Effect of drought stress on chlorophyll a fluorescence and electrical admittance of shoots in Norway spruce seedlings

Effects of mild and severe soil drought on the water status of needles, chlorophyll a fluorescence, shoot electrical admittance, and concentrations of photosynthetic pigments in needles of seedlings of Picea abies (L.) Karst. were examined under controlled greenhouse conditions. Drought stress reduced shoot admittance linearly with a decrease in shoot water potential (_w) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot _w=–2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (F_v/F_m), and the vitality index (R_fd). Variations in these parameters suggest an inhibition of the photosynthetic electron transport in spruce needles. Water stress led to a decrease in the mobility of electrolytes in tissues, which was reflected by decreased shoot electrical admittance. After re-watering for 21 days the WD in needles decreased and the shoot water potential increased. In the re-watered plants, the chloroplast function was restored and chlorophyll a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in the seedlings triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. We conclude that the shoot electrical admittance and photosynthetic electron transport in leaves are closely linked to changes in water status and their decrease is among the initial responses of seedlings to water stress.     

Cyanobacterial NADPH dehydrogenase complexes

Cyanobacteria possess functionally distinct multiple NADPH dehydrogenase (NDH-1) complexes that are essential to CO₂ uptake, photosystem-1 cyclic electron transport and respiration. The unique nature of cyanobacterial NDH-1 complexes is the presence of subunits involved in CO₂ uptake. Other than CO₂ uptake, chloroplastic NDH-1 complex has a similar role as cyanobacterial NDH-1 complexes in photosystem-1 cyclic electron transport and respiration (chlororespiration). In this mini-review we focus on the structure and function of cyanobacterial NDH-1 complexes and their phylogeny. The function of chloroplastic NDH-1 complex and characteristics of plants defective in NDH-1 are also described for comparison.     

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.     

INFLUENCE OF ULTRAVIOLET RADIATION ON GROWTH AND PHOTOSYNTHESIS OF TWO COLD OCEAN DIATOMS1

The influence of chronic exposure to UV-B and UV-A radiation on growth and photosynthesis of two polar marine diatoms (Pseudonitzschia seriata and Nitzschia sp.) was investigated in cultures exposed to moderate photon fluences for 3–7 days. Population growth rates were diminished 50% by UV-B. Fluorescence induction kinetics of photo-system II (PSII) revealed that UV-B caused lower F_v/F_m ratios and half-rise times, indicating damage to the reaction center of PSII and to related elements of the photosynthetic electron transport chain. Carbon assimilation rates per cell and per chlorophyll a were nonetheless highest for UV-B—exposed populations, which also had the highest chlorophyll a content per cell. The UV-B—exposed cells were, however, more vulnerable to visible light-induced photoinhibition. Exposure to UV-A in the absence of UV-B had little effect on growth, fluorescence induction of PSII, or chlorophyll a contents but did have some inhibitory effects on carbon assimilation per chlorophyll a and per cell. The increased photosynthetic capacity of UV-B-exposed cells suggested some ability to compensate for damage to the photosynthetic apparatus.     

Effects of O(2) and CO(2) Concentration on the Steady-State Fluorescence Yield of Single Guard Cell Pairs in Intact Leaf Discs of Tradescantia albiflora: Evidence for Rubisco-Mediated CO(2) Fixation and Photorespiration in Guard Cells

A procedure for following changes in the steady-state yield of chlorophyll a fluorescence (F_s) from single guard cell pairs in variegated leaves of Tradescantia albiflora is described. As an indicator of photosynthetic electron transport, F_s is a very sensitive indirect measure of the balance of adenosine 5′-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), producing reactions with the sink reactions that utilize those light-generated products. We found that F_s under constant light is sensitive to manipulation of ambient CO₂ concentrations, as would be expected if either phosphoenolpyruvate carboxylase or ribulose-1, 5 bisphosphate carboxylase/oxygenase (Rubisco)-dependent CO₂ fixation is the sink for photosynthetic ATP and NADPH in guard cells. However, we also found that changing O₂ concentration had a strong effect on fluorescence yield, and that O₂ sensitivity was only evident when the concentration of CO₂ was low. This finding provides evidence that both O₂ and CO₂ can serve as sinks for ATP and NADPH produced by photosynthetic electron transport in guard cell chloroplasts. Identical responses were observed with mesophyll cell chloroplasts in intact leaves. This finding is difficult to reconcile with the view that guard cell chloroplasts have fundamentally different pathways of photosynthetic metabolism from other chloroplasts in C₃ plants. Indeed, Rubisco has been detected at low levels in guard cell chloroplasts, and our studies indicate that it is active in the pathways for photosynthetic carbon reduction and photorespiration in guard cells.     

Fluorescence and Photosynthetic Activity of Chloroplasts in Acid and Alkaline Zones of Chara corallina

Continuous profiles of local pH near the cell surface of Chara corallinawere recorded during uniform longitudinal movement of an internodal cell relative to a stationary pH microelectrode. Under illumination, the pH profile consisted of alternating acid and alkaline bands with a pH difference of up to 3 pH units. After darkening, the bands disappeared and pH became uniformly distributed along the cell length. Chlorophyll fluorescence of chloroplasts was measured by microfluorometry at different locations within one cell, and significant differences were observed in close relation to light-dependent pH banding. The chlorophyll fluorescence yield was lower in zones of low external pH than in alkaline zones both under actinic and saturating light. The fluorescence parameters Fand F" _m and the quantum yield of photosystem II (PSII) displayed variations along the cell length in accordance with pH changes in unstirred layers of the medium. The results show that PSII photochemical efficiency and the rate of noncyclic electron transport are higher in the chloroplasts of acid zones (zones of H⁺extrusion from the cell) than in alkaline zones. The dependence of photosynthetic electron transport on local pH near the cell surface may result from different contents of CO₂in acid and alkaline regions. The acid zones are enriched with CO₂that readily permeates through the membrane providing the substrate for the Calvin cycle. Conversely, a poorly permeating form, HCO^– ₃is predominant in alkaline zones, which may restrict the dark reactions and photosynthetic electron flow.     

兰科菌根真菌对铁皮石斛光合性能及pepc基因表达的影响

为了阐明兰科菌根真菌对铁皮石斛光合作用的影响及机制,采用盆栽方式研究了兰科菌根真菌对铁皮石斛幼苗生长的影响,并分析了叶片中叶绿素含量、光合参数、叶绿素荧光参数以及pepc基因表达的变化。结果表明：（1）兰科菌根真菌促进了铁皮石斛幼苗生长,接种兰科菌根真菌的铁皮石斛的株高、根重、茎叶重和总生物量分别是未接种对照组的1.21、1.54、1.71和1.68倍;而且可显著提高叶片中叶绿素含量、叶片净光合速率（P_n）、蒸腾速率（G_s）和气孔导度（T_r）。（2）接种兰科菌根真菌的铁皮石斛叶片潜在光化学效率（F_v/F₀）、最大光化学效率（F_v/F_m）、光化学猝灭系数（q_P）、非光化学猝灭系数（q_N）、实际光化学反应量子效率（Yield）和表观光合电子传递速率（ETR）均高于未接种对照组。（3）菌根真菌能促进pepc基因的表达,增强PEPC活性,提高铁皮石斛叶片的光合碳同化能力。研究表明,菌根的形成可以提高铁皮石斛叶片光合性能和pepc基因的表达水平,促进铁皮石斛幼苗的生长。     

