Ultrastructural indications for coexistence of symplastic and apoplastic phloem loading in Commelina benghalensis leaves

The ultrastructural ontogeny of Commelina benghalensis minor-vein elements was followed. The mature minor vein has a restricted number of elements: a sheath of six to eight mestome cells encloses one xylem vessel, three to five vascular parenchyma cells, a companion cell, a thin-walled protophloem sieve-tube member and a thick-walled metaphloem sieve-tube member. The protophloem sieve-tube member (diameter 4–5 m; wall thickness 0.12 m) and the companion cell originated from a common mother cell. The metaphloem sieve-tube member (diameter 3 m; wall thickness 0.2 m) developed from the same precursor cell as the phloem parenchyma cells. Counting the plasmodesmatal frequencies demonstrated a symplastic continuum from mesophyll to the minor-vein phloem. The metaphloem sievetube member and the phloem parenchyma cells are the termini of this symplast. The protophloem sieve-tube member and companion cell constitute an insulated symplastic domain. The symplastic route, mesophyll to metaphloem sieve tube, appears to offer a path for symplastic loading; the protophloem sieve tube may be capable of accumulation from the apoplast. A similar two-way system of loading may exist in a number of plant families. Plasmodesmograms (a novel way to depict cell elements, plasmodesmatal frequencies and vein architecture) of some other species also displayed the anatomical requirements for two routes from mesophyll to sieve tube and indicate the potential coexistence of symplastic and apoplastic loading.     

Cytochemical localization of ATPase activity in phloem tissues ofRicinus communis

Summary Standard lead precipitation procedures have been used to examine the localization of ATPase activity in phloem tissues ofRicinus communis. Reaction product was localized on the plasma membrane of the companion cells associated with sieve elements and of parenchyma cells in phloem tissues from the leaf, petiole, stem and root. ATPase activity was also present on the plasma membrane and dispersed P-protein of sieve elements in petiole, stem and root tissue, but was absent from the plasma membrane of these cells in the leaf minor veins. Substitution of-glycerophosphate for ATP produced no change in the localization of reaction product in leaf tissue. These findings are discussed in relation to current theories on the mechanism of sugar transport and phloem loading.     

Development and morphology of stone cells in phloem of Toxicodendron vernicifluum

Key message

The morphology and development of sumac phloem sclereid were observed, sclereid was developed from phloem parenchyma and lignin was deposited in the cell wall of parenchyma and formation sclereid.

Abstract

Sumac [Toxicodendron vernicifluum (Stokes) F.A.Barkley] is a unique economic tree species in China. Raw lacquer is the sap flowing from the phloem of sumac. Stone cell clusters exist in the secondary phloem of sumac stem. In the present study, the morphology and development of stone cell clusters in sumac phloem were observed with optical microscope and transmission electron microscope. The distribution of lignin in the composition molecules of secondary phloem was observed with histochemistry method and fluorescence microscope. The results showed that phloem stone cells of sumac were developed from phloem parenchyma cells, and that lignin was deposited in layers in the cell wall of phloem parenchyma cells which cause the formation of stone cell clusters and which have the secondary wall. Studies on the ultrastructure of stone cells indicated that there was an obvious stratification and pits during the process of lignin deposition.     

The distribution of P-Protein in mature sieve elements of celery

Summary The extent of blocking of sieve-plate pores caused by release of cell turgor was investigated by fixing and processing for electron microscopy a long length of celery (Apium graveolens L.) phloem. Differences in distribution of P-protein within the pores were observed between those cells near the two cut ends, and the central cells.To assess the effect of chemical fixation on the distribution of P-protein, strands of celery phloem (fixed or unfixed, and not treated with cryoprotectants) were frozen in Freon 12 and then freeze-substituted. In sieve elements from unfixed tissue there were a greater number of sieve plates displaying partially open pores.Direct freezing of unprotected phloem tissue in Freon 12 resulted in the formation of ice crystals within the lumen of the sieve elements. Freezing of tissue at rates fast enough to avoid the formation of damaging ice crystals resulted in sieve-plate pores having an unoccluded central channel with a peripheral lining of P-protein. In the lumen of the sieve elements the P-protein filaments occurred as discrete bundles ca. 0.5 m in diameter, and as a parietal layer varying in thickness from 0.1 to 0.5 m.     

