Electron Transport Flux Research Articles

Effects of drought stress on fluorescence characteristics of photosystem II in leaves of Plectranthus scutellarioides

Drought stress has multiple effects on the photosynthetic apparatus. Herein, we aimed to study the effect of drought stress on fluorescence characteristics of PSII in leaves of Plectranthus scutellarioides and explore potentially underlying mechanisms. Plants of P. scutellarioides were grown in a greenhouse and subjected to drought (DS, drought-stressed) or daily irrigation (control group). Leaf chlorophyll (Chl) index and induction kinetics curves of Chl a fluorescence and the JIP-test were used to evaluate effects of drought lasting for 20 d. Our results showed that both the leaf and soil relative water content decreased with increasing treatment duration. The leaf Chl index was reduced to half in the DS plants compared with the control group after 20 d. The minimal fluorescence in the DS plants was higher than that in the control plants after 10 d of the treatment. Maximum photochemical efficiency and lateral reactivity decreased with increasing treatment duration in the DS plants. With the continuing treatment, values of absorption flux per reaction center (RC), trapped energy flux per RC, dissipated energy flux per RC, and electron transport flux per RC increased in the earlier stage in the DS plants, while obviously decreased at the later stage of the treatment. In conclusion, drought stress inhibited the electron transport and reduced PSII photochemical activity in leaves of P. scutellarioides.

Effects of ethylene on photosystem II and antioxidant enzyme activity in Bermuda grass under low temperature.

The phytohormone ethylene has been reported to mediate plant response to cold stress. However, it is still debated whether the effect of ethylene on plant response to cold stress is negative or positive. The objective of the present study was to explore the role of ethylene in the cold resistance of Bermuda grass (Cynodon dactylon (L).Pers.). Under control (warm) condition, there was no obvious effect of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) or the antagonist Ag(+) of ethylene signaling on electrolyte leakage (EL) and malondialdehyde (MDA) content. Under cold stress conditions, ACC-treated plant leaves had a greater level of EL and MDA than the untreated leaves. However, the EL and MDA values were lower in the Ag(+) regime versus the untreated. In addition, after 3 days of cold treatment, ACC remarkably reduced the content of soluble protein and also altered antioxidant enzyme activity. Under control (warm) condition, there was no significant effect of ACC on the performance of photosystem II (PS II) as monitored by chlorophyll α fluorescence transients. However, under cold stress, ACC inhibited the performance of PS II. Under cold condition, ACC remarkably reduced the performance index for energy conservation from excitation to the reduction of intersystem electron acceptors (PI(ABS)), the maximum quantum yield of primary photochemistry (φP0), the quantum yield of electron transport flux from Q(A) to Q(B) (φE0), and the efficiency/probability of electron transport (ΨE0). Simultaneously, ACC increased the values of specific energy fluxes for absorption (ABS/RC) and dissipation (DI0/RC) after 3 days of cold treatment. Additionally, under cold condition, exogenous ACC altered the expressions of several related genes implicated in the induction of cold tolerance (LEA, SOD, POD-1 and CBF1, EIN3-1, and EIN3-2). The present study thus suggests that ethylene affects the cold tolerance of Bermuda grass by impacting the antioxidant system, photosystem II, as well as the CBF transcriptional regulatory cascade.

Kaolin-based, foliar reflective film protects photosystem II structure and function in grapevine leaves exposed to heat and high solar radiation

Extreme conditions, such as drought, high temperature, and solar irradiance intensity, are major factors limiting growth and productivity of grapevines. In a field experiment, kaolin particle film application on grapevine leaves was examined during two different summer conditions (in 2012 and 2013) with the aim to evaluate benefits of this practice against stressful conditions hindering photochemical processes. We used chlorophyll a fluorescence to investigate attached leaves. Two months after the application, during the hottest midday, the kaolin-treated plants showed by the JIP test significantly higher quantum yield of PSII photochemistry, flux ratios, maximum trapped excitation flux of PSI, absorption flux, electron transport flux, maximum trapped energy flux per cross section, and performance index than plants under control conditions in the warmer year. On the contrary, the treated plants showed a lower initial slope of relative variable fluorescence and a decrease in the absorption and electron transport per cross section. The JIP test showed higher efficiency of PSII in the plants treated with kaolin mainly in 2013 (higher temperature and drought). Our results supported the hypothesis that the accumulation of active PSII reaction centres was associated with decreased susceptibility to photoinhibition in the kaolin-treated plants and with more efficient photochemical quenching. Grapevines in the Douro Region seems to profit from the kaolin application.

옥수수의 파종시기 및 질소수준별 광화학적 반응 해석

본 연구는 파종시기와 질소시비 수준에 대한 옥수수의 광화학적 반응을 분석하여 광생리적 특성을 해석하였다. 1. 5월 22일 파종시 질소 함량이 증가함에 따라 생육후기까지 엽록소 형광량 또한 증가하여 질소 배량처리에서 광이용효율이 더 높은 것으로 확인되었다. 2. 생육초기의 형광량이 가장 높았으며, 이후 모든 처리구에서 엽록소 형광량이 50% 가량 감소하여 전자전달을 위한 광화학 반응이 크게 감소했음을 알 수 있었다. 3. 질소 반량구에서 활성화된 RC (RC/ABS)가 감소하며 13% 이상의 에너지가 손실되었으며, 결국 전자전달시 에너지 전환효율(PI, DF 등)이 감소해 광이용효율이 낮았다. 4. 배량 처리구는 생육후기로 갈수록 전자전달 효율(ET2o/CS) 및 단면당 활성화된 RC(RC/CS) 등 광이용 효율이 대조구 대비 크게 증가하여 광합성기구 사이의 전자전달이 잘 이루어지는 것으로 확인되었다. The photochemical characteristics were analyzed in the context of sowing time and different levels of fertilized nitrogen during the maize (Zea mays L.) growth. When maize was early sawn, the fluorescence parameters related with electron-transport, in photosystem II (PSII) and PSI, were effectively enhanced with the higher level of fertilized nitrogen. Highest values were observed in maize leaves grown in double N-fertilized plot. The photochemical parameters were declined in the progress of growth stage. In early growth stage, the fluorescence parameters were highest, and then reduced to about half of the parameters related with electron transport on PSII and PSI at middle and late growth stages. In 1/2 N plot, the photochemical energy dissipation was measured to 13% in term of active reaction center per absorbed photon resulting in decrease in performance index and driving force of electron. This decrease induced to lower the photochemical effectiveness. In 2 N plots, the electron transport flux from <TEX>$Q_A$</TEX> to <TEX>$Q_B$</TEX> per cross section and the number of active PSII RCs per cross section were considerably enhanced. It was clearly indicated that the connectivity between photosynthetic PSII and PSI, i.e. electron transport, was far effective.

Cell size and the blockage of electron transfer in photosynthesis: Proposed endpoints for algal assays and its application to soil alga Chlorococcum infusionum

This study evaluated multiple endpoints of algal assays to identify sensitive and easy to use endpoints that could be applied to evaluate algal toxicity in metal-polluted soil extracts. Soil algae play an important role in trophic levels; thus, Chlorococcum infusionum was selected as the test species. Soil extracts were used because they might help identify potential soil retention and ecological hazards caused by pollutants that are present in the soil aqueous phase. The multi-endpoints measured were growth yield, photosynthetic activities, and cell viabilities. Nine parameters were measured to evaluate photosynthetic activity; namely, specific energy fluxes per quinone A-reducing photosystem II reaction center (absorption flux, trapped energy flux, electron transport flux, and dissipated energy flux per reaction center), quantum yields (maximum quantum yield of primary photochemistry, quantum yield of electron transport, quantum yield of energy dissipation, and average quantum yield of primary photochemistry), and the blockage of electron transfer from the reaction center to the quinone pool. Cell viability was evaluated by measuring cell size, cell granularity, and the autofluorescence of chlorophyll using flow cytometry. The results showed that heavy metals reduced growth yield, cell viability, and the photosynthetic activity of C. infusionum in soil extracts. Out of the 13 tested endpoints, the blockage of electron transfer from the reaction center to the quinone pool and cell size represented the most sensitive endpoints. We propose that both endpoints should be measured, along with conventional growth yield, to determine the effect of soil pollutants and to lower pollutant concentrations in soils.

Quantitative trait locus mapping of chlorophyll a fluorescence parameters using a recombinant inbred line population in maize

Chlorophyll a fluorescence parameters, derived from its induction, known as the OJIP rise, are often used to characterize the structure and function of photosystem II (PSII). Under field conditions, structure and function of photosystems of crops can be affected by various environmental factors. This study was conducted to identify quantitative trait loci (QTLs) associated with chlorophyll fluorescence parameters in field-grown maize. During three growing seasons, a recombinant inbred line population, consisting of 228 lines, was evaluated for five chlorophyll fluorescence parameters, which indirectly measure: absorbed photon flux per cross section of leaf (ABS/CSo), maximum trapped exciton flux per cross section of leaf (TRo/CSo), electron transport flux from QA to QB per cross section of leaf (ETo/CSo), number of active PSII reaction centers (RCs) per cross section of leaf (RC/CSo), and the performance index on a cross section of leaf (PICS). Significant correlations were frequently observed among these chlorophyll fluorescence parameters. Three major genomic regions dispersed across chromosomes 1, 5, and 9 were detected to be associated with chlorophyll fluorescence parameters. The genomic region in chromosome bins 9.06–9.07 contained QTLs that not only showed pleiotropy for multiple chlorophyll fluorescence parameters but also showed stable expression and explained a large proportion of the phenotypic variation. Additionally, a QTL for grain yield was also detected in this important genomic region. The identified QTLs for chlorophyll fluorescence parameters in the present study may help elucidate plants’ responses to environmental cues and assist in developing marker-assisted selection breeding programs in maize.

Mechanisms of sensing and adaptive responses to low oxygen conditions in mammals and yeasts.

Oxygen is required for effective production of ATP and plays a key role in the maintenance of life for all organisms, excepting strict anaerobes. The ability of aerobic organisms to sense and respond to changes in oxygen level is a basic requirement for their survival. Eukaryotes have developed adaptive mechanisms to sense and respond to decreased oxygen concentrations (hypoxia) through adjustment of oxygen homeostasis by upregulating hypoxic and downregulating aerobic nuclear genes. This review summarizes recent data on mechanisms of cells sensing and responding to changes in oxygen availability in mammals and in yeasts. In the first part of the review, prominence is given to functional regulation and stabilization of hypoxia-inducible factors (HIFs), HIF-mediated regulation of electron transport flux and repression of lipogenesis, as well as to hypoxia-induced mitochondrial permeability transition (pore) opening, cell death, and autophagy. In the second part of the review emphasis is placed on oxygen sensing in nonpathogenic yeasts by heme, unsaturated fatty acids, and sterols, as well as on responses to hypoxia in fungal pathogens.

Interactive effects of elevated CO&lt;sub&gt;2&lt;/sub&gt; and drought on photosynthetic capacity and PSII performance in maize

Elevated atmospheric CO2 concentration [CO2] and the change of water distribution in arid and semiarid areas affect plant physiology and ecosystem processes. The interaction of elevated [CO2] and drought results in the complex response such as changes in the energy flux of photosynthesis. The performance of photosystem (PS) II and the electron transport were evaluated by using OJIP induction curves of chlorophyll a fluorescence and the P N-C i curves in the two-factor controlled experiment with [CO2] of 380 (AC) or 750 (EC) [&mu;mol mol-1] and water stress by 10% polyethylene glycol 6000. Compared to water-stressed maize (Zea mays L.) under AC, the EC treatment combined with water stress decreased the number of active reaction centers but it increased the antenna size and the energy flux (absorbed photon flux, trapping flux, and electron transport flux) of each reaction center in PSII. Thus, the electron transport rate was enhanced, despite the indistinctively changed quantum yield of the electron transport and energy dissipation. The combination of EC and the water-stress treatment resulted in the robust carboxylation rate without elevating the saturated photosynthetic rate (P max). This study demonstrated that maize was capable of transporting more electrons into the carboxylation reaction, but this could not be used to increase P max under EC.

