Modified Clearing Method to Show Sieve Tubes in Minor Veins of Leaves

Leaf samples of Glycine max and numerous other dicotyledonous species were cleared by a common, well established procedure modified by using more concentrated (10% w/v) aqueous NaOH, and by leaving samples in NaOH for 2-4 weeks and in chloral hydrate for 3 days, all at mom temperature. A single dye, chlorazol black E (1g/100 ml absolute ethanol), is used to stain for 3-6 min. Samples are mounted with the lower epidermis upward. Sieve tubes in favorable material can be seen in minor veins and vein endings.     

Distinct Cell Wall Architectures in Seed Endosperms in Representatives of the Brassicaceae and Solanaceae

In some species, a crucial role has been demonstrated for the seed endosperm during germination. The endosperm has been shown to integrate environmental cues with hormonal networks that underpin dormancy and seed germination, a process that involves the action of cell wall remodeling enzymes (CWREs). Here, we examine the cell wall architectures of the endosperms of two related Brassicaceae, Arabidopsis (Arabidopsis thaliana) and the close relative Lepidium (Lepidium sativum), and that of the Solanaceous species, tobacco (Nicotiana tabacum). The Brassicaceae species have a similar cell wall architecture that is rich in pectic homogalacturonan, arabinan, and xyloglucan. Distinctive features of the tobacco endosperm that are absent in the Brassicaceae representatives are major tissue asymmetries in cell wall structural components that reflect the future site of radicle emergence and abundant heteromannan. Cell wall architecture of the micropylar endosperm of tobacco seeds has structural components similar to those seen in Arabidopsis and Lepidium endosperms. In situ and biomechanical analyses were used to study changes in endosperms during seed germination and suggest a role for mannan degradation in tobacco. In the case of the Brassicaceae representatives, the structurally homogeneous cell walls of the endosperm can be acted on by spatially regulated CWRE expression. Genetic manipulations of cell wall components present in the Arabidopsis seed endosperm demonstrate the impact of cell wall architectural changes on germination kinetics.Angiosperms are a diverse group of seed plants that reproduce by a double fertilization event; the first produces a zygote and the second a specialized nutritive tissue known as the endosperm. The endosperm and the maternally derived testa (seed coat) evolved to protect the embryo until conditions are favorable for germination and establishment of the next generation (Rajjou and Debeaujon, 2008; Linkies et al., 2010). Endosperm from cereals/grasses, such as maize (Zea mays), barley (Hordeum vulgare), and wheat (Triticum aestivum), is vital for human and animal nutrition and is therefore of global economic importance (Olsen, 2007). In many seeds, such as some representatives of the Brassicaceae, the endosperm is entirely absent at seed maturity, the storage reserves having been absorbed by the cotyledons during embryo development. Arabidopsis (Arabidopsis thaliana) and Lepidium (Lepidium sativum) are notable exceptions in that they have retained a thin layer of endosperm tissue in the mature seed (Müller et al., 2006; Linkies and Leubner-Metzger, 2012).Some seeds exhibit primary dormancy at maturity that has been induced by abscisic acid (ABA; Hilhorst, 1995; Kucera et al., 2005). In its simplest sense, dormancy can be thought of as a block to germination of an intact viable seed under favorable conditions (Hilhorst, 1995; Bewley, 1997). A more sophisticated definition was proposed by Baskin and Baskin (2004), who state that a dormant seed does not have the capacity to germinate in a specified period of time under any combination of normal physical environmental factors that are otherwise favorable for its germination. Seed dormancy can be imposed by the embryo, the seed coat (including the endosperm), or a combination of both depending on the plant species (Bewley, 1997).The endosperm has been shown to be an important regulator of germination potential in several systems, including tomato (Solanum lycopersicum; Groot et al., 1988; Toorop et al., 2000), tobacco (Nicotiana tabacum; Leubner-Metzger et al., 1995; Petruzzelli et al., 2003), Arabidopsis (Bethke et al., 2007), and Lepidium (Müller et al., 2006; Linkies et al., 2009; Voegele et al., 2011). Arabidopsis continues to be an important model for elucidating the hormonal and genetic networks that regulate dormancy and germination (Kucera et al., 2005; Holdsworth et al., 2008), and