Short-term responses of Photosystem I to heat stress

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350–600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl₂ and saturated at a rather high photon flux density of around 1200 E m^–2 s^–1. Upon transition from far-red light to darkness, the oxidized reaction center P700⁺ of PS I was re-reduced very slowly in control leaves (with a half time t_1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700⁺ reduction, with t_1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t_1/2 was sensitive to HgCl₂ and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700⁺ reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.Abbreviations PFD photon flux density - PS Photosystem - Apt and Aox amplitude of the photothermal and photobaric components of the photoacoustic signal, respectively - P700 reaction center pigment of PS I - Fo and Fm initial and maximal levels of chlorophyll fluorescence, respectively - Fv=Fm Fo-variable chlorophyll fluorescence - Q_A primary (stable) electron acceptor of PS II - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Cyt cytochrome     

Redox states of photosystems I and II in irradiated leaves of wheat seedlings grown under different conditions of nitrogen nutrition

Changes in the redox states of photosystem I (PSI) and PSII in irradiated wheat leaves were studied after growing seedlings on a nitrogen-free medium or media containing either nitrate or ammonium. The content of P700, the primary electron donor of PSI was quantified using the maximum magnitude of absorbance changes at 830 nm induced by saturating white light. The highest content of P700 in leaves was found for seedlings grown on the ammonium-containing medium, whereas its lowest content was observed on seedlings grown in the presence of nitrate. At all irradiances of actinic light, the smallest accumulation of reduced Q_A was observed in leaves of ammonium-grown plants. Despite variations in light-response curves of P700 photooxidation and Q_A photoreduction, the leaves of all plants exposed to different treatments demonstrated similar relationships between steady-state levels of P700⁺ and Q_A ^–. The accumulation of oxidized P700 up to 40% of total P700 content was not accompanied by significant Q_A photoreduction. At higher extents of P700 photooxidation, a linear relationship was found between the steady-state levels of P700⁺ and Q_A ^–. The leaves of all treatments demonstrated biphasic patterns of the kinetics of P700⁺ dark reduction after irradiation by far-red light exciting specifically PSI. The halftimes of corresponding kinetic components were found to be 2.6–4 s (fast component) and 17–22 s (slow component). The two components of P700⁺ dark reduction were related to the existence of two PSI populations with different rates of electron input from stromal reductants. The magnitudes of these components differed for plants grown in the presence of nitrate, on the one hand, and plants grown either in the presence of ammonium or in the absence of nitrogen, on the other hand. This indicates the possible influence of nitrogen nutrition on synthesis of different populations of PSI in wheat leaves. The decrease in far-red light irradiance reduced the relative contribution of the fast component to P700⁺ reduction. The fast component completely disappeared at low irradiances. This finding indicates that the saturating far-red light must be applied to determine correctly the relative content of each PSI population in wheat leaves.Translated from Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 165–171.Original Russian Text Copyright © 2005 by Dzhibladze, Polesskaya, Alekhina, Egorova, Bukhov.This revised version was published online in April 2005 with a corrected cover date.     

Flexible coupling between light-dependent electron and vectorial proton transport in illuminated leaves of C3 plants. Role of photosystem I-dependent proton pumping

The role of cyclic electron transport has been re-examined in leaves of C₃ plants because the bioenergetics of chloroplasts (H⁺/e = 3 in the presence of a Q-cycle; H⁺/ATP = 4 of ATP synthesis) had suggested that cyclic electron flow has no function in C₃ photosynthesis. After light activation of pea leaves, the dark reduction of P700 (the donor pigment of PSI) following far-red oxidation was much accelerated. This corresponded to loss of sensitivity of P700 to oxidation by far-red light and a large increase in the number of electrons available to reduce P700⁺ in the dark. At low CO₂ and O₂ molar ratios, far-red light was capable of decreasing the activity of photosystem II (measured as the ratio of variable to maximal chlorophyll fluorescence, F_v/F_m) and of increasing light scattering at 535 nm and zeaxanthin synthesis, indicating formation of a transthylakoid pH gradient. Both the light-induced increase in the number of electrons capable of reducing far-red-oxidised P700 and the decline in F_v/F_m brought about by far-red in leaves were prevented by methyl viologen. Antimycin A inhibited CO₂-dependent O₂ evolution of pea leaves at saturating but not under limiting light; in its presence, far-red light failed to decrease F_v/F_m. The results indicate that cyclic electron flow regulates the quantum yield of photosystem II by decreasing the intrathylakoid pH when there is a reduction in the availability of electron acceptors at the PSI level (e.g. during drought or cold stresses). It also provides ATP for the carbon-reduction cycle under high light. Under these conditions, the Q-cycle is not able to maintain a H⁺/e ratio of 3 for ATP synthesis: we suggest that the ratio is flexible, not obligatory. Received: 23 February 1999 / Accepted: 19 August 1999     

干旱胁迫对不同葡萄砧木光合特性和荧光参数的影响

干旱胁迫导致葡萄砧木实生苗叶片光合能力下降.在正常供水和轻度干旱下,砧木的P_n 以3309C最高,其次是1103P,420A较低,各砧木的G_s和T_r差异不显著;中度干旱下,则以1103P的P_n最高,3309C最低;而严重干旱胁迫下,1103P的P_n比3309C高出124%,水分利用效率是3309C的1.95倍.干旱胁迫下,3种砧木的共同趋势是可变荧光 (F_o) 升高,最大荧光 (F_m)、实际光能转化效率 (ФPSⅡ）和可变荧光与最大荧光比 (F_v/F_m) 降低,但品种变幅不同.中度干旱使3309C的F _o升高17.1%,F_v/F_m降低了8.5%,而1103P的F_o升高6.8%,F_v/F_m降低了5.8%;严重干旱则使3309C的F_o升高36.2%,F_v/F_m降低了20.1%,而1103P的F_o升高9.9%,F_v/F_m降低了10.2%.干旱胁迫对不同葡萄砧木光合和荧光参数的影响与其抗旱性密切相关,其中F_v/F_m和P_n的相关系数最大（r=0.9883）.     

Inhibition of photosynthetic electron transport and formation of inactive chlorophyll in winter stressed Pinus silvestris

Kinetics of fluorescence at room temperature, electron transport and photooxidation of P700 and cytochrome f have been studied in chloroplasts isolated from active and winter stressed Pinus silvestris. The winter stress induced block in the electron transport chain between the two photosystems is close to the site of plastoquinone, since winter stress and DCMU caused the same type of inhibition of the reoxidation of the primary electron acceptor Q of photosystem II. No winter inhibition of the electron transport between cytochrome f and P700 was observed. Time course studies of P700 photooxidation in chloroplasts of active and winter stressed pine have shown that the photosynthetic unit size must be about equal in the two types of chloroplasts. An apparent increase of the photosynthetic unit size was induced by winter stress, as revealed by the high chlorophyll/P700 ratio of winter stressed pine. The phenomenon is explained by the formation of photosynthetically inactive chlorophyll. Low-temperature fluorescence emission spectra were recorded when either chlorophyll a (433 nm) or chlorophyll b (477 nm) were preferentially excited. Winter stress induced the formation of a chlorophyll a fraction emitting at 673 nm. This chlorophyll is most likely derived from the chlorophyll a antennae of the two photosystems, and it probably contributes to the photosynthetically inactive pool of chlorophyll in winter stressed pine. The light harvesting chlorophyll a/b complex is relatively resistant to winter stress.     

Water stress induces different levels of photosynthesis and electron transport rate regulation in grapevines

A, net CO₂ assimilation rate
E, leaf transpiration
ETR, electron transport rate
F_s, fluorescence yield at steady state
F_m and F_m', maximal fluorescence levels when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively
F_o and F_o', initial fluorescence levels when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively
F_v/F_m, efficiency of excitation capture by open PSII in dark-adapted leaves
ΔF/F_m', actual photochemical efficiency of PSII
g, stomatal conductance
NPQ, non-photochemical quenching of chlorophyll fluorescence
PPFD, photosynthetic photon flux density
Ψ_PD and Ψ_MD, leaf water potential at pre-dawn and midday, respectively
Rl, estimated photorespiration rate
I₁ and I₂, Irrigation treatments
R, Recovery treatment
D₁ and D₂, drought treatments
HD₁ and HD₂, hard drought treatments

Diurnal time courses of chlorophyll fluorescence and gas-exchange rates were measured in young potted grapevines (Vitis vinifera L. cv. Tempranillo) subjected to different conditions of water supply under Mediterranean summer conditions. The irrigated plants exhibited typical diurnal patterns for all measured parameters, showing a correspondence between electron transport rate, net CO₂ assimilation and stomatal conductance. Mild decreases in soil-water availability led to different degrees of down-regulation of photosynthesis and increased nonphotochemical quenching of chlorophyll fluorescence. A good correspondence between electron transport rate and CO₂ assimilation was still maintained, suggesting a coregulation of both photosynthetic processes. In contrast, a severe water deficit induced a drastic down-regulation of photosynthesis and breakage of the above-mentioned link. Both midday net CO₂ assimilation and electron transport rate significantly correlated with pre-dawn water potential (Ψ_PD) (r² = 0·65 and r² = 0·92, P < 0·001, respectively). However, when field data were analysed, the relationship between electron transport rate and Ψ_PD was not maintained, although net CO₂ assimilation was similarly correlated with Ψ_PD. Interestingly, the steady-state chlorophyll fluorescence yield was a good indicator of plant water stress.     