POTASSIUM ALLEVIATES THE INHIBITORY ACTION OF AMMONIUM UPON THE PHOTOSYNTHETIC RECOVERY OF NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of ammonium on the photosynthetic recovery of Nostoc flagelliforme Berk. et M. A. Curtis were assayed when being rehydrated in low‐K⁺ or high‐K⁺ medium. Its photosynthetic recovery was K⁺ limited after 3 years of dry storage. The potassium absorption of N. flagelliforme reached the maximum after 3 h rehydration in low‐K⁺ medium but at 5 min in high‐K⁺ medium. The K⁺ content of N. flagelliforme rehydrated in high‐K⁺ medium was much higher than that in low‐K⁺ medium. The maximal PSII quantum yield (F_v/F_m) value of N. flagelliforme decreased significantly when samples were rehydrated in low‐K⁺ medium treated with 5 mM NH₄Cl. However, the treatment of 20 mM NH₄Cl had little effect on its F_v/F_m value in high‐K⁺ medium. The relative F_v/F_m 24 h EC₅₀ (concentration at which 50% inhibition occurred) value of NH₄⁺ in high‐K⁺ medium (64.35 mM) was much higher than that in low‐K⁺ medium (22.17 mM). This finding indicated that high K⁺ could alleviate the inhibitory action of NH₄⁺ upon the photosynthetic recovery of N. flagelliforme during rehydration. In the presence of 10 mM tetraethylammonium chloride (TEACl), the relative F_v/F_m 24 h EC₅₀ value of NH₄⁺ was increased to 46.34 and 70.78 mM, respectively, in low‐K⁺ and high‐K⁺ media. This observation suggested that NH₄⁺ entered into N. flagelliforme cells via the K⁺ channel. Furthermore, NH₄⁺ could decrease K⁺ absorption in high‐K⁺ medium.     

The kinetics of photoinhibition of the photosynthetic apparatus in pea chloroplasts

Abstract The kinetics of a range of chlorophyll fluorescence parameters, non-cyclic electron transport and the capacity of the thylakoids to bind Atrazine were examined during photoinhibition treatment of intact pea chloroplasts. Parameters of fluorescence induction of chloroplasts in the presence and absence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea at 20 °C and at 77 K were determined. The contributions of photochemical and non-photochemical quenching processes to the loss of fluorescence during photoinhibitory treatment were assessed. Two distinct phases of photoinhibitory damage were observed. During the initial 5 min period of exposure to light the minimal fluorescence level (F_o) increased, whilst the maximal fluorescence level (F_P) decreased, both coupled and uncoupled non-cyclic electron transport to methyl viologen decreased and the ability to bind Atrazine to the thylakoids decreased. Fluorescence analyses demonstrated that during this period thylakoids were becoming increasingly less efficient at generating and maintaining a transmembrane proton electrochemical gradient. Photoinhibitory damage that occurred at later times between 5 and 20 min was of a very different nature. Both F_o and F_P declined, a loss of coupled and uncoupled non-cyclic electron transport was observed together with a loss of the capacity to photo-oxidize water. However, no further loss of Atrazine-binding was associated with such changes. A consistent decrease in the quantum yield of non-cyclic electron transport was also observed throughout photoinhibition treatment. The possibility of two distinct mechanisms of photoinhibitory damage to the photosynthetic apparatus is discussed.     

Early Toxic Effects of Zinc, Cobalt, and Cadmium on Photosynthetic Activity of the Green AlgaChlorella pyrenoidosaChick S-39

Early toxic effects of heavy metals (HMs) Zn, Co, and Cd at concentrations from 0.01 to 100 mM on photosynthetic activity of the green algaChlorella pyrenoidosaChick S-39 were studied. The early effect of HMs was manifested as a rapid (within 0.5–2 h) reduction of photoinduced oxygen release by the algal cells. The suppressed relative yield of variable chlorophyll fluorescence (F _V /F _m ) by HMs as well as its dynamics inC. pyrenoidosaprovided evidence for rapid inactivation of photosystem II (PS II). Analysis of the induction curve of delayed chlorophyll fluorescence inChlorellacells suggested that the early toxic effects of Zn, Co, and Cd at the above concentrations manifested itself not only in inhibited electron transport in PS II, but also in reduced energization of photosynthetic membranes. Hence, the early toxic effect of Zn, Co, and Cd was primarily related to the decreased efficiency of the light reactions of photosynthesis, which subsequently resulted in reduced productivity of the alga.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

