Short-term effects of aluminium at alkaline pH on the structure and function of the photosynthetic apparatus

A 24 h exposure of the salt-tolerant grass Thinopyrum bessarabicum (Savul. and Rayss) A. Love seedlings to 1 mM aluminium (Al) in nutrient solution at pH of 9.0 resulted in a significant reduction of the biomass. In control samples the mesophyll chloroplasts exhibited the usual lens shape with most grana arranged in straight or slightly curving lines, and only 6.5 % of the grana were out of order. In Al-treated plants the mesophyll chloroplasts displayed a slightly distorted shape and distended size with most grana arranged in bow-like lines, while in the central region of the organelle as many as 26.7 % of the grana were independent and out of order in relation to the long axis. The morphological changes in the chloroplast shape and grana arrangement were probably due to swelling and distension of the chloroplasts in consequence to the altered membrane permeability. The initial in vivo chlorophyll (Chl) fluorescence FO, as well as the intermediate FI and peak fluorescence FP were increased under the Al stress: this indicated a destruction of photosystem (PS) 2 reaction centres and increased reduction of QA. The (FI-FO)/(FP-FO) ratio exhibited a significant increase indicating higher proportion of PS2 centres unable to reduce QB. Changes in the chloroplast ultrastructure seemed to be the reason of photosynthetic electron transport inhibition. Yet all these changes in the photosynthetic performance and chloroplast ultrastructure were considered as indirect effects of Al treatment since Al concentration in the leaves was undetectable. Disturbances in the chloroplast ultrastructure could be caused by a reduced uptake and/or transport of other nutrients.     

花粒期光照对夏玉米光合特性和叶绿体超微结构的影响

在大田条件下,以夏玉米品种‘登海605’为试验材料,研究花粒期不同光照强度(正常光照、开花至收获期遮阴和开花至收获期增光)对夏玉米叶片光合、荧光性能和叶绿体超微结构的影响.结果表明:与对照相比,花粒期遮阴影响叶绿体排布及内部结构发育,基粒个数和基粒片层数均有不同程度减少,叶片的净光合速率、蒸腾速率、气孔导度、叶绿素含量下降,PSⅡ反应中心的实际光化学效率和最大光化学效率降低,非光化学淬灭系数数值增加,导致产量降低;增光后叶绿体结构良好,基粒片层排列紧致、清晰且数量增加,PSⅡ反应中心的实际光化学效率增加,净光合速率、蒸腾速率、气孔导度、叶绿素含量上升,叶片光合性能增强,产量增加.即花粒期遮阴破坏了夏玉米叶片叶绿体超微结构,降低了叶片光合能力,产量下降;花粒期增光增加了叶肉细胞中叶绿体的基粒和基粒片层,导致基粒片层排列紧密有序,有利于增加作物产量潜力.     

The coleoptile chloroplast: Distinct distribution of xanthophyll cycle pigments,and enrichment in Photosystem II

