饱和白光引起的光系统II捕光复合体(LHCII)从反应中心复合体脱离不同于弱红光引起的状态1向状态2的转换

我们观测了不同光照预处理对拟南芥、小麦和大豆叶片光合作用和低温(77K) 叶绿素荧光参数F685、F735和F685/F735的影响.野生型拟南芥叶片光合作用对饱和光到有限光转变的响应曲线是V型,而缺乏叶绿体蛋白激酶的突变体STN7的这一曲线为L型. 饱和白光可以引起拟南芥叶片F685/F735的明显降低,但是F735没有明显增高,而弱红光可以导致拟南芥叶片F685/F735的明显降低和F735的明显增高,表明弱红光可以引起状态1向状态2的转变,同时伴随从光系统II脱离的LHC II与光系统I的结合,而饱和白光只能引起LHC II从光系统II反应中心复合体脱离.并且,低温叶绿素荧光分析结果证明,饱和白光可以引起大豆叶片LHC II脱离,但是不能引起小麦叶片LHC II脱离,而弱红光可以引起小麦叶片的这种状态转换,却不能引起大豆叶片的这种状态转换.因此,饱和白光引起的野生型拟南芥和大豆叶片的LHC II脱离不是一个典型的状态转换现象.     

Species-dependence of the pattern of plant photosynthetic rate response to light intensity transition from saturating to limiting one]

By observing the photosynthetic responses of leaves to changes in light intensity and CO(2) concentration it was found that among the more than 50 plant species examined 32 species and 25 species showed respectively the V pattern and L pattern of the photosynthetic response to light intensity transition from saturating to limiting one (Figs.1 and 2 and Table 1). The pattern of photosynthetic response to light intensity transition is species-dependent but not leaf developmental stage-dependent (Fig.3). The species-dependence was not related to classification in taxonomy because the photosynthetic response might display the two different patterns (V and L) in plants of the same family, for example, rice and wheat (Gramineae), soybean and peanut (Leguminosae). It seemed to be related to the pathway of photosynthetic carbon assimilation because all of the C(4) plants examined (maize, green bristlegrass and thorny amaranth) displayed the L pattern. It might be related to light environment where the plants originated. The V pattern of photosynthetic response to light intensity transition was often observed in some plants grown in shade habitats, for example, sweet viburnum and Japan fatsia, while the L pattern was frequently observed in those plants grown in sunny habitats, for example, ginkgo and cotton. Furthermore, the ratio of electron transport rate to carboxylation rate in vivo measured at limiting light was far higher in the V pattern plants (mostly higher than 10) than in the L pattern plants (mostly lower than 5), but the ratio measured at saturating light had no significant difference between the two kinds of plants (Table 2). These results can be explained in part by that the V pattern plant species have larger light-harvesting complex (LHCII) and at saturating light the reversible dissociation of some LHCIIs from PSII reaction center complex occurs. The pattern of photosynthetic response to light intensity transition and the ratio of electron transport rate to carboxylation rate in vivo measured at limiting light can probably be used as a criterion to distinguish sun plants from shade plants. In the observation of photosynthetic response to light intensity transition the use of saturating light is very important because using non-saturating light can form an artifact, which leads to incorrect conclusion (Fig.4).     

Novel Evidence for a Reversible Dissociation of Light-harvesting Complex II from Photosystem II Reaction Center Complex Induced by Saturating Light Illumination in Soybean Leaves

After saturating light illumination for 3 h the potential photochemical efficiency of photosystem Ⅱ （PSII） （FJF,, the ratio of variable to maximal fluorescence） decreased markedly and recovered basically to the level before saturating light illumination after dark recovery for 3 h in both soybean and wheat leaves, indicating that the decline in FJ/Fm is a reversible down-regulation. Also, the saturating light illumination led to significant decreases in the low temperature （77 K） chlorophyll fluorescence parameters F685 （chlorophyll a fluorescence peaked at 685 nm） and F685/F735 （F735, chlorophyll a fluorescence peaked at 735 nm） in soybean leaves but not in wheat leaves. Moreover, trypsin （a protease） treatment resulted in a remarkable decrease in the amounts of PsbS protein （a nuclear gene psbS-encoded 22 kDa protein） in the thylakoids from saturating light-illuminated （SI）, but not in those from darkadapted （DT） and dark-recovered （DRT） soybean leaves. However, the treatment did not cause such a decrease in amounts of the PsbS protein in the thylakoids from saturating light-illuminated wheat leaves. These results support the conclusion that saturating light illumination induces a reversible dissociation of some light-harvesting complex Ⅱ （LHClI） from PSII reaction center complex in soybean leaf but not in wheat leaf.     

