Summary. Calcium distribution and pectin esterification patterns inthe cambial zone of poplar branches were studied with ionic microscopy and immunological tools respectively. Dynamic changes correlating with cell growth and cell differentiation were observed both on the xylem and on the phloem sides. In expanding cell walls of xylem derivatives, unesterified pectins were restricted to cell junctions and middle lamellae, occasionally accompanied by calcium ions. In contrast, in differentiating and mature phloem cells, acidic pectins and Ca 2+ were present all over the walls leading to early stiffening of the polysaccharide network. Significant labelling was detected with JIM5 antibodies in some dictyosomes suggesting exocytosis of low methylated polymers towards the cell walls. At cell junctions, unesterified pectins might originate from the activity of pectinmethylesterases localized in these areas. Thus un-and deesterified pectins might be located in different cell wall domains whose distribution, varying with cell type, will confer specific extensibility to the wall matrix.Keywords: Calcium; Cambium; Ionic microscopy; Immunolocalization; Pectins; Populus.Abbreviations: BSA bovine serum albumin; DM degree of methylation; FITC fluorescein isothiocyanate; HM highly methylated pectins; LM low methylated pectins; PME pectin methylesterase; SIMS secondary ