Modeling and simulation of the initial phases of chlorophyll fluorescence from Photosystem II

A simple kinetic model structure for chlorophyll fluorescence (ChlF) from Photosystem II (PSII) offers practical usefulness in quantitative analysis and extraction of information from measured ChlF. In this work, the major PSII phototransduction kinetics was represented with only five state variables. Parameters were estimated through a least-squares algorithm. The developed model structure could produce the well-known OJIP pattern and fit measured ChlF. Influences of PQ pool size, active Q_B sites, and Q_A reduction rate on ChlF emission were simulated and discussed in light of the existing literature.     

Do all chlorophyll fluorescence emission wavelengths capture the spring recovery of photosynthesis in boreal evergreen foliage?

Chlorophyll a fluorescence (ChlF) is closely related to photosynthesis and can be measured remotely using multiple spectral features as solar‐induced fluorescence (SIF). In boreal regions, SIF shows particular promise as an indicator of photosynthesis, in part because of the limited variation of seasonal light absorption in these ecosystems. Seasonal spectral changes in ChlF could yield new information on processes such as sustained nonphotochemical quenching (NPQ_S) but also disrupt the relationship between SIF and photosynthesis. We followed ChlF and functional and biochemical properties of Pinus sylvestris needles during the photosynthetic spring recovery period to answer the following: (a) How ChlF spectra change over seasonal timescales? (b) How pigments, NPQ_S, and total photosynthetically active radiation (PAR) absorption drive changes of ChlF spectra? (c) Do all ChlF wavelengths track photosynthetic seasonality? We found seasonal ChlF variation in the red and far‐red wavelengths, which was strongly correlated with NPQ_S, carotenoid content, and photosynthesis (enhanced in the red), but not with PAR absorption. Furthermore, a rapid decrease in red/far‐red ChlF ratio occurred in response to a cold spell, potentially relating to the structural reorganization of the photosystems. We conclude that all current SIF retrieval features can track seasonal photosynthetic dynamics in boreal evergreens, but the full SIF spectra provides additional insight.     

Long‐term measurements of chlorophyll a fluorescence using the JIP‐test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland

Several experiments have highlighted the complexity of stress interactions involved in plant response. The impact in field conditions of combined environmental constraints on the mechanisms involved in plant photosynthetic response, however, remains understudied. In a long‐term field study performed in a managed grassland, we investigated the photosynthetic apparatus response of the perennial ryegrass (Lolium perenne L.) to environmental constraints and its ability to recover and acclimatize. Frequent field measurements of chlorophyll a fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a population of L. perenne. Strong midday declines in the maximum quantum yield of primary photochemistry (F_VF_M) were observed in summer, when a combination of heat and high light intensity increased photosynthetic inhibition. During this period, increase in photosystem I (PSI) activity efficiency was also recorded, suggesting an increase in the photochemical pathway for de‐excitation in summer. Strong climatic events (e.g. heat waves) were shown to reduce electron transport between photosystem II (PSII) and PSI. This reduction might have preserved the PSI from photo‐oxidation. Periods of low soil moisture and high levels of sun irradiance increased PSII sensitivity to heat stress, suggesting increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of photosynthetic functionality in summer, the L. perenne population showed increased PSII tolerance to environmental stresses in August. This might have been a response to earlier environmental constraints. It could also be linked to the selection and/or emergence of well‐adapted individuals.     

