Dissimilar phloem loading in leaves with symplasmic or apoplasmic minor-vein configurations |
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Authors: | Aart J E van Bel Yuri V Gamalei Ankie Ammerlaan Leonard P M Bik |
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Institution: | (1) Transport Physiology Research Group, Department of Plant Ecology and Evolutionary Biology, University of Utrecht, Sorbonnelaan 16, NL-3584 CA Utrecht, The Netherlands;(2) Komarov Botanical Institute, 2 Popov Street, 197376 St. Petersburg, USSR |
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Abstract: | Plant species were selected on the basis of abundant or no symplasmic continuity between sieveelement-companion-cell (SE-CC) complexes and adjacent cells in the minor veins. Symplasmic continuity and discontinuity are denoted, respectively, as symplasmic and apoplasmic minor-vein configurations. Discs of predarkened leaves from which the lower epidermis had been removed, were exposed to 14CO2. After 2 h of subsequent incubation, phloem loading in control discs and discs treated with p-chloromercuribenzenesulfonic acid (PCMBS) was recorded by autoradiography. Phloem loading was strongly suppressed by PCMBS in minor veins with symplasmically isolated SE-CC complexes (Centaurea, Impatiens, Ligularia, Pelargonium, Pisum, Symphytum). No significant inhibition of phloem loading by PCMBS was observed in minor veins containing sieve elements with abundant symplasmic connections (Epilobium, Fuchsia, Hydrangea, Oenothera, Origanum, Stachys). Phloem loading in minor veins with both types of SE-CC complex (Acanthus) had apoplasmic features. The results provide strong evidence for coincidence between the mode of phloem loading and the minor-vein configuration. The widespread occurrence of a symplasmic mode of phloem loading is postulated.Abbreviations PCMBS
p-chloromercuribenzenesulfonic acid
- SE-CC complex
sieve-element-companion-cell complex |
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Keywords: | Leaf (source/sink relations) Minor-vein configuration Phloem loading (apoplasmic symplas-mic) Plasmodesmatal connectivity |
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