Recent studies have shown that coleoptile chloroplasts operate the xanthophyll cycle, and that their zeaxanthin concentration co-varies with their sensitivity to blue light. The present study characterized the distribution of photosynthetic pigments in thylakoid pigment–protein complexes from dark-adapted and light-treated coleoptile and mesophyll chloroplasts, the low temperature fluorescence emission spectra, and the rates of PS I and PS II electron transport in both types of chloroplasts from 5-day-old corn seedlings. Pigments were extracted from isolated PS I holocomplex, LHC IIb trimeric and LHC II monomeric complexes and analyzed by HPLC. Chlorophyll distribution in coleoptile thylakoids showed 31% of the total collected Chl in PS I and 65% in the light harvesting complexes of PS II. In mesophyll thylakoids, the values were 44% and 54%, respectively. Mesophyll and coleoptile PS I holocomplexes differed in their Chl t a/Chl t b ratios (8.1 and 6.1, respectively) and -carotene content. In contrast, mesophyll and coleoptile LHC IIb trimers and LHC II monomers had similar Chl t a/Chl t b ratios and -carotene content. The three analyzed pigment–protein complexes from dark-adapted coleoptile chloroplasts contained zeaxanthin, whereas there was no detectable zeaxanthin in the complexes from dark-adapted mesophyll chloroplasts. In both chloroplast types, zeaxanthin and antheraxanthin increased markedly in the three pigment–protein complexes upon illumination, while violaxanthin decreased. In mesophyll thylakoids, zeaxanthin distribution as a percentage of the xanthophyll cycle pool was: LHC II monomers > LHC IIb trimers > PS I holocomplex, and in coleoptile thylakoids, it was: LHC IIb trimers > LHC II monomers = PS I holocomplex. Low temperature (77 K) fluorescence emission spectra showed that the 686 nm emission of coleoptile chloroplasts was approximately 50% larger than that of mesophyll chloroplasts when normalized at 734 nm. The pigment and fluorescence analysis data suggest that there is relatively more PS II per PS I and more LHC I per CC I in coleoptile chloroplasts than in mesophyll chloroplasts. Measurements of t in vitro uncoupled photosynthetic electron transport showed approximately 60% higher rates of electron flow through PS II in coleoptile chloroplasts than in mesophyll chloroplasts. Electron transport rates through PS I were similar in both chloroplast types. Thus, when compared to mesophyll chloroplasts, coleoptile chloroplasts have a distinct PS I pigment composition, a distinct chlorophyll distribution between PS I and PS II, a distinct zeaxanthin percentage distribution among thylakoid pigment–protein complexes, a higher PS II-related fluorescence emission, and higher PS II electron transport capacity. These characteristics may be associated with a sensory transducing role of coleoptile chloroplasts.     

Structure and distribution of chloroplasts and other organelles in leaves with various rates of photosynthesis

The ultrastructure and distribution of chloroplasts, mitochondria, peroxisomes, and other cellular constituents have been examined in cross sections of leaves from plants with either high or low photosynthetic capacity. Photosynthetic capacity of a given plant cannot be correlated with the presence or absence of grana in bundle sheath cell chloroplasts, the presence or absence of starch grains in bundle sheath or mesophyll cell chloroplasts, the chloroplast size in bundle sheath or mesophyll cells, or the location of chloroplasts within bundle sheath cells. We conclude that the number and concentration of chloroplasts, mitochondria, and peroxisomes in bundle sheath cells is the most reliable anatomical criterion presently available for determining the photosynthetic capacity of a given plant.     

Na2CO3胁迫对星星草叶肉细胞超微结构的影响

利用透射电镜技术对Na2CO3胁迫下星星草叶肉细胞超微结构进行了观察。结果表明：未胁迫的叶肉细胞排列疏松,各种细胞器结构完整,叶绿体含少量淀粉粒和脂质球。轻度盐胁迫（2g/L,4g／LNa2CO3）对叶肉细胞超微结构影响较小。中度盐胁迫（6g／L,8g／L Na2CO3）引起叶肉细胞超微结构的变化,叶绿体类囊体肿胀,基粒紊乱,不含淀粉粒,脂质球数量增加,叶绿体由原来的梭形或椭球形变成圆球状;部分线粒体嵴消失,出现晶体结构;中央大液泡破裂;核逐渐降解。高度盐胁迫（10g／L,12g／LNa2CO3）下,叶绿体片层结构消失,脂质球数量增加,体积变大,被大量的膜片层所包围,叶绿体内、外膜消失,叶肉细胞中看不到叶绿体的存在;膜片层包围线粒体;叶肉细胞中可见大量的泡状结构和膜片层,叶肉细胞死亡。上述结果表明,细胞器特别是叶绿体膜结构的破坏与盐胁迫叶肉细胞最终死亡密切相关。     

Structural and functional properties of the coleoptile chloroplast: Photosynthesis and photosensory transduction