Optically detected magnetic resonance and mutational analysis reveal significant differences in the photochemistry and structure of chlorophyll f synthase and photosystem II

In cyanobacteria that undergo far red light photoacclimation (FaRLiP), chlorophyll (Chl) f is produced by the ChlF synthase enzyme, probably by photo-oxidation of Chl a. The enzyme forms homodimeric complexes and the primary amino acid sequence of ChlF shows a high degree of homology with the D1 subunit of photosystem II (PSII). However, few details of the photochemistry of ChlF are known. The results of a mutational analysis and optically detected magnetic resonance (ODMR) data from ChlF are presented. Both sets of data show that there are significant differences in the photochemistry of ChlF and PSII. Mutation of residues that would disrupt the donor side primary electron transfer pathway in PSII do not inhibit the production of Chl f, while alteration of the putative Chl_Z, P680 and Q_A binding sites rendered ChlF non-functional. Together with previously published transient EPR and flash photolysis data, the ODMR data show that in untreated ChlF samples, the triplet state of P680 formed by intersystem crossing is the primary species generated by light excitation. This is in contrast to PSII, in which ³P680 is only formed by charge recombination when the quinone acceptors are removed or chemically reduced. The triplet states of a carotenoid (³Car) and a small amount of ³Chl f are also observed by ODMR. The polarization pattern of ³Car is consistent with its formation by triplet energy transfer from Chl_Z if the carotenoid molecule is rotated by 15° about its long axis compared to the orientation in PSII. It is proposed that the singlet oxygen formed by the interaction between molecular oxygen and ³P680 might be involved in the oxidation of Chl a to Chl f.     

Growth,fluorescence, photosynthetic O<Subscript>2</Subscript> production and pigment content of salt adapted cultures of <Emphasis Type="Italic">Arthrospira</Emphasis> (<Emphasis Type="Italic">Spirulina</Emphasis>) <Emphasis Type="Italic">platensis</Emphasis>

The effect of salt concentration (NaCl) on growth, fluorescence, photosynthetic activities and pigment content of the cyanobacterium Arthrospira platensis has been investigated over 15 days. It has been observed that high NaCl concentration induces an increase of the growth, photosynthetic efficiency (α), phycobilin/chlorophyll ratio and a slight decrease of dark respiration and compensation points. Moreover, high NaCl concentration enhances photosystem II (PSII) activity compared to photosystem I (PSI). Results show that the phycobilin-PSII energy transfer compared to the chlorophyll-PSII (F_695,600/F_695,440) increases. However, data obtained about the maximal efficiency of PSII photochemistry are controversial. Indeed, the Fv/Fm ratio decreases in salt adapted cultures, while at the same time the trapping flux per PSII reaction center (TR₀/RC) and the probability of electron transport beyond Q_A (₀) remain unchanged at the level of the donor and the acceptor sites of PSII. This effect can be attributed to the interference of phycobilin fluorescence with Chl a when performing polyphasic transient measurements.     

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

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

Changes in photosynthetic capacity and polypeptide patterns during natural senescence and rejuvenation of <Emphasis Type="Italic">Cucurbita pepo</Emphasis> L. (zucchini) cotyledons

Natural senescence of Cucurbita pepo (zucchini) cotyledons was accompanied by a gradual degradation of reserve proteins (globulins) and an intensive decrease in the content of both large subunit (LSU) and small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The net photosynthetic rate, the primary photochemical activity of PSII, estimated by the variable fluorescence (F_v)/maximal fluorescence (F_m) ratio (F_v/F_m) and the actual quantum yield of PSII electron transport in the light-adapted state (ΦPSII) also progressively decreased during natural senescence. In contrast, the fraction of the absorbed light energy, which is not used for photochemistry (LNU) increased with progression of senescence. The decline in the photosynthetic rate started earlier in ontogenesis compared with the down-regulation of the functional activity of PSII, thus suggesting the existence of protective mechanisms which maintain higher efficiency of the photochemical electron transport reactions of photosynthesis compared with the dark reactions of the Calvin cycle during earlier stages of natural senescence. Decapitation of the epicotyl above the senescing cotyledons resulted in full recovery of the polypeptide profile in the rejuvenated cotyledons. In addition, the photosynthetic rate increased reaching values that exceeded those measured in juvenile cotyledons. The photochemical efficiency of PSII also gradually recovered, although it did not reach the maximum values measured in the presenescent cotyledons.     