尖叶拟船叶藓的77K荧光光谱及对强光照的短期适应

报道了东亚特有濒危植物尖叶拟船叶藓(Dolichomitriopsis diversiformis)在不同光质的光照诱导下的低温77K荧光光谱及状态转移的初步研究结果,实验中,尖叶拟船叶藓在77K下出现了3条发射带,分别是F680、F685、F720nm,并没有出现存在于大部分高等植物中的F695nm和F740nm两个峰．经过PSⅡ光诱导后、在77K下出现了F680nm,这个峰在77K下出现是首次报道,而以前的研究认为只在4K下才出现这一条光谱带,这一结果表明尖叶拟船叶藓叶绿体的两个光系统结构与其他高等植物存在着差异。在自然光下,PSⅡ与PSⅠ的总能量比是2.04,经过15min的PSⅡ光(670nm)诱导后,PSⅡ与PSⅠ的总能量比变成了1．28(状态2),当用15min的PSⅠ光(716nm)照射后,PSⅡ与PSⅠ的总能量比从2．04变成了3．4l(状态1)。在自然光下,由尖叶拟船叶藓的光系统的外部LHCⅡ所吸收的激发能是整个光系统激发能的21．19％．这说明尖叶拟船叶藓对光的短期调节能力是21．19％．尖叶拟船叶藓的光系统的外部LHCⅡ有51．7％位于PSⅡ中,48．3％在PSⅠ中.     

Structural variability of plant photosystem II megacomplexes in thylakoid membranes

Plant photosystem II (PSII) is organized into large supercomplexes with variable levels of membrane‐bound light‐harvesting proteins (LHCIIs). The largest stable form of the PSII supercomplex involves four LHCII trimers, which are specifically connected to the PSII core dimer via monomeric antenna proteins. The PSII supercomplexes can further interact in the thylakoid membrane, forming PSII megacomplexes. So far, only megacomplexes consisting of two PSII supercomplexes associated in parallel have been observed. Here we show that the forms of PSII megacomplexes can be much more variable. We performed single particle electron microscopy (EM) analysis of PSII megacomplexes isolated from Arabidopsis thaliana using clear‐native polyacrylamide gel electrophoresis. Extensive image analysis of a large data set revealed that besides the known PSII megacomplexes, there are distinct groups of megacomplexes with non‐parallel association of supercomplexes. In some of them, we have found additional LHCII trimers, which appear to stabilize the non‐parallel assemblies. We also performed EM analysis of the PSII supercomplexes on the level of whole grana membranes and successfully identified several types of megacomplexes, including those with non‐parallel supercomplexes, which strongly supports their natural origin. Our data demonstrate a remarkable ability of plant PSII to form various larger assemblies, which may control photochemical usage of absorbed light energy in plants in a changing environment.     

缺铁对大豆叶片光合作用和光系统Ⅱ功能的影响

通过气体交换和叶绿素荧光测定研究了缺铁对大豆叶片碳同化和光系统Ⅱ的影响。缺铁条件下大豆光合速率(Pn)大幅下降;最大光化学效率(po)下降幅度较小;荧光诱导动力学曲线发生明显的变化,其中电子传递活性明显下降,K相(VK)相对荧光产量提高。缺铁大豆的天线转化效率(Fv'/Fm')、光化学猝灭系数(qP)和光系统Ⅱ实际光化学效率(ΦPSⅡ)降低,而非光化学猝灭(NPQ)则明显增加。此外,缺铁大豆的光后荧光上升增强。据此,认为铁缺乏伤害了光系统Ⅱ复合物供体侧和受体侧的电子传递;缺铁条件下光系统I环式电子传递的增强可能在维持激发能耗散和ATP供给方面起一定作用。     

Functional Rearrangement of the Light-Harvesting Antenna upon State Transitions in a Green Alga