海盐对绿竹叶片反射光谱及叶绿素荧光参数的影响

沿海防护林是海岸生态系统中最基本的生物资源，由于沿海地区地下水位高，土壤含盐量高，肥力低，生态环境恶劣等特点，所以优良植物种选择是加快沿海优良防护林体系建设的关键。为了探讨绿竹耐盐性以及为沿海地区防护林选择优良植物种，以2 年生绿竹 （<em>Bambusa oldhamii</em>） 为材料，采用水培法进行不同浓度的海盐处理，利用 Unispec-SC 型单通道光纤光谱仪和非调制式叶绿素荧光仪对绿竹叶片反射光谱和叶绿素荧光参数等进行测定。结果表明：当海盐浓度小于1.2%时，绿竹叶片中叶绿素a、叶绿素b和类胡萝卜素含量，以及"三边"参数与对照无显著差异，当海盐浓度升高到1.6%时，色素与对照相比分别降低了63.2%、62.8%和47.2% （<em>P</em> < 0.01），红边位置 （<em>λ</em>_red） 和红边面积 （<em>S</em>_red） 相比对照显著减小 （<em>P</em> < 0.05）。1.2%浓度的海盐处理，红边归一化指数 （<em>rNDVI</em>）、绿度归一化指数 （<em>gNDVI</em>）、类胡萝卜素反射指数2 （<em>CRI</em>₇₀₀） 和光化学反射指数 （<em>PRI</em>） 显著降低 （<em>P</em> < 0.05），与对照相比分别降低了27.3%、23.3%、19.5%和43.9%；当海盐浓度增加到1.6%时，<em>rNDVI</em> 、改良红边比值指数 （<em>mND</em>）、<em>gNDVI</em> 、改良归一化差值指数 （<em>mSR</em>₇₀₀）、类胡萝卜素反射指数1 （<em>CRI</em>₅₅₀）、<em>CRI</em>₇₀₀和<em>PRI</em> 等参数与对照相比分别下降了42.4%、43.9%、32.6%、21.5%、47.2%、49.9%和58.5%。绿竹叶片PSⅡ最大量子产率 （<em>F</em>_v/<em>F</em>_m）、电子传递的量子产额 （<em>ΦE</em><em>_o</em>）、单位反应中心捕获的用于电子传递的能量 （<em>ET</em>_<em>o</em>/<em>RC</em>）、单位面积反应中心数目 （<em>RC/CS</em>） 和叶片性能指数 （<em>PI</em>_ABS） 等参数，1.6%海盐处理与对照相比分别降低了50.8%、28.6%、21.7%、52.1%和92.3%，单位反应中心复合体吸收的能量 （<em>ABS/RC</em>） 比对照提高了96.9%。说明绿竹具有一定的耐盐性。;The coastal protection forest is the most basic living resources in the coastal ecosystem. Due to the high underground water level, high soil salt content, low fertility, and deteriorated ecological environment in the coastal area, the selection of plant species is the key to speed up the construction of coastal protection forest system. To evaluate the salt tolerance of <em>Bambusa oldhamii</em> and provide plant species for coastal protection forest in the coastal area, we investigated reflectance spectra and chlorophyll fluorescence parameters in the leaves of 2 years <em>B. oldhamii</em> under sea salt stress by hydroponics using Unispec-SC spectrometer and non-modulated chlorophyll fluorometer, respectively. Results showed that when the sea salt concentration was under 1.2%, the contents of chlorophyll a, chlorophyll b and carotenoids, and reflectance spectra parameters were not significantly different with that under control. However, when the sea salt concentration was 1.6%, the contents of chlorophyll a, chlorophyll b and carotenoids were decreased by 63.2%, 62.8%, and 47.2% (<em>P</em> < 0.01), respectively and reflectance spectra parameters red wavelength (<em>λ</em>_red) and red area (<em>S</em>_red) were significantly reduced (<em>P</em> < 0.05) compared with control. The red-edge normalized difference vegetation index (<em>rNDVI</em>), greenness normalized index (<em>gNDVI</em>), carotenoids reflectance indexes Ⅱ (<em>CRI</em>₇₀₀) and photochemical reflectance index (<em>PRI</em>) under 1.2% sea salt concentration was reduced by 27.3%、23.3%、19.5% and 43.9%, respectively compared with control. The <em>rNDVI</em>, modified red-edge normalized difference vegetation index (<em>mND</em>), <em>gNDVI</em>, modified red-edge ratio (<em>mSR</em>₇₀₀), carotenoids reflectance indexesⅠ (<em>CRI</em>₅₅₀),<em>CRI</em>₇₀₀ and <em>PRI</em> under 1.6% sea salt concentration was reduced by 42.4%, 43.9%, 32.6%, 21.5%, 47.2%, 49.9% and 58.5%, respectively compared with control. The maximum quantum yield of photosystemⅡ (<em>F</em>_v<em>/F</em>_m), quantum yield for electron transport (<em>ΦE</em>_o), electron transport flux per reaction center (<em>ET</em>_o<em>/RC</em>), density of reaction center (QA-reducing PSⅡ reaction centers) (<em>RC/CS</em>) and performance index on absorption basis (<em>PI</em>_ABS) under 1.6% sea salt concentration was reduced by 50.8%, 28.6%, 21.7%, 52.1%, and 92.3% respectively while the absorption flux per reaction center (<em>ABS/RC</em>) increased 96.9% compared with control. The above results showed that high salt stress inhibited the synthesis of the chlorophyll, reduced the absorption of the light energy, resulted in light inhibition and the damage of the acceptor side from PSⅡ, reaction center degradation or inactivation, and inhibited directly growth and development of bamboo. While under low salt stress, the salt tolerance of bamboo was improved by increasing chlorophyll content, light absorption, <em>F</em>_v/<em>F</em>_m, <em>ΦE</em>_o, <em>ET</em>_o/<em>RC</em>, <em>RC</em>/<em>CS</em> and <em>PI</em>_ABS.

Photochemical Response Analysis on Drought Stress for Red Pepper (Capsiumannuum L.)

The aim of this study is to determine the drought stress index through photochemical analysis in red pepper (Capsiumannuum L.). The photochemical interpretation was performed in the basis of the relation between Kautsky effect and Photosystem II (PSII) following the measurement of chlorophyll, pheophytin contents, and CO₂ assimilation in drought stressed 5-week-old red pepper plants. The CO₂ assimilation rate was severely lowered with almost 77% reduction of chlorophyll and pheophytin contents at four days after non-irrigation. It was clearly observed that the chlorophyll fluorescence intensity rose from a minimum level (the O level), in less than one second, to a maximum level (the P-level) via two intermediate steps labeled J and I (OJIP process). Drought factor index (DFI) was also calculated using measured OJIP parameters. The DFI was -0.22, meaning not only the initial inhibition of PSII but also sequential inhibition of PSI . In real, most of all photochemical parameters such as quantum yield of the electron transport flux from Quinone A (QA) to Quinone B (QB), quantum yield of the electron transport flux until the PSI electron acceptors, quantum yield of the electron transport flux until the PSI electron acceptors, average absorbed photon flux per PSII reaction center, and electron transport flux until PSI acceptors per cross section were profoundly reduced except number of QA reducing reaction centers (RCs) per PSII antenna chlorophyll (RC/ABS). It was illuminated that at least 6 parameters related with quantum yield/efficiency and specific energy fluxes (per active PSII RC) could be applied to be used as the drought stress index. Furthermore, in the combination of parameters, driving forces (DF) for photochemical activity could be deduced from the performance index (PI) for energy conservation from photons absorbed by PSII antenna until the reduction of PSI acceptors. In conclusion, photochemical responses and their related parameters can be used as physiological DFI.

Photosynthetic performance and growth traits in Pennisetum centrasiaticum exposed to drought and rewatering under different soil nutrient regimes

Responses of plants exposed to drought and rewatering have been well documented; however, little is known concerning strategies of psammophyte to drought and rewatering under different soil nutrient regimes. For this study, Pennisetum centrasiaticum under two soil nutrient regimes was subjected to progressive drought and subsequent rewatering. Soil water status, gas exchange characteristics, chlorophyll a fluorescence characteristics as well as biomass traits were measured to investigate ecophysiological responses. Net photosynthesis rate (P n), stomatal conductance (g s), water use efficiency, maximum quantum efficiency of photosynthesis system II (PSII, F V/F M), electron transport flux per cross section (ET0/CS0), and performance index on cross section basis (PICS) were suppressed during drought periods for both nutrient regimes. Meanwhile, leaf intercellular CO2 concentration (C i ), minimal fluorescence intensity (F 0), and dissipated energy flux per cross section (DI0/CS0) increased. Reversible downregulation of PSII photochemistry and enhanced thermal dissipation of excess excitation energy (DI0/CS0) contributed to enhanced photo-protection in drought-stressed plants. Thus, the results indicate that P. centrasiaticum is capable of withstanding and surviving extreme drought events, and the recovery pattern of stressed P. centrasiaticum under both nutrient regimes was similar. However, fertilization increased the biomass and the variation in gas exchange and chlorophyll a fluorescence characteristics during drought periods. Additionally, fertilization accelerated the process of drought and aggravated stress under extreme drought events. Thus, the fertilization strategy used in P. centrasiaticum restoration should be carefully selected—fertilization may not always be beneficial.