Microautoradiographic study of the incorporation of labelled amino acids into insoluble compounds of the shoot of a higher plant

Summary Radioactive amino acids are fed singly to intact shoots of field pea (Pisum arvense L.) via the transpiration stream. The range of radiosubstrates used is chosen to be representative of those compounds exported in quantity from the root system of field pea. The distribution of radioactivity from each substrate suggests that mature organs actively assimilate materials which enter the shoot through the xylem. During a four hour period 5–20% of the label recovered from mature tissues is present in insoluble form, much of this as protein.A microautoradiographic technique is used to localize insoluble labelled materials in thin (1–2 ) sections of the plant tissues. Certain cell types (xylem parenchyma, cambial initials, mesophyll of leaves, chlorenchyma of stem and petiole, and all elements of the phloem) appear to be particularly active in elaborating protein and other structural constituents from the labelled substrates. Preferential labelling of chloroplasts is observed in cells of mesophyll, while in all cells the wall components appear to be less readily labelled than their contained protoplasts.     

LEAF OF SPINACIA OLERACEA (SPINACH): ULTRASTRUCTURE,AND PLASMODESMATAL DISTRIBUTION AND FREQUENCY,IN RELATION TO SIEVE-TUBE LOADING

Minor veins and contiguous tissues of the Spinacia oleracea leaf were analyzed by electron microscopy to determine the characteristics of the component cells and the structure, distribution, and frequency of plasmodesmata between the various cell types of the leaf. Mesophyll and bundle-sheath cells contain components typical of photosynthetic cells although the latter cell type contains smaller chloroplasts and fewer mitochondria and microbodies than the mesophyll cells. In addition, the mesophyll cells contain numerous invaginations of the plasmalemma bordering the chloroplasts and evaginations of the outer membrane of the opposing chloroplast envelope. In places, these membranes appear continuous with each other. The minor veins consist of tracheary elements, xylem parenchyma cells, sieve-tube members, companion and phloem parenchyma cells, and other cells simply designated vascular parenchyma cells. The companion and phloem parenchyma cells are typically larger than the sieve-tube members with the companion cells containing a much denser cytoplasm that the phloem parenchyma. Cytoplasmic connections occur along all possible routes from the mesophyll to the sieve-tube members and consist of either simple or branched plasmodesmata between parenchymatic elements or pore-plasmodesmata between the sieve-tube members and parenchyma cells. The highest frequency of plasmodesmata occurs between the sieve-tube members and companion cells, although the value is essentially the same as between the various parenchymatic elements of the phloem. Compared to several previously studied species, the frequency of plasmodesmata between cell types of the spinach leaf is low. These results are discussed in relation to apoplastic vs. symplastic solute transport and sieve-tube loading in this species.     

Microelectrode-recorded development of the symplasmic autonomy of the sieve element/companion cell complex in the stem phloem of Lupinus luteus L.