State transitions in the green alga Chlamydomonas reinhardtii serve to balance excitation energy transfer to photosystem I (PSI) and to photosystem II (PSII) and possibly play a role as a photoprotective mechanism. Thus, light-harvesting complex II (LHCII) can switch between the photosystems consequently transferring more excitation energy to PSII (state 1) or to PSI (state 2) or can end up in LHCII-only domains. In this study, low-temperature (77 K) steady-state and time-resolved fluorescence measured on intact cells of Chlamydomonas reinhardtii shows that independently of the state excitation energy transfer from LHCII to PSI or to PSII occurs on two main timescales of <15 ps and ∼100 ps. Moreover, in state 1 almost all LHCIIs are functionally connected to PSII, whereas the transition from state 1 to a state 2 chemically locked by 0.1 M sodium fluoride leads to an almost complete functional release of LHCIIs from PSII. About 2/3 of the released LHCIIs transfer energy to PSI and ∼1/3 of the released LHCIIs form a component designated X-685 peaking at 685 nm that decays with time constants of 0.28 and 5.8 ns and does not transfer energy to PSI or to PSII. A less complete state 2 was obtained in cells incubated under anaerobic conditions without chemical locking. In this state about half of all LHCIIs remained functionally connected to PSII, whereas the remaining half became functionally connected to PSI or formed X-685 in similar amounts as with chemical locking. We demonstrate that X-685 originates from LHCII domains not connected to a photosystem and that its presence introduces a change in the interpretation of 77 K steady-state fluorescence emission measured upon state transitions in Chalamydomonas reinhardtii.     

Adaptation to shade of the light-harvesting apparatus in Silene dioica

Abstract. The physiological characteristics and photo-system composition of the photosynthetic apparatus of Silene dioica , a woodland plant, grown in sun and natural shade are examined. As expected, shade leaves exhibited lower chlorophyll a/b ratios, light saturated rates of CO₂ assimilation (A_sat), dark respiration (R_d,) and light compensation points ( Г ), with both sun and shade leaves having similar absorptances and quantum yields of CO₂ assimilation (φ). Shade leaves were able to utilize far-red light for electron transport and carbon assimilation and reach the compensation point. Sun leaves in far-red light had a rate of carbon assimilation equivalent to their dark respiration rate. Chlorophyll fluorescence kinetics from leaves at 77 K together with analyses of thylakoid contents of photosystems (PS) I and II and the light-harvesting cholorphyll a/b protein complex associated with PSII (LHCII) demonstrated that the antenna size of PSII was similar in thylakoids of sun and shade leaves, but shade leaves contained ca. 20% more PSII and ca. 12% less PSI complexes. The increased PSII/PSI ratio in shade leaves accounted for their ability to achieve the compensation point in far-red light. An important feature of photosynethic shade adaptation in S. dioica is an increase in the PSII/PSI ratio and not an increase in the antenna size of PSII. The adaptive response of sun leaves when placed in a shade environment was rapid and had a half-time of ca. 18h.     

饱和白光引起的光系统Ⅱ捕光复合体（LHCⅡ）从反应中心复合体脱离不同于弱红光引起的状态1向状态2的转换

我们观测了不同光照预处理对拟南芥、小麦和大豆叶片光合作用和低温（77K）叶绿素荧光参数F685、F735和F685／F735的影响。野生型拟南芥叶片光合作用对饱和光到有限光转变的响应曲线是V型,而缺乏叶绿体蛋白激酶的突变体STN7的这一曲线为L型。饱和白光可以引起拟南芥叶片F685／F735的明显降低,但是F735没有明显增高,而弱红光可以导致拟南芥叶片F685／F735的明显降低和F735的明显增高,表明弱红光可以引起状态1向状态2的转变,同时伴随从光系统Ⅱ脱离的LHCⅡ与光系统Ⅰ的结合,而饱和白光只能引起LHCⅡ从光系统Ⅱ反应中心复合体脱离。并且,低温叶绿素荧光分析结果证明,饱和白光可以引起大豆叶片LHCⅡ脱离,但是不能引起小麦叶片LHCⅡ脱离,而弱红光可以引起小麦叶片的这种状态转换,却不能引起大豆叶片的这种状态转换。因此,饱和白光引起的野生型拟南芥和大豆叶片的LHCⅡ脱离不是一个典型的状态转换现象。     

Changes in activity of energy dissipating mechanisms in wheat flag leaves during senescence

