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.     

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.     

Ferredoxin limits cyclic electron flow around PSI (CEF-PSI) in higher plants--stimulation of CEF-PSI enhances non-photochemical quenching of Chl fluorescence in transplastomic tobacco

We tested the hypothesis that ferredoxin (Fd) limits the activity of cyclic electron flow around PSI (CEF-PSI) in vivo and that the relief of this limitation promotes the non-photochemical quenching (NPQ) of Chl fluorescence. In transplastomic tobacco (Nicotiana tabacum cv Xanthi) expressing Fd from Arabidopsis (Arabidopsis thaliana) in its chloroplasts, the minimum yield (F(o)) of Chl fluorescence was higher than in the wild type. F(o) was suppressed to the wild-type level upon illumination with far-red light, implying that the transfer of electrons by Fd-quinone oxidoreductase (FQR) from the chloroplast stroma to plastoquinone was enhanced in transplastomic plants. The activity of CEF-PSI became higher in transplastomic than in wild-type plants under conditions limiting photosynthetic linear electron flow. Similarly, the NPQ of Chl fluorescence was enhanced in transplastomic plants. On the other hand, pool sizes of the pigments of the xanthophyll cycle and the amounts of PsbS protein were the same in all plants. All these results supported the hypothesis strongly. We conclude that breeding plants with an NPQ of Chl fluorescence increased by an enhancement of CEF-PSI activity might lead to improved tolerance for abiotic stresses, particularly under conditions of low light use efficiency.     

Effect of light quality on chloroplast-membrane organization and function in pea

The effect of light quality during plant growth of chloroplast membrane organization and function in peas (Pisum sativum L. cv. Alaska) was investigated. In plants grown under photosystem (PS) I-enriched (far-red enriched) illumination both the PSII/PSI stoichiometry and the electrontransport capacity ratios were high, about 1.9. In plants grown under PSII-enriched (far-red depleted) illumination both the PSII/PSI stoichiometry and the electron-transport capacity ratios were significantly lower, about 1.3. In agreement, steady-state electron-transport measurements under synchronous illumination of PSII and PSI demonstrated an excess of PSII in plants grown under far-red-enriched light. Sodium dodecylsulfate polyacrylamide gel electrophoretic analysis of chlorophyll-containing complexes showed greater relative amounts of the PSII reaction center chlorophyll-protein complex in plants grown under farred-enriched light. Additional changes were observed in the ratio of light-harvesting chlorophyll a/b protein to PSII reaction center chlorophyll-protein under the two different light-quality regimes. The results demonstrate the dynamic nature of chloroplast structure and support the notion that light quality is an important factor in the regulation of chloroplast membrane organization and-function.Abbreviations and symbols Chl chlorophyll - CPa PSII reaction center chlorophyll protein complex - CPI PSI chlorophyll protein complex - FR-D light depleted in far-red sensitizing primarily PSII - FR-E light enriched in far-red sensitizing primarily PSI - LHCP PSII light-harvesting chlorophyll a/b protein complex - P ₇₀₀ primary electron donor of PSI - PSI, PSII photosystems I and II, respectively - Q primary electron acceptor of PSII     

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.     

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).     

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to −196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.     

Variations in light energy dissipation in <Emphasis Type="Italic">Woodfordia fruticosa</Emphasis> leaves during expansion

Young leaves of tropical trees frequently appear red in color, with the redness disappearing as the leaves mature. During leaf expansion, plants may employ photoprotective mechanisms to cope with high light intensities; however, the variations in anthocyanin contents, nonphotochemical quenching (NPQ), and photorespiration during leaf expansion are poorly understood. Here, we investigated pigment contents, gas exchange, and chlorophyll (Chl) fluorescence in Woodfordia fruticosa leaves during their expansion. Young red leaves had significantly lower Chl content than that of expanding or mature leaves, but they accumulated significantly higher anthocyanins and dissipated more excited light energy through NPQ. As the leaves matured, net photosynthetic rate, total electron flow through PSII, and electron flow for ribulose-1,5-bisphosphate oxygenation gradually increased. Our results provided evidence that photorespiration is of fundamental importance in regulating the photosynthetic electron flow and CO₂ assimilation during leaf expansion.     

Physiological functions of the water-water cycle (Mehler reaction) and the cyclic electron flow around PSI in rice leaves

Changes in chlorophyll fluorescence, P700(+)-absorbance and gas exchange during the induction phase and steady state of photosynthesis were simultaneously examined in rice (Oryza sativa L.), including the rbcS antisense plants. The quantum yield of photosystem II (PhiPSII) increased more rapidly than CO(2) assimilation in 20% O(2). This rapid increase in PhiPSII resulted from the electron flux through the water-water cycle (WWC) because of its dependency on O(2). The electron flux of WWC reached a maximum just after illumination, and rapidly generated non-photochemical quenching (NPQ). With increasing CO(2) assimilation, the electron flux of WWC and NPQ decreased. In 2% O(2), WWC scarcely operated and PhiPSI was always higher than PhiPSII. This suggested that cyclic electron flow around PSI resulted in the formation of NPQ, which remained at higher levels in 2% O(2). The electron flux of WWC in the rbcS antisense plants was lower, but these plants always showed a higher NPQ. This was also caused by the operation of the cyclic electron flow around PSI because of a higher ratio of PhiPSI/PhiPSII, irrespective of O(2) concentration. The results indicate that WWC functions as a starter of photosynthesis by generating DeltapH across thylakoid membranes for NPQ formation, supplying ATP for carbon assimilation. However, WWC does not act to maintain a high NPQ, and PhiPSII is down-regulated by DeltapH generated via the cyclic electron flow around PSI.     

Chlorophyll a fluorescence of typical desert plant Alhagi sparsifolia Shap. at two light levels

Alhagi sparsifolia Shap. is exposed to a high-irradiance environment as the main vegetation found in the forelands of the Taklamakan Desert. We investigated chlorophyll a fluorescence emission of A. sparsifolia seedlings grown under ambient (HL) and shade (LL) conditions. Our results indicated that the fluorescence intensity in the leaves was significantly higher for LL-grown plants than that under HL. High values of the maximum quantum yield of PSII for primary photochemistry (φP_o) and the quantum yield that an electron moves further than Q_A - (φE_o) in the plants under LL conditions suggested that the electron flow from Q_A - (primary quinone electron acceptors of PSII) to Q_B (secondary quinone acceptor of PSII) or Q_B - was enhanced at LL compared to natural HL conditions. The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side and the quantum yield for the reduction of end electron acceptors at the PSI acceptor side were opposite to φP_o, and φE_o. Thus, we concluded that the electron transport on the donor side of PSII was blocked under LL conditions, while acceptor side was inhibited at the HL conditions. The PSII activity of electron transport in the plants grown in shade was enhanced, while the energy transport from PSII to PSI was blocked compared to the plants grown at HL conditions. Furthermore, PSII activity under HL was seriously affected in midday, while the plants grown in shade enhanced their energy transport.     

Low temperature delays development of photosystem II activity in winter rye leaves

The ability of leaves to acclimate photosynthetically to low temperature was examined during leaf development in winter rye plants ( Secale cereale L. cv. Puma) grown at 20°C or at 6°C. All leaves grown at 6°C exhibit increased chlorophyll (Chl) levels per leaf area, higher rates of uncoupled, light-saturated photosystem I (PSI) electron transport, and slower increases in photosystem II (PSII) electron transport capacity, when compared with 20°C leaves. The stoiehiometry of PSI and PSII was estimated for each leaf age class by quantifying Chl in elcctrophorctic separations of Chl-protein complexes. The ratio of PSII/PSI electron transport in 20°C leaves is highly correlated with the ratio of core Chl a -proteins associated with PSII (CPa) to those associated with PSI (CP1). In contrast, PSII/PSI electron transport in 6°C leaves is not as well correlated with CPa/CP1 and is related, in part, to the amount and organization of light-harvesting Chl a/b -proteins associated with PSII. CPa/CP1 increases slowly in 6°C leaves, although the ratio of CPa/CP1 in mature 20°C and 6°C leaves is not different. The results suggest that increased PSI activity at low temperature is not related to an increase in the relative proportion of PSI and may reflect, instead, a regulatory change. Photosynthetic acclimation to low environmental temperature involves increased PSI activity in mature leaves shifted to 6°C. In leaves grown entirely at 6°C, however, acclimation includes both increased PSI activity and modifications in the rate of accumlation of PSII and in the organization of LHCII.     

