-Volmer law. The values of Stern-Volmer quenching coefficients were found to be 3.28 x 10(5) M(-1) (DBMIB), 1.31 x 10(4) M(-1) (DCBQ), and 3.7 x 10(3) M(-1) (duroquinone). The relative quenching capacities of the various exogenous quinones in PSI thus strictly coincided to those found for the quenching of Fo level of Chl fluorescence in isolated thylakoids, which is emitted largely by Photosystem II (PSII) [Biochim. Biophys. Acta (2003) 1604, 115-123]. Quenching of Chl excited states in PSI submembrane fractions by exogenous quinones slowed down the rate of P700, primary electron donor of PSI, photooxidation measured at limiting actinic light irradiances thus revealing a reduced photochemical capacity of absorbed quanta. The possible involvement of non-photochemical quenching of excited Chl states by oxidized phylloquinones, electron acceptors of PSI, and oxidized plastoquinones, mobile electron carriers between PSII and the cytochrome b(6)/f complex, into the control of photochemical activity of PSI is discussed.     

Interactions between thylakoid electron transfer complexes. II. Modification studies with glutaraldehyde

Photosystem I (PSI) and photosystem II (PSII) complexes have been isolated from stacked spinach thylakoid membranes that had been treated with varying amounts of glutaraldehyde. The concentrations of cytochrome f, Q, and P700 have been determined by spectrophotometric methods. It was found that at low concentrations of glutaraldehyde, the amount of cytochrome f associated with either PSII or PSI increased significantly while the amounts of Q and P700 stayed relatively constant. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting analyses indicated the presence of cytochrome f and other components of the cytochrome b6-f complex in the PSII and PSI preparations after glutaraldehyde treatment, but no intermolecular cross-linked polypeptides could be detected. Solubilization of the cytochrome b6-f complex was also inhibited after thylakoid membranes were treated with low concentrations of glutaraldehyde. These results are discussed in relation to current models for the organization of the membrane complexes, and relate to the location of the cytochrome b6-f complex in appressed and nonappressed membrane regions of thylakoids.     

Photosystem II proteins PsbL and PsbJ regulate electron flow to the plastoquinone pool

The psbEFLJ operon of tobacco plastids encodes four bitopic low molecular mass transmembrane components of photosystem II. Here, we report the effect of inactivation of psbL on the directional forward electron flow of photosystem II as compared to that of the wild type and the psbJ deletion mutant, which is impaired in PSII electron flow to plastoquinone [Regel et al. (2001) J. Biol. Chem. 276, 41473-41478]. Exposure of Delta psbL plants to a saturating light pulse gives rise to the maximal fluorescence emission, Fm(L), which is followed within 4-6 s by a broader hitherto not observed second fluorescence peak in darkness, Fm(D). Conditions either facilitating oxidation or avoiding reduction of the plastoquinone pool do not affect the Fm(L) level of Delta psbL plants but prevent the appearance of Fm(D). The level of Fm(D) is proportional to the intensity and duration of the light pulse allowing reduction of the plastoquinone pool in dark-adapted leaves prior to the activation of PSI and oxidation of plastoquinol. Lowering the temperature decreases the Fm(D) level in the Delta psbL mutant, whereas it increases considerably the lifetime of Q(A)*- in the Delta psbJ mutant. The thermoluminescence signal generated by Q(A)*-/S(2) charge recombination is not affected; on the other hand, charge recombination of Q(B)*-/S(2,3) could not be detected in Delta psbL plants. PSII is highly sensitive to photoinhibition in Delta psbL. We conclude that PsbL prevents reduction of PSII by back electron flow from plastoquinol protecting PSII from photoinactivation, whereas PsbJ regulates forward electron flow from Q(A)*- to the plastoquinone pool. Therefore, both proteins contribute substantially to ensure unidirectional forward electron flow from PSII to the plastoquinone pool.     

Effects of selective inactivation of individual genes for low-molecular-mass subunits on the assembly of photosystem II,as revealed by chloroplast transformation: the <Emphasis Type="Italic"> psbEFLJ </Emphasis>operon in <Emphasis Type="Italic"> Nicotiana tabacum</Emphasis>

Photosystem (PSII) is a supramolecular polypeptide complex found in oxygenic photosynthetic membranes, which is capable of extracting electrons from water for the reduction of plastoquinone. An intriguing feature of this assembly is the fact that it includes more than a dozen low-mass polypeptides of generally unknown function. Using a transplastomic approach, we have individually disrupted the genes of the psbEFLJoperon in Nicotiana tabacum, which encode four such polypeptides, without impairing expression of downstream loci of the operon. All four mutants exhibited distinct phenotypes; none of them was capable of photoautotrophic growth. All mutants bleached rapidly in the light. Disruption of psbEand psbF, which code for the alpha and beta apoproteins of cytochrome b(559), abolished PSII activity, as expected; Delta psbL and Delta psbJ plants displayed residual PSII activity in young leaves. Controlled partial solubilisation of thylakoid membranes uncovered surprisingly severe impairment of PSII structure, with subunit and assembly patterns varying depending on the mutant considered. In the Delta psbL mutant PSII was assembled primarily in a monomeric form, the homodimeric form was preponderant in Delta psbJ, and, unlike the case in Delta psbZ, the thylakoids of both mutants released some PSII supercomplexes. On the other hand, Photosystem I (PSI), the cytochrome b(6)f complex, ATP synthase, LHCII, and CP24/CP26/CP29 antennae were present in near wild-type levels. The data are discussed in terms of their implications for structural, biogenetic and functional aspects of PSII.     

