Genome-wide identification and expression profiling of photosystem II(PsbX)gene family in upland cotton(Gossypium hirsutum L.)

Background Photosystem II(PSII)constitutes an intricate assembly of protein pigments,featuring extrinsic and intrinsic polypeptides within the photosynthetic membrane.The low-molecular-weight transmembrane protein PsbX has been identified in PSII,which is associated with the oxygen-evolving complex.The expression of PsbX gene protein is regulated by light.PsbX’s central role involves the regulation of PSII,facilitating the binding of quinone molecules to the Qb(PsbA)site,and it additionally plays a crucial role in optimizing the efficiency of photosynthesis.Despite these insights,a comprehensive understanding of the PsbX gene’s functions has remained elusive.Results In this study,we identified ten PsbX genes in Gossypium hirsutum L.The phylogenetic analysis results showed that 40 genes from nine species were classified into one clade.The resulting sequence logos exhibited substantial conservation across the N and C terminals at multiple sites among all Gossypium species.Furthermore,the ortholo-gous/paralogous,Ka/Ks ratio revealed that cotton PsbX genes subjected to positive as well as purifying selection pressure might lead to limited divergence,which resulted in the whole genome and segmental duplication.The expression patterns of GhPsbX genes exhibited variations across specific tissues,as indicated by the analysis.Moreover,the expression of GhPsbX genes could potentially be regulated in response to salt,intense light,and drought stresses.Therefore,GhPsbX genes may play a significant role in the modulation of photosynthesis under adverse abiotic conditions.Conclusion We examined the structure and function of PsbX gene family very first by using comparative genom-ics and systems biology approaches in cotton.It seems that PsbX gene family plays a vital role during the growth and development of cotton under stress conditions.Collectively,the results of this study provide basic information to unveil the molecular and physiological function of PsbX genes of cotton plants.

Effects of drought treatment on photosystemⅡactivity in the ephemeral plant Erodium oxyrhinchum

Drought is a critical limiting factor affecting the growth and development of plants in arid and semi-arid areas.Photosynthesis,one of the most important physiological processes of plants,can be significantly inhibited by drought.PhotosystemⅡ(PSⅡ)is considered the main attack target when photosynthesis is affected by drought.To clarify how PSⅡcomponents of the ephemeral plant Erodium oxyrhinchum(grown in the Gurbantunggut Desert,China)respond to drought treatment,we evaluated the functional activity of PSII by determining chlorophyll fluorescence and gas exchange parameters under different drought treatment levels(control(400 mL),moderate drought(200 mL),and severe drought(100 m L)).Under moderate drought treatment,significant decreases were found in net photosynthetic rate(Pn),effective quantum yield of PSII(Y(Ⅱ)),relative electron transfer rate of PSII(rETR(Ⅱ)),oxygen-releasing complex,probability of an absorbed exciton moving an electron into the electron transport chain beyond primary quinone receptor Q_(A)-(Φ(E_(o))),probability of a trapped exciton moving an electron into the electron transport chain beyond primary quinone receptor Q_(A)-(ψ(E_(o))),and performance index of PSⅡ(PI_(abs)).Compared to control treatment,marked increases were observed in water use efficiency(WUE),relative variable fluorescence at the J step(V_(J)),initial fluorescence(F_(o)),and dissipated energy per active reaction center(DI_(o)/RC)under moderate drought treatment,but there were no substantial changes in semi-saturated light intensity(I_(K)),active reaction centers per cross-section(RC/CS),and total performance index of PSII and PSI(PI_(total),where PSI is the photosystemⅠ).The changes of the above parameters under severe drought treatment were more significant than those under moderate drought treatment.In addition,severe drought treatment significantly increased the absorbed energy per active reaction center(ABS/RC)and trapping energy per active reaction center(TR_(o)/RC)but decreased the energy transmission connectivity of PSⅡcomponents,RC/CS,and PI_(total),compared to moderate drought and control treatments.Principle component analysis(PCA)revealed similar information according to the grouping of parameters.Moderate drought treatment was obviously characterized by RC/CS parameter,and the values of F_(o),V_(J),ABS/RC,DI_(o)/RC,and TR_(o)/RC showed specific reactions to severe drought treatment.These results demonstrated that moderate drought treatment reduced the photochemical activity of PSII to a certain extent but E.oxyrhinchum still showed strong adaptation against drought treatment,while severe drought treatment seriously damaged the structure of PSⅡ.The results of this study are useful for further understanding the adaptations of ephemeral plants to different water conditions and can provide a reference for the selection of relevant parameters for photosynthesis measurements of large samples in the field.