Recent studies have shown that guard cell and coleoptile chloroplasts appear to be involved in blue light photoreception during blue light-dependent stomatal opening and phototropic bending. The guard cell chloroplast has been studied in detail but the coleoptile chloroplast is poorly understood. The present study was aimed at the characterization of the corn coleoptile chloroplast, and its comparison with mesophyll and guard cell chloroplasts. Coleoptile chloroplasts operated the xanthophyll cycle, and their zeaxanthin content tracked incident rates of solar radiation throughout the day. Zeaxanthin formation was very sensitive to low incident fluence rates, and saturated at around 800–1000 mol m^–2 s^–1. Zeaxanthin formation in corn mesophyll chloroplasts was insensitive to low fluence rates and saturated at around 1800 mol m^–2 s^–1. Quenching rates of chlorophyll a fluorescence transients from coleoptile chloroplasts induced by saturating fluence rates of actinic red light increased as a function of zeaxanthin content. This implies that zeaxanthin plays a photoprotective role in the coleoptile chloroplast. Addition of low fluence rates of blue light to saturating red light also increased quenching rates in a zeaxanthin-dependent fashion. This blue light response of the coleoptile chloroplast is analogous to that of the guard cell chloroplast, and implicates these organelles in the sensory transduction of blue light. On a chlorophyll basis, coleoptile chloroplasts had high rates of photosynthetic oxygen evolution and low rates of photosynthetic carbon fixation, as compared with mesophyll chloroplasts. In contrast with the uniform chloroplast distribution in the leaf, coleoptile chloroplasts were predominately found in the outer cell layers of the coleoptile cortex, and had large starch grains and a moderate amount of stacked grana and stroma lamellae. Several key properties of the coleoptile chloroplast were different from those of mesophyll chloroplasts and resembled those of guard cell chloroplasts. We propose that the common properties of guard cell and coleoptile chloroplasts define a functional pattern characteristic of chloroplasts specialized in photosensory transduction.Abbreviations Ant or A antheraxanthin - dv/dt fluorescence quenching rate - Fm maximum yield of fluorescence with all PS II reaction centers closed - Fo yield of instantaneous fluorescence with all PS II reaction centers open - Vio or V violaxanthin - Zea or Z zeaxanthin     

Effect of Doubled-CO2 Concentration on the Ultrastructure of Chloroplasts from Medicago sativa and Setaria italica

It has been reported in quite a number of literatures that doubled CO2 concentration increased the photosynthetic rate and dry matter production of C3 plants, but substantially affected C4 plants little. However, why may CO2 enrichment promote growth and either no change or decrease reproductive allocation of the C3 species, but havinag no effects on growth characteristics of the C4 plants? So far, there has been no satisfactory explanation on that mentioned above, except the differences in their CO2 compensatory points. In the past, although some studies on ultrastructure of the chloroplasts under doubled CO2 concentration were limitedly conducted. Almost all the relevant experimental materials were only from C3 plants not from C4 plants, and even though the results were of inconsistancy. Thereby, it needs to verify whether the differences in photosynthesis of C3 and C4 plants at doubled CO2 level is caused by the difference in their chloroplast deterioration. Experiments to this subject were conducted at the Botanical Garden of Institute of Botany, Academia Sinica in 1993 and 1994. Both experimental materials from C3 plant alfalfa (Medicago sativa) and C4 plant foxtail millet (Setaria italica) were cultivated in the cylindrical open-top chambers (2.2 m in diameter × 2.4 m in height) with aluminum frames covered by polyethylene film. Natural air or air with 350× 10-6 CO2 were blown from the bottom of the chamber space with constant temperature between inside and outside of the chamber 〈0.2℃〉. Electron microscopic observation revealed that the ultrastructure of the chloroplasts from C3 plant Medicago sativa and C4 plant Seteria italica growing under the same doubled CO2 concentration were quite different from each other. The differential characteristics in ultrastructure of chloro plasts displayed mainly in the configuration of thylakoid membrances and the accumulation of starch grains. They were as follows: 1. The most striking feature was the building up of starch grains in the chloroplasts of the bundle sheath cells (BSCs) and the mesophyll cells (MCs) at doubled CO2 concentra tion. The starch grains appeared centrifugally first in the BSCs and then in the chloroplast of the other MCs. It was worthy to note that the starch grains in the chloroplasts of C4 plant Setaria ira/ica were much more than those of the C3 plant Medicago sativa . The decline of photosynthesis in the doubled CO2-grown C4 plants might be caused by an over accumulation of starch grains, that deformed the chloroplast even demaged the stroma thylakoids and grana. There might exsist a correlation between the comformation of thylakoid system and starch grain accumulation, namely conversion and transfer of starch need energy from ATP, and coupling factor (CF) for ATP formation distributed mainly on protoplastic surface (PSu) of stroma thylakoid membranes, as well as end and margin membranes of grana thylakoids. Thereby, these results could provide a conclusive evidence for the reason of non effectiveness on growth characteristics of C4 plant. 2. Under normal condition , the mature chlolroplats of higher plants usually develop complete and regularly arranged photosynthetic membrane systems . Chloroplasts from the C4 plant Setaria italica, however, exerted significant changes on stacking degree, grana width and stroma thylakoid length under doubled CO2 concentration; In these changes, the grana stacks were smaller and more numerous, and the number of thylakoids per granum was greatly increased, and the stroma thylakoid was greatly lengthened as compared to those of the control chloroplasts. But the grana were mutually intertwined by stroma thylakoid. The integrity of some of the grana were damaged due to the augmentation of the intrathylakoid space . Similarly, the stroma thylakoids were also expanded. In case. the plant was seriously effected by doubled CO2 concentration as observed in C4 plant Setaria italica , its chloroplasts contained merely the stroma (matrix) with abundant starch grains, while grana and stroma thylakoid membranes were unrecognizable, or occasionally a few residuous pieces of thylakoid membranes could be visualized, leaving a situation which appeared likely to be chloroplast deterioration. However, under the same condition the C3 plant Medicago sativa possessed normally developed chloroplasts, with intact grana and stroma thylakoid membranes. Its chloroplasts contained grana intertwined with stroma thylakoid membranes, and increased in stacking degree and granum width, in spite of more accumulated starch grains within the chloroplasts. These configuration changes of the thylakoid system were in consistant with the results of the authors another study on chloroplast function, viz. the increased capacity of chloroplasts for light absorption and efficiency of PSⅡ.     