Effect of low growth temperature on coupling between electron transport and proton flux in Vicia faba thylakoids

Coupling between electron transport and proton flux has been compared in chloroplasts from Vicia faba (cv. Windsor) plants grown at 20 and 5°C. Proton uptake by warm-grown thylakoids was sensitive to external pH and stimulated by micromolar adenine nucleotide above pH 7.0. Electron transport was modulated by pH, adenine nucleotide and energy transfer inhibitors (triphenyltin and Hg²⁺). By contrast, proton uptake by cold-grown thylakoids was generally lower and was insensitive to micromolar ATP. The rate of non-phosphorylating electron flow in cold-grown thylakoids was relatively insensitive to pH and Hg²⁺ and was not modulated by adenine nucleotides or triphenyltin. Stimulation of electron transport by phosphorylating conditions in cold-grown thylakoids was generally lower and insensitive to pH. It is concluded that the control of proton efflux through CF₀-CF₁ differs in thylakoids of V. faba grown at warm and cold temperatures.     

The response of electron transport mediated by active NADPH dehydrogenase complexes to heat stress in the cyanobacterium Synechocystis 6803

The electron-transport machinery in photosynthetic membranes is known to be very sensitive to heat. In this study, the rate of electron transport (ETR) driven by photosystem I (PSI) and photosystem II (PSII) during heat stress in the wild-type Synechocystis sp. strain PCC 6803 (WT) and its ndh gene inactiva-tion mutants △ndhB (M55) and △ndhD1/ndhD2 (D1/D2) was simultaneously assessed by using the novel Dual-PAM-100 measuring system. The rate of electron transport driven by the photosystems (ETRPSs) in the WT, M55, and D1/D2 cells incubated at 30℃ and at 55℃ for 10 min was compared. Incubation at 55 ℃ for 10 min significantly inhibited PSII-driven ETR (ETRPSII) in the WT, M55 and D1/D2 cells, and the ex-tent of inhibition in both the M55 and D1/D2 cells was greater than that in the WT cells. Further, PSI-driven ETR (ETRPSI) was stimulated in both the WT and D1/D2 cells, and this rate was increased to a greater extent in the D1/D2 than in the WT cells. However, ETRPSI was considerably inhibited in the M55 cells. Analysis of the effect of heat stress on ETRPSs with regard to the alterations in the 2 active NDH-1 complexes in the WT, M55, and D1/D2 cells indicated that the active NDH-1 supercomplex and medi-umcomplex are essential for alleviating the heat-induced inhibition of ETRPSII and for accelerating the heat-induced stimulation of ETRPSI, respectively. Further, it is believed that these effects are most likely brought about by the electron transport mediated by each of these 2 active NDH-1 complexes.     

Photosynthetic electron transport and establishment of an associated trans-thylakoid proton electrochemical gradient during development of the wheat leaf

Abstract Photosynthetic electron transport activities and the ability to generate and maintain a trans-thylakoid proton electrochemical gradient were examined during chloroplast development in 4-day-old wheat leaves grown under a diurnal light regime. Polarographic and spectropholometric studies on leaf tissue demonstrated that poorly developed chloroplasls at the leaf base could photo-oxidize water and transfer electrons from photosystem 2 to photosystem 1. The capacity for non-cyclic whole-chain electron transport increased during chloroplast development. Thylakoids isolated from the leaf base, although capable of pumping protons into the inlrathylakoid space, could not maintain a trans-membrane proton electrochemical gradient; this ability developed at later stages of chloroplast biogenesis in the leaf. The implications of these results for the energetics of the developing leaf are discussed.     

Photosynthetic electron transport and carbon-reduction-cycle enzyme activities under long-term drought stress in Casuarina equisetifolia Forst. & Forst.