From the cambial stage onwards, the symplasmic autonomy of sieve element/companion cell complexes (SE/CC-complexes) was followed in stems of Lupinus luteus L. by microinjection techniques. The membrane potential and the symplasmic autonomy of the mature SE/CC-complex was measured in successive internodes. A microelectrode was inserted into SE/CC-complexes or phloem parenchyma cells (PPs) and, after stabilization of the membrane potential, the membrane-impermeant fluorescent dye Lucifer Yellow CH (LYCH) was injected intracellullary. The plasmodesmata of the cambial SE/ CC precursor were gradually shut off at all interfaces beginning at the walls to be transformed into sieve plates. In the course of maturation, symplasmic discontinuity was maintained at the longitudinal walls of the complex. In the transverse walls of the SE, wide sieve pores were formed giving rise to longitudinal multicellular symplasmic domains of SE/CC-complexes. Symplasmic isolation of the files of mature SE/CC-complexes was demonstrated in several ways: (i) the membrane potential of the SE/CC-complexes (between -100 mV and -130 mV) was consistently more negative than that of the PPs (between-50 and -100 mV), (ii) No exchange of LYCH was observed between SE/CC-complexes and the PPs. Lucifer Yellow CH injected into the SEs exclusively moved to the associated CCs and to other SE/CC-complexes whereas LYCH injected into the PPs was only displaced to other PPs. (iii) The electrical coupling ratio between adjacent PPs was ten times higher than that between SE/CC-complex and PP. A gradient in the membrane potential of the SE/CC-complexes along the stem was not conclusively demonstrated.Abbreviations LYCH Lucifer Yellow CH - membrane potential - PMF proton-motive force - PP phloem parenchyma cell - SE/CC-complex sieve element/companion cell complex - SR-G sulphorhodamine G     

Morphological changes and transversal growth kinetics along the apical meristem in the pericycle cell types of the onion adventitious root

Summary In the apical meristem of the adventitious root ofAllium cepa, all pericycle cells show a marked increase in cross-sectional area between 400 and 800 m behind the tip, this transversal growth ceasing in the 1,200–1,400 m interval. However, different pericycle cell types (opposite xylem, intervening and opposite phloem) show different transversal growth kinetics. Along the meristem, the opposite xylem cells are narrower than both the intervening and opposite phloem cells, and these latter are similar in cross-sectional area. Another relevant difference is in the polarity of the transversal expansion, which in turn gives rise to changes in cell shape. In fact, in apical most portions of the meristem, the opposite phloem cells mainly expand tangentially, while the intervening cells do so radially, and the opposite xylem cells undergo a similar tangential and radial expansion. By contrast, in basal most portions of the meristem, radial expansion continues in the opposite phloem cells when it has ceased in the intervening cells. These latter expand tangentially once again when tangential expansion has ceased in the opposite phloem cells. As a consequence of this transversal growth, the opposite xylem cells, which can initiate lateral root primordia, retain their isodiametric transversal shape along the meristem, whereas the transversal shape of the opposite phloem and intervening cells initially changes from isodiametric to markedly enlarged tangentially (opposite phloem) or radially (intervening), after which both cell types tend to become more rounded in shape.     

宁夏枸杞果实韧皮部及其周围细胞超微结构研究

应用透射电镜技术研究了宁夏枸杞果实韧皮部细胞的超微结构变化。结果表明:(1)随着枸杞果实的发育成熟,果实维管组织中的韧皮部筛分子筛域逐渐变宽,筛孔大而多,通过筛孔的物质运输十分活跃;筛分子和伴胞间有胞间连丝联系,伴胞属传递细胞类型,与其相邻韧皮薄壁细胞和果肉薄壁细胞连接处的细胞界面发生质膜内突,整个筛分子/伴胞复合体与韧皮薄壁细胞之间形成共质体隔离,韧皮部糖分的卸载方式主要以质外体途径进行。(2)韧皮薄壁细胞间的胞间连丝较多,而韧皮薄壁细胞与果肉薄壁细胞的胞间连丝相对较少,但果肉薄壁细胞间几乎无胞间连丝;果肉薄壁细胞之间胞间隙较大,细胞壁和质膜内突间形成较大的质外体空间,为质外体的糖分运输创造了条件。(3)筛管、伴胞、韧皮薄壁细胞和果肉薄壁细胞中丰富的囊泡以及活跃的囊泡运输现象,暗示囊泡也参与了果实糖分的运输过程。研究推测,枸杞果实韧皮部同化物的卸载方式以及卸载后的同化物运输主要以质外体途径为主。     