Photosynthetic adaptation of pea plants grown at different light intensities: State 1 - State 2 transitions and associated chlorophyll fluorescence changes

A study of pea plants grown at different light intensities has been made. Using a leaf oxygen electrode, it was shown that plants grown under low light intensities had lower saturated rates of photosynthesis than high-light-grown plants however, at low light intensities the photosynthetic rates were similar for both types of plants. State 1- State 2 transitions have been monitored with attached leaves using a modulated fluorescence technique. It is shown that peas grown under low light intensities (20 W m^-2) had a faster State 1 to State 2 transition when compared with medium-(50 W m^-2) and high-(70 W m^-2) light-grown plants. Measurement of fast-fluorescence-induction curves in the absence of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) have shown that low-light plants are, when in State 1, more effective at using Photosystem-two (PSII) light to reduce their plastoquinone pool than high-light plants. Transition from State 1 to State 2 for all plants led to a decrease in the reduction level of the plastoquinone pool inidcating that the transition had increased electron flow through Photosystem one (PSI) relative to PSII. Analyses of fast fluorescence induction in the presence of DCMU indicate that low-light-grown plants have a higher PSII-α/PSII-β ratio than high-light-grown plants. Such a difference is in line with the increase in the PSII/PSI ratio of low-light plants and is reflected in their high chlorophyll b/chlorophyll a ratio and their larger appressed to non-appressed thylakoid-membrane areas. It is suggested that these two latter factors give rise to the faster State 1 - State 2 transitions in low-light plants.     

Characterization of factors affecting the activity of photosystem I cyclic electron transport in chloroplasts

PSI cyclic electron transport is essential for photosynthesis and photoprotection. In higher plants, the antimycin A-sensitive pathway is the main route of electrons in PSI cyclic electron transport. Although a small thylakoid protein, PGR5 (PROTON GRADIENT REGULATION 5), is essential for this pathway, its function is still unclear, and there are numerous debates on the rate of electron transport in vivo and its regulation. To assess how PGR5-dependent PSI cyclic electron transport is regulated in vivo, we characterized its activity in ruptured chloroplasts isolated from Arabidopsis thaliana. The activity of ferredoxin (Fd)-dependent plastoquinone (PQ) reduction in the dark is impaired in the pgr5 mutant. Alkalinization of the reaction medium enhanced the activity of Fd-dependent PQ reduction in the wild type. Even weak actinic light (AL) illumination also markedly activated PGR5-dependent PSI cyclic electron transport in ruptured chloroplasts. Even in the presence of linear electron transport [11 mumol O2 (mg Chl)(-1) h(-1)], PGR5-dependent PSI electron transport was detected as a difference in Chl fluorescence levels in ruptured chloroplasts. In the wild type, PGR5-dependent PSI cyclic electron transport competed with NADP+ photoreduction. These results suggest that the rate of PGR5-dependent PSI cyclic electron transport is high enough to balance the production ratio of ATP and NADPH during steady-state photosynthesis, consistently with the pgr5 mutant phenotype. Our results also suggest that the activity of PGR5-dependent PSI cyclic electron transport is regulated by the redox state of the NADPH pool.     

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.     