Effect of high light and desiccation on photosystem Ⅱ in the seedlings and mature plants of tropical seagrass Enhalus acoroides during low tide

During low tide,the intertidal seagrass Enhalus acoroides is often exposed to high light and desiccation,which can seriously threaten its survival,at least partly by inhibiting photosystem Ⅱ(PSⅡ)activity.The response of leaves of E.acoroides to high light and desiccation was compared for seedlings and mature plants.Results show that the resistance of seedling and mature leaves to high light was quite similar,but to desiccation was very different.Seedling leaves were more sensitive to desiccation than the mature plant leaves,but had better water retention.The damage of desiccation to seedling leaves was mainly caused by dehydration,whereas that to mature plant leaves was caused by hypersaline toxicity.The recovery rate of PSⅡ of seedling leaves was significantly slower than that of the mature plants after the stresses disappeared,which may at least partly contribute to seedling mortality in the wild.In addition,compared to high light,desiccation seriously inhibited the recovery rate of PSⅡ activities even if the leaves became fully rehydrated to their normal relative water content(RWC)in the following re-immersion.Desiccation inhibited the recovery rate of RC/CS_(M)(reaction center per cross section(at t=t_(Fm)))to decrease the production of assimilatory power,which maybe the cause of the slower PSⅡ recovery in desiccation treatments.This study demonstrates that desiccation particularly coupling with high light have a very negative ef fect on the PSⅡ of E.acoroides during low tide and the sensitivity of seedlings and mature plants to desiccation is significantly different,which have important reference significance to choose an appropriate transplanting depth where seedlings and mature plants of E.acoroides not only receive sufficient light for growth,but also that minimize desiccation stress during low tide.

Blue light is more essential than red light for maintaining the activities of photosystem Ⅱ and Ⅰ and photosynthetic electron transport capacity in cucumber leaves

Blue and red lights differently regulate leaf photosynthesis. Previous studies indicated that plants under blue light generally exhibit better photosynthetic characteristics than those under red light. However, the regulation mechanism of related photosynthesis characteristics remains largely unclear. Here, four light qualities treatments （300 μmol m-2 s-1） including white fluorescent light （FL）, blue monochromatic light （B, 440 nm）, red monochromatic light （R, 660 nm）, and a combination of red and blue light （RB, R：B=8：1） were carried out to investigate their effects on the activity of photosystem II （PSII） and photosystem I （PSI）, and photosynthetic electron transport capacity in the leaves of cucumber （Cucumis sativus L.） seedlings. The results showed that compared to the FL treatment, the R treatment significantly limited electron transport rate in PSII （ETR11） and in PSI （ETR1） by 79.4 and 66.3%, respectively, increased non-light induced non-photochemical quenching in PSII （q^No） and limitation of donor side in PSI （φND） and reduced most JIP-test parameters, suggesting that the R treatment induced suboptimal activity of photosystems and inhibited electron transport from PSII donor side up to PSI. However, these suppressions were effectively alleviated by blue light addition （RB）. Compared with the R treatment, the RB treatment significantly increased ETR, and ETR1 by 176.9 and 127.0%, respectively, promoted photosystems activity and enhanced linear electron transport by elevating electron transport from QA to PSI. The B treatment plants exhibited normal photosystems activity and photosynthetic electron transport capacity similar to that of the FL treatment. It was concluded that blue light is more essential than red light for normal photosynthesis by mediating photosystems activity and photosynthetic electron transport capacity.

Effects of CeCl_3 on Energy Transfer and Oxygen Evolution in Spinach Photosystem Ⅱ

Due to 4f electron characteristics and alternation valence, cerium involved in an oxidation-reduction reaction in plant, closely relating to photosynthesis. Our studies proved that cerium could promote photosynthesis and greatly improve spinach growth. However, the mechanism of promoting energy transfer and conversion by cerium remains unclear. Here we reported that the effects of Ce^3＋ on energy transfer and oxygen evolution in photosystem Ⅱ （PS Ⅱ ） isolated from spinach, which was related to 4f electron characteristics and alternation valence in Ce molecule. The methods of absorption spectrum, fluorescence spectrum were used in the research. Results showed that Ce^3＋ treatment at low concentration could suitably change PS Ⅱ mieroenvironment, increase the absorbance of visible light, improve the energy transfer among amino acids within PS Ⅱ protein-pigment complex, and accelerate energy transport from tyrosine residue to chlorophyll a. In summary, the photochemical activity of PS Ⅱ （fluorescence quantum yield） and its oxygen evolving rate were enhanced by Ce^3＋.