new bioinformatic methods are providing insights into the evolutionary conservation of such networks in angiosperms (Bassel et al., 2011). Research using the close relative Lepidium, whose larger size makes it amenable to biomechanical techniques, has given insight into the hormonal control of endosperm weakening during germination and established that the mechanism of control is conserved between Arabidopsis, Lepidium, and tobacco (Müller et al., 2006; Linkies et al., 2009; Voegele et al., 2011). It has been reported that ABA is a key regulator of germination in tobacco, Arabidopsis, and Lepidium, controlling the process of endosperm rupture but not testa rupture (Leubner-Metzger et al., 1995; Petruzzelli et al., 2003; Müller et al., 2006). Microarray analyses of ABA-treated Arabidopsis and Lepidium seeds revealed that many cell wall remodeling enzyme (CWRE) genes are down-regulated upon exogenous application of ABA (Penfield et al., 2006; Linkies et al., 2009). Therefore, it follows that ABA impacts cell wall remodeling, which influences germination kinetics. The endosperm is therefore an important control tissue for seed germination and represents a useful model to investigate cell wall architectures and their remodeling.Cell walls are robust, multifunctional structures that not only protect cells from biotic and abiotic stresses, but also regulate growth, physiology and development (Albersheim et al., 2010). Cell walls are fibrous composites in which cellulose microfibrils are coextensive with/cross-linked by noncellulosic polysaccharides. In dicotyledonous plants, xyloglucan (XG) is a major polymer that can cross-link cellulose (Cosgrove, 2000). Load-bearing fibrous networks impart tensile strength to cell walls and are embedded in more soluble, gel-like matrices of pectic polysaccharides, glycoproteins, proteins, ions, and water. The constituent pectic polymers are currently classified as homogalacturonan (HG), rhamnogalacturonan I [RG-I; also comprising arabinans and type 1 (arabino)galactans as side branches] and rhamnogalacturonan II, and xylogalacturonan (XGA) (Willats et al., 2001; Caffall and Mohnen, 2009). Pectins are involved in a diverse range of processes, including the regulation of intercellular adhesion/cell separation at the middle lamella, regulating the ionic status, and the porosity of cell walls that influences the access of CWREs to substrates (Willats et al., 2001). Noncellulosic polysaccharides exhibit numerous structural elaborations and differ in their glycan, methyl, and acetyl substitution (Caffall and Mohnen, 2009; Burton et al., 2010). Such modifications have the potential to impact their functionality, including their ability to interact with other wall components and their susceptibility to degradation and modification by CWREs.Studies using Arabidopsis (Iglesias-Fernández et al., 2011), Lepidium (Morris et al., 2011), and tomato (Groot et al., 1988) have highlighted a role for endo-β-mannanases (EBMs), enzymes that degrade heteromannan polysaccharides, during seed germination. In hard seeds with heteromannan-rich endosperms, such as carob (Ceratonia siliqua), date (Phoenix dactylifera), Chinese senna (Senna obtusifolia), and fenugreek (Trigonella foenum-graecum), however, it has been proposed that thinner walls in the micropylar endosperm (ME) and not EBM activity are responsible for allowing radicle protrusion during germination (Gong et al., 2005). Therefore, enzymatic cell wall remodeling and native cell wall architectural asymmetries both have the potential to impact on germination.Although studies on the molecular networks controlling germination have indicated a role for several classes of CWREs in endosperm remodeling and the promotion of germination (Penfield et al., 2006; Kanai et al., 2010; Morris et al., 2011), there is a paucity of information relating to the characterization of such changes at the cell wall level and, indeed, cell wall structures themselves. This study focuses on the targets of CWRE genes currently thought to be involved in seed germination (i.e. cellulose, XG, heteromannan, and pectic polysaccharides). We show that all three seeds possess a similar core cell wall architecture containing unesterified HG, arabinan, and XG. In tobacco, the core cell wall architecture is restricted to the ME, whereas in Arabidopsis and Lepidium, this architecture is observed throughout the endosperm. A further unique feature of the tobacco endosperm is abundant heteromannan. We also outline, using Arabidopsis, to what extent cell wall components contribute to the regulation of seed germination.     

