Ultrastructure of Mesophyll Cells and Pigment Content in Senescing Leaves of Maize and Barley

Leaf senescence is a genetically regulated stage in the plant life cycle leading to death. Ultrastructural analysis of a particular region of the leaf and even of a particular mesophyll cell can give a clear picture of the time development of the process. In this study we found relations between changes in mesophyll cell ultrastructure and pigment concentration in every region of the leaf during leaf senescence in maize and barley. Our observations demonstrated that each mesophyll cell undergoes a similar senescence sequence of events: a) chromatin condensation, b) degradation of thylakoid membranes and an increase in the number of plastoglobules, c) damage to internal mitochondrial membrane and chloroplast destruction. Degradation of chloroplast structure is not fully correlated with changes in photosynthetic pigment content; chlorophyll and carotenoid content remained at a rather high level in the final stage of chloroplast destruction. We also compared the dynamics of leaf senescence between maize and barley. We showed that changes to the mesophyll cells do not occur at the same time in different parts of the leaf. The senescence damage begins at the base and moves to the top of the leaf. The dynamics of mesophyll cell senescence is different in leaves of both analyzed plant species; in the initial stages, the process was faster in barley whereas in the later stages the process occurred more quickly in maize. At the final stage, the oldest barley mesophyll cells were more damaged than maize cells of the same age.     

A large population of small chloroplasts in tobacco leaf cells allows more effective chloroplast movement than a few enlarged chloroplasts

We generated transgenic tobacco (Nicotiana tabacum cv Xanthi) plants that contained only one to three enlarged chloroplasts per leaf mesophyll cell by introducing NtFtsZ1-2, a cDNA for plastid division. These plants were used to investigate the advantages of having a large population of small chloroplasts rather than a few enlarged chloroplasts in a leaf mesophyll cell. Despite the similarities in photosynthetic components and ultrastructure of photosynthetic machinery between wild-type and transgenic plants, the overall growth of transgenic plants under low- and high-light conditions was retarded. In wild-type plants, the chloroplasts moved toward the face position under low light and toward the profile position under high-light conditions. However, chloroplast rearrangement in transgenic plants in response to light conditions was not evident. In addition, transgenic plant leaves showed greatly diminished changes in leaf transmittance values under both light conditions, indicating that chloroplast rearrangement was severely retarded. Therefore, under low-light conditions the incomplete face position of the enlarged chloroplasts results in decreased absorbance of light energy. This, in turn, reduces plant growth. Under high-light conditions, the amount of absorbed light exceeds the photosynthetic utilization capacity due to the incomplete profile position of the enlarged chloroplasts, resulting in photodamage to the photosynthetic machinery, and decreased growth. The presence of a large number of small and/or rapidly moving chloroplasts in the cells of higher land plants permits more effective chloroplast phototaxis and, hence, allows more efficient utilization of low-incident photon flux densities. The photosynthetic apparatus is, consequently, protected from damage under high-incident photon flux densities.     

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.     

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.     

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.     

Leaf permease1 gene of maize is required for chloroplast development.

Adjacent bundle sheath and mesophyll cells cooperate for carbon fixation in the leaves of C4 plants. Mutants with compromised plastid development should reveal the degree to which this cooperation is obligatory, because one can assay whether mesophyll cells with defective bundle sheath neighbors retain C4 characteristics or revert to C3 photosynthesis. The leaf permease1-mutable1 (lpe1-m1) mutant of maize exhibits disrupted chloroplast ultrastructure, preferentially affecting bundle sheath choroplasts under lower light. Despite the disrupted ultrastructure, the metabolic cooperation of bundle sheath and mesophyll cells for C4 photosynthesis remains intact. To investigate this novel mutation, the Activator transposon-tagged allele and cDNAs corresponding to the Lpe1 mRNA from wild-type plants were cloned. The Lpe1 gene encodes a polypeptide with significant similarity to microbial pyrimidine and purine transport proteins. An analysis of revertant sectors generated by Activator excision suggests that the Lpe1 gene product is cell autonomous and can be absent up to the last cell divisions in the leaf primordium without blocking bundle sheath chloroplast development.     

The Development of Chloroplast Structure During Leaf Ontogeny

Advances achieved during last fifteen years in the understanding the development of chloroplast ultrastructure during natural leaf ontogeny are summarized. Life span of a typical C3 mesophyll cell chloroplast is outlined and placed into the scheme of cyclic plastid interrelationships. Possible modifications of this development by stresses, environmental factors or experimental treatments are also shown.     

The leaf anatomy of a broad-leaved evergreen allows an increase in leaf nitrogen content in winter

In temperate regions, evergreen species are exposed to large seasonal changes in air temperature and irradiance. They change photosynthetic characteristics of leaves responding to such environmental changes. Recent studies have suggested that photosynthetic acclimation is strongly constrained by leaf anatomy such as leaf thickness, mesophyll and chloroplast surface facing the intercellular space, and the chloroplast volume. We studied how these parameters of leaf anatomy are related with photosynthetic seasonal acclimation. We evaluated differential effects of winter and summer irradiance on leaf anatomy and photosynthesis. Using a broad-leaved evergreen Aucuba japonica , we performed a transfer experiment in which irradiance regimes were changed at the beginning of autumn and of spring. We found that a vacant space on mesophyll surface in summer enabled chloroplast volume to increase in winter. The leaf nitrogen and Rubisco content were higher in winter than in summer. They were correlated significantly with chloroplast volume and with chloroplast surface area facing the intercellular space. Thus, summer leaves were thicker than needed to accommodate mesophyll surface chloroplasts at this time of year but this allowed for increases in mesophyll surface chloroplasts in the winter. It appears that summer leaf anatomical characteristics help facilitate photosynthetic acclimation to winter conditions. Photosynthetic capacity and photosynthetic nitrogen use efficiency were lower in winter than in summer but it appears that these reductions were partially compensated by higher Rubisco contents and mesophyll surface chloroplast area in winter foliage.     

Effect of N6-benzyladenine and indole-3-butyric acid on photosynthetic apparatus of Orthosiphon stamineus plants grown in vitro

The leaf structure and chloroplast ultrastructure of kidney tea (Orthosiphon stamineus Benth.) was studied in in vitro culture on standard MS medium supplemented with or without plant growth regulators (PGRs). The cytokinin N⁶-benzyladenine (BA) negatively affected the structure of the palisade parenchyma and chloroplast ultrastructure and increased the stomatal frequency of the adaxial epidermis. The auxin indole-3-butyric acid (IBA) did not modify the morphology of regenerated leaf tissues as well as the chloroplast ultrastructure. The effect of both PGRs applied in combination was manifested in well-differentiated mesophyll parenchyma, typical chloroplast ultrastructure and increased stomatal frequency on both leaf surfaces. This protocol can be suggested for further ex vitro propagation.     

Changes in photosynthetic apparatus of mustard (<Emphasis Type="Italic">Sinapis alba</Emphasis>) cotyledons following cycloheximide and kinetin treatments

Biochemical and accompanying structural characteristics of the photosynthetic process were studied in mustard seedlings cultivated on medium with increasing concentrations of cycloheximide alone as well as in combination with various kinetin concentrations. After 7 days of cultivation the contents of total chlorophyll, carotenoids and content of Rubisco in mustard cotyledons were determined. The content of chlorophyll pigments and carotenoids decreased in dependence of cycloheximide concentration. Following antibiotic treatment the content of both Rubisco subunits markedly decreased. In addition cycloheximide caused disturbance in mesophyll organization and chloroplast ultrastructure. Kinetin applied with cycloheximide increased the amount of photosynthetic pigments as well as of Rubisco, compared to the cycloheximide alone. In the seedlings treated with cycloheximide+kinetin the structure of leaf mesophyll and chloroplast membrane system was similar to control. Our results indicate that kinetin diminished the negative effects of cycloheximide on photosynthetic pigments and Rubisco as well as on the structural traits of the cotyledons.     

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.     

Membrane-bound Plastid Inclusions and Chloroplast Thylakoid Formation in Sunflower (Helianthus annuus L.)

The ultrastructure, distribution and frequency of membrane-boundplastid inclusions present in the epidermal cells of leavesof intact sunflower plants (Helianthus annuus L.) and in theepidermal and mesophyll cells of sunflower leaf discs culturedin darkness have been studied. These inclusions appear to bedilated thylakoids containing a granular material which, undernormal conditions, is probably involved in chloroplast membraneformation. It is suggested that this material accumulates, andinclusions form, in the chloroplasts of sunflower leaves intwo specific situations. Firstly, in the completely differentiatedcells of the epidermis where the chloroplasts, although at arelatively immature stage, have nevertheless reached a terminalstage of development. Secondly, in the mesophyll cells of youngleaves when chloroplast development has been arrested at animmature stage by a 5-day dark period. In the latter situationthe material can be remobilized if plastid development is restimulated.The plastids of sunflower leaf discs cultured in darkness containboth membrane-bound inclusions and prolamellar bodies, indicatingthat they are separate and distinct structures possibly containingdifferent membrane components. Helianthus annuus L., sunflower, chloroplast, ultrastructure, plastid inclusions, thylakoid formation     

叶肉细胞导度研究进展

叶肉细胞导度指叶片叶肉细胞内部的CO2扩散能力,在植物生理生态及全球气候变化和陆地生态系统相互关系的研究中具有重要作用。系统介绍了叶肉细胞导度的发现、发展过程及其研究进展、几种目前国际上常用的叶肉细胞导度测度方法的原理、计算过程;强调了叶肉细胞导度作为光合作用扩散过程一部分的重要意义,明确了叶肉细胞导度的定义及分布范围。并探讨了不同方法的优缺点及注意事项。总结分析了叶肉细胞导度对不同环境因子(温度、水分及环境中CO2和O3浓度等)的响应,从不同角度对叶肉细胞导度的生态学意义进行了简单的概括。对叶肉细胞导度的未来研究进行了展望。     

Shading-induced changes in the leaf mesophyll of plants of different functional types

Changes in the structural characteristics of mesophyll induced by shading were investigated in ten species of wild plants of diverse functional types. In all plant types, shading reduced leaf thickness and density by 30–50% and total surface of mesophyll, by 30–70%. The extent and mechanisms of mesophyll structural rearrangement depended on the plant functional type. In the ruderal plants, integral parameters of mesophyll, such as the surface of cells and chloroplasts and mesophyll resistance, changed threefold predominantly because of changes in the dimensions of the cells and chloroplasts. In these plants, shading reduced the volume of chloroplasts by 30%, and the chloroplast numbers per cell declined. The competitor plants showed a twofold increase in mesophyll resistance due to a decrease in the number of photosynthesizing cells per leaf area unit. Moreover, these plants maintained constant dimensions of mesophyll cells, ratios mesophyll surface/mesophyll volume and chloroplast surface/cell surface. In stress-tolerant plants, diffusion resistance of mesophyll remained the same irrespective of the growing conditions, and mesophyll rearrangement was associated with inversely proportional changes in the dimensions of the cells and cell volume per chloroplast. Noteworthy of these plants were relatively constant chloroplasts number per cell, per leaf area unit and total surface area of chloroplasts. The nature of relationship between the mesophyll diffusion resistance and structural parameters of leaf mesophyll differed in plants of diverse functional types.     

Epidermal Micromorphology and Mesophyll Structure of Populus euphratica Heteromorphic Leaves at Different Development Stages

Leaf epidermal micromorphology and mesophyll structure during the development of Populus euphratica heteromorphic leaves, including linear, lanceolate, ovate, dentate ovate, dentate rhombic, dentate broad-ovate and dentate fan-shaped leaves, were studied by using electron and light microscopy. During development of heteromorphic leaves, epidermal appendages (wax crystals and trichomes) and special cells (mucilage cells and crystal idioblasts) increased in all leaf types while chloroplast ultrastructure and stomatal characters show maximum photosynthetic activity in dentate ovate and rhombic leaves. Also, functional analysis by subordinate function values shows that the maximum adaptability to adverse stress was exhibited in the broad type of mature leaves. The 12 heteromorphic leaf types are classified into three major groups by hierarchical cluster analysis: young, developing and mature leaves. Mature leaves can effectively obtain the highest stress resistance by combining the protection of xerophytic anatomy from drought stress, regulation of water uptake in micro-environment by mucilage and crystal idioblasts, and assistant defense of transpiration reduction through leaf epidermal appendages, which improves photosynthetic activity under arid desert conditions. Our data confirms that the main leaf function is differentiated during the developing process of heteromorphic leaves.     

烟草NtFtsZ2-1基因参与叶绿体分裂和扩大的调节

叶绿体虽然是植物细胞内一种极其重要的细胞器,但其分裂的分子机制尚不很清楚。已经证明FtsZ蛋白作为真核细胞分裂装置的一个关键成分,参与叶绿体的分裂过程。烟草的FtsZ基因属于2个不同的家族,在对NtFtsZ1家族成员研究的基础上,用正义和反义表达技术研究了NtFtsZ2家族成员NtFtsZ2-1基因在转基因烟草中的功能。显微分析结果表明NtFtsZ2-1基因的表达水平异常增强或减弱都会严重干扰叶绿体的正常分裂过程,导致叶绿体在形态和数目上的异常（体积明显增大,数目显著减少）,而单个叶肉细胞中叶绿体的总表面积在正反义转基因烟草和野生型烟草之间保持了相对稳定,没有发生明显的变化。同时还证明NtFtsZ2-1基因表达的变化对叶绿素含量和叶绿体的光合作用能力没有直接的影响。据此我们认为NtFtsZ2-1基因参与叶绿体的分裂和体积的扩大,其表达水平的波动会改变植物中叶绿体的数目和大小,而且在叶绿体的数目与体积之间可能存在一种补偿机制,保证叶绿体能最大限度地吸收光能,从而使光合作用得以正常进行。     

Maize F<Subscript>1</Subscript> hybrid differs from its maternal parent in the development of chloroplasts in bundle sheath,but not in mesophyll cells: Quantitative analysis of chloroplast ultrastructure and dimensions in different parts of leaf blade at the beginning of its senescence

The quantitative changes of chloroplast ultrastructure and dimensions in mesophyll (MC) and bundle sheath (BSC) cells, associated with the onset of leaf senescence, were followed along the developmental leaf blade gradient of the third leaf of maize (Zea mays L.). To ascertain whether the rapidity of structural changes associated with the transition of chloroplasts from mature to senescent state is a heritable trait, the parental and the first filial generations of plants were used. The heterogeneity of leaf blade, associated with the development of maize leaf (with the oldest regions at the apex and the youngest ones at the base) was clearly discernible in the ultrastructure and dimensions of chloroplasts; however, there were differences in the actual pattern of chloroplast development between both genotypes as well as between both cell types examined. While the course of MC chloroplasts’ development at the onset of leaf senescence in maize hybrid followed that of its parent rather well, this did not apply for the BSC chloroplasts. In this case, each genotype was characterized by its own distinguishable developmental pattern, particularly as regards the accumulation of starch inclusions and the associated changes of the size and shape of BSC chloroplasts.     

Structure of the Photosynthetic Apparatus in Leaves of Freshwater Hydrophytes: 2. Quantitative Characterization of Leaf Mesophyll and the Functional Activity of Leaves with Different Degrees of Submersion

The structure of leaf photosynthetic elements was investigated on 42 boreal plant species characterized by different degrees of submergence (helophytes, neustophytes, and hydatophytes). Six main types of mesophyll structures were identified. Quantitative characteristics for the mesostructure of the photosynthetic apparatus in these groups were determined, such as the size and abundance of cells and chloroplasts in the mesophyll and epidermis, the number of plastids per cell in each tissue, the total surface area of the mesophyll cells, epidermal cells, and chloroplasts per unit leaf area. Analysis showed that quantitative characteristics of the photosynthetic apparatus in hydrophytes are determined by two factors: (a) the degree of leaf submergence and (b) the type of mesophyll structure. With an increasing degree of immersion in water, the mesophyll types change in a sequence isopalisade dorsoventral homogeneous. The leaves become thinner, their weight per unit area diminishes, cells and chloroplasts become less numerous (on a per unit leaf area basis), but their dimensions become larger. Adaptation to aquatic medium is also manifested in the increasing contribution of the epidermal tissue to the overall photosynthesis: in submerged leaves, the epidermis accounts for more than 50% of the photosynthetic activity. The occurrence of six structural types of leaves contrasting in their characteristics was confirmed by discriminatory analysis according to the qualitative parameters of mesophyll.