NdhP Is an Exclusive Subunit of Large Complex of NADPH Dehydrogenase Essential to Stabilize the Complex in Synechocystis sp. Strain PCC 6803

Two major complexes of NADPH dehydrogenase (NDH-1) have been identified in cyanobacteria. A large complex (NDH-1L) contains NdhD1 and NdhF1, which are absent in a medium size complex (NDH-1M). They play important roles in respiration, cyclic electron transport around photosystem I, and CO₂ acquisition. Two mutants sensitive to high light for growth and impaired in NDH-1-mediated cyclic electron transfer were isolated from Synechocystis sp. strain PCC 6803 transformed with a transposon-bearing library. Both mutants had a tag in sml0013 encoding NdhP, a single transmembrane small subunit of the NDH-1 complex. During prolonged incubation of the wild type thylakoid membrane with n-dodecyl β-d-maltoside (DM), about half of the NDH-1L was disassembled to NDH-1M and the rest decomposed completely without forming NDH-1M. In the ndhP deletion mutant (ΔndhP), disassembling of NDH-1L to NDH-1M occurred even on ice, and decomposition to a small piece occurred at room temperature much faster than in the wild type. Deletion of the C-terminal tail of NdhP gave the same result. The C terminus of NdhP was tagged by YFP-His₆. Blue native gel electrophoresis of the DM-treated thylakoid membrane of this strain and Western analysis using the antibody against GFP revealed that NdhP-YFP-His₆ was exclusively confined to NDH-1L. During prolonged incubation of the thylakoid membrane of the tagged strain with DM at room temperature, NDH-1L was partially disassembled to NDH-1M and the 160-kDa band containing NdhP-YFP-His₆ and possibly NdhD1 and NdhF1. We therefore conclude that NdhP, especially its C-terminal tail, is essential to assemble NdhD1 and NdhF1 and stabilize the NDH-1L complex.     

NdhM Subunit Is Required for the Stability and the Function of NAD(P)H Dehydrogenase Complexes Involved in CO2 Uptake in Synechocystis sp. Strain PCC 6803

The cyanobacterial type I NAD(P)H dehydrogenase (NDH-1) complexes play a crucial role in a variety of bioenergetic reactions such as respiration, CO₂ uptake, and cyclic electron transport around photosystem I. Two types of NDH-1 complexes, NDH-1MS and NDH-1MS′, are involved in the CO₂ uptake system. However, the composition and function of the complexes still remain largely unknown. Here, we found that deletion of ndhM caused inactivation of NDH-1-dependent cyclic electron transport around photosystem I and abolishment of CO₂ uptake, resulting in a lethal phenotype under air CO₂ condition. The mutation of NdhM abolished the accumulation of the hydrophilic subunits of the NDH-1, such as NdhH, NdhI, NdhJ, and NdhK, in the thylakoid membrane, resulting in disassembly of NDH-1MS and NDH-1MS′ as well as NDH-1L. In contrast, the accumulation of the hydrophobic subunits was not affected in the absence of NdhM. In the cytoplasm, the NDH-1 subcomplex assembly intermediates including NdhH and NdhK were seriously affected in the ΔndhM mutant but not in the NdhI-deleted mutant ΔndhI. In vitro protein interaction analysis demonstrated that NdhM interacts with NdhK, NdhH, NdhI, and NdhJ but not with other hydrophilic subunits of the NDH-1 complex. These results suggest that NdhM localizes in the hydrophilic subcomplex of NDH-1 complexes as a core subunit and is essential for the function of NDH-1MS and NDH-1MS′ involved in CO₂ uptake in Synechocystis sp. strain PCC 6803.     