不同叶色水稻叶绿体密度及基粒结构的计算机图象分析

水稻叶绿体计算机图象分析表明,随着叶片色级的提高,叶绿体表面积密度、体积密度以及两者的比值都相应增加。深色稻叶基粒堆直径与高度、类囊体垛叠数与类囊体厚度、叶绿素与类胡萝卜素含量、气孔导度与净光合率均大于浅色叶片。深色叶片基粒堆密集,有些基粒类囊体出现沿叶绿体长轴方向排列整齐现象;浅色叶片基粒堆稀疏,其中较大的基粒类囊体与长轴呈倾斜排列。     

盐胁迫下芦苇叶肉细胞超微结构的研究

对青藏高原柴达木盆地柯柯盐湖边盐碱地上生长的芦苇叶肉细胞的超微结构进行了研究,并以西宁地区非盐碱地上生长的芦苇作对照。结果表明：西宁地区的芦苇叶肉细胞的叶绿体呈椭圆形,其膜系统完整,基粒片层和基质片层发育良好。在盐碱地上生长的芦苇叶肉细胞的叶绿体呈圆形,叶绿体内出现较大的淀粉粒,并发现有线粒体嵌入叶绿体的现象。叶绿体的类囊体膨大,线粒体的嵴也有膨大的现象。在盐湖水中生长的芦苇叶肉细胞,叶绿体的类囊体排列紊乱、扭曲、松散。类囊体膜局部被破坏,部分类囊体膜解体,空泡化,甚至消失,一些溶解了的类囊体流进细胞质中。综上所述,芦苇叶肉细胞超微结构的变化是该植物适应柯柯盐湖地区盐渍、低温、低气压、强辐射等环境因子的结果。     

THE EFFECT OF LIGHT INTENSITY AND TEMPERATURE ON PLANT GROWTH AND CHLOROPLAST ULTRASTRUCTURE IN SOYBEAN

Soybean plants grown in controlled environment cabinets under light intensities of 220 w/m² or 90 w/m² (400–700 nm) and day to night temperatures of 27.5–22.5 C or 20.0–12.5 C in all combinations, exhibited differences in growth rate, leaf anatomy, chloroplast ultrastructure, and leaf starch, chlorophyll, and chloroplast lipid contents. Leaves grown under the lower light intensity at both temperatures had palisade mesophyll chloroplasts containing well-formed grana. The corresponding leaves developed under the higher light intensity had very rudimentary grana. Chloroplasts from high temperature and high light had grana consisting of two or three appressed thylakoids, while grana from the low temperature were confined to occasional thylakoid overlap. Spongy mesophyll chloroplasts were less sensitive to growth conditions. Transfer experiments showed that the ultrastructure of chloroplasts from mature leaves could be modified by changing the conditions, though the effect was less marked than when the leaf was growing.     