Deoxynivalenol and Acetyldeoxynivalenol Accumulation in Field Maize Inoculated with Fusarium graminearum

Seven selected maize hybrids (Smolimag, Ruten, KLG 2210, Zenit, Betulisa, RAH BE 86101 and RAH BE 90102) were toothpick inoculated in the ear (at ∼ 10 days after midsilk) with a single isolate of Fusarium graminearum in order to determine their reaction to infection and susceptibility to mycotoxin accumulation in the tissues. Deoxynivalenol (DON), 15-acetyldeoxynivalenol (15-AcDON), and 3-acetyldeoxynivalenol (3-AcDON) were detected in differing amounts in kernels, husks, stalks (nodes and internodes) and roots. DON was recovered from stems, husks and damaged kernels at levels of 864, 278 and 93mg/kg, respectively, while 15-AcDON was recovered at 235, 55 and 11mg/kg and 3-AcDON was recovered at 3, 2 and 0.5 mg/kg, respectively. Maize hybrids Smolimag and Ruten were most resistant to infection and contained the lowest amounts of mycotoxins in the tissues while Zenit, RAH BE 90102, RAH BE 86101 and Betulisa were most susceptible to infection and accumulated higher amounts of mycotoxins.     

Preferential Accumulation by Mesophyll Cells at Low and by Veins at High Exogenous Amino Acid and Sugar Concentrations in Commelina benghalensis L. Leaves

The contribution to solute uptake by mesophyll cells and veinsin leaf discs, was assessed through a study of uptake in relationto concentration for ¹⁴C-labelled substrates (sucrose, glucose,arginine, proline, valine and -aminoisobutyric acid) using isolatedmesophyll cells and stripped leaf discs of Commelina benghalensisL. Uptake per unit fresh weight was higher in mesophyll cellsthan in discs at low substrate concentrations (lower than about0·5 mol m^–3). At higher concentrations, uptakeby discs exceeded that by mesophyll cells except for glucoseuptake which was higher in mesophyll cells over the whole concentrationrange. The profiles of uptake versus concentration displayedbiphasic kinetics in mesophyll cells and discs. Comparison ofthe uptake characteristics obtained by iterative fitting confirmedthat the high-affinity systems of uptake prevail in the mesophyllcells, whereas the low-affinity systems are dominant in theveins. The results provide good evidence that, supplementaryto direct vein loading, a pathway via the mesophyll contributesstrongly to the photosynthate loading by veins in stripped discs. Key words: Commelina benghalensis L., amino acid uptake, mesophyll, minor veins, phloem loading, sugar uptake     

Proline Accumulation in Leaves of NaCl-sensitive and NaCl-tolerant Tomatoes

Proline (Pro) accumulation in leaf discs of the NaCl-treated salt sensitive Lycopersicon esculentum was higher than in the salt tolerant L. pennellii. The magnitude of Pro accumulation differed when leaf discs of both species were floated in the light or under darkness, and in various incubation media: buffer solutions (pH from 3.9 to 7.8), abscisic acid, isobutyric acid, NH4Cl, malate, citrate, and mixtures of NaCl and KCl, NaCl and CaCl2, and NaCl and mannitol. Under darkness, Pro accumulation in L. esculentum was not regulated by salts. Conversely, the light-independent NaCl-induced Pro accumulation observed in L. pennellii became light-dependent when CaCl2 was added. The different expression patterns of Pro accumulation reinforce the proposal of two Pro metabolic pathways in the domestic and the wild tomatoes.     