Photosynthetic electron transport and specific photoprotective responses in wheat leaves under drought stress

The photosynthetic responses of wheat (Triticum aestivum L.) leaves to different levels of drought stress were analyzed in potted plants cultivated in growth chamber under moderate light. Low-to-medium drought stress was induced by limiting irrigation, maintaining 20 % of soil water holding capacity for 14 days followed by 3 days without water supply to induce severe stress. Measurements of CO₂ exchange and photosystem II (PSII) yield (by chlorophyll fluorescence) were followed by simultaneous measurements of yield of PSI (by P700 absorbance changes) and that of PSII. Drought stress gradually decreased PSII electron transport, but the capacity for nonphotochemical quenching increased more slowly until there was a large decrease in leaf relative water content (where the photosynthetic rate had decreased by half or more). We identified a substantial part of PSII electron transport, which was not used by carbon assimilation or by photorespiration, which clearly indicates activities of alternative electron sinks. Decreasing the fraction of light absorbed by PSII and increasing the fraction absorbed by PSI with increasing drought stress (rather than assuming equal absorption by the two photosystems) support a proposed function of PSI cyclic electron flow to generate a proton-motive force to activate nonphotochemical dissipation of energy, and it is consistent with the observed accumulation of oxidized P700 which causes a decrease in PSI electron acceptors. Our results support the roles of alternative electron sinks (either from PSII or PSI) and cyclic electron flow in photoprotection of PSII and PSI in drought stress conditions. In future studies on plant stress, analyses of the partitioning of absorbed energy between photosystems are needed for interpreting flux through linear electron flow, PSI cyclic electron flow, along with alternative electron sinks.     

Effect of the anthocyanic epidermal layer on Photosystem II and I energy dissipation processes in Tradescantia pallida (Rose) Hunt

The effect of anthocyanic cells of the epidermal layer was investigated on photosynthetic activity of the higher plant Tradescantia pallida. To determine the possible indirect role of anthocyanin in photosynthesis, analysis was done on intact leaves and leaves where anthocyanic epidermal layer was removed. Energy dissipation processes related to Photosystem II (PSII) and Photosystem I (PSI) activity was done using simultaneously Chlorophyll a (Chl a) fluorescence and P700 transmittance signals change. In anthocyanic epidermal-less leaves, PSII photochemical activity was more decreased in dependence to increasing light irradiance exposure. We found that photoinhibition of PSII decreased PSI activity by reducing the electron flow toward PSI, especially under high light intensities. Under those conditions, it resulted in the accumulation of oxidized PSI reaction centers, which was stronger in leaves where the anthocyanic epidermal layer was removed. In conclusion, our results showed that the anthocyanic epidermal layer had a photoprotective effect only on the PSII and not on the PSI of T. pallida leaves, supporting the role of anthocyanin pigments in the regulation of photosynthesis for excess absorbed light irradiance.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Organization and activity of photosystems in the mesophyll and bundle sheath chloroplasts of maize

Photosystem I and Photosystem II activities, as well as polypeptide content of chlorophyll (Chl)-protein complexes were analyzed in mesophyll (M) and bundle sheath (BS) chloroplasts of maize (Zea mays L.) growing under moderate and very low irradiance. This paper discusses the application of two techniques: mechanical and enzymatic, for separation of M and BS chloroplasts. The enzymatic isolation method resulted in depletion of polypeptides of oxygen evolving complex (OEC) and alphaCF1 subunit of coupling factor; D1 and D2 polypeptides of PSII were reduced by 50%, whereas light harvesting complex of photosystem II (LHCII) proteins were still detectable. Loss of PSII polypeptides correlated with the decreasing of Chl fluorescence measured at room temperature. Using mechanical isolation of chloroplasts from BS cells, all tested polypeptides could be detected. We found a total lack of O2 evolution in BS chloroplasts, but dichlorophenolindophenol (DCPIP) was photoreduced. PSI activity of chloroplasts isolated from 14- and 28-day-old plants was similar in BS chloroplasts in moderate light (ML), but in low light (LL) it was reduced by about 20%. PSI and PSII activities in M chloroplasts of plants growing in ML decreased with aging of plants. In older LL-grown plants, activities of both photosystems were higher than those observed in chloroplasts from ML-grown plants. We suggest that in BS chloroplasts of maize, PSII complex is assembled typically for the agranal membranes (containing mainly stroma thylakoids) and is able to perform very limited electron transport activity. This in turn suggests the role of PSII for poising the redox state of PSI.     

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q_B-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.     