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.     

Localization of photosynthetic electron transport components in mesophyll and bundle sheath chloroplasts of Zea mays

The organization of the electron transport components in mesophyll and bundle sheath chloroplasts of Zea mays was investigated. Grana-containing mesophyll chloroplasts (chlorophyll a to chlorophyll b ratio of about 3.0) possessed the full complement of the various electron transport components, comparable to chloroplasts from C₃ plants. Agranal bundle sheath chloroplasts ($\text{Chl}\text{a}\text{Chl}\text{b}\text{> 5.0}$) contained the full complement of photosystem (PS) I and of cytochrome (cyt) f but lacked a major portion of PS II and its associated Chl $\text{a}\text{b}$ light-harvesting complex (LHC), and most of the cyt b₅₅₉. The kinetic analysis of system I photoactivity revealed that the functional photosynthetic unit size of PS I was unchanged and identical in mesophyll and bundle sheath chloroplasts. The results suggest that PS I is contained in stroma-exposed thylakoids and that it does not receive excitation energy from the Chl $\text{a}\text{b}$ LHC present in the grana. A stoichiometric parity between PS I and cyt f in mesophyll and bundle sheath chloroplasts indicates that biosynthetic and functional properties of cyt f and P₇₀₀ are closely coordinated. Thus, it is likely that both cyt f and P₇₀₀ are located in the membrane of the intergrana thylakoids only. The kinetic analysis of PS II photoactivity revealed the absence of PS II_αfrom the bundle sheath chloroplasts and helped identify the small complement of system II in bundle sheath chloroplasts as PS II_β. The distribution of the main electron transport components in grana and stroma thylakoids is presented in a model of the higher plant chloroplast membrane system.     

Palisade Tissue Chloroplasts and Spongy Tissue Chloroplasts in Spinach: Biochemical and Ultrastructural Differences

Palisade tissue chloroplasts (P-Chlts) and spongy tissue chloroplasts(S-Chlts) were separately isolated from spinach leaves, andtheir photosynthetic properties were compared. The followingresults were obtained: (1) At saturating light, the activities of overall electrontransport and CO₂ fixation in P-Chlts were respectively 1.6–2.0and 2.5–3.0 times higher than those in S-Chlts on a Chlbasis. (2) The contents of PS I and PS II reaction centers (P700 and47 kDa polypeptide, respectively) were slightly higher in P-Chltsthan in S-Chlts, while the contents of plastoquinone, Cyt f,plastocyanin, ferredoxin, ferredoxin-NADP⁺ reductase, couplingfactor and ribulose-bisphosphate carboxylase were 1.6–2.2times higher in P-Chlts than in S-Chlts on a Chl basis. (3) Electron microscopic examination of chloroplast ultrastructureshowed that S-Chlts have highly stacked grana accompanied byhigher proportion of appressed thylakoids relative to non-appressedthylakoids, while P-Chlts have poorly stacked grana. The volumeratio of thylakoids to stroma was higher in S-Chlts than inP-Chlts. These results indicate that mesophyll chloroplasts adapt tothe light environment within a leaf in a similar way that thesun and shade plant chloroplasts adapt to the light environmentwithin a canopy. (Received July 19, 1984; Accepted October 13, 1984)     

Recovery of maize seedling growth, development and photosynthetic efficiency after initial growth at low temperature