Alleviation of heat damage to photosystem II by nitric oxide in tall fescue

Nitric oxide (NO) has been found to mediate plant responses to heat stress. The objective of this study was to investigate the protective role of NO in the recovery process of photosystem II (PSII) in tall fescue (Festuca arundinacea) against heat stress. Treatment of tall fescue leaves with NO donor sodium nitroprusside significantly improved the overall behavior of PSII probed by the chlorophyll a fluorescence transients, while the inhibition of NO accumulation by 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (PTIO, a NO scavenger) plus N (G)-nitro-L-arginine-methyl ester (L-NAME, NO synthase inhibitor) dramatically disrupted the operation of PSII. Specifically, under heat stress, the exogenous NO reduced the initial fluorescence (F 0), increased the maximal quantum yield (F V/F M), and disappeared the K-step of 0.3ms. By the analysis of the JIP-test, the exogenous NO improved the quantum yield of the electron transport flux from Q A to Q B (ET0/ABS), and decreased the trapped excitation flux per reaction center (RC) (TR0/RC), electron transport flux per RC (ET0/RC), and electron flux reducing end electron acceptors per RC (RE0/RC). In addition, the exogenous NO reduced the content of H2O2, O 2 (•-) , and malondialdehyde and electrolyte leakage of tall fescue leaves. These data suggest that exogenous NO could protect plants, increase the amount of activated RC and improve the electron transport from oxygen evolving complex to D1 protein. Moreover, quantitative RT-PCR revealed that, in the presence of hydrogen peroxide, NO induced the gene expression of psbA, psbB, and psbC, which encode proteins belonging to subunits of PSII core reaction center (Psb) complex. These findings indicate that, as an important strategy to protect plants against heat stress, NO could improve the recovery process of PSII by the up regulation of the transcriptions of genes encoding PSII core proteins.

A novel Osmium-based compound targets the mitochondria and triggers ROS-dependent apoptosis in colon carcinoma

Engagement of the mitochondrial-death amplification pathway is an essential component in chemotherapeutic execution of cancer cells. Therefore, identification of mitochondria-targeting agents has become an attractive avenue for novel drug discovery. Here, we report the anticancer activity of a novel Osmium-based organometallic compound (hereafter named Os) on different colorectal carcinoma cell lines. HCT116 cell line was highly sensitive to Os and displayed characteristic features of autophagy and apoptosis; however, inhibition of autophagy did not rescue cell death unlike the pan-caspase inhibitor z-VAD-fmk. Furthermore, Os significantly altered mitochondrial morphology, disrupted electron transport flux, decreased mitochondrial transmembrane potential and ATP levels, and triggered a significant increase in reactive oxygen species (ROS) production. Interestingly, the sensitivity of cell lines to Os was linked to its ability to induce mitochondrial ROS production (HCT116 and RKO) as HT29 and SW620 cell lines that failed to show an increase in ROS were resistant to the death-inducing activity of Os. Finally, intra-peritoneal injections of Os significantly inhibited tumor formation in a murine model of HCT116 carcinogenesis, and pretreatment with Os significantly enhanced tumor cell sensitivity to cisplatin and doxorubicin. These data highlight the mitochondria-targeting activity of this novel compound with potent anticancer effect in vitro and in vivo, which could have potential implications for strategic therapeutic drug design.

Decreased photochemical efficiency of photosystem II following sunlight exposure of shade-grown leaves of avocado: because of, or in spite of, two kinetically distinct xanthophyll cycles?