Acclimations to light quality on plant and leaf level affect the vulnerability of pepper (<Emphasis Type="Italic">Capsicum annuum</Emphasis> L.) to water deficit

We investigated the influence of light quality on the vulnerability of pepper plants to water deficit. For this purpose plants were cultivated either under compact fluorescence lamps (CFL) or light-emitting diodes (LED) providing similar photon fluence rates (95 µmol m^?2 s^?1) but distinct light quality. CFL emit a wide-band spectrum with dominant peaks in the green and red spectral region, whereas LEDs offer narrow band spectra with dominant peaks at blue (445 nm) and red (665 nm) regions. After one-week acclimation to light conditions plants were exposed to water deficit by withholding irrigation; this period was followed by a one-week regeneration period and a second water deficit cycle. In general, plants grown under CFL suffered more from water deficit than plants grown under LED modules, as indicated by the impairment of the photosynthetic efficiency of PSII, resulting in less biomass accumulation compared to respective control plants. As affected by water shortage, plants grown under CFL had a stronger decrease in the electron transport rate (ETR) and more pronounced increase in heat dissipation (NPQ). The higher amount of blue light suppressed plant growth and biomass formation, and consequently reduced the water demand of plants grown under LEDs. Moreover, pepper plants exposed to high blue light underwent adjustments at chloroplast level (e.g., higher Chl a/Chl b ratio), increasing the photosynthetic performance under the LED spectrum. Differently than expected, stomatal conductance was comparable for water-deficit and control plants in both light conditions during the stress and recovery phases, indicating only minor adjustments at the stomatal level. Our results highlight the potential of the target-use of light quality to induce structural and functional acclimations improving plant performance under stress situations.     

Alternative electron flows (water-water cycle and cyclic electron flow around PSI) in photosynthesis: molecular mechanisms and physiological functions

An electron flow in addition to the major electron sinks in C(3) plants [both photosynthetic carbon reduction (PCR) and photorespiratory carbon oxidation (PCO) cycles] is termed an alternative electron flow (AEF) and functions in the chloroplasts of leaves. The water-water cycle (WWC; Mehler-ascorbate peroxidase pathway) and cyclic electron flow around PSI (CEF-PSI) have been studied as the main AEFs in chloroplasts and are proposed to play a physiologically important role in both the regulation of photosynthesis and the alleviation of photoinhibition. In the present review, I discuss the molecular mechanisms of both AEFs and their functions in vivo. To determine their physiological function, accurate measurement of the electron flux of AEFs in vivo are required. Methods to assay electron flux in CEF-PSI have been developed recently and their problematic points are discussed. The common physiological function of both the WWC and CEF-PSI is the supply of ATP to drive net CO(2) assimilation. The requirement for ATP depends on the activities of both PCR and PCO cycles, and changes in both WWC and CEF-PSI were compared with the data obtained in intact leaves. Furthermore, the fact that CEF-PSI cannot function independently has been demonstrated. I propose a model for the regulation of CEF-PSI by WWC, in which WWC is indispensable as an electron sink for the expression of CEF-PSI activity.     

田间条件下棉花幼叶光合特性及光保护机制

通过比较棉花(Gossypium hirsutum)幼叶和完全展开叶气体交换参数及叶绿素荧光特性的差异, 探讨高光强下幼叶的光抑制程度及明确光保护机制间的协调机理。在田间自然条件下, 以棉花刚展平的幼嫩叶片(幼叶)和面积已达到最大的完全展开叶片为研究对象, 通过测定不同发育阶段叶片气体交换参数及叶绿素a荧光参数的变化, 并运用Dual-PAM100对不同发育阶段的叶片进行快速光响应曲线的拟合。结果表明: 幼叶和完全展开叶片在光合、荧光特性方面表现出明显的差异。与完全展开叶相比, 较低的叶绿素(Chl)含量和气孔导度(G_s)是幼叶较低净光合速率(P_n)的限制因素, 从而直接导致其光系统II (PSII)实际光化学效率(Φ_PSII)和光化学猝灭系数(q_P)的降低。在1800 μmol·m^-2·s^-1光强以下, 完全展开叶具有较强的围绕PSI循环的电子流(CEF), 有利于合成ATP, 是其具有较高光合能力的原因之一。相同光强下, 幼叶较低的光饱和点(LSP)更易受光抑制, 但其PSII原初光化学效率(F_v/F_m)的日变化幅度显著小于完全展开叶, 说明强光下幼叶通过类胡萝卜素(Car)猝灭单线态氧、光呼吸(P_r)、热耗散(NPQ)以及PSI-CEF等光保护机制能有效地耗散过剩的光能, 从而避免其光合机构发生光抑制。     