Cadmium-Induced Accumulation of Putrescine in Oat and Bean Leaves

The effects of Cd²⁺ on putrescine (Put), spermidine (Spd), and spermine (Spm) titers were studied in oat and bean leaves. Treatment with Cd²⁺ for up to 16 hours in the light or dark resulted in a large increase in Put titer, but had little or no effect on Spd or Spm. The activity of arginine decarboxylase (ADC) followed the pattern of Put accumulation, and experiments with α-difluoromethylarginine established that ADC was the enzyme responsible for Put increase. Concentrations of Cd²⁺ as low as 10 micromolar increased Put titer in oat segments. In bean leaves, there was a Cd²⁺-induced accumulation of Put in the free and soluble conjugated fractions, but not in the insoluble fraction. This suggests a rapid exchange between Put that exists in the free form and Put found in acid soluble conjugated forms. It is concluded that Cd²⁺ can act like certain other stresses (K⁺ and Mg²⁺ deficiency, excess NH₄⁺, low pH, salinity, osmotic stress, wilting) to induce substantial increases in Put in plant cells.     

Moniliformin Accumulation in Kernels of Triticale Accessions Inoculated with Fusarium avenaceum, in Poland

Twelve Polish winter triticale cultivars and 14 doubled haploid lines (DH) (derived from the cv. Lasko × line SZD 366 hybrids) were inoculated with Fusarium avenaceum isolate ATCC 64451, mycotoxin moniliformin (MON) producer to evaluate their susceptibility to Fusarium head blight (FHB). Chemical analysis revealed MON accumulation in kernels of all inoculated cultivars in three consecutive years with the following averages and ranges: 1.50 mg/kg (0.47–2.67 mg/kg) in 1996, 2.63 mg/kg (0.11–8.14 mg/kg) in 1997 and 0.25 mg/kg (0.07–0.47 mg/kg) in 1998. Cultivar Malno kernels accumulated a low level of MON in all 3 years of the experiment. In most of the genotypes examined the reaction to the pathogen and MON content changed significantly from season to season. DH lines accumulated on average 2.62 and 0.85 mg/kg of MON in 1997 and 1998, respectively. Yield parameter reductions (1000 kernel weight, kernel number per head and kernel weight per head) were higher in 1997 than in 1998. The correlation coefficient for MON content/ Fusarium damaged kernels percentage was 0.539 in cultivars and 0.548 in the DH lines. This is the first report of FHB of a segregating population in triticale.     

Site of Fluoride Accumulation in Navel Orange Leaves

Fluoride-polluted navel orange leaves, Citrus sinensis (Linn.) Osbeck, were fractionated into the subcellular components in hexane/carbon tetrachloride mixtures having various densities. Fluoride was determined at each fraction. Analyses were also made for the subcellular distribution of chlorophyll, nitrogen, and DNA to assess the extent of cross-contamination of each component.The fraction containing cell wall, nuclei, and partly broken cells apparently contained a major amount of fluoride. However, if allowance was made for the cross-contamination of chloroplasts and chloroplast fragments, the fraction of chloroplasts was found to be the site of the highest fluoride accumulation. When each particulate component was washed with water after drying, the combined washings contained more than 50% of the total fluoride of the isolated fractions.The usual method of subcellular fractionation with aqueous solvent shifted the major site of fluoride accumulation from the fraction of chloroplasts to that of the supernatant.     