Effects of short-term low temperatures on photosystem Ⅱ function of samara and leaf of Siberian maple (Acer ginnala) and subsequent recovery

Samara is the reproductive organ (seed) for many tree species in arid land in northwestern China. It is ecologically important in population development due to its dispersal function. However, information on its photosynthesis and effect of environmental stresses on its photosynthesis is still very limited. In the present study, responses of photosystem II (PSII) activity in samara and leaf of Siberian maple to short-term chilling/freezing and subsequent recovery potential were comparatively investigated by using polyphasic fluorescence test. The samara had more efficient photosynthesis (Fv/Fm and PIABS) and more efficient electron transport (φEo) but lower energy dis- sipation (DIo/RC) than leaf. Generally, the PSII performance and the electron transport for both samara and leaf were inhibited under low temperature stress, accompanied by an increase of energy dissipation in PSII reaction centers (RCs). PSII of both samara and leaf was not markedly affected by chilling and could acclimate to chilling stress. Short-term freezing could completely inhibit PSII activity in both samara and leaf, indicated by the drop of values of Fv/Fm, PIABS, φEo to zero. PSII functional parameters of short-term dark frozen samara could be largely recovered whereas those of frozen leaf could not be recovered. The higher tolerance of samara to short-term low temperature stress than leaf is of great ecological significance for seed development, population establishment of Siberian maple.

Effect of Nano-anatase TiO_2 on Spectral Characterization of PhotosystemⅡParticles from Spinach

The photosystem Ⅱ(PSⅡ) particles were purified by means of nano-anatase TiO_2 treatment of spinach and studied by spectroscopy. The results show that the electron transport and the oxygen-evolving rate of PSⅡ are accelerated after it has been treated with nano-anatase TiO_2; the UV-Vis absorption spectrum of PSⅡ particles is increased; the red shift of fluorescence emission peak of PSⅡ is 2 nm; the peak intensity is decreased; the PSⅡ signal Ⅱs of low temperature electron paramagnetic resonanace(EPR) spectrum is intensified under light, and the PSⅡ circular dichroism(CD) spectrum is similar to that of control. It is suggested that nano-anatase TiO_2 might bind to the PSⅡ reaction center complex and intensify the function of the PSⅡ electron donor, however, nano-anatase TiO_2 treatment does not change the configuration of the PSⅡ reaction center complex.

EXCITATION ENERGY TRANSFER IN VITRO BETWEEN PHYCOBILIPROTEINS AND THYLAKOID PHOTOSYSTEM Ⅱ OF HIGHER PLANTS

The excitation energy transfer from phycobiliproteins to thylakoid PSII of higher plants was investigated. When incubated with spinach thylakoids, phycobiliproteins isolated from red and blue- green algae transferred light energy absorbed to spinach PSII. The efficiency of energy transfer was dependent on the kind of phycobiliproteins used. If spinach thylakoids were replaced by the thylakoids of Brassica chinensis, R phycoerythin or C- phycocyanin did not transfer their excitation energy to PSII of Brassica chinensis unless allophycocyanin was present.

Reduced salinity interacts with ultraviolet radiation to alter photosystem Ⅱ function in diatom Skeletonema costatum

To investigate the effect of reduced salinity on diatoms’ capacity to cope with changing ultraviolet radiation(U VR) and photosynthetically active radiation(PAR),Skeletonema costatum was grown in a range of salinity(15,25,and 35).The photo system Ⅱ(PSⅡ) function was analyzed by increasing PAR and UVR to mimic a mixing event in turbulent waters.The re sults show that high UVR exposure significantly reduced PSII activity,especially in cells grown at low salinity.UVR,but not salinity,stimulated the ’removal’ rate of PSII protein PsbA.Salinity alone,in the range of 15 to 35,did not regulate PSⅡ acceptor region;however,the low salinity+UVR treatment decreased the energy flux for electron transport per PSⅡ reaction center in S.costatum.It showed that low salinity exacerbated the damaging effect of UVR on PSⅡ function in S.costatum by suppressing Psb A protein synthe sis and modifying the photochemistry of PSⅡ.Although higher catalase(CAT) activity and NPQs were induced,they were unable to prevent the combined damage effect of low salinity+UVR.Our findings indicate that reduced salinity and increased UVR potentially affect the abundance and distribution of S.costatum with the escalation of climate disturbances.