Alterations in photosynthesis and pigment distributions in pea leaves following UV-B exposure

We compared photosynthetic and UV-B-absorbing pigment concentrations, gas-exchange rates and photosystem II (PSII) electron transport rates in leaves of pea (Pisum sativum mutant Argenteum) grown without UV-B or under an enhanced UV-B treatment (18 kJ m^?2 biologically effective daily dose) in a greenhouse. We also compared the distribution of chlorophyll by depth within leaves of each treatment by using image analysis of chlorophyll autofluorescence. Ultraviolet-B treatment elicited putative protective responses such as an 80% increase in UV-B-absorbing compound concentrations (leaf-area basis), and a slight increase in mesophyll thickness (178 in controls compared to 191 μm in UV-B-treated leaves). However, photosynthetic rates of UV-B-treated leaves were only 80% of those of controls. This was paralleled by reductions in leaf conductance to water vapor (50% of controls) and intercellular CO₂ concentrations, suggesting that stomatal limitations were at least partly responsible for lower photosynthetic rates under the UV-B treatment. Total chlorophyll concentrations (leaf-area basis) in UV-B-treated leaves were only 70% of controls, and there was a shift in the relative distribution of chlorophyll with depth in UV-B-treated leaves. In control leaves chlorophyll concentrations were highest near the adaxial surface of the upper palisade, dropped with depth and then increased slightly in the bottom of the spongy mesophyll nearest the abaxial surface. In contrast, in UV-B-treated leaves chlorophyll concentrations were lowest at the adaxial surface of the upper palisade and increased with depth through the leaf. The most notable treatment difference in chlorophyll concentrations was in the upper palisade near the adaxial surface of leaves, where we estimate that chlorophyll concentrations in each 1-μm-thick paradermal layer were about 50% lower in UV-B-treated leaves than in controls. We found reduced electron transport capacity in UV-B-treated leaves, based on lower maximum fluorescence (F_m), variable to maximum fluorescence ratios (F,/F_m) and quantum yield of PSII electron transport (Y). However, the above were assessed from fluorometer measurements on the adaxial leaf surface and may reflect the markedly lower chlorophyll concentrations in the upper palisade of UV-B-treated leaves.     

Growth and photosynthetic efficiency promotion of sugar beet (<Emphasis Type="Italic">Beta vulgaris</Emphasis> L.) by endophytic bacteria

Very little is known about the physiological interactions between plants and endophytic bacteria. We investigated the impact of three endophytic bacteria, Bacillus pumilus 2-1, Chryseobacterium indologene 2-2, and Acinetobacter johnsonii 3-1, on the photosynthetic capacity and growth of sugar beet. Endophyte-free plants were obtained first and infected with the bacteria. Measurements of total chlorophyll content revealed very significant differences between endophyte-free beet plants and some infected by endophytic bacteria. The maximum photochemical yield (Fv/Fm) was used to determine any photosynthetic effect on plants caused by biotic or abiotic factors. After 30 days of growth, there was significantly higher Fv/Fm for endophyte-infected than endophyte-free plants. The light response curves of beet showed that photosynthetic capacity was significantly increased in endophyte-infected plants. Photosynthesis of endophyte-free plants was saturated at 1,300 μmol m⁻² s⁻¹, whereas endophyte-infected plants were not saturated at the irradiance used. The effect seemed to be due to promotion of electron transport in the thylakoid membranes. Promotion of photosynthetic capacity in sugar beet was due to increased chlorophyll content, leading to a consequent increased carbohydrate synthesis. It is possible that the increased maximum yield of photosynthesis in sugar beet was promoted by phytohormones and produced by the bacteria.     