The inhibition of photosynthetic electron transport and the activity of photosynthetic carbon reduction cycle (PCR) enzymes under long-term water stress after slow dehydration was studied in non-nodulated Casuarina equisetifolia Forst. & Forst. plants. Initially, drought increased the fraction of closed Photosystem II (PS II) reaction centres (lowered qP) and decreased the quantum yield of PS II electron transport (_PSII) with no enhancement of non-radiative dissipation of light energy (qN) because it increased the efficiency of electron capture by open PS II centres (F_v/F_m). As drought progressed, F_v/F_m fell and the decrease in _PSII was associated with an increased qN. The kinetics of dark relaxation of fluorescence quenching pointed to an increase in a slowly-relaxing component under drought, in association with increased contents of zeaxanthin and antheraxanthin. Total NADP-dependent malate dehydrogenase activity increased and total stromal fructose-1,6-bisphosphatase activity decreased under drought, while the activation state of these enzymes remained unchanged. Water stress did not alter the activity and the activation state of ribulose bisphosphate carboxylase oxygenase.     

Electron transport activities of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> mutants with impaired chloroplastic NAD(P)H dehydrogenase

The activities of electron transport are compared between wild-type Arabidopsis and two Arabidopsis mutants deficient for the chloroplastic NAD(P)H dehydrogenase (NDH) which catalyzes cyclic electron transport around photosystem I. The quantum yield of photosystem II and the degree of non-photochemical quenching of chlorophyll fluorescence were of similar levels in the two NDH-deficient mutants and the wild type under non-stressed standard growth conditions. Stromal over-reduction was induced in Arabidopsis NDH mutants with high light treatment, as is the case in tobacco NDH mutants. However, unlike tobacco mutants, photoinhibition was not observed in the Arabidopsis NDH mutants.     

Regulation of electron transport in maize mesophyll chloroplasts: The relationship between chlorophyll a fluorescence quenching and O2 evolution

The relationship between the redox state of the primary electron acceptor of photosystem II (Q_A) and the rate of O₂ evolution in isolated mesophyll chloroplasts from Zea mays L. is examined using pulse-modulated chlorophyll a fluorescence techniques. A linear relationship between photochemical quenching of chlorophyll fluorescence (q_Q) and the rate of O₂ evolution is evident under most conditions with either glycerate 3-phosphate or oxaloacetate as substrates. There appears to be no effect of the transthylakoid pH gradient on the rate of electron transfer from photosystem II into Q_A in these chloroplasts. However, the proportion of electron transport occurring through cyclic-pseudocyclic pathways relative to the non-cyclic pathway appears to be regulated by metabolic demand for ATP. The majority of non-photochemical quenching in these chloroplasts at moderate irradiances appeared to be energy-dependent quenching.Abbreviations and symbols PSII photosystem II - Fm maximum fluorescence obtained on application of a saturating light pulse - Fo basal fluorescence recorded in the absence of actinic light (i.e. all PSII traps are open) - Fv Fm-Fo - q_Q photochemical quenching - q_NP non-photochemical quenching - q_E energy-dependent quenching of chlorophyll fluorescence     

The response of electron transport mediated by active NADPH dehydrogenase complexes to heat stress in the cyanobacterium Synechocystis 6803

The electron-transport machinery in photosynthetic membranes is known to be very sensitive to heat. In this study, the rate of electron transport (ETR) driven by photosystem I (PSI) and photosystem II (PSII) during heat stress in the wild-type Synechocystis sp. strain PCC 6803 (WT) and its ndh gene inactiva-tion mutants ΔndhB (M55) and ΔndhD1/ndhD2 (D1/D2) was simultaneously assessed by using the novel Dual-PAM-100 measuring system. The rate of electron transport driven by the photosystems (ETR_PSs) in the WT, M55, and D1/D2 cells incubated at 30℃ and at 55℃ for 10 min was compared. Incubation at 55℃ for 10 min significantly inhibited PSII-driven ETR (ETR^_PSII) in the WT, M55 and D1/D2 cells, and the ex-tent of inhibition in both the M55 and D1/D2 cells was greater than that in the WT cells. Further, PSI-driven ETR (ETR_PSI) was stimulated in both the WT and D1/D2 cells, and this rate was increased to a greater extent in the D1/D2 than in the WT cells. However, ETR_PSI was considerably inhibited in the M55 cells. Analysis of the effect of heat stress on ETR_PSs with regard to the alterations in the 2 active NDH-1 complexes in the WT, M55, and D1/D2 cells indicated that the active NDH-1 supercomplex and medi-umcomplex are essential for alleviating the heat-induced inhibition of ETRPSII and for accelerating the heat-induced stimulation of ETR_PSI, respectively. Further, it is believed that these effects are most likely brought about by the electron transport mediated by each of these 2 active NDH-1 complexes.     