Abstract: Excitation energy dissipation, including the xanthophyll cycle, during senescence in wheat flag leaves grown in the field was investigated at midday and in the morning. With progress of senescence, photosynthesis (Pn) and actual PSII photochemical efficiency (ΦPSII) decreased markedly at midday. The decrease in extent of Pn was greater than that of ΦPSII. However, there was no significant decline in Pn and ΦPSII observed in the morning, except in leaves 60 days after anthesis. The kinetics of xanthophyll cycle activity, thermal dissipation (NPQ), and q_f observed at midday during senescence exhibited two distinct phases. The first phase was characterized by an increase of xanthophyll cycle activity, NPQ, and q_f during the first 45 days after anthesis. The second phase took place 45 days after anthesis, characterized by a dramatic decline in the above parameters. However, the qI, observed both at midday and in the morning, always increased along with senescence. A larger proportion of NPQ insensitive to DTT (an inhibitor of the de-epoxidation of V to Z) was also observed in severely senescent leaves. In the morning, only severely senescent leaves showed higher xanthophyll cycle activity, NPQ, q_f, and qI. It was demonstrated that, at the beginning of senescence or under low light, wheat leaves were able to dissipate excess light energy via NPQ, depending on the xanthophyll cycle. However, the xanthophyll cycle was insufficient to protect leaves against photodamage under high light, when leaves became severely senescent. The ratio of (Fj - Fo)/(Fp - Fo) increased gradually during the first 45 days after anthesis, but dramatically increased 45 days after anthesis. We propose that another photoprotection mechanism might exist around reaction centres, activated in severely senescent leaves to protect leaves from photodamage.     

Temperature-dependent changes in Photosystem II heterogeneity of attached leaves under high light

Attached leaves of pumpkin ( Cucurbita pepo L. cv. Jattiläismeloni) were exposed to high light intensity at room temperature (ca 23°C) and at 1°C. Fluorescence parameters and electron transport activities measured from isolated thylakoids indicated faster photoinhibition of PSII at low temperature. Separation of the α and β components of the complementary area above the fluorescence induction curve of dichlorophenyl-dimethylurea-poisoned thylakoids revealed that at low temperature only the α-centers declined during exposure to high light intensity while the content of functional β-centers remained constant. Freeze-fracture electron microscopy showed no decrease in the density of particles on the appressed exoplasmic fracture face, indicating that the photoinhibited α-centers remained in the appressed membranes at 1°C. Because of the function of the repair and protective mechanisms of PSII, strong light induced less photoinhibition at room temperature, but more complicated changes occurred in the α/β-heterogeneity of PSII. During the first 30 min at high light intensity the decrease in α-centers was almost as large as at 1°C, but in contrast to the situation at low temperature the decrease in α-centers was compensated for by a significant increase in PSIIβ-centers. Changes in the density and size of freeze-fracture particles suggest that this increase in β-centers was due to migration of phosphorylated light-harvesting complex from appressed to non-appressed thylakoid membranes while the PSII core remained in the appressed membranes. This situation, however, was only transient and was followed by a rapid decrease in the functionalβ-centers.     

Photodamage to Pigment in the Photosystem Ⅱ Reaction Center D1/D2/Cytochrome b559 Complex

Strong light （800μmol photons/m^2 per s）-induced bleaching of the pigment in the isolated photosystem Ⅱ reaction center （PSII RC） under aerobic conditions （in the absence of electron donors or acceptors） was studied using high-pressure liquid chromatography （HPLC）, absorption spectra, 77K fluorescence spectra and resonance Raman spectra. Changes in pigment composition of the PSII RC as determined by HPLC after light treatment were as follows： with Increasing illumination time chlorophyll （Chl） a and β-carotene （β-car） content decreased. However, decreases in pheophytin （Pheo） could not be observed because of the mixture of the Pheo formed by degraded chlorophyll possibly. On the basis of absorption spectra, it was determined that, with a short time of illuminatlon, the initial bleaching occurred maximally at 680 nm but that with Increasing Illumination time there was a blue shift to 678 nm. It was suggested that P680 was destroyed Initially, followed by the accessory chlorophyll. The activity of P680 was almost lost after 10 mln light treatment. Moreover, the bleaching of Pheo and β-car was observed at the beginning of illumination. After Illumination, the fluorescence emission Intensity changed and the fluorescence maximum blue shifted, showing that energy transfer was disturbed. Resonance Raman spectra of the PSII RC excited at 488.0 and 514.5 nm showed four main bands, peaking at 1 527 cm^-1 （υ101）, 1 159 cm^-1 （υ2）, 1 006 cm^-1 （υ3）, 966 cm^-1 （υ4） for 488.0 nm excitation and 1 525 cm^-1 （υ1）, 1 159 cm^-1 （υ2）, 1 007 cm^-1 （υ3）, 968 cm^-1 （υ4） for 514.5 nm excitation. It was confirmed that two spectroscopically different β-car molecules exist In the PSII RC. After light treatment for 20 mln, band positions and bandwidths were unchanged. This indicates that carotenoid configuration Is not the parameter that regulates photoprotectlon in the PSII RC.     

