Effect of Different Light Quality on Photosynthetic Characteristics of Cucumber Leaves

Cucumber (Cucumis sativus L. cultivar "Changchun Mici") seedlings were cultured in Hoagland solution under irradiation with different light spectra (8 h per day) for 20 days. The red light (λmax 658 nm, λ1/2 25 nm), blue light (λmax 450 nm, λ1/2 43 nm) and white fluorescent light possessed the same fluent rate (20 μmol· m-2·s-1 ). The experimental results showed that chlorophyll content of the leaves grown under white light was 7 % and 22.4% higher than those in red and blue light, respectively. Compared with white and blue light, red light induced a lower Chl a/b ratio and a higher level of Chl b in the cucumber leaves. Measurements of the low temperature (77 K) fluorescence emission spectra and kinetics of Chl a fluorescence induction of the leaves proved that the leaves grown under red light expressed the highest PSⅡ and the lowest PSⅠactivities while the leaves under blue light had the lowest PSⅡand the highest PSⅠ activities. The O2 evolution rate of red light-grown leaves was 44.9% higher than that of the white light-grown leaves, while blue light effect was similar to that of white in respect of O2 evolution. It is concluded that light quality is an important factor in regulating the development and activities of PSⅡ and PSⅡand the O2 evolution of photosynthesis in cucumber leaves.     

Comparative Study of Chlorophyll-Protein Complexes of Thylakoids of Bryopsis and Spinacia

properties, pigment compositions, Chl a/b ratios and apparent molecular weights of chlorophyll-protein complexes were compared between spinach and a marine green alga, Bryopsis corticulans. The results are as follows: 1. Ten chlorophyll-protein complexes were resolved from spinach thylakoid membranes solubilized by SDS in a final SDS/Chl weight ratio of 10:1, and subjected to SDS-PAGE with 11% resolution gel. CPIa 1–3 and CPI belonged to photosystem Ⅰ, and the rest to phorosystem Ⅱ. The maximum absorption of CPIa2, CPIas and CPI were all at 674nm, but that of CPIa1 at 670nm, and those of LHCII and D2 at 670 and 673nm, respectively. Chlorophyll ia PSⅡ was 63% of the total. In PSⅡ, most of chlorophyll was in LHCII which contained 86% of the chlorophyll in PSⅡ. In PSⅠ, chlorophyll in CPla was 72% of the total. Chlorophyll a was the main pigment in PSⅠ components which have Chl a/b ratio over 15. 2. Eight chlorophyll-protein complexes were isolated from B. corticulans with a SDS/Chi weight ratio of 8:1 and 8% resolution gel. The maximum absorption of CPIa, CPI, LHCII and D2 were respectively at 671nm, 673nm, 669nm and 664nm. PSⅡ contained 77% of the total chlorophyll. LHCII chlorophyll was 95% of the PSⅡ chlorophyll. CPI held 77% of PSⅠ chloro~ phyll. There was more chlorophyll b in Bryopsis complexes, especially in LHCI1 (Chl a/b< 0.8). The molecular weights of Bryopsis complexes were higher than those of the spinach complexes. Bryopsis LHCII contained siphoxanthin and siphothin, the marked pigments of Siphohales, as functional pigments. The above results revealed three points of difference between these two plants. Firstly, Chl a is the main pigment in spinach, whereas in Bryopsis the main pigments are Chl b and siphoxanthin. This is in accordance with the suggestion that plants may change their pigment composition to adapt light regime in the environment during evolution. Secondly, in Bryopsis, chlorophyll is concentrated in photosystem Ⅱ, but in spinach chlorophyll is shared evenly by two photosystems. Finally, CPI in Bryopsis contained the major part of chlorophyll in PSⅠ, yet in spinach CPIa is the superior.     