In order to investigate the mechanisms of maize adaptation to temperate climate, we studied photosynthetic efficiency, as evaluated by means of phiPSII and chloroplast ultrastructure, as well as growth and development of two inbred lines (the chilling-tolerant KW 1074 and the chilling-sensitive CM 109) under laboratory conditions. Plants were grown from seed to the 3rd leaf stage at a suboptimal temperature (14 degrees C/ 12 degrees C) and then the temperature was increased to 24 degrees C/22 degrees C. To verify the results obtained with the two model lines, twelve inbred lines were tested under both laboratory and field conditions. Initial growth at low temperature affected the chloroplast ultrastructure and photosynthetic efficiency, and this was more pronounced in CM 109 than in KW 1074 plants. The differences between the two lines were particularly pronounced in leaf 5. One week after the onset of favourable conditions, mesophyll chloroplast grana in the CM 109 line were small and thylakoids were developed only poorly. Also, thylakoids in bundle sheath chloroplasts were less frequent in CM 109 than in KW 1074. However, two weeks after the temperature increase, the ultrastructure of chloroplasts of the 5th leaf no longer differed distinctly between the two lines. One should note that in both lines, only the 7th and younger leaves reached a chloroplast ultrastructure and phiPSII indistinguishable from those of control plants. In general, the recovery of photosynthetic efficiency followed the development of leaves. It was delayed in the CM 109 more than in the KW 1074 inbred line relative to control plants grown continuously at the optimal temperature. The growth difference of 2-3 days between the two lines persisted even after the growth temperature was elevated. This suggested that the primary factor responsible for the different chilling-sensitivities of the two model lines was leaf development and the differences in development of the photosynthetic apparatus had only a secondary role. The delay in leaf development appeared as early as the stage of the 1st leaf. The same delay was observed when only the shoot apex was cooled. The importance for further recovery of the early stages of morphogenesis was confirmed by a correlation of Laboratory and field data that were obtained using a set of 12 inbred lines. Our results suggest that early stages of shoot morphogenesis determine the duration of the vegetative phase in cool regions, since the delay in growth at a low temperature cannot be compensated for during later growth at a higher temperature.     

Na2CO3胁迫对星星草叶肉细胞超微结构的影响

利用透射电镜技术对Na₂CO₃胁迫下星星草叶肉细胞超微结构进行了观察。结果表明：未胁迫的叶肉细胞排列疏松,各种细胞器结构完整,叶绿体含少量淀粉粒和脂质球。轻度盐胁迫（2g/L,4g/L Na₂CO₃）对叶肉细胞超微结构影响较小。中度盐胁迫（6g/L,8g/L Na₂CO₃）引起叶肉细胞超微结构的变化,叶绿体类囊体肿胀,基粒紊乱,不含淀粉粒,脂质球数量增加,叶绿体由原来的梭形或椭球形变成圆球状;部分线粒体嵴消失,出现晶体结构;中央大液泡破裂;核逐渐降解。高度盐胁迫（10g/L,12g/L Na₂CO₃）下,叶绿体片层结构消失,脂质球数量增加,体积变大,被大量的膜片层所包围,叶绿体内、外膜消失,叶肉细胞中看不到叶绿体的存在;膜片层包围线粒体;叶肉细胞中可见大量的泡状结构和膜片层,叶肉细胞死亡。上述结果表明,细胞器特别是叶绿体膜结构的破坏与盐胁迫叶肉细胞最终死亡密切相关     

Characterization of target site of aluminum phytotoxicity in photosynthetic electron transport by fluorescence techniques in tobacco leaves

Aluminum (Al) toxicity limits crop yield in acidic soil through affecting diverse metabolic processes, especially photosynthesis. The aim of this work was to examine the effect of Al on photosynthetic electron transport in vivo as determined by chlorophyll fluorescence and delayed fluorescence of tobacco leaves. Results showed that Al treatment inhibited the photosynthetic rate and electron transfer, and decreased photosystem (PS) II photochemical activity in a time- and concentration-dependent manner, which could not be obviously alleviated by the addition of the reactive oxygen species (ROS) scavenger ascorbic acid (AsA). These results suggested that photosynthetic electron transfer chain components, especially PSII, might be directly damaged by Al instead of in an ROS-dependent manner. Furthermore, the fluorescence imaging and biochemical analysis exhibited that Al, after entering the cells, could accumulate in the chloroplasts, which paralleled the decreased content of Fe in the chloroplast. The changes in the chlorophyll fluorescence decay curve, the delayed fluorescence decay curve and the chlorophyll fluorescence parameters indicated that Al, through interacting with or replacing the non-heme iron between Q(A) and Q(B), caused the inhibition of electron transfer between Q(A) and Q(B), resulting in PSII photochemical damage and inhibition of the photosynthetic rate. In summary, our results characterized the target site of Al phytotoxicity in photosynthetic electron transport, providing new insight into the mechanism of Al phytotoxicity-induced chloroplast dysfunction and photosynthetic damage.     