Essential role of the PSI–LHCII supercomplex in photosystem acclimation to light and/or heat conditions by state transitions

Light and temperature affect state transitions through changes in the plastoquinone (PQ) redox state in photosynthetic organisms. We demonstrated that light and/or heat treatment induced preferential photosystem (PS) I excitation by binding light-harvesting complex II (LHCII) proteins. The photosystem of wheat was in state 1 after dark overnight treatment, wherein PQ was oxidized and most of LHCII was not bound to PSI. At the onset of the light treatment [25 °C in the light (100 µmol photons m^?2 s^?1)], two major LHCIIs, Lhcb1 and Lhcb2 were phosphorylated, and the PSI–LHCII supercomplex formed within 5 min, which coincided with an increase in the PQ oxidation rate. Heat treatment at 40 °C of light-adapted wheat led to further LHCII protein phosphorylation of, resultant cyclic electron flow promotion, which was accompanied by ultrafast excitation of PSI and structural changes of thylakoid membranes, thereby protecting PSII from heat damage. These results suggest that LHCIIs are required for the functionality of wheat plant PSI, as it keeps PQ oxidized by regulating photochemical electron flow, thereby helping acclimation to environmental changes.     

Phosphorylation-dependent regulation of excitation energy distribution between the two photosystems in higher plants

Phosphorylation-dependent movement of the light-harvesting complex II (LHCII) between photosystem II (PSII) and photosystem I (PSI) takes place in order to balance the function of the two photosystems. Traditionally, the phosphorylatable fraction of LHCII has been considered as the functional unit of this dynamic regulation. Here, a mechanical fractionation of the thylakoid membrane of Spinacia oleracea was performed from leaves both in the phosphorylated state (low light, LL) and in the dephosphorylated state (dark, D) in order to compare the phosphorylation-dependent protein movements with the excitation changes occurring in the two photosystems upon LHCII phosphorylation. Despite the fact that several LHCII proteins migrate to stroma lamellae when LHCII is phosphorylated, no increase occurs in the 77 K fluorescence emitted from PSI in this membrane fraction. On the contrary, such an increase in fluorescence occurs in the grana margin fraction, and the functionally important mobile unit is the PSI-LHCI complex. A new model for LHCII phosphorylation driven regulation of relative PSII/PSI excitation thus emphasises an increase in PSI absorption cross-section occurring in grana margins upon LHCII phosphorylation and resulting from the movement of PSI-LHCI complexes from stroma lamellae and subsequent co-operation with the P-LHCII antenna from the grana. The grana margins probably give a flexibility for regulation of linear and cyclic electron flow in plant chloroplasts.     

饱和白光引起的光系统II 捕光复合体(LHCII ) 从反应中心复合体脱离不同于弱红光引起的状态1 向状态2 的转换

我们观测了不同光照预处理对拟南芥、小麦和大豆叶片光合作用和低温( 77K) 叶绿素荧光参数F685、F735 和F685&#1089839;F735 的影响。野生型拟南芥叶片光合作用对饱和光到有限光转变的响应曲线是V 型,而缺乏叶绿体蛋白激酶的突变体STN7 的这一曲线为L 型。饱和白光可以引起拟南芥叶片F685&#1089839;F735 的明显降低, 但是F735 没有明显增高, 而弱红光可以导致拟南芥叶片F685&#1089839;F735 的明显降低和F735 的明显增高, 表明弱红光可以引起状态1 向状态2 的转变, 同时伴随从光系统II 脱离的LHC II 与光系统I 的结合, 而饱和白光只能引起LHC II 从光系统II 反应中心复合体脱离。并且, 低温叶绿素荧光分析结果证明, 饱和白光可以引起大豆叶片LHC II 脱离, 但是不能引起小麦叶片LHC II 脱离, 而弱红光可以引起小麦叶片的这种状态转换, 却不能引起大豆叶片的这种状态转换。因此, 饱和白光引起的野生型拟南芥和大豆叶片的LHC II 脱离不是一个典型的状态转换现象。     