Photoinduced hydrogen evolution in an artificial system containing photosystem I, hydrogenase, methy1 viologen and mercaptoacetic acid

Hydrogen evolution was detected in an artificial system composed of light-harvesting unit of purified photosystem I, catalyst of hydrogenase, methyl viologen and electron donor under radiation. Absorption spectral features confirmed that electron transfer from electron donors to proton was via a photoinduced reductive process of methyl viologen.

Effects of Severe Drought and Glyphosate Stress on Physiological Characteristics and Protein Expression of Photosystem Ⅱ in Genentically Modified Soybean

[Objective] The paper was to investigate effects of glyphosate stress on physiological characteristics and protein expression of photosystem Ⅱ(PSⅡ) in genentically modified soybean GTS 40-3-2 seedlings under severe drought condition. [Method] A pot experiment was carried out in growth chamber to determine the response of genetically modified soybean treated by severe drought stress and different concentrations of glyphosate at the third compound leaf stage. [Result] Severe drought treatment increased the electrolyte leakage(EL), superoxide dismutase(SOD) and peroxidase(POD) activities, and decreased the relative water content(RWC), chlorophyll content, and catalase(CAT) activity. The EL, SOD and POD activities were significantly increased in severe drought and glyphosate treatments, which were related to glyphosate concentrations. The chlorophyll content decreased, which was also related to glyphosate concentrations. But the BWC and CAT activity were not affected by glyphosate concentrations. Western blot displayed that PSⅡ protein Lhcb2 was not affected by stress conditions and stably expressed. D1, D2 and Lhcb4 protein level decreased, and there was no significant change in Lhcb1 expression under severe drought stress. The protein levels of D1, D2, Lhcb1 and Lhcb4 decreased with the increase of glyphosate concentrations under severe drought and glyphosate stress. When the glyphosate concentrations were 0.92 and 1.84 kg·ai/hm^2, the protein levels of D1, D2 and Lhcb4 were slightly higher than those in severe drought stress. When the glyphosate concentrations were 3.68 and 7.36 kg·ai/hm^2, the protein level of D1, D2, Lhcb1 and Lhcb4 decreased sharply. [Conclusion] This research provides a theoretical basis for production of genetically modified soybean.

Analysis of Chloroplast Ultrastructure,Photosystem Ⅱ Light Harvesting Complexes and Chlorophyll Synthesis in a Chlorophyll-Less Rice Mutant W2555

A comparative study on chloroplast ultrastructure and light harvesting complex of photosystem Ⅱ （LHC Ⅱ） was conducted between a new rice mutant （W2555） and its wild type （WT）. The chloroplasts of W2555 had less thylakoids and grana stacks compared with the wild type. There was no significant change in the composition of LHC Ⅱ polypeptide in W2555, while a decline had been noted in LHC Ⅱ content. Northern blot analysis with a specific cab gene probe showed no appreciable difference in the LHC Ⅱ mRNA level between the W2555and its wild type. The precursors of chlorophyll synthesis, 6-aminolevulinic acid （ALA） and porphobilinogen （PBG） were over accumulated in W2555, but the other precursors were all decreased. These results indicated that the decreased level of LHC Ⅱ in the mutant W2555 was attributed to the change of cab gene transcription, but a blockage in chlorophyll biosynthesis due to the formation of uroporphyrinogen Ⅲ （Urogen Ⅲ）.

Responses of Maximum Photosystem H PhotochemicalEfficiency of Phytoplankton Communities to NutrientLimitation in the Coastal Sea of Qingdao, China

Nutrient enrichment experiments with nitrogen （N） and phosphorus （P） were conducted with samples from two stationsin the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations andthe maximum photochemical efficiency of photosystem II （Fv/Fm） in the initial samples together with Fv/Fm and chlorophyll a （Chl a）responses to P addition indicated P limitation at the two stations. In early August, low P levels still limited phytoplankton growth atstation A. Fv/Fm and Chl a were the highest in the NP treatments at station B, suggesting an N/P co-limitation. In mid-September,nutrient concentrations and Fv/Fm were elevated and phytoplankton communities were healthy. Greater Fv/Fm and Chl a in the treat-ments with added P than those without the addition suggested potential P limitation at station A. Lack of Fv/Fm and Chl a responsesfollowing nutrient additions indicated N and P repletion at station B. At the end of July 2010, neither N nor P was limited at station B.Additionally, Fv/Fm coupled with 24-h-long nutrient enrichment experiments can be used to detect P limitation and N/P co-limitationto natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrationsand ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indica-tor, when a population is potentially limited. Compared with the two conventional methods, the results based on F,/F~ can also pro-vide more detailed information about physiological states of the populations.