Effects of inorganic carbon accumulation on photosynthetic oxygen reduction and cyclic electron flow in the cyanobacterium Synechococcus PCC7942

This paper examines the effect of inorganic carbon transport and accumulation in Synechococcus PCC7942 on fluorescence quenching, photosynthetic oxygen reduction and both linear and cyclic electron flow. The data presented support the previous findings of Miller et al. (1991) that the accumulation of Ci by the CO₂ concentrating mechanism is able to stimulate oxygen photoreduction, particularly so when CO₂ fixation is inhibited by PCR cycle inhibitors such as glycolaldehyde. This effect is found with both high and low-Ci grown cells, but the potential for oxygen photoreduction is about two-fold higher in low-Ci grown cells. This greater potential for O₂ photoreduction is also correlated with a higher ability of low-Ci cells to photoreduce H₂O₂. Experiments with a mutant which transports Ci but does not accumulate it internally, indicates that the stimulation of O₂ photoreduction appears to be a direct effect of the internal accumulation of Ci rather than from its participation in the transport process. In the absence of Ci, no specific partial reactions of photosynthetic electron transport appear to be inhibited, and the PS 1 acceptors PNDA and MV as well as the PS 2 acceptor DMQ can all run electron transport at levels approaching those during active CO₂ fixation. Measurements of P700⁺ show that when the cells are depleted of Ci during photosynthesis, P700 becomes more oxidised. This indicates that the resupply of electrons from the intersystem chain is relatively more restricted under conditions of Ci limitation than is the availability of PS 1 electron acceptors. It is proposed that the accumulated Ci pool can directly stimulate the ability of O₂ to act as a PS 1 acceptor and that the ability of PS 1 acceptors, such as O₂, to relieve restrictions on intersystem electron transfer is perhaps a result of a reduction in cyclic electron flow and a subsequent increase in the oxidation state of the plastoquinone pool.Abbreviations BTP 1,3-bis[tris(hydroxymethyl)-methylaminopropane] - CA carbonic anhydrase' - Ci inorganic carbon (CO₂+HCO₃ ^–+CO₃ ^2–) - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMQ 2,6-dimethylbenzoquinone - EZ ethoxyzolamide or 6-ethoxy-2-benzothiazole-sulfonamide - FCCP carbonyl cyanide p-trifluoro methoxyphenyl-hydrazone - F steady-state chlorophyll fluorescence - F_m chlorophyll fluorescence during a saturating light pulse - F_o chlorophyll fluorescence in the dark, prior to illumination by actinic light - MV methyl viologen or 1,1-dimethyl-4,4-bipyridinium dichloride - PCR cycle photosynthetic carbon reduction cycle - PNDA N,N-dimethyl-p-nitrosoaniline - _{PS 1} the quantum yield of Photosystem 1 - _{PS 2} the quantum yield of Photosystem 2     

Photosynthetic electron transport is differentially affected during early stages of cultivar/race-specific interactions between potato andPhytophthora infestans

The influence of the early stages of fungal infection on chloroplast metabolism was studied in cultivar/race-specific interactions between potato (Solanum tuberosum L. cv. Datura) and the late-blight fungusPhytophthora infestans. The accumulation of several mRNAs encoding components of the photosynthetic apparatus was not affected, either in compatible or in incompatible interactions. However, within 3 h after inoculation of potato leaves with fungal spores, a change in the photochemistry of photosystem II was detectable by measuring chlorophylla fluorescence. Characteristic fluorescence parameters, such as maximum fluorescence yield (F_m), variable fluorescence yield (F_v) and photochemical efficiency (F_v/F_m), were specifically reduced in the compatible host/pathogen interaction. Analyses of photochemical and nonphotochemical fluorescence quenching showed an increase in the photochemical fraction. The amounts of two selected thylakoid membrane proteins and of total chlorophyll remained unchanged during this process, suggesting that the functional modification of the electron-transport system was not correlated with a change in the composition of the photosynthetic apparatus. The alterations of photosynthetic electron transport represent a rapidly detectable and sensitive physiological marker for compatible interactions in the potato/Phytophthora infestans pathosystem.     