Cell wall structure in the xylem parenchyma ofCryptomeria

Summary Cell wall structure in ray and axial parenchyma cells in the wood ofCryptomeria was shown to be typically crossed polylamellate and dissimilar to the characteristically layered wall of fibers and tracheids. Ray cells differed from axial cells in terms of form and also in the relative inclination of crossed microfibrillar helices in the cell wall. This feature was reflected by positive birefringence in ray cells and negative birefringence in axial cells. Localized wall thickenings,viz. transverse bars in ray cells and longitudinal ribs in axial cells, also displayed crossed polylamellate structure. This observation contrasts with the exclusively longitudinal microfibrillar orientation previously reported for longitudinal ribs in elongated parenchyma cells of primary tissue. On the basis of similar microfibrillar orientations between outer and inner wall lamellae, the cell walls ofCryptomeria parenchyma were judged to be predominantly secondary.Lignin was heterogeneously distributed in lamellate fashion and a high concentration characterized the thin middle lamella. Both types of parenchyma suggested a higher lignin content than adjacent longitudinal tracheids.     

Significance of minor-vein anatomy to carbohydrate transport

Plant species which translocate distinct combinations of carbohydrates in the phloem were investigated to assess whether differences in minor-vein anatomy were associated with differences in carbohydrate composition of the phloem sap. In Vicia faba L., a species in which the minor-vein companion cells are modified into transfer cells, sucrose alone was found to be the translocated form of carbohydrate. In Vicia, phloem transport of sucrose was inhibited by pretreatment of leaves with p-chloromercuribenzenesulfonic acid (PCMBS), a known inhibitor of the sucrose carrier. In contrast, in Ocimum basilicum L., a species in which the minor-vein companion cells are of the symplasmically linked intermediary cell type, both sucrose- and raffinose-family oligosaccharides were exported in the phloem. In this species, no PCMBS sensitivity was observed for phloem transport of either sucrose- or raffinose-family oligosaccharides, although a PCMBS-sensitive sucrose carrier was detected in leaf tissues. This carrier did not appear to be involved in phloem loading, rather, it appeared that phloem loading occurred via the symplasm in this species. In the polyoltranslocating species Petroselinum crispum L., the same insensitivity to PCMBS was seen, suggesting that symplasmic phloem loading also occurred. The companion cells were symplasmically connected to the surrounding bundle-sheath cells by numerous H-shaped plasmodesmata but were not intermediary cells, and no raffinose oligosaccharides were exported by Petroselinum. Taken together, the data indicate that apoplasmic transport may be responsible for phloem loading in species in which sucrose alone is exported. However, in those plant species in which a combination of sucrose and any other carbohydrate, including the polyols, is translocated, symplasmic phloem loading may predominate.Abbreviation PCMBS p-chloromercuribenzenesulfonic acid This work was supported by National Science Foundation Grant DCB 8901785 to M.A.M. and by a National Science Foundation Graduate Minority Fellowship to L.L.F. The authors gratefully acknowledge the help of Dr. William W. Thomson in preparing the micrograph.     

Minor vein structure and sugar transport in Arabidopsis thaliana

Leaf and minor vein structure were studied in Arabidopsis thaliana (L.) Heynh. to gain insight into the mechanism(s) of phloem loading. Vein density (length of veins per unit leaf area) is extremely low. Almost all veins are intimately associated with the mesophyll and are probably involved in loading. In transverse sections of veins there are, on average, two companion cells for each sieve element. Phloem parenchyma cells appear to be specialized for delivery of photoassimilate from the bundle sheath to sieve element-companion cell complexes: they make numerous contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact with sieve elements. Plasmodesmatal frequencies are high at interfaces involving phloem parenchyma cells. The plasmodesmata between phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem parenchyma cell side of the wall and only one branch on the companion cell side. Most of the translocated sugar in A. thaliana is sucrose, but raffinose is also transported. Based on structural evidence, the most likely route of sucrose transport is from bundle sheath to phloem parenchyma cells through plasmodesmata, followed by efflux into the apoplasm across wall ingrowths and carrier-mediated uptake into the sieve element-companion cell complex. Received: 5 October 1999 / Accepted: 20 November 1999     

Structural and ultrastructural characteristics of the vascular apparatus of the sensitive plant (Mimosa pudica L.)