Moderate heat stress reduces the pH component of the transthylakoid proton motive force in light-adapted, intact tobacco leaves

We measured the Δ Ψ and ΔpH components of the transthylakoid proton motive force ( pmf ) in light-adapted, intact tobacco leaves in response to moderate heat. The Δ Ψ causes an electrochromic shift (ECS) in carotenoid absorbance spectra. The light–dark difference spectrum has a peak at 518 nm and the two components of the pmf were separated by following the ECS for 25 s after turning the light off. The ECS signal was deconvoluted by subtracting the effects of zeaxanthin formation (peak at 505 nm) and the qE-related absorbance changes (peak at 535 nm) from a signal measured at 520 nm. Heat reduced ΔpH while Δ Ψ slightly increased. Elevated temperature accelerated ECS decay kinetics likely reflecting heat-induced increases in proton conductance and ion movement. Energy-dependent quenching (qE) was reduced by heat. However, the reduction of qE was less than expected given the loss of ΔpH. Zeaxanthin did not increase with heat in light-adapted leaves but it was higher than would be predicted given the reduced ΔpH found at high temperature. The results indicate that moderate heat stress can have very large effects on thylakoid reactions.     

冬小麦农田暂时水分胁迫状况下水、热通量日变化

以冬小麦农田尺度为研究对象,采用涡度相关技术测定农田能茸平衡各分量,研究暂时水分胁迫状况下农田蒸散通量和蒸发比值（evaporative fraction）日变化特征。结果表明,冬小麦在农田郁闭（LAI≥3）且土壤含水量为田间持水量的55％-65％时,晴天日农田潜热通量日变化在正午前后存在明显的“蒸散高地（evapotranspiration plateau）”现象,持续时间达2．5—4h,表现为蒸散通量增量日变化突然极显著降低,即蒸散通量呈相对稳定、甚至下降的变化趋势;反映在蒸发比值日变化过程方面,夜间和日出日落前后时刻蒸发比值变化较大,7：00-18：00时段内蒸发比值曲线近似呈倒“s”型,9：00～16：00时段内蒸发比值比较稳定,正午前后蒸散高地出现时,蒸发比值有所下降大约在0．5—0．65范围内,12：00—13：30时段内蒸发比值平均值接近9：00—16：00时段内蒸发比值平均值。     

Effect of heat stress and feeding phosphorus levels on pig electron transport chain gene expression

The purpose of this study was to evaluate the effect of temperature and different levels of available phosphorus (aP) on the expression of nine genes encoding electron transport chain proteins in the Longissimus dorsi (LD) muscle of pigs. Two trials were carried out using 48 high-lean growth pigs from two different growth phases: from 15 to 30 kg (phase 1) and from 30 to 60 kg (phase 2). Pigs from growth phase 1 were fed with three different levels of dietary aP (0.107%, 0.321% or 0.535%) and submitted either to a thermoneutral (24°C and RH at 76%) or to a heat stress (34°C and RH at 70%) environment. Pigs from growth phase 2 were fed with three different levels of dietary aP (0.116%, 0.306% or 0.496%) and submitted either to a thermoneutral (22ºC and RH at 77%) or to a heat stress (32ºC and RH at 73%) environment. Heat stress decreased (P<0.001) average daily feed intake at both growth phases. At 24°C, pigs in phase 1 fed the 0.321% aP diet had greater average daily gain and feed conversion (P<0.05) than those fed the 0.107% or 0.535% while, at 34°C pigs fed the 0.535% aP had the best performance (P<0.05). Pigs from phase 2 fed the 0.306% aP had best performance in both thermal environments. Gene expression profile was analyzed by quantitative real-time polymerase chain reaction. Irrespective of growing phase, the expression of six genes was lower (P<0.05) at high temperature than at thermoneutrality. The lower expression of these genes under high temperatures evidences the effects of heat stress by decreasing oxidative metabolism, through adaptive physiological mechanisms in order to reduce heat production. In pigs from phase 1, six genes were differentially expressed across aP levels (P<0.05) in the thermoneutral and one gene in the heat stress. In pigs from phase 2, two genes were differentially expressed across aP levels (P<0.05) in both thermal environments. These data revealed strong evidence that phosphorus and thermal environments are key factors to regulate oxidative phosphorylation with direct implications on animal performance.     