Light‐harvesting superstructures of green plant chloroplasts lacking photosystems

The light‐harvesting antenna of higher plant photosystem II (LHCII) is the major photosynthetic membrane component encoded by an entire family of homologous nuclear genes. On the contrary, the great majority of proteins of photosystems and electron transport components are encoded by the chloroplast genome. In this work, we succeeded in gradually inhibiting the expression of the chloroplast genes that led to the disappearance of the photosystem complexes, mimicking almost total photoinhibition. The treated plants, despite displaying only some early signs of senescence, sustained their metabolism and growth for several weeks. The only major remaining membrane component was LHCII antenna that formed superstructures – stacks of dozens of thylakoids or supergrana. Freeze‐fracture electron microscopy revealed specific organization, directly displaying frequently bifurcated membranes with reduced or totally absent photosystem II (PSII) reaction centre complexes. Our findings show that it is possible to accumulate large amounts of light‐harvesting membranes, organized into three‐dimensional structures, in the absence of reaction centre complexes. This points to the reciprocal role of LHCII and PSII in self‐assembly of the three‐dimensional matrix of the photosynthetic membrane, dictating its size and flexible adaptation to the light environment.     

The original basal stem section influences rooting in Pinus banksiana cuttings

Photosystem II (PSII) from Cu-deficient pea plants ( Pisum sativum L., cv. Lincoln) has been investigated for electron transport activity, Cu content, and changes in some lipid components. Total fatty acid content was lower that in control plants, with an additional shift in the C₁₈ fatty acid patterns. Less α-linolenic and more linoleic and oleic acids were found. PSII preparations from Cu-depleted plants showed a decreased carotenoid content in light harvesting chlorophyll a/b complex of photosystem II (LHCII) and additional variations in pigment composition of pigment-protein complexes. In the green alga Dunaliella the effect of Cu deficiency on fatty acid composition was similar to that in pea plants, but the influence on the carotenoid pattern was much less pronounced.     

Effect of copper deficiency on photosynthetic electron transport in wheat plants

Copper deficiency in wheat ( Triticum aestivum L. cv. Nazareno Stramppeli) markedly affects photosynthetic activity. Flag leaves of copper-deficient plants showed a 50% reduction of the photosynthetic rate expressed as mg CO₂ dm⁻²h⁻¹. The activities of PSI and PSII, determined for isolated chloroplasts, as well as fluorescence measurements on intact leaves of copper-deficient plants, indicated a low activity of photosynthetic electron transport. Ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity was not affected by copper deficiency but copper deficiency affected the chloroplast ultrastructure, especially at the level of grana, where a disorganization of thylakoids is evident.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.     

Novel Evidence for a Reversible Dissociation of Light-harvesting Complex Ⅱ from Photosystem Ⅱ Reaction Center Complex Induced by Saturating Light Illumination in Soybean Leaves

After saturating light illumination for 3 h the potential photochemical efficiency of photosystem Ⅱ (PSⅡ) (Fv/Fm, the ratio of variable to maximal fluorescence) decreased markedly and recovered basically to the level before saturating light illumination after dark recovery for 3 h in both soybean and wheat leaves, indicating that the decline in Fv/Fm is a reversible down-regulation. Also, the saturating light illumination led to significant decreases in the low temperature (77 K) chlorophyll fluorescence parameters F685 (chlorophyll a fluorescence peaked at 685 nm ) and F685/F735 (F735, chlorophyll a fluorescence peaked at 735 nm) in soybean leaves but not in wheat leaves. Moreover,trypsin (a protease) treatment resulted in a remarkable decrease in the amounts of PsbS protein (a nuclear gene psbS-encoded 22 kDa protein) in the thylakoids from saturating light-illuminated (SI), but not in those from darkadapted (DT) and dark-recovered (DRT) soybean leaves. However, the treatment did not cause such a decrease in amounts of the PsbS protein in the thylakoids from saturating light-illuminated wheat leaves. These results support the conclusion that saturating light illumination induces a reversible dissociation of some light-harvesting complex Ⅱ (LHCⅡ) from PSⅡ reaction center complex in soybean leaf but not in wheat leaf.