This study resolved correlations between changes in xanthophyll pigments and photosynthetic properties in attached and detached shade-grown avocado (Persea americana) leaves upon sun exposure. Lutein epoxide (Lx) was deepoxidized to lutein (L), increasing the total pool by ΔL over 5 h, whereas violaxanthin (V) conversion to antheraxanthin (A) and zeaxanthin (Z) ceased after 1 h. During subsequent dark or shade recovery, de novo synthesis of L and Z continued, followed by epoxidation of A and Z but not of L. Light-saturated nonphotochemical quenching (NPQ) was strongly and linearly correlated with decreasing [Lx] and increasing [L] but showed a biphasic correlation with declining [V] and increasing [A+Z] separated when V deepoxidation ceased. When considering [ΔL+Z], the monophasic linear correlation was restored. Photochemical efficiency of photosystem II (PSII) and photosystem (PSI; deduced from the delivery of electrons to PSI in saturating single-turnover flashes) showed a strong correlation in their continuous decline in sunlight and an increase in NPQ capacity. This decrease was also reflected in the initial reduction of the slope of photosynthetic electron transport versus photon flux density. Generally longer, stronger sun exposures enhanced declines in both slope and maximum photosynthetic electron transport rates as well as photochemical efficiency of PSII and PSII/PSI more severely and prevented full recovery. Interestingly, increased NPQ capacity was accompanied by slower relaxation. This was more prominent in detached leaves with closed stomata, indicating that photorespiratory recycling of CO(2) provided little photoprotection to avocado shade leaves. Sun exposure of these shade leaves initiates a continuum of photoprotection, beyond full engagement of the Lx and V cycle in the antenna, but ultimately photoinactivated PSII reaction centers.

Effects of Artemisinin on Photosystem II Performance of Microcystis aeruginosa by In Vivo Chlorophyll Fluorescence

Effects of artemisinin (derived from Artemisia annua) on the photosynthetic activity of Microcystis aeruginosa was investigated by using chlorophyll a (Chl a) fluorescence transient O-J-I-P and JIP-test after exposure to elevated artemisinin concentration. High artemisinin concentration resulted in a significant suppression in photosynthesis and respiration. Results showed that the OJIP curves flattened and the maximal fluorescence yield reached at the J step under artemisinin stress. The decreased values of the energy needed for the RCs' closure (Sm) and the number of oxidation and reduction (N) suggested that the reduction times of primary bound plastoquinone (Q(A)) was also decreased. The absorption flux (ABS/RC) per photosystem II (PSII) reaction center and the electron transport flux (ET(0)/RC) decreased with increasing artemisinin concentration. Excess artemisinin had little effect on the trapping flux (TR(0)/RC). The results showed that the decrease of photosynthesis in exposure to excess artemisinin may be a result of the inactivation of PSII reaction centers and the inhibition of electron transport in the acceptor side.

Age-specific responses to elevated salinity in the coastal marsh plant black needlerush (Juncus roemerianus Scheele) as determined through polyphasic chlorophyll a fluorescence transients (OJIP)

This study employed polyphasic chlorophyll a fluorescence transients (OJIP), a non-invasive marker of environmental stress in plants, to evaluate salt tolerance in three different Juncus roemerianus age classifications (6-, 24-, and 60-months). Following exposure to elevated salts (30 psu), the younger plants sustained growth, which was comparable to freshwater controls. While older (60-month) plants receiving only freshwater also grew over the 8-week study, the older salt-treated plants did not increase in size. Similarly, there were significant declines in variable chlorophyll a fluorescence parameters (Fv/Fm and Fv/Fo), electron transport flux per reaction center (ETo/RC), and photosystem II performance index (PIABS) for 60-month J. roemerianus following salt treatment. These responses were not evident in the two younger salt-treated age classifications. Our results suggest that older J. roemerianus are less tolerant to rapid and sudden increases in salinity relative to younger plants and that this age-specific response may help explain observed discrepancies in salt tolerance in J. roemerianus.

Nonequilibrium density-matrix description of steady-state quantum transport

With this work we investigate the stationary nonequilibrium density matrix of current carrying nonequilibrium steady states of in-between quantum systems that are connected to reservoirs. We describe the analytical procedure to obtain the explicit result for the reduced density matrix of quantum transport when the system, the connecting reservoirs, and the system-reservoir interactions are described by quadratic Hamiltonians. Our procedure is detailed for both electronic transport described by the tight-binding Hamiltonian and for phonon transport described by harmonic Hamiltonians. For the special case of weak system-reservoir couplings, a more detailed description of the steady-state density matrix is obtained. Several paradigm transport setups for interelectrode electron transport and low-dimensional phonon heat flux are elucidated.

Effect of water deficit and potassium fertilization on photosynthetic activity in cotton plants

Physiological mechanisms that can contribute to drought tolerance and the role of potassium fertilization in cotton were studied by evaluation of parameters describing photosynthetic performance. Gas-exchange and chlorophyll fluorescence characteristics were measured on leaves of two cotton genotypes, one drought sensitive (Nazilli 84-S) and the other drought tolerant (Sahin 2000), grown in field conditions in the Aegean region of Turkey under different regimes of water and potassium supply. It was shown that under drought conditions without potassium fertilization Sahin 2000 had a higher photosynthetic rate and stomatal conductance than Nazilli 84-S. Potassium fertilization to a great extent compensated for the inhibitory effect of drought on photosynthesis. Application of the JIP test by using chlorophyll fluorescence data revealed that the drought sensitive Nazilli 84-S was more responsive to potassium fertilization than Sahin 2000, as judged by a number of parameters representing quantum efficiency of the processes and energy fluxes in photosystem (PS) II. The observed decrease in photosynthetic CO2 assimilation in both cotton cultivars under drought conditions was not accompanied by any significant decrease in the electron transport flux in PSII and maximum quantum yield of primary photochemistry.

Effects of Short-Term High Temperature on Photosynthesis and Photosystem II Performance in Sorghum

Gas exchange and chlorophyll a fluorescence transient were examined in leaves of sorghum at high temperatures. No changes were found in photosynthetic rate (Pn) and photosystem II (PS II) performance index on absorption base (PI(abs)) at 40 degrees C for 1 h. But transpiration rate was enhanced significantly, which served as a self-protection response for dissipating heat. The Pn decreased significantly at 40 degrees C for 3 h, and the decrease became greater at 45 degrees C. Decrease in Pn mainly resulted from stomatal limitation at 40 degrees C for 3 h, whereas it was due to non-stomatal limitation at 45 degrees C. Decline in PS II function indicated by the significant decrease in PI(abs), trapped energy flux and electron transport flux were responsible for the decrease in Pn at 45 degrees C. PS II reaction centre and oxygen-evolving complex in the donor side were not affected at high temperatures, but electron transport in the acceptor side was sensitive to high temperature. The PS II function recovered completely 1 day after high temperature stress even as high as 45 degrees C, which is favourable for sorghum to meet the challenge of global warming. However, Pn did not completely recover possibly due to heat-induced irreversible damage to CO(2) fixation process.

Two distinct strategies of cotton and soybean differing in leaf movement to perform photosynthesis under drought in the field.

This paper reports an experimental test of the hypothesis that cotton and soybean differing in leaf movement have distinct strategies to perform photosynthesis under drought. Cotton and soybean were exposed to two water regimes: drought stressed and well watered. Drought-stressed cotton and soybean had lower maximum CO2 assimilation rates than well-watered (control) plants. Drought reduced the light saturation point and photorespiration of both species - especially in soybean. Area-based leaf nitrogen decreased in drought-stressed soybean but increased in drought-stressed cotton. Drought decreased PSII quantum yield (ΦPSII) in soybean leaves, but increased ΦPSII in cotton leaves. Drought induced an increase in light absorbed by the PSII antennae that is dissipated thermally via ΔpH- and xanthophylls-regulated processes in soybean leaves, but a decrease in cotton leaves. Soybean leaves appeared to have greater cyclic electron flow (CEF) around PSI than cotton leaves and drought further increased CEF in soybean leaves. In contrast, CEF slightly decreased in cotton under drought. These results suggest that the difference in leaf movement between cotton and soybean leaves gives rise to different strategies to perform photosynthesis and to contrasting photoprotective mechanisms for utilisation or dissipation of excess light energy. We suggest that soybean preferentially uses light-regulated non-photochemical energy dissipation, which may have been enhanced by the higher CEF in drought-stressed leaves. In contrast, cotton appears to rely on enhanced electron transport flux for light energy utilisation under drought, for example, in enhanced nitrogen assimilation.