不同海拔火绒草叶绿体超微结构的比较

利用透射电镜对生长于青藏高原东北部3个不同海拔地区(2300m、2700m和3800m)的火绒草叶绿体超微结构进行了比较观察。结果发现,随着海拔的升高,叶绿体结构差异明显。海拔2300m处,叶绿体呈扁船形,沿细胞壁分布,基粒片层排列整齐,片层可达32层;海拔2700m处,叶绿体呈扁船形,沿细胞壁分布,基粒片层排列不规则,片层下降到十几层,类囊体出现轻微膨大;海拔3800m处,叶绿体呈圆形,位于细胞中央,基粒片层则严重扭曲,片层只有几层,类囊体膨大严重,出现脂质小球。研究表明,火绒草叶绿体结构的变化是对逆境的一种适应,是青藏高原特殊生态条件长期胁迫的结果。     

Chloroplast ultrastructure and fluorescence response of oilseed rape containing male-sterile radish cytoplasm

Summary The production of hybrid seed is facilitated if one parent possesses a male-sterile cytoplasm. Introduction of the cytoplasm of male-sterile radish (Raphanus sativus L.) into rapeseed (Brassica napus L.) results not only in transfer of the desirable male-sterile trait but induces a chlorophyll defect in the backcrossed male-sterile plants. In this study we show that the defect manifests itself in two different ways in the alloplasmic plants: a) smaller and fewer chloroplasts with an impaired ultrastructure and b) an increase in chlorophyll fluorescence. Defective chloroplasts were characterized by a reduction in both the number and size of grana, the latter due to poor stacking of thylakoids and with frequent discontinuity in the intergranal thylakoid systems. The changed chloroplast morphology and the increase in chlorophyll fluorescence are probably the cause of the lowered photosynthetic efficiency associated with the alloplasmic plants. We propose that the deficiency is the result of incompatibility between the genomes of the radish chloroplast and the rapeseed nucleus. Supporting this hypothesis are studies of male-sterile rapeseed plants in which, by protoplast fusion, the radish chloroplasts were substituted by those of normal male-fertile rapeseed. Such plants showed complete restoration of their photosynthetic potential and displayed both normal chloroplast ultrastructure and normal levels of chlorophyll fluorescence.     

Fluorescence and Delayed Light Emission from Mesophyll and Bundle Sheath Cells in Leaves of Normal and Salt-Treated Panicum miliaceum

A modified fluorescence microscope system was used to measure chlorophyll fluorescence and delayed light emission from mesophyll and bundle sheath cells in situ in fresh-cut sections from leaves of Panicum miliaceum L. The fluorescence rise in 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU)-treated leaves and the slow fluorescence kinetics in untreated leaves show that mesophyll chloroplasts have larger photosystem II unit sizes than do bundle sheath chloroplasts. The larger photosystem II units imply more efficient noncyclic electron transport in mesophyll chloroplasts. Quenching of slow fluorescence also differs between the cell types with mesophyll chloroplasts showing complex kinetics and bundle sheath chloroplasts showing a relatively simple decline. Properties of the photosynthetic system were also investigated in leaves from plants grown in soil containing elevated NaCl levels. As judged by changes in both fluorescence kinetics in DCMU-treated leaves and delayed light emission in leaves not exposed to DCMU, salinity altered photosystem II in bundle sheath cells but not in mesophyll cells. This result may indicate different ionic distributions in the two cell types or, alternatively, different responses of the two chloroplast types to environmental change.     

Structural and Functional Changes of Mesophyll Cells during Leaf Growth in Two Species of Spring Ephemers

The chloroplast ultrastructure, plastid pigments, and potential photosynthesis of leaf mesophyll cells were examined during the vegetative season of two spring ephemers Scilla sibirica Haw. and Chionodoxa luciliae Boiss. The development of chloroplasts was shown to precede the appearance of photosynthesis. The earliest stage of leaf growth was marked by the synthesis of carotenoids that play a structural and organizational role in the formation of chloroplast grana and protect the photosynthetic apparatus from photodynamic destruction under high insolation and low temperature conditions. Chlorophyll synthesis was closely correlated with the dynamics of potential photosynthesis. All these structural and functional features of mesophyll cells reflect the evolutionary strategies of adaptation in spring ephemers, which enable these plants to complete their short life cycle in the environment combining low temperature and high insolation.