Where Is the Site of the "Oxygen Burst" Located During Light Induction in Dark-Adapted Leaves? A Study Using Photoacoustic Techniques

The "oxygen burst" phenomenon that appeared during the light-induction period of intact leaves could be monitored using a photoacoustic technique high time resolution. The relationship between oxygen bursts and dark-adapted time, far-red light pretreatment, photothermal signal, and chlorophyll a (Chl a) fluorescence kinetics were investigated in the present study. Using extraneous inhibitors or cofactors of electron transport, a modified vacuum-infiltration method was undertaken to locate directly the site at which oxygen bursts of intact leaves occurred. We found that the photothermal signal showed little evidence of oscillation during the light-induction period. The oxygen burst was resolved into two components if darkadapted time lasted longer than 20 min. Methyl viologen (MV) or far-red light could not eliminate the first component, whereas formate-Na (pH 7.0, 20 μmol/L) eliminated the first component but had no effect on the second one. Furthermore, the photochemical quenching, the electron transport rate of Chl a fluorescence,and the first component of the oxygen bursts approached lowest values simultaneously. This evidence indicates that the site at which the first component of oxygen bursts occurred was located between photosystem (PS)Ⅰ and PSⅡ (i.e. the PQ pool). The formate-Na experiment also showed a linkage between the first component and the S state of oxygen evolution at the donor side of PSⅡ. Furthermore, elimination of the second component by far-red light and absorption of the second component by MV indicated that the site at which the second component of oxygen bursts may be located at the acceptor side of PSⅡ.     

Studies on the Chlorophyll,protein Complexes and Polypeptides of Chloroplast Membrane of Cator Bean

The chlorophyll protien complexes separated from several plants were resolved by SDS-PAGE. It showed that the percentage (18%) of complex Ⅳ in total amount of chlorophyll of castor bean (Ricinus communis L.) was obviously higher than that ( 13% ) of snake gourd (Luffa cylindrka (L.) Roem) and the percentage (41%) of LHCP complex in total amount of chlorophyll of castor bean was lower than that (48%) of snake gourd. The light-induced variable fluorescence change of castor bean (Fv/Fo=2.6) was much higher than that of snake gourd (Fv/Fo = 1.08) and other plants. The photochemical activities of chloroplasts of castor bean were very high. Its oxygen evolution activity (H20 → DMBQ) was 575 μ moles O2·mg-1Chl·h-1, and the oxygen absorbtion activity of PSⅠ (DCIPH2 → WV) was 1274μ moles O2·mg-1Chl·h-1, the Chl/P700 ratio was 373, but those of snake gourd were 210, 148 and 751 respectively. There were distinct difference between the polypeptide pattern of castor bean and those of other plants. It was observed that 43, 47 KD polypeptide bands (polypeptides of PSⅡ) of castor bean were darker than those of other plants. From the results mentioned above, it was considered that the high photochemical activities possessed by castor bean chloroplasts were partly due to its small photosynthetic units, e.g. its LHCP/RCCHL ratio yas smaller than those of other plants.     

Light-induced Damage of Photosystem Ⅱ Primary Electron Donor P680: a High Performance Liquid Chromatographic Analysis of Pigment Content in D1/D2/Cytochrome b559 Complex Under Photoinhibitory Conditions

By HPLC analytical method, the change of PS Ⅱ RC' s pigment content in the process of photodamage under strong illumination from spinach ( Spinacia oleracea Mill. ) was comparatively studied. The experimental results show that: (1) In authors' analytical conditions, (of which, [Chl] = 150 µg/mL, and the illumination strength was put at 2.3 ×10 6 mJ·m－2·s－1 ), 45 rain of illumination could cause almost the whole loss of A680 in the fourth derivative absorption spectra, while A670 decreased to about one half of its original intensity; the absorption maximum in red, concurrently, was shifted from 676 nm to 671 nm, representing the loss of more than 90% of the photochemical activities of the PS Il RC. (2) During the period of continuous illumination, the Chl concentration decreased in a 3-period style, which meant that the first [Chl] decreased to the 2/3 of its original amount from 20 min to 40 rain after illumination had started, then became stabilized up to about 60 min of illumination, there after a second decrease of [ Chl ] in another about 20 min until it reached about 30 % of the original level and remained unchanged from about 80 min on. The original pigment components of D1/D2/Cyt b559 was approximately as 6 Chl a:2 Pheo:2β-Car which are in support of authors' previous proposal about the minimum Chl/Pheo ratio of 4: 2 in PS Ⅱ RC’s pigment contents. (3) After about 40 min of illumination, a newly appeared elution peak was found between the Pheo andβ-Car peaks in HPLC profile, at the retention time of 7.2 min, a little later than that (6.9 rain) of Pheo molecules, the newly appeared elution peak was supposed to be a kind of accumulated and stable product of the PS II RC's photodamage process and very much possible the Pheo-like molecules.     