Optimum Dark Adaptation Period for Evaluating the Maximum Quantum Efficiency of Photosystem II in Ozone-Exposed Rice Leaves

Because the transient O3 injury of leaves is lost with time, the evaluation of O3 effect on the maximum quantum efficiency of PSII (Fv/Fm) is difficult. Thus, the authors examined Fv/Fm in rice leaves exposed to different O3 concentrations (0, 0.1, and 0.3 cm3·m-3, expressed as O0, O0.1, and O0.3) under different dark adaptation periods (0, 1, 5, 10, 20, and 30 min, expressed as D0, D1, D5, D10, D20, and D30) to ascertain its optimum time span. Fv/Fm was inhibited by O3;however in the O0 and O0.1 plants, it recovered during dark adaptation. In the O0.3 plants, Fv/Fm decreased gradually with time. F0 was found to be increased by O3, and it increased further in the O0.3 plants during dark adaptation. Under a high light intensity, Fm was decreased by O3, and the O3-induced damage to Fv/Fm was therefore more pronounced. However, the sensitivity of

Different Effects of Malate on the Activities of Photosystem II in Detached Leaves of Maize and Tobacco

Malate is the first stable product after CO2 is fixed in NADP-dependent malic enzyme (NADP-ME) type of C4 plants, which transfers CO2 and the reducing equivalent from mesophyll cell (MC) to vascular bundle sheath cell (BSC) chloroplasts and affects the redox state of BSC. The aim of this experiment is to investigate the effect of exogenous malate on the activity of photosystem II (PS II) in C4 and C3 plants. The leaf discs from the 5th fully expanded leaves of maize (NADP-ME type C4 plants) and the 10th fully expanded leaves of tobacco (C3 plants) were treated with malate of 50, 100 μM and the chlorophyll fluorescence parameters were measured. Malate treatments decreased the photochemical reaction efficiency (FV/FM) in maize leaves, as a result of rising in initial fluorescence (FO) and decreasing in maximal fluorescence (FM). The number of active PS II reaction center (RC) per excited cross section (RC/CS) declined in malate-treated maize, suggesting that malate inactivated PS II RC. Malate treatments also increased Wk, representing the severity of oxygen-evolving complex (OEC) damage, and decreased the rate of photosynthetic oxygen evolution. We conclude that exogenous malate regulates the activity and structure of PS II in C4 plant maize. No significant changes in the activity of PS II were observed in malate-treated C3 plant tobacco. It is suggested that the short term malate treatment will inhibit PS II of leaves which have C4 anatomy and C4 enzymes.

Bio-inspired photosystem for green energy

Science,2016,351,74-77;Angew.Chem.Int.Ed.,2016,55,12503-12507Over billions years of evolution,biosystems have developed elaborate architectures to realize efficient substance metabolism and energy conversion.Using and/or mimicking biological process has provided us an elegant strategy for design and fabrication of biologically functional systems,such as photosystem.To realize efficient energy production,it

不同温度和胁迫时长下蓝云杉的光合响应

[目的]全球气候持续变暖将不利于蓝云杉生长,研究蓝云杉在高温胁迫下的生长、光合响应机制,为蓝云杉引种栽培提供参考。[方法]以五年生蓝云杉苗为试验材料,设定45、50、55℃3种温度,处理6 h;其中45℃处理还有12、18、和24 h不同时长,常温(25℃)作为对照。研究处理后、恢复7和28 d的生长表型、针叶含水量和叶绿素荧光参数,分析高温、胁迫时长对蓝云杉生长和光合作用的影响。[结果]45℃处理6 h蓝云杉基本可以恢复正常生长,而50℃和55℃处理的蓝云杉整株针叶都变黄,干枯掉落,伴随着相对含水量的降低,且光系统Ⅰ(PSⅠ)有效量子产量实际光化学效率(Y(Ⅰ))、PSⅠ相对电子传递速率(ETR(Ⅰ))、由供体侧限制引起的PSI处非光化学能量耗散的量子产量(Y(ND))、受体侧非光化学能量耗散产生的量子产量(Y(NA))、光系统Ⅱ(PSⅡ)有效量子产量实际光化学效率(Y(Ⅱ))、PSⅡ相对电子传递速率(ETR(Ⅱ))、PSⅡ的最大光化学效率(F_(v)/F_(m))和用于电子传递的量子产额(φE_(o))均显著低于45℃处理和对照。尽管45℃处理6、12、18、24 h后从表型上看不出蓝云杉的形态发生明显变化,但是恢复期发现12、18 h针叶继续失水、变黄,24 h处理整株针叶都变黄,基本干枯掉落,针叶相对含水量降至最低。恢复28 d时,18、24 h的荧光参数φE_(o)几乎为0,电子传递受到了抑制。[结论]胁迫时长为6 h,蓝云杉可以耐45℃高温处理,不耐50、55℃高温。45℃高温处理时长超过12 h苗木失水严重,针叶干枯,对光合特性产生了显著影响,难恢复正常生长。