Loss of the precise control of photosynthesis and increased yield of non‐radiative dissipation of excitation energy after mild heat treatment of barley leaves

The after effects of a short exposure of intact barley leaves to moderately elevated temperature (40°C, 5 min) on the induction transients and the irradiance dependencies of photosynthesis and chlorophyll fluorescence are presented. This mild heat treatment strongly reduced the oscillations in the rate of photosynthesis and in the yield of chlorophyll fluorescence. However, only a 25% irreversible inhibition of maximum photosynthetic capacity of photosystem II (PSII) measured by oxygen evolution was produced and the intrinsic quantum yield of PSII measured by the chlorophyll fluorescence ratio (F_m‐ F_o)/F_m decreased by only 15%. In contrast, the above treatment increased radiationless dissipation processes in PSII by a factor of two. In heat‐treated leaves, photosynthesis was not saturated even by strong light. Both ΔpH‐dependent quenching of excitons in PSII (including formation of zeaxanthin) and state 1/state 2 transition were found to be stimulated. Heat exposure enhanced the control of PSII activity by PSI, as evidenced by a significant increase in the quenching effect of far‐red light on the maximum yield of chlorophyll fluorescence. It was deduced that after mild heat treatment, the photosynthetic apparatus in leaves lacks the precise coordinating control of electron transport and carbon metabolism owing to the inability of PSII to support electron transport at a level adequate for carbon metabolism. This effect was not related to the small irreversible thermal damage to PSII, but was rather due to a significant increase in non‐photochemical quenching of excitation energy.     

Enhancement of cyclic electron flow around PSI at high light and its contribution to the induction of non-photochemical quenching of chl fluorescence in intact leaves of tobacco plants

Non-photochemical quenching (NPQ) of Chl fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O2-evolution rate [V(O2)], quantum yield of both PSII and PSI [Phi(PSII) and Phi(PSI)], and Chl fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO2-saturated air. Although increases in light intensity raised V(O2) and the relative electron fluxes through both PSII and PSI [Phi(PSII) x PFD and Phi(PSI) x PFD] only Phi(PSI) x PFD continued to increase after V(O2) and Phi(PSII) x PFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chl fluorescence drastically increased after Phi(PSII) x PFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of Phi(PSII) x PFD was lower than that of Phi(PSI) x PFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII) x PFD and Phi(PSI) x PFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chl fluorescence.     

Responses to desiccation stress in bryophytes and an important role of dithiothreitol-insensitive non-photochemical quenching against photoinhibition in dehydrated states

The effects of air drying and hypertonic treatments in the dark on seven bryophytes, which had grown under different water environments, were studied. All the desiccation-tolerant species tested lost most of their PSII photochemical activity when photosynthetic electron transport was inhibited by air drying, while, in all the sensitive species, the PSII photochemical activity remained at a high level even when photosynthesis was totally inhibited. The PSI reaction center remained active under drying conditions in both sensitive and tolerant species, but the activity became non-detectable in the light only in tolerant species due to deactivation of the cyclic electron flow around PSI and of the back reaction in PSI. Light-induced non-photochemical quenching (NPQ) was found to be induced not only by the xanthophyll cycle but also by a DeltapH-induced, dithiothreitol-insensitive mechanism in both the desiccation-tolerant and -intolerant bryophytes. Both mechanisms are thought to have an important role in protecting desiccation-tolerant species from photoinhibition under drying conditions. Fluorescence emission spectra at 77K showed that dehydration-induced quenching of PSII fluorescence was observed only in tolerant species and was due to neither state 1-state 2 transition nor detachment of light-harvesting chlorophyll protein complexes from PSII core complexes.The presence of dehydration-induced quenching of PSI fluorescence was also suggested.     

Photosynthetic acclimation to drought stress in Agave salmiana Otto ex Salm-Dyck seedlings is largely dependent on thermal dissipation and enhanced electron flux to photosystem I

Agave salmiana Otto ex Salm-Dyck, a crassulacean acid metabolism plant that is adapted to water-limited environments, has great potential for bioenergy production. However, drought stress decreases the requirement for light energy, and if the amount of incident light exceeds energy consumption, the photosynthetic apparatus can be injured, thereby limiting plant growth. The objective of this study was to evaluate the effects of drought and re-watering on the photosynthetic efficiency of A. salmiana seedlings. The leaf relative water content and leaf water potential decreased to 39.6 % and ?1.1 MPa, respectively, over 115 days of water withholding and recovered after re-watering. Drought caused a direct effect on photosystem II (PSII) photochemistry in light-acclimated leaves, as indicated by a decrease in the photosynthetic electron transport rate. Additionally, down-regulation of photochemical activity occurred mainly through the inactivation of PSII reaction centres and an increased thermal dissipation capacity of the leaves. Prompt fluorescence kinetics also showed a larger pool of terminal electron acceptors in photosystem I (PSI) as well as an increase in some JIP-test parameters compared to controls, reflecting an enhanced efficiency and specific fluxes for electron transport from the plastoquinone pool to the PSI terminal acceptors. All the above parameters showed similar levels after re-watering. These results suggest that the thermal dissipation of excess energy and the increased energy conservation from photons absorbed by PSII to the reduction of PSI end acceptors may be an important acclimation mechanism to protect the photosynthetic apparatus from over-excitation in Agave plants.