A Comparative Study on PSⅡ Light Harvesting Chlorophyll a/b Protein Complexes Between Spinach and Cucumber

PS Ⅱ light harvesting chlorophyll a/b protein complexes (LHC Ⅱ ) were isolated from chloroplast of spinach (Spinacia oleracea Mill. ) and cucumber (Cucumis sativus L. ). Comparative studies were made on the polymerized forms. Chl a/b ratio, spectral characteristics and polypeptide components of these two kinds of LHC Ⅱ. Experimental results showed that the LHC Ⅱ from spinach had a Chl a/b ratio of 1.33 and the LHC Ⅱ from cucumber had a Chl a/b ratio of 1.77. The spectral characteristics of the LHC Ⅱ from cucumber also indicated the enrichment of Chl b in this LHC Ⅱ . There was also obvious differences in the polypeptide components between these two kinds of LHC Ⅱ, the LHC Ⅱ of spinach contained a 27 kD and a 25 kD polypeptides, while the LHC Ⅱ of cucumber contained only a 27 kD polypeptide. This showed that the 25 kD polypeptide contained less Chl b. The analysis of the chlorophyll protein complexes showed that the monomer, dimer and trimer of the LHC Ⅱ of spinach were composed of two polypeptides, while all the polymerized forms of cucumber’s LHC Ⅱ were composed of one polypeptide.     

套袋对番茄果实表面光系统Ⅱ光能吸收利用的影响

用白纸淋膜袋对温室番茄‘保罗塔’果实进行套袋处理,采用光纤光谱仪和叶绿素成像荧光仪测定了番茄果实的吸收光谱和叶绿素荧光参数,分析了套袋对番茄果实光系统Ⅱ光能利用效率的影响.结果表明： 在套袋后的前20 d内,与对照(CK)相比,套袋果实表面的叶绿素a (Chl a)含量和光系统Ⅱ最大光化学效率(F_v/F_m)无明显变化,但是套袋降低了果实表面的相对吸光系数A_670/780和光系统Ⅱ实际光化学效率（Y_Ⅱ）,此时番茄果实主要以PSⅡ调节性能量耗散机制为主.随后,番茄果实表面的Chl a和Chl b含量开始明显下降,但是套袋果实的F_v/F_m、Y_Ⅱ和A_670/780与CK无显著差异.在套袋后的第40天,套袋果实表面的Chl a和Chl b含量分别比CK降低了35.2%和52.8%,F_v/F_m和Y_Ⅱ仍然维持较高水平,分别比CK增加了24.5%和35.4%,表明此时番茄果实PSⅡ具有较高的光能利用效率,通过进一步降低非调节性能量耗散量子产额Y_NO为果实的早熟奠定了能量基础.     