Antioxidant Systems and O2.−/H2O2 Production in the Apoplast of Pea Leaves. Its Relation with Salt-Induced Necrotic Lesions in Minor Veins

The present work describes, for the first time, the changes that take place in the leaf apoplastic antioxidant defenses in response to NaCl stress in two pea (Pisum sativum) cultivars (cv Lincoln and cv Puget) showing different degrees of sensitivity to high NaCl concentrations. The results showed that only superoxide dismutase, and probably dehydroascorbate reductase (DHAR), were present in the leaf apoplastic space, whereas ascorbate (ASC) peroxidase, monodehydroascorbate reductase (MDHAR), and glutathione (GSH) reductase (GR) seemed to be absent. Both ASC and GSH were detected in the leaf apoplastic space and although their absolute levels did not change in response to salt stress, the ASC/dehydroascorbate and GSH to GSH oxidized form ratios decreased progressively with the severity of the stress. Apoplastic superoxide dismutase activity was induced in NaCl-treated pea cv Puget but decreased in NaCl-treated pea cv Lincoln. An increase in DHAR and GR and a decrease in ASC peroxidase, MDHAR, ASC, and GSH levels was observed in the symplast from NaCl-treated pea cv Lincoln, whereas in pea cv Puget an increase in DHAR, GR, and MDHAR occurred. The results suggest a strong interaction between both cell compartments in the control of the apoplastic ASC content in pea leaves. However, this anti-oxidative response does not seem to be sufficient to remove the harmful effects of high salinity. This finding is more evident in pea cv Lincoln, which is characterized by a greater inhibition of the growth response and by a higher rise in the apoplastic hydrogen peroxide content, O(2)(.-) production and thiobarbituric acid-reactive substances, and CO protein levels. This NaCl-induced oxidative stress in the apoplasts might be related to the appearance of highly localized O(2)(.-)/H(2)O(2)-induced necrotic lesions in the minor veins in NaCl-treated pea plants. It is possible that both the different anti-oxidative capacity and the NaCl-induced response in the apoplast and in the symplast from pea cv Puget in comparison with pea cv Lincoln contributes to a better protection of pea cv Puget against salt stress.     

Symplastic Transfer of Fluorescent Dyes from Mesophyll to Sieve Tube in Stripped Leaf Tissue and Partly Isolated Minor Veins of Commelina benghalensis

We have stripped small (3 × 3 mm) fields of the upper and the opposite lower epidermis of Commelina benghalensis leaves. Pectinase treatment of the resulting chlorenchyma windows produced free-lying viable minor veins with small lumps of mesophyll cells attached. These veins were still connected with the intact remainder of the leaf. Fluorescent dyes were injected into mesophyll cells or mestome sheath cells. Continuous following of the dye from the moment of injection and use of the simple vein system allowed an unhindered and precise assessment of the cell-to-cell route of dye transfer. Disodium fluorescein and Lucifer Yellow CH injected into mesophyll or mestome sheath cells readily moved to the sieve tube. This symplastic dye transfer from mesophyll to sieve tube was also observed after injection into unmacerated stripped leaf tissue. The displacement of fluorescent dyes substantiates a symplastic continuity between mesophyll and sieve tube and therefore supports the possibility of symplastic phloem loading.     

Accumulation of Maltose during Photosynthesis in Protoplasts Isolated from Spinach Leaves Treated with Mannose

When mannose was included in the enzyme incubation medium during the preparation of protoplasts from leaves of spinach, maltose was an early product of protoplast photosynthesis and, after 12 minutes, accounted for up to 15% of the ¹⁴C incorporated from ¹⁴CO₂. Maltose was not detected in protoplasts prepared in the normal enzyme medium. Rapid separation of cytoplasm and chloroplasts following exposure to ¹⁴CO₂ showed that maltose was present in both fractions. Direct measurements of [¹⁴C]maltose uptake indicated transport across the chloroplast envelope at rates similar to the transport of glucose. The source of maltose and site of its initial formation are discussed.     