Stimulation of isoprene emissions and electron transport rates as key mechanisms of thermal tolerance in the tropical species Vismia guianensis

Tropical forests absorb large amounts of atmospheric CO₂ through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to photosynthetic electron transport (ETR) as a function of temperature, direct field observations of this phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here we demonstrate that in the early successional species Vismia guianensis in the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as stomatal conductance (g_s) and net photosynthetic carbon fixation (P_n) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While P_n showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (q_L) increased with leaf temperature with strong linear correlations for ETR (? = 0.98) and q_L (? = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non‐photochemical quenching, were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed P_n at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease g_s and P_n in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary “ground‐truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.     

Evidence for the operation of alternative electron transport routes through photosystem I in intact barley leaves under weak and moderate white light

The functioning of alternative routes of photosynthetic electron transport was analyzed from the kinetics of dark reduction of P700⁺ , an oxidized primary donor of PSI, in barley (Hordeum vulgare L.) leaves irradiated by white light of various intensities. Redox changes of P700 were monitored as absorbance changes at 830 nm using PAM 101 specialized device. Irradiation of dark-adapted leaves caused a gradual P700⁺ accumulation, and the steady-state level of oxidized P700 increased with intensity of actinic light. The kinetics of P700⁺ dark reduction after a pulse of strong actinic light, assayed from the absorbance changes at 830 nm, was fitted by a single exponential term with a halftime of 10–12 ms. Two slower components were observed in the kinetics of P700⁺ dark reduction after leaf irradiation by attenuated actinic light. The contribution of slow components to P700⁺ reduction increased with the decrease in actinic light intensity. Two slow components characterized by halftimes similar to those observed after leaf irradiation by weak white light were found in the kinetics of dark reduction of P700⁺ oxidized in leaves with far-red light specifically absorbed by PSI. The treatment of leaves with methyl viologen, an artificial PSI electron acceptor, significantly accelerated the accumulation of P700⁺ under light. At the same time, the presence of methyl viologen, which inhibits ferredoxin-dependent electron transport around PSI, did not affect three components of the kinetics of P700⁺ dark reduction obtained after irradiations with various actinic light intensities. It was concluded that some part of PSI reaction centers was not reduced by electron transfer from PSII under weak or moderate intensities of actinic light. In this population of PSI centers, P700⁺ was reduced via alternative electron transport routes. Insensitivity of the kinetics of P700⁺ dark reduction to methyl viologen evidences that the input of electrons to PSI from the reductants (NADPH or NADH) localized in the chloroplast stroma was effective under those light conditions.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 5–11.Original Russian Text Copyright © 2005 by Bukhov, Egorova.     

Growth,photosynthetic electron transport,and antioxidant responses of young soybean seedlings to simultaneous exposure of nickel and UV-B stress

The effects of enhanced ultraviolet-B (UV-B, 0.4 W m⁻²) irradiance and nickel (Ni, 0.01, 0.10 and 1.00 mM; Ni_0.01, Ni_0.10, Ni_1.00, respectively) treatment, singly and in combination, on growth, photosynthetic electron transport activity, the contents of reactive oxygen species (ROS), antioxidants, lipid peroxidation, and membrane leakage in soybean seedlings were evaluated. Ni_0.10 and Ni_1.00 and UV-B declined the growth and chlorophyll content, which were further reduced following combined exposure. Contrary to this, Ni_0.01 stimulated growth, however, the effect together with UV-B was inhibitory. Carotenoids showed varied response to both the stresses. Simultaneous exposure of UV-B and Ni as well as UV-B alone reduced the activities of photosystems 1 and 2 (PS1 and PS2) and whole chain activity significantly, while Ni individually, besides strongly inhibiting PS2 and whole chain activity, stimulated the PS1 activity. Both the stresses, alone and together, enhanced the contents of superoxide radical (O ₂ ^⋅− ), hydrogen peroxide (H₂O₂), malondialdehyde (MDA), electrolyte leakage, and proline content, while ascorbate content declined over control. Individual treatments increased the activities of catalase (CAT), peroxidase, and superoxide dismutase (SOD), but Ni_1.00 declined SOD activity significantly. Combined exposure exhibited similar response, however, CAT activity declined even more than in control. Compared to individual effects of UV-B and Ni, the simultaneous exposure resulted in strong inhibition of photosynthetic electron transport and excessive accumulation of ROS, thereby causing severe damage to soybean seedlings.This work was supported by the CSIR, New Delhi, India in the form of JRF to Rajiv Dwivedi.