Calcium contributes to photoprotection and repair of photosystem II in peanut leaves during heat and high irradiance

In this study, we investigated the effects of exogenous calcium nitrate on photoinhibition and thylakoid protein level in peanut plants under heat （40 ℃） and high irradiance （HI） （1,200 mmol/m2 per s） stress. Compared with control seedlings （cultivated in 0 mmol/L Ca（NO3）2 medium）, the maximal photochemical efficiency of photosystem II （PSII） in Ca2t‐treated plants showed a slight decrease after 5 h stress, accompanied by lower degree of PSII closure （1‐qP）, higher non‐photochemical quenching, and lower level of membrane damage. Ca2t inhibitors were used to analyze the varieties of antioxidant enzymes activity and PSII proteins. These results indicated that Ca2t could protect the subunits of PSII reaction centers from photoinhibition by reducing the generation of reactive oxygen species. In the presence of both ethyleneglycol‐bis（2‐aminoethylether）‐tetraacetic acid and ascorbic acid （AsA）, the net degradation of the damaged D1 protein was faster than that only treated with AsA. Our previous study showed that either the transcriptional or the translational level of calmodulin was obviously higher in Ca2t‐treated plants. These results suggested that, under＆amp;nbsp;heat and HI stress, the Ca2t signal transduction pathway can al eviate the photoinhibition through regulating the protein repair process besides an enhanced capacity for scavenging reactive oxygen species.

Effects of Exogenous Spermidine on PhotosystemⅡof Wheat Seedlings Under Water Stress

The effects of exogenous spermidine （Spd） on lipid peroxidation, relative plasma membrane permeability, photosystem Ⅱ （PSII） gene expression and PSII photochemical activity in water-stressed wheat seedlings were investigated. The decrease in relative water content （RWC）, Chl content, and 2,6-dichlorophenol indophenol （DCIP） photoreduction of PSII, and increases in electrolyte leakage of plasma membranes and malonyldialdehyde （MDA） in water-stressed leaves was alleviated by Spd pretreatment. Furthermore, Western and Northern blot analysis showed that decreases in the PSll major proteins D1, D2 and LHCII and the transcripts of corresponding genes psbA, psbD and cab were also alleviated by Spd pretreatment under water stress. These results suggest that the application of exogenous Spd protects PSII against water stress at both the transcriptional level and the translational level, and allows PSII to retain a higher activity level during water stress. The protective role of Spd in the photosynthetic apparatus also is discussed.

Thermal stability of oxygen evolution in photosystem Ⅱ core complex in the presence of digalactosyl diacylglycerol

The influence of digalactosyldiacylglycerol (DGDG), one of the photosynthetic membrane lipids, on heat inactivation of the process of oxygen evolution has been studied in vitro in photosystem Ⅱ(PS Ⅱ) core complex. It was found that the temperature of semi-inactivation of oxygen evolution in the complex increased from 40.0 to about 43.0℃ in the presence of DGDG with 5-min heat treatment in the dark. Furthermore, when PS Ⅱ core complex was incubated for 5 min at 45.0℃, the oxygen evolution in the complex was completely lost, whilst the DGDG-complexed PS Ⅱ core complex still retained a 16% of activity (100% for 25.0℃). In addition, a 1-h incubation at 38.0℃ inactivated absolutely the oxygen evolution for the PS Ⅱ core complex. By contrast, there remained about 20% of activity (zero time for 100%) for the complex in the presence of DGDG under the same condition. These results indicate a new role of DGDG in the protection of PS Ⅱ core complex against the deleterious effects of temperature. It was