Summary 1. In motor organs ofMimosa pudica xylem contains living fibriform elements limited by a thick lignified highly pitted wall, whereas in other parts of the plant (stem, petiole, rachis), xylem and protoxylem vessels are closely associated with parenchyma cells which possess wall ingrowths. These ingrowths, at the apex of which the plasmalemma and the tonoplast touch, are localized like those of transfer cells of C type described byGunning andPate. Nevertheless, xylem parenchyma cells differ from cells of C type in several characteristics. Moreover, in motor organs, phloem contains cells characterized by wall ingrowths, less abundant on the parts adjacent to the sieve tubes; these cells which are localized near collenchyma cells of primary phloem, look like transfer cells of A type defined byGunning andPate; they are absent from internodes, petioles and rachides. 2. In motor organs, three types of vascular cells (companion cells, living xylem fibriform elements and protoxylem parenchyma cells) are characterized by reduced vacuolar volumes and well developed membrane systems, as compared with homologuous cells belonging to other parts of the plant. 3. A symplastic continuity holds from the middle of motor organs to their cortex: it is provided by the presence, in xylem and phloem respectively, of living fibriform elements and collenchyma cells bearing numerous pit fields containing large numbers of plasmodesmata. Several ultrastructural features suggest that the vascular apparatus ofMimosa pudica would be the site of intensive lateral transfer at different levels, specially in motor organs. Possible functions of certain structures observed are discussed in relation to some hypotheses relative to excitatory conduction pathways.     

Characteristics of protein-storing cells associated with a 67 kDa protein in Hevea brasiliensis

 The protein-storing cells (PSCs) in Hevea brasiliensis were studied by using light- and electron-microscopy and SDS-PAGE. The cells were found in stem and root where secondary phloem was developed. They are a special kind of phloem parenchyma cell which accumulate in their central vacuoles large amounts of protein, fibril-like under an electron microscope, and have few plastids with very small starch grains. Their distribution is strictly restricted to the secondary phloem axial system where they exactly sequestered in functional phloem or slightly over it. A 67 kDa protein was always found in the tissues where the PSCs were observed. During the first seasonal growth flush, the 67 kDa protein in the terminal branchlet exhibits marked quantitative fluctuation which is consistent with the change of the vacuole protein inclusion of the PSCs in the branchlet. These facts suggested that the 67 kDa protein might be the major part of the vacuole protein of the PSCs. Considering the differences between the PSCs in Hevea and the PSCs in the other trees studied, we define two types of PSCs: Hevea-type, which are the cells specialized for protein storage and Populus-type, which are ordinary parenchyma cells accumulating protein and starch. Received: 11 April 1997 / Accepted: 28 July 1997     

Distribution and frequency of plasmodesmata in relation to photoassimilate pathways and phloem loading in the barley leaf