毕氏海蓬子类囊体膜与卵磷脂重组脂质体的耐热性研究

采用卵磷脂(PC)构建脂质体,然后将毕氏海蓬子类囊体膜蛋白复合物重组到脂质体中.分析不同温度(25℃、35℃、45℃和55℃)处理后蛋白脂质体的电子传递活性、吸收光谱和荧光光谱的变化,以探讨膜脂与膜蛋白在高温胁迫下的交互作用.结果显示:蛋白脂质体光系统Ⅱ(PSⅡ)的放氧活性和光系统Ⅰ(PSⅠ)的耗氧活性随着PC比例的提高而增加,在PC与类囊体膜比例为4∶1(Lipid∶Chl,w/w)时达到最高,同时蛋白脂质体的吸收光谱和荧光光谱也呈上升趋势;在PC与类囊体膜重组比例为4∶1条件下,高温处理后的蛋白脂质体的PSⅡ放氧活性和PSⅠ耗氧活性显著大于未经重组的,其吸收光谱和荧光光谱峰值下降幅度低于未经重组的,且峰位基本没有变化.研究表明,PC可能通过增加结合天线的大小来促进蛋白脂质体对光能的吸收和能量从外周天线到PSⅡ和PSⅠ核心复合物的传递;在脂质体中,PC与类囊体膜的交互作用提高了PSⅡ和PSⅠ在高温胁迫下的光化学效率,增强了PSⅡ和PSⅠ的耐热性.     

Molecular mechanism of high-temperature-induced inhibition of acceptor side of Photosystem II

High-temperature-induced inhibition of the acceptor side of Photosystem II (PS II) was studied in tobacco thylakoids using oxygen evolution, chlorophyll a (Chl a) fluorescence and redox potential measurements. When thylakoids were heated at 2 °C/min from 25 to 50 °C, the oxygen evolving complex became inhibited between 32 and 45 °C, whereas the acceptor side of PS II tolerated higher temperatures. Variable Chl a fluorescence decreased more slowly than oxygen evolution, suggesting that transitions between some S-states occurred even after heat-induced inhibition of the oxygen evolving activity. 77 K emission spectroscopy reveals that heating does not cause detachment of the light-harvesting complex II from PS II, and thus the heat-induced increase in the initial F₀ fluorescence is due to loss of exciton trapping in the heated PS II centers. Redox titrations showed a heat-induced increase in the midpoint potential of the Q_A/Q_A ^-) couple from the control value of –80 mV to +40 mV at 50 °C, indicating a loss of the reducing power of Q_A ^-). When its driving force thus decreased, electron transfer from Q_A ^-) to Q_B in the PS II centers that still could reduce Q_A became gradually inhibited, as shown by measurements of the decay of Chl a fluorescence yield after a single turnover flash. Interestingly, the heat-induced loss of variable fluorescence and inhibition of electron transfer from Q_A ^-) to Q_B could be partially prevented by the presence of 5 mM bicarbonate during heating, suggesting that high temperatures cause release of the bicarbonate bound to PS II. We speculate that both the upshift in the redox potential of the Q_A/Q_A ^-) couple and the release of bicarbonate may be caused by a heat-induced structural change in the transmembrane D1 or D2 proteins. This structural change may, in turn, be caused by the inhibition of the oxygen evolving complex during heating.     

Kinetics of the oxygen evolution step in plants determined from flash-induced chlorophyll t a fluorescence

Photosystem II (PS II) of plants and cyanobacteria, which catalyzes the light-induced splitting of water and the release of oxygen, is the primary source of oxygen in the earth atmosphere. When activated by short light flashes, oxygen release in PS II occurs periodically with maxima after the third and the seventh flashes. Many other processes, including chlorophyll (Chl) t a fluorescence, are also modulated with period of four, reflecting their sensitivity to the activity of Photosystem II. A new approach has been developed for the analysis of the flash-induced fluorescence of Chl t a in plants, which is based on the use of the generalized Stern–Volmer equation for multiple quenchers. When applied to spinach thylakoids, this analysis reveals the presence of a new quencher of fluorescence whose amplitude is characterized by a periodicity of four with maxima after the third and the seventh flashes, in phase with oxygen release. The quencher appears with a delay of 0.5 ms followed by a rise time of 1.2–2 ms at pH 7, also in agreement with the expected time for oxygen evolution. It is concluded that the quencher is a product of the reaction leading to the oxygen evolution in PS II. The same quenching activity, maximal after the third flash, could be seen in dark adapted leaves, and provides the first fully time-resolved measurement of the kinetics of the oxygen evolution step in the leaf. Thus, the non-invasive probe of Chl t a fluorescence provides a new and sensitive method for measuring the kinetics of oxygen evolution with potential for use in plants and cyanobacteria t in vivo.