Dilated Endoplasmic Reticulum Cisternae in Differentiating Xylem of Minor Veins of Mimosa pudica L. Leaf

The differentiating tracheary elements in the xylem of minorveins in Mimosa pudica L. contain, as is usual, complete nucleateprotoplasts. Within the latter, dictyosomes with associatedvesicles and endoplasmic reticulum (ER) are prominent components.The ER cisternae show an uncommon feature of developing voluminousdilations accumulating finely fibrous material. Similar dilatedER cisternae occur in parenchyma cells associated with the trachearyelements. Most of the dilated cisternae are elongated, taperingat both ends, and almost circular in transections. They varyin size. The largest measured was 10 µm long and 3 µmwide. In the tracheary elements the cisternae break down asthe protoplast disintegrates. For a time, mature cells containfibrous material, apparently the product of the dilated cisternae,at least in part. In the parenchyma cells the dilated cisternaeare released into the vacuoles after the associated trachearyelements reach maturity. They become structurally modified anddisintegrate. The timing in the appearance and disintegrationof the dilated ER cisternae suggests that these structures havesome function with reference to the differentiation of trachearyelements in the xylem.     

Proline Oxidase and Water Stress-induced Proline Accumulation in Spinach Leaves

Spinach (Spinacia oleracea L.) leaf discs accumulated free proline when exposed to polyethylene glycol solutions of water potential less than −10 bars. At −20 bars, the accumulation was 11 micromoles per gram original fresh weight in a 24-hour period.     

植物叶片中抗坏血酸含量与草酸积累的关系

不同植物和不同生长期烟草叶片中抗坏血酸含量变化与相应的草酸含量变化之间都无显著的相关性;喂饲外源抗坏血酸后的水稻和荞麦叶中草酸含量提高不明显。据此推测：尽管不能排除抗坏血酸可能是植物草酸合成的前体,但其内源含量高低不一定影响植物中草酸积累。     

Volatile Products of the Lipoxygenase Pathway Evolved from Phaseolus vulgaris (L.) Leaves Inoculated with Pseudomonas syringae pv phaseolicola

Activation of the "lipoxygenase pathway" in plants gives rise to a series of products derived from fatty acids. Analysis by gas chromatography-mass spectroscopy of volatile products produced by Phaseolus vulgaris (L.) cv Red Mexican leaves during a hypersensitive resistance response (HR) to the plant pathogenic bacterium Pseudomonas syringae pv phaseolicola showed evolution of several lipid-derived volatiles, including cis-3-hexenol and trans-2-hexenal, which arise from the 13-hydroperoxide of linolenic acid. These compounds were not produced in detectable amounts by buffer-inoculated leaves, nor did they evolve to such a high degree during comparable stages of the susceptible response. The absence of trans-2,cis-6-nonadienal, a product expected from 9-hydroperoxide of linolenic acid, suggests that lipid peroxidation during the HR proceeded primarily enzymically via bean lipoxygenase, which produces the 13-hydroperoxide, and not via autoxidative processes. The effects of trans-2-hexenal, cis-3-hexenol, and traumatic acid on P.s pv phaseolicola were investigaed. trans-2-Hexenal appeared to be highly bactericidal at low concentrations, whereas cis-3-hexenol was bactericidal only at much higher concentrations. Traumatic acid appeared to have no effect on P.s. pv. phaseolicola at the concentrations tested. These results demonstrate that during plant defense responses against microbial attack, several lipid-derived compounds are produced by the plant, some of which possess antimicrobial activity and conceivably are involved in plant disease resistance. The time of production of these substances, in amounts that would be expected to be antibacterial in vitro, correlated with a slowing down of the growth rate of bacteria in the leaves and was seen at a time before the accumulation of isoflavonoid phytoalexins in the host.     

