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  • Articles  (4)
  • Phloem loading  (4)
  • 1
    Electronic Resource
    Electronic Resource
    Microautoradiographic studies of phloem loading and transport in the leaf of Zea mays L. (1983)
    Springer
    Planta 159 (1983), S. 193-206 
    Details
    ISSN: 1432-2048
    Keywords: Assimilate transport ; Leaf (14C transport) ; Phloem loading ; Sieve tube ; Vascular bundle ; Zea (14C transport)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Microautoradiographs showed that [14C]sucrose taken up in the xylem of small and intermediate (longitudinal) vascular bundles of Zea mays leaf strips was quickly accumulated by vascular parenchyma cells abutting the vessels. The first sieve tubes to exhibit 14C-labeling during the [14C]sucrose experiments were thick-walled sieve tubes contiguous to the more heavily labeled vascular parenchyma cells. (These two cell types typically have numerous plasmodesmatal connections.) With increasing [14C]sucrose feeding periods, greater proportions of thick- and thin-walled sieve tubes became labeled, but few of the labeled thin-walled sieve tubes were associated with labeled companion cells. (Only the thin-walled sieve tubes are associated with companion cells.) When portions of leaf strips were exposed to 14CO2 for 5 min, the vascular parenchyma cells-regardless of their location in relation to the vessels or sieve tubes-were the most consistently labeled cells of small and intermediate bundles, and label (14C-photosynthate) appeared in a greater proportion of thin-walled sieve tubes than thick-walled sieve tubes. After a 5-min chase with 12CO2, the thin-walled sieve tubes were more heavily labeled than any other cell type of the leaf. After a 10-min chase with 12CO2, the thin-walled sieve tubes were even more heavily labeled. The companion cells generally were less heavily labeled than their associated thin-walled sieve tubes. Although all of the thick-walled sieve tubes were labeled in portions of leaf strips fed 14CO2 for 5 min and given a 10-min 12CO2 chase, only five of 72 vascular bundles below the 14CO2-exposed portions contained labeled thick-walled sieve tubes. Moreover, the few labeled thick-walledsieve tubes of the “transport region” always abutted 14C-labeled vascular parenchyma cells. The results of this study indicate that (1) the vascular parenchyma cells are able to retrieve at least sucrose from the vessels and transfer it to the thick-walled sieve tubes, (2) the thick-walled sieve tubes are not involved in long-distance transport, and (3) the thin-walled sieve tubes are capable themselves of accumulating sucrose and photosynthates from the apoplast, without the companion cells serving as intermediary cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00397525
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  • 2
    Electronic Resource
    Electronic Resource
    Loading and transport of assimilates in different maize leaf bundles (1989)
    Springer
    Planta 178 (1989), S. 1-9 
    Details
    ISSN: 1432-2048
    Keywords: Leaf (assimilate transport) ; Phloem loading ; Phloem transport ; Sieve tube ; Vascular bundle ; Zea (assimilate transport)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The loading and transport functions of vascular bundles in maize (Zea mays L.) leaf strips were investigated by microautoradiography after application of 14CO2. The concentrations of 14C-contents in thin-walled sieve tubes of individual bundles in the loading and transport regions were determined by digital image analysis of silver-grain density over the sieve tubes and compared. In the loading region, relatively high concentrations of 14C-contents were found in the thin-walled sieve tubes of small bundles and in the small, thin-walled sieve tubes of the intermediate bundles; the concentration of 14C-label in large bundles was very low. In the transport region, at a transport distance of 2 cm, all of the small bundles contained 14C-assimilates, but generally less than the same bundles did in the loading region; by comparison, at that distance intermediate and large bundles contained two-to threefold more 14C-assimilates than the same bundles in the loading region. The lateral transfer of assimilates from smaller to larger bundles via transverse veins could be demonstrated directly in microautoradiographs. A reverse transport from larger to smaller bundles was not found. At a transport distance of 4 cm, all large and intermediate bundles were 14C-labeled, but many of the small bundles were not. Although all longitudinal bundles were able to transport 14C-asimilates longitudinally down the blade, it was the large bundles that were primarily involved with longitudinal transport and the small bundles that were primarily involved with loading.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00392520
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  • 3
    Electronic Resource
    Electronic Resource
    Ultrastructure of and plasmodesmatal frequency in mature leaves of sugarcane (1991)
    Springer
    Planta 184 (1991), S. 291-306 
    Details
    ISSN: 1432-2048
    Keywords: C4 plant ; Leaf ultrastucture ; Mestome ; sheath ; Phloem loading ; Plasmodesma ; Saccharum (plasmodesmata) ; Vascular bundle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Vascular bundles and contiguous tissues of leaf blades of sugarcane (Saccharum interspecific hybrid L62–96) were examined with light and transmission electron microscopes to determine their cellular composition and the frequency of plasmodesmata between the various cell combinations. The large vascular bundles typically are surrounded by two bundle sheaths, an outer chlorenchymatous bundle sheath and an inner mestome sheath. In addition to a chlorenchymatous bundle sheath, a partial mestome sheath borders the phloem of the intermediate vascular bundles, and at least some mestome-sheath cells border the phloem of the small vascular bundles. Both the walls of the chlorenchymatous bundlesheath cells and of the mestome-sheath cells possess suberin lamellae. The phloem of all small and intermediate vascular bundles contains both thick- and thin-walled sieve tubes. Only the thin-walled sieve tubes have companion cells, with which they are united symplastically by pore-plasmodesmata connections. Plasmodesmata are abundant at the Kranz mesophyll-cell-bundlesheath-cell interface associated with all sized bundles. Plasmodesmata are also abundant at the bundle-sheathcell-vascular-parenchyma-cell, vascular-parenchyma-cellvascular-parenchyma-cell, and mestome-sheath-cell-vascular-parenchyma-cell interfaces in small and intermediate bundles. The thin-walled sieve tubes and companion cells of the large vascular bundles are symplastically isolated from all other cell types of the leaf. The same condition is essentially present in the sieve-tube-companion-cell complexes of the small and intermediate vascular bundles. Although few plasmodesmata connect either the thin-walled sieve tubes or their companion cells to the mestome sheath of small and intermediate bundles, plasmodesmata are somewhat more numerous between the companion cells and vascular-parenchyma cells. The thick-walled sieve tubes are united with vascular-parenchyma cells by pore-plasmodesmata connections. The vascular-parenchyma cells, in turn, have numerous plasmodesmatal connections with the bundle-sheath cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00195330
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  • 4
    Electronic Resource
    Electronic Resource
    Studies on the leaf of Amaranthus retroflexus (Amaranthaceae): ultrastructure, plasmodesmatal frequency, and solute concentration in relation to phloem loading (1982)
    Springer
    Planta 155 (1982), S. 377-387 
    Details
    ISSN: 1432-2048
    Keywords: Amaranthus ; Leaf ultrastructure ; Phloem loading ; Plasmodesmata ; Veins (minor)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Both the mesophyll and bundle-sheath cells associated with the minor veins in the leaf of Amaranthus retroflexus L. contain abundant tubular endoplasmic reticulum, which is continuous between the two cell types via numerous plasmodesmata in their common walls. In bundle-sheath cells, the tubular endoplasmic reticulum forms an extensive network that permeates the cytoplasm, and is closely associated, if not continuous, with the delimiting membranes of the chloroplasts, mitochondria, and microbodies. Both the number and frequency of plasmodesmata between various cell types decrease markedly from the bundle-sheath — vascular-parenchyma cell interface to the sicve-tube member — companion-cell interface. For plants taken directly from lighted growth chambers, a stronger mannitol solution (1.4 M) was required to plasmolyze the companion cells and sieve-tube members than that (0.6 M) necessary to plasmolyze the mesophyll, bundle-sheath, and vascular-parenchyma cells. Placing plants in the dark for 48 h reduced the solute concentration in all cell types. Judging from the frequency of plasmodesmata between the various cell types of the vascular bundles, and from the solute concentrations of the various cell types, it appears that assimilates are actively accumulated by the sieve-tube — companion-cell complex from the apoplast.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00394465
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