Is electron transport to oxygen an important mechanism in photoprotection? Contrasting responses from Antarctic vascular plants

Photoreduction of oxygen by the photosynthetic electron transport chain has been suggested to be an important process in protecting leaves from excess light under conditions of stress; however, there is little evidence that this process occurs significantly except when plants are exposed to conditions outside their normal tolerance range. We have examined the oxygen dependency of photosynthetic electron transport in the two vascular plants found growing in Antarctica – Colobanthus quitensis and Deschampsia antarctica. Photosynthetic electron transport in C. quitensis is insensitive to changes in oxygen concentration under non-photorespiratory conditions, indicating that electron transport to oxygen is negligible; however, it has a substantial capacity for non-photochemical quenching (NPQ) of chlorophyll fluorescence. In contrast, D. antarctica has up to 30% of its photosynthetic electron transport being linked to oxygen, but has a substantially lower capacity for NPQ. Thus, these plants rely on contrasting photoprotective mechanisms to cope with the Antarctic environment. Both plants seem to use cyclic electron flow associated with PSI, however, this is activated at a lower irradiance in C. quitensis than in D. antarctica.     

High irradiance-induced changes in carotenoid composition and increase in non-photochemical quenching of Chl a fluorescence in primary wheat leaves

The effect of acclimation to high irradiance stress (HIS, 250 Wm-2) in wheat leaves grown under three different irradiances was investigated by HPLC analyses of pigments, chlorophyll a fluorescence parameters and photochemical activities of chloroplasts. Significant loss of beta-carotene was observed compared to the xanthophylls in all three types of seedlings exposed to HIS. However, the effect of HIS on neoxanthin and lutein contents was not significant. The loss of partial electron transport (Asc-DCPIP to MV, PSI activity) was less than the whole chain (H2O to MV) and PS II activity (H2O to DCPIP) suggesting that PS I is less susceptible to HIS compared to PS II. The percent of reductions in Fv/Fm and phi PS II were less in plants grown under high irradiance (HI-1, 30 Wm-2 and HI-2, 45 Wm-2) compared to those grown under moderate irradiance (MI, 15 Wm-2). On the other hand, the percent of NPQ increased more in the leaves of HI plants compared to the leaves of MI when exposed to HIS which suggests a more efficient non-radiative dissipation of excess excitation energy in HI plants compared to MI. These observations suggest that plants grown under relatively high irradiance are better adapted to HIS condition.     

Distinct roles of the cytochrome pathway and alternative oxidase in leaf photosynthesis

In illuminated leaves, mitochondria are thought to play roles in optimizing photosynthesis. However, the roles of the cytochrome pathway (CP) and alternative oxidase (AOX) in photosynthesis, in particular in the redox state of the photosynthetic electron transport chain, are not separately characterized. We examined the effects of specific inhibition of two respiratory pathways, CP and AOX, on photosynthetic oxygen evolution and the redox state of the photosynthetic electron transport chain in broad bean (Vicia faba L.) leaves under various light intensities. Under saturating photosynthetic photon flux density (PPFD; 700 micromol photon m(-2) s(-1)), inhibition of either pathway caused a decrease in the steady-state levels of the photosynthetic O(2) evolution rate and the PSII operating efficiency, Phi(II). Because these inhibitors, at the concentrations applied to the leaves, had little effect on photosynthesis in the intact chloroplasts, two respiratory pathways are essential for maintenance of high photosynthetic rates at saturating PPFD. CP or AOX inhibition affected to Chl fluorescence parameters (e.g. photochemical quenching and non-photochemical quenching) differently, suggesting that CP and AOX contribute to photosynthesis in different ways. At low PPFD (100 micromol photon m(-2) s(-1)), only the inhibition of AOX, not CP, lowered the photosynthetic rate and Phi(II). AOX inhibition also decreased the Phi(II)/Phi(I) ratio even at low PPFD levels. These data suggest that AOX inhibition caused the over-reduction of the photosynthetic electron transport chain and induced the cyclic electron flow around PSI (CEF-PSI) even at the low PPFD. Based on these results, we discuss possible roles for CP and AOX in the light.