Pterocarpan Accumulation in Pod Infection-droplets and Pod Tissues of Phaseolus vulgaris Inoculated with Colletotrichum lindemuthianum1

Following inoculation of pods of several cultivars of French bean (Red Kidney Selections W 243, 244, 245), with Colletotrichum lindemuthianum (Sacc. & Magn.) Bri. and Cav. ANZ races 1 and 2, phaseollin, phaseollidin and 6-α-hydroxyphaseollin were estimated in infection-droplets and tissue extracts by u.v. spectrophotometry and HPLC. The examples represented both compatible and incompatible cultivar-race interactions and confirmed a positive correlation between degree of incompatibility andphaseollin concentration or content. The data supported the concept that a phytoalexin index may be of value in selecting for disease resistance or studying the quantitative inheritance of phaseollin in studies of anthracnose resistance in French beans.Concentrations of phaseollidin and 6-α-hydroxyphaseollin were low and were not correlated with degree of compatibility of cultivar-race interactions. The evidence did not support the putative role of 6-α-hydroxyphaseollin as an in planta degra, dative phaseollin metabolite associated with French bean –C. lindemuthianum compatible interactions.     

Immunolocalization of the Plasma Membrane H+ -ATPase in Minor Veins of Vicia faba in Relation to Phloem Loading

The phytotoxin fusicoccin (FC), after binding to a plasma membrane-localized receptor, causes higher plant cells to excrete protons. Ligand-binding analysis has been used to show that the plasma membrane of mung bean (Vigna radiata L.) hypocotyls contains both high-affinity (HA) and low-affinity (LA) binding sites for FC. The effect of tissue maturation on these sites was determined on isolated membrane vesicles from the meristematic region (hook) and the elongation zone and from mature hypocotyl tissues. In the meristematic region the HA:LA ratio was 1:20. As hypocotyl tissues matured, the site density of HA increased and there was no change in LA density, so that the HA:LA ratio increased to 1:2 in maturet issues. FC-induced proton excretion correlates with the HA density, not the LA density. When sections isolated from each region were incubated with FC prior to isolation of membranes, there was an apparent conversion of LA to HA sites during the first 90 min in all regions. During the next 1 to 3 h there was a further 2.5- to 3- fold increase in binding sites in all regions, accompanied by a slight decline in dissociation constant. The increase in binding sites, but not the apparent conversion of LA to HA, was partly blocked by cycloheximide. These data suggest that FC alters FC-binding protein activity in two ways: first, by causing an increase in affinity for FC of preexisting LA receptors, and second by inducing the synthesis of additional FC receptors. This apparent up-regulation of a phytotoxin receptor by its ligand in plants has not previously been reported.     

A Comparative Study of the Structure-Function Relationship of Cross Veins in Leaves of Digitaria eriantha and Zea mays

Ultrastructural and functional differences between the crossveins of Digitaria eriantha and Zea mays were investigated.Cross veins of both genera possess similar conducting tissues,namely one tracheary element and one sieve cell. In Digitariaeriantha these conducting elements are associated with onlytwo parenchyma cells, and, those in Zea mays are completelysurrounded by chJorenchyma cells. The protein ribulose 1,5-bisphosphatecarboxylase/oxygenase (rubisco) was used as a probe for CO₂fixation sites by comparing its distribution in the varioustissue types in the leaves of the two genera. The protein wasfound to be equally and uniformly distributed in the stromalregions of the chlorenchyma sheath cell chloroplasts of longitudinalveins of both genera. The chlorenchyma sheath cells in crossveins of Zea mays show a similar distribution of the enzymeas the longitudinal bundles. However, this enzyme was shownto be absent in the cross vein parenchyma cells of Digitariaeriantha and in the mesophyll cells of both genera. Cross veins, immuno-gold labelling, ribulose 1,5-bisphosphate carboxylase/oxygenase, Digitaria eriantha, Zea mays     

ABA诱致叶绿体膜脂变化和脂褐素类似物积累

酰脂是叶绿体膜结构的主要组分,它们的种类和在膜上的分布与膜的功能有关(Rawyler和Siegenthaler 1980)。Henry等(1982)曾证明,叶绿体在体外老化时膜的半乳糖脂和磷脂的相对含量发生变化。ABA一般能加速叶片的衰老反应,它在叶片衰老时积累(Wright和 Hiron 1970)。在气孔开启状态下,激动素有防止衰老的效应(Thimann和Satler 1979)。在衰老的菜豆(Pha-