不同剂量~(137)Cs-γ辐射对毛竹幼苗叶片叶绿素荧光参数的影响

利用不同剂量的137Cs-γ射线对毛竹(Phyllostachys heterocycla ‘Pubescens’)种子进行辐射,测定实生苗叶片中的光合色素含量和叶绿素荧光参数等指标,探讨辐射对毛竹幼苗生长的影响,为筛选有利的突变单株奠定良好基础。结果表明:30或60Gy的137Cs-γ射线辐射后,毛竹幼苗的光合色素含量以及最大荧光强度(Fm)、可变荧光强度(Fv)、PSII最大光化学效率(Fv/Fm)、PSII的潜在活性(Fv/Fo)、PSII实际光化学效率(Yield)和表观光合电子传递速率(ETR)等荧光参数值均高于90Gy辐射处理,说明较低剂量辐射后PSII反应中心的能量捕获效率高,且具有较强的光合能力;而90Gy的137Cs-γ射线辐射对毛竹的影响则与之相反。不同处理剂量之间叶片光能耗散程度以及表观光合电子传递速率-光合有效辐射(ETR-PAR)响应曲线的分析结果也进一步证实了以上结论。     

Effect of light intensity on partitioning of photosynthetic electron transport to photorespiration in four subtropical forest plants

Photosynthetic rate (P_n) and the partitioning of noncyclic photosynthetic electron transport to photorespiration (J_O) in seedlings of four subtropical woody plants growing at three light intensities were studied in the summer time by measurements of chlorophyll fluorescence and CO₂ exchange. ExceptSchima superba, an upper canopy tree species, the tree speciesCastanopsis fissa and two understory shrubsPsychotria rubra, Ardisia quinquegona had the highestP _n at 36% of sunlight intensity. The total photosynthetic electron transport rate (J_F) and the ratio ofJ _O/J_F were elevated in leaves under full sunlight.J _O/J_F ratio reached 0.5–0.6 and coincided with the increasing of oxygenation rate of Rubisco (V_O), the activity of glycolate oxidase and photorespiration rate at full sunlight. It is suggested that an increasing partitioning proportion of photosynthetic electron transport to photorespiration might be one of the protective regulation mechanisms in forest plant under strong summer light and high temperature conditions.     

Moderate water stress causes different stomatal and non‐stomatal changes in the photosynthetic functioning of Phaseolus vulgaris L. genotypes

The impact of moderate water deficit on the photosynthetic apparatus of three Phaseolus vulgaris L. cultivars, Plovdiv 10 (P10), Dobrudjanski Ran (DR) and Prelom (Prel), was investigated. Water shortage had less impact on leaf hydration, RWC (predawn and midday) and predawn water potential in Prel. RWC and Ψ_p were more reduced in P10, while there was no osmotic adjustment in any cultivar. Although drought drastically reduced stomatal opening in P10 and DR, reduced A_max indicated non‐stomatal limitations that contributed to the negligible P_n. These limitations were on potential thylakoid electron transport rates of PSI and II, pointing to photosystem functioning as a major limiting step in photosynthesis. This agrees with decreases in actual photochemical efficiency of PSII (F_v′/F_m′), quantum yield of photosynthetic non‐cyclic electron transport (?_e) and energy‐driven photochemical events (q_P), although the impact on these parameters would also include down‐regulation processes. When compared to DR, Prel retained a higher functional state of the photosynthetic machinery, justifying reduced need for photoprotective mechanisms (non‐photochemical quenching, zeaxanthin, lutein, β‐carotene) and maintenance of the balance between energy capture and dissipative pigments. The highest increases in fructose, glucose, arabinose and sorbitol in Prel might be related to tolerance to a lower oxidative state. All cultivars had reduced A_max due to daytime stomatal closure in well‐watered conditions. Under moderate drought, Prel had highest tolerance, higher leaf hydration and maintenance of important photochemical use of energy. However, water shortage caused appreciable non‐stomatal limitations to photosynthesis linked to regulation/imbalance at the metabolic level (and growth) in all cultivars. This included damage, as reflected in decreased potential photosystem functioning, pointing to higher sensitivity of photosynthesis to drought than is commonly assumed.