Large, intermediate, and small bundles and contiguous tissues of the leaf blade of Hordeum tvulgare L. ‘Morex’ were examined with the transmission electron microscope to determine their cellular composition and the distribution and frequency of the plasmodesmata between the various cell combinations. Plasmodesmata are abundant at the mesophyll/parenchymatous bundle sheath, parenchymatous bundle sheath/mestome sheath, and mestome sheath/vascular parenchyma cell interfaces. Within the bundles, plasmodesmata are also abundant between vascular parenchyma cells, which occupy most of the interface between the sieve tube-companion cell complexes and the mestome sheath. Other vascular parenchyma cells commonly separate the thick-walled sieve tubes from the sieve tube-companion cell complexes. Plasmodesmatal frequencies between all remaining cell combinations of the vascular tissues are very low, even between the thin-walled sieve tubes and their associated companion cells. Both the sieve tube-companion cell complexes and the thick-walled sieve tubes, which lack companion cells, are virtually isolated symplastically from the rest of the leaf. Data on plamodesmatal frequency between protophloem sieve tubes and other cell types in intermediate and large bundles indicate that they (and their associated companion cells, when present) are also isolated symplastically from the rest of the leaf. Collectively, these data indicate that both phloem loading and unloading in the barley leaf involve apoplastic mechanisms.     

Changes in cell length and mitotic index in vascular pattern-related pericycle cell types along the apical meristem and elongation zone of the onion root

Summary Three pericycle cell types (opposite xylem, opposite phloem and intervening) distinguished by their location in relation to different elements of the vascular system were studied in the adventitious root ofAllium cepa L. Changes in cell length and mitotic index were analysed in these cells along the apical meristem and elongation zone of the root. The opposite phloem and intervening pericycle cells are significantly shorter than the opposite xylem pericycle cells in the apical half of the meristem. Between 1,200 and 1,400 m behind the tip, length became similar in all three pericycle cell types, while in more proximal zones the opposite phloem cells were significantly longer. These results suggest that the number of transverse divisions is different in the three types of pericycle cells. In the apical half of the meristem, mitotic index increased in intervening and opposite xylem cells but remained unchanged in opposite phloem cells, a fact likely to account for the relative lengthening of the latter. In the proximal half of the meristem, mitotic index fell in all three cell types until cell division had ceased. However, mitotic index in opposite xylem cells remained high for longer than in the other two cell types, implying that increase of the mean cell length in the former was slower. These results suggest that differences in mean cell length between the three pericycle cell types are due to different rates of proliferation.     

THE PHLOEM OF ETAPTERIS LECLERCQII AND BOTRYOPTERIS TRIDENTATA

The phloem of Etapteris leclercqii and Botryopteris tridentata petioles is described from Lower Pennsylvanian coal balls. Petioles of B. tridentata are characterized in transverse section by an omega-shaped xylem trace, a phloem zone which extends from 2-10 cells in width, and 2-parted cortex. Etapteris leclercqii petioles exhibit a 4–9 cell-wide phloem zone surrounding the central clepsydroid xylem mass, and a 3-parted cortex. In both taxa a 1–2 cell layer parenchyma sheath separates the xylem from the extra-xylary tissues. The phloem of both species consists of sieve elements that average about 20 μm in diam by 200 μm in length in Botryopteris, and 100 μm in length in Etapteris, with horizontal-slightly oblique end walls. In transmitted light, the radial walls of the sieve elements form an irregular reticulate pattern enclosing elliptical lighter areas. With the scanning electron microscope, these areas appear as horizontal-slightly oblique furrows on the cell wall, with many small indentations lining the furrows. These indentations, because of their regular occurrence and size (from a few fractions of a micron up to 1.0 μm in diam), are interpreted as sieve pores, and the elliptical areas that enclose them as sieve areas. The phloem of E. leclercqii and B. tridentata is compared with that described for other fossil genera and with that of extant ferns.     

Seasonal variation in formation,structure, and chemical properties of phloem in <Emphasis Type="Italic">Picea abies</Emphasis> as studied by novel microtechniques

Main conclusion

Phloem production and structural development were interlinked with seasonal variation in the primary and secondary metabolites of phloem. Novel microtechniques provided new perspectives on understanding phloem structure and chemistry. To gain new insights into phloem formation in Norway spruce (Picea abies), we monitored phloem cell production and seasonal variation in the primary and secondary metabolites of inner bark (non-structural carbohydrates and phenolic stilbene glucosides) during the 2012 growing season in southern and northern Finland. The structure of developing phloem was visualised in 3D by synchrotron X-ray microtomography. The chemical features of developing phloem tissues isolated by laser microdissection were analysed by chemical microanalysis. Within-year phloem formation was associated with seasonal changes in non-structural carbohydrates and phenolic extractive contents of inner bark. The onset of phloem cell production occurred in early and mid-May in southern and northern Finland, respectively. The maximal rate of phloem production and formation of a tangential band of axial phloem parenchyma occurred in mid-June, when total non-structural carbohydrates peaked (due to the high amount of starch). In contrast, soluble sugar content dropped during the most active growth period and increased in late summer and winter. The 3D visualisation showed that the new axial parenchyma clearly enlarged from June to August. Sub-cellular changes appeared to be associated with accumulation of stilbene glucosides and soluble sugars in the newest phloem. Stilbene glucosides also increased in inner bark during late summer and winter. Our findings may indicate that stilbene biosynthesis in older phloem predominantly occurs after the formation of the new band(s) of axial parenchyma. The complementary use of novel microtechniques provides new perspectives on the formation, structure, and chemistry of phloem.

     

Acid invertase is predominantly localized to cell walls of both the practically symplasmically isolated sieve element/companion cell complex and parenchyma cells in developing apple fruits

The acid invertase (β‐fructosidase, EC 3·2·1·26) was localized at subcellular level via immunogold electron microscopy in the phloem‐unloading zone of developing apple fruit. The enzyme (immunogold particles) was found to reside predominantly in the cell walls of the sieve element/companion cell (SE/CC) complex, phloem parenchyma cells and other parenchyma cells. There was almost no gold particle found in cytoplasm and vacuole. This distribution pattern remained unchanged throughout the growing season, but the enzyme numbers varied. The density of immunogold particles increased during fruit development. The immunoblotting of soluble and insoluble acid invertases provided a supporting proof for the assays of immunolocalization. The biochemical analysis showed a predominantly cell‐wall‐distributed activity of acid invertase that corresponds essentially with its amount distribution. The ultrastructural observations showed that there were numerous plasmodesmata between the parenchyma cells, but almost no plasmodesmium between the SE/CC complex and its surrounding parenchyma cells, practically resulting in the symplasmic isolation of the SE/CC complex. It is therefore suggested that the unloading pathway of sucrose from the SE/CC complex may be predominantly apoplasmic in the developing apple fruit, and that the unloaded sucrose may be hydrolysed by the functional acid invertase localized in the cell wall before it is loaded in sink cells.     

Histochemical localization of nucleoside triphosphatase activity in assimilate conducting plant tissue

Summary For the histochemical localization of nucleoside triphosphatases at the electron microscopic level, prefixed tissues were incubated with lead nitrate in addition to substrate (GOMORI reaction). While ATP and UTP as substrates gave electron-dense reaction products at the plasmalemma of sieve tubes, companion cells and phloem parenchyma cells, and at plasmodesmata in primary pitfields, AMP gave reaction products only at the tonoplast of parenchyma cells. Since electron-dense deposits also occur in cell walls and vacuoles, energy dispersive X-ray microanalysis was used to distinguish between lead deposits and lead-phosphate deposits. The latter were restricted to the symplast. Among the three plant species used, the leaf bundle phloem ofHordeum distichon showed ATPase activity largely restricted to the phloem cells, except for the thickwalled sieve tubes. Some activity also bordered the chloroplasts of the bundle sheath cells. In the C₄ plantGomphrena globosa, ATPase and UTPase activities appeared to be the greater in phloem parenchyma cells than in sieve tubes. In the phloem of youngMonstera deliciosa roots, ATPase occurred not only at the plasmalemma of sieve tubes, but also around sieve-tube plastids. When compared with AMP as substrate, it appears that nucleoside triphosphates are the natural substrates of the enzyme(s) in the plasmalemma of sieve tubes and phloem parenchyma cells.