Modeling chlorophyll a fluorescence transient: Relation to photosynthesis

To honor Academician Alexander Abramovitch Krasnovsky, we present here an educational review on the relation of chlorophyll a fluorescence transient to various processes in photosynthesis. The initial event in oxygenic photosynthesis is light absorption by chlorophylls (Chls), carotenoids, and, in some cases, phycobilins; these pigments form the antenna. Most of the energy is transferred to reaction centers where it is used for charge separation. The small part of energy that is not used in photochemistry is dissipated as heat or re-emitted as fluorescence. When a photosynthetic sample is transferred from dark to light, Chl a fluorescence (ChlF) intensity shows characteristic changes in time called fluorescence transient, the OJIPSMT transient, where O (the origin) is for the first measured minimum fluorescence level; J and I for intermediate inflections; P for peak; S for semi-steady state level; M for maximum; and T for terminal steady state level. This transient is a real signature of photosynthesis, since diverse events can be related to it, such as: changes in redox states of components of the linear electron transport flow, involvement of alternative electron routes, the build-up of a transmembrane pH gradient and membrane potential, activation of different nonphotochemical quenching processes, activation of the Calvin-Benson cycle, and other processes. In this review, we present our views on how different segments of the OJIPSMT transient are influenced by various photosynthetic processes, and discuss a number of studies involving mathematical modeling and simulation of the ChlF transient. A special emphasis is given to the slower PSMT phase, for which many studies have been recently published, but they are less known than on the faster OJIP phase.     

Chlorophyll a fluorescence during flag leaf senescence of field-grown winter wheat plants under drought conditions

Improving wheat grain yield plays a significant role in ensuring global food security. Wheat production could be increased by the genetic improvement of wheat genotypes where delayed senescence with enhanced post-anthesis capacity and staygreen traits could have an important role. In this study, chlorophyll a fluorescence (ChlF) rise kinetics from the early until late senescence of flag leaves, grain yield and other agro-morphological characteristics were compared for three winter wheat advanced lines (Osk.4.312/10-18, Osk.4.330/6-18 and Osk.4.354/12-18) under natural drought conditions. The differences between lines were observed when considering the heading date which was 1 and 4 days earlier for the line Osk.4.354/12-18, than lines Osk.4.312/10-18 and Osk.4.330/6-18, respectively. Furthermore, line Osk.4.354/12-18 had the highest test weight (kg hl⁻¹), while line Osk.4.330/6-18 showed a tendency of decreased grain yield, compared to the other two lines. Analysis of ChlF transients and several JIP-test parameters indicated that all three lines had a generally similar course of changes in the photosynthetic performance of flag leaves during senescence under drought conditions. However, at the point when a decrease in photosynthetic performance was initiated, it was slightly less intensive in line Osk.4.354/12-18 accompanied by longer preservation of functionality and connectivity of PSII units, than in the other two lines, which contributed to its better agronomical performance. These results indicated that even delicate variations in the functioning of the photosynthetic apparatus of the flag leaf during grain filling were agronomically important, especially when plants were exposed to drought stress, and could be used to differentiate otherwise similar wheat genotypes. Even small genotype-specific differences in the photosynthetic performance of senescing flag leaves, along with earlier heading dates, could assist in the selection of genotypes with a better ability to cope with unfavourable environmental conditions.     

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

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

Photosynthetic performance of regenerated protoplasts from disintegrated cells of <Emphasis Type="Italic">Bryopsis hypnoides</Emphasis> (Chlorophyta)

The photosynthetic performances of regenerated protoplasts of Bryopsis hypnoides, which were incubated in seawater for 1, 6, 12, and 24 h, were studied using chlorophyll (Chl) fluorescence and oxygen measurements. Results showed that for the regenerated protoplasts, the pigment content, the ratios of photosynthetic rate to respiration rate, the maximal photosystem II (PSII) quantum yield (F_v/F_m), and the effective PSII quantum yield (Φ_PSII) decreased gradually along with the regeneration progress, indicated that during 24 h of regeneration there was a remarkable reduction in PSII activity of those newly formed protoplasts. We assumed that during the cultivation progress the regenerated protoplasts had different photosynthetic vigor, with only some of them able to germinate and develop into mature thalli. The above results only reflected the photosynthetic features of the regenerated protoplasts at each time point as a whole, rather than the actual photosynthetic activity of individual aggregations. Further investigation suggested a relationship between the size of regenerated protoplasts and their viability. The results showed that the middle-sized group (diameter 20–60 μm) retained the largest number of protoplasts for 24 h of growth. The changes in F_v/F_m and Φ_PSII of the four groups of differently sized protoplasts (i.e. < 20, 20–60, 60–100, and > 100 μm) revealed that the protoplasts 20–60 μm in diameter had the highest potential activity of the photosynthetic light energy absorption and conversion for several hours.     

The influence of antimycin A on pigment composition and functional activity of photosynthetic apparatus in <Emphasis Type="Italic">Triticum aestivum</Emphasis> L. under high temperature

The purpose of the current investigation was to evaluate the influence of antimycin A (AA) as an activator of the alternative respiratory pathway (AP) on photosynthetic pigment composition and functional activity of the photosynthetic apparatus of wheat seedlings (Triticum aestivum L.) under exposure to high temperature as well as their acclimation. Our results indicated that a significant decrease (44–74%) of photosynthetic pigment contents was caused by a long-term exposure to high temperature (42°C), while the short-term exposure resulted in 20–46% decline. However, a combined effect of AA and long-term high temperature reduced the total pigment contents by 28–41%. Our results demonstrated that the reduction of the chlorophyll a/b ratio was less significant under the combined effect of AA and high temperature than that under the stressful condition without AA. We observed that short-term and long-term high temperature modified PSII functionality of the first leaves in wheat seedlings, which was manifested by the low maximal quantum yield of PSII photochemistry, maximum fluorescence yield in the dark-adapted state, and by high minimum fluorescence yield in the dark-adapted state. The quantum yield of PSII photochemistry decreased rapidly by 16–24% under the combination of AA and high temperature. Overall, these results suggest that the activation of the alternative pathway, induced by AA, contributed to the stabilization of the photosynthetic apparatus in wheat seedlings under high temperature.     

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

叶绿素荧光主动与被动联合观测应用前景

叶绿素荧光是研究植物光合生理机制、量化植被光合作用时空格局以及准确理解气候变化背景下陆地生态系统生产力的关键。然而, 目前对于叶绿素荧光主动与被动联合观测的研究还较少。该文对比了叶绿素荧光主动观测与被动观测的优缺点, 展示了叶片尺度和冠层尺度主动与被动联合观测的仪器设备组成, 探讨了主动与被动联合观测在探索叶绿体尺度-叶片尺度-冠层尺度能量在光合、荧光以及热耗散中的分配, 阐明叶绿素荧光与总初级生产力的关联机制, 验证星基日光诱导叶绿素荧光, 解译叶绿素荧光光谱形状4个方面的应用前景。综上, 叶绿素荧光的主动与被动联合观测对于揭示各尺度上荧光与光合作用之间的关联机制, 改善全球尺度植被生产力模型至关重要。     

Responses of photosynthetic apparatus of the halotolerant microalga <Emphasis Type="Italic">Dunalliella maritima</Emphasis> to hyperosmotic salt shock

Chlorophyll fluorescence induction curves were used as a means to assess the functional condition of the photosynthetic apparatus in cells of the halotolerant green microalga Dunaliella maritima (Massjuk) (division Chlorophyta) exposed to hyperosmotic salt shock of various intensities. The shock was caused by the transfer of algal cells grown in the medium with 0.5 M NaCl to the media with elevated NaCl concentrations (1.0, 1.5, and 2.0 M). Parameters of chlorophyll fluorescence (F ₀, F _m, F ₀′, F _t′) were measured by means of a specialized pulse-amplitude-modulation fluorometer PAM 2100. In addition, the rate of photosynthetic oxygen evolution as well as the intracellular Na⁺ and glycerol content (the main osmolyte in this microalga) were determined. The hyperosmotic salt shock was found to elevate the intracellular Na+ content and reduce the functional activity of PSII in D. maritima. The suppression of PSII activity was evident from the decrease in the maximal quantum yield of photochemical energy conversion in PSII, the decreased rate of linear electron transport, the increased reduction of the primary acceptor Q_A, and the suppression of photosynthetic O₂ evolution. The functional activity of PSII recovered gradually along with restoration of osmotic and ionic balance in algal cells. It is proposed that PSI ensures energy supply during cell responses of D. maritima to hyperosmotic salt shock.     

<Emphasis Type="Italic">GmFtsH9</Emphasis> expression correlates with in vivo photosystem II function: chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient analysis and eQTL mapping in soybean

Filamentation temperature-sensitive H (FtsH) is an ATP-dependent zinc metalloprotease involved in diverse biological functions. There are 12 FtsH proteins in Arabidopsis, among which AtFtsH2 plays an important role in regulating the turnover of photosystem II (PSII) reaction center D1 protein and the development of the photosynthetic apparatus. Here, we have identified 11 FtsH genes in the soybean genome by a bioinformatics approach. These soybean FtsH genes corresponded to seven Arabidopsis FtsH genes, suggesting that the main characteristics of soybean FtsH genes were formed before the evolutionary split of soybean and Arabidopsis. Phylogenetic analyses allowed us to clone a soybean AtFtsH2-like gene designated as GmFtsH9. The predicted protein of GmFtsH9 consists of 690 amino acids and contains three typical FtsH proteins conserved domains. The expression level of GmFtsH9 was determined in a soybean recombinant inbred line population under a pot experiment conducted for measuring chlorophyll a fluorescence transient parameters, photosynthetic CO₂ fixation rate (P _N), and seed yield. Expression quantitative trait loci (eQTL) mapping revealed two trans-acting eQTLs for GmFtsH9. The significant correlation of gene expression level with chlorophyll a fluorescence transient parameters and the presence of overlapping eQTL (QTL) between gene expression level and chlorophyll a fluorescence transient parameters indicated that GmFtsH9 could be involved in regulating PSII function. These results further lead to the understanding of the mechanism underlying FtsH gene expression, and contribute to the development of marker-assisted selection breeding programs for modulating soybean FtsH gene expression.     

Direct impact of silicate on the photosynthetic performance of the diatom Thalassiosira weissflogii assessed by on- and off-line PAM fluorescence measurements

Pulse-amplitude-modulation (PAM) fluorometry was used to investigate the effects of varying the silicate concentration on different fluorescence characteristics of batch and chemostat cultures of the diatom Thalassiosira weissflogii. The fluorescence signals, measured both on- and off-line, were used to calculate the actual (P) and potential (P0) photochemical efficiencies of the reaction centres of photosystem II (PSII). Also the relative electron transport rate of the reaction centres of PSII (Je) was calculated. Fluorescence decreased in silicate-limited cells and increased rapidly after silicate addition. The decrease was caused by strong non-photochemical quenching (qN) in silicate-limited cells. Continuous recording (on-line) of the minimum and maximum fluorescence provided data with high temporal resolution, revealing that the first recovery of silicate-limited cells occurred 20 min after the addition of silicate. Based on these observations, we assume a strong influence of silicate metabolism on the photosynthetic efficiency of the reaction centres of PSII assessed by PAM fluorescence. This implies that silicate, as well as other nutrients, has to be considered as a possible cause for variable photosynthetic efficiency of diatoms.      

Bicarbonate protects the water-oxidizing complex of photosystem II against thermoinactivation in intact <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> cells

Currently available data about bicarbonate (BC) action on the Mn-containing water-oxidizing complex (WOC) of the photosystem II (PSII) were obtained almost solely in vitro, e.g. on subchloroplast membrane fragments enriched with PSII. To investigate the in vivo BC effect on the PSII donor side, we used the method of dark thermoinactivation of intact Chlamydomonas reinhardtii cells. Photosynthetic activity of PSII was measured as photoinduced changes in the PSII chlorophyll fluorescence yield and as the rate of photosynthetic oxygen evolution. To exclude a “direct” effect of the absence of BC on the PSII activity, before measurements of the photosynthetic activity, the concentration of BC in all samples was equalized by addition of NaHCO₃ to each of them (except for those that contained 5 mM of NaHCO₃ during thermoinactivation) to reach the final concentration of 5 mM. This allowed registering only so-called “irreversible” (i.e., not reversible by subsequent addition of BC) effect of the absence of BC during thermoinactivation. It was shown that, if 5 mM NaHCO₃ was added to the medium before thermoinactivation, the rate of inactivation of the PSII donor side was lower than in BC-depleted medium 1.5-to 2-fold. The obtained results are interpreted as an indication that BC protects the donor side of PSII against thermoinactivation in vivo, in intact C. reinhardtii cells. This proves the correctness of the earlier proposition that BC is an integral constituent of the Mn-containing water-oxidizing complex of PSII. Published in Russian in Fiziologiya Rastenii, 2007, Vol. 54, No. 3, pp. 342–349. The article was translated by the authors.     

Effects of salt stress on photosynthesis,PSII photochemistry and thermal energy dissipation in leaves of two corn (<Emphasis Type="Italic">Zea mays</Emphasis> L.) varieties

The effect of four different NaCl concentrations (from 0 to 102 mM NaCl) on seedlings leaves of two corn (Zea mays L.) varieties (Aristo and Arper) was investigated through chlorophyll (Chl) a fluorescence parameters, photosynthesis, stomatal conductance, photosynthetic pigments concentration, tissue hydration and ionic accumulation. Salinity treatments showed a decrease in maximal efficiency of PSII photochemistry (F_v/F_m) in dark-adapted leaves. Moreover, the actual PSII efficiency (ϕ_PSII), photochemical quenching coefficient (q_p), proportion of PSII centers effectively reoxidized, and the fraction of light used in PSII photochemistry (%P) were also dropped with increasing salinity in light-adapted leaves. Reductions in these parameters were greater in Aristo than in Arper. The tissue hydration decreased in salt-treated leaves as did the photosynthesis, stomatal conductance (g _s) and photosynthetic pigments concentration essentially at 68 and 102 mM NaCl. In both varieties the reduction of photosynthesis was mainly due to stomatal closure and partially to PSII photoinhibition. The differences between the two varieties indicate that Aristo was more susceptible to salt-stress damage than Arper which revealed a moderate regulation of the leaf ionic accumulation.     

土壤水分对湿地松幼苗光合特征的影响

通过设置常规供水(CK)、轻度干旱(T1)、水饱和(T2)、水淹(T3)4个处理组,研究湿地松当年实生幼苗在不同土壤水分条件下的光合生理响应及叶绿素荧光特性。结果表明:不同水分处理对湿地松幼苗的叶片气体交换参数、水分利用效率(WUE)、光合色素、叶绿素荧光参数等指标有不同的影响;其中,T3的光合色素含量最低;T2、T3组的湿地松幼苗表现出较低的净光合速率(Pn)、电子传递速率(ETR)和PSⅡ光化学的量子效应(ΦPSⅡ),但与其他耐水淹植物相比,T3条件下的湿地松幼苗仍具有较高的Pn,说明湿地松幼苗具有较强的耐水淹能力;在T1条件下,湿地松幼苗具有较高的WUE和较低的蒸腾速率(Tr)、气孔导度(Gs)以对抗干旱的逆境,其Pn、PSⅡ最大光化学效率(Fv/Fm)、ETR和ΦPSII均有所下降,但仍能维持在相对正常的水平。研究证明,湿地松幼苗具有一定的耐淹耐旱特性,可运用于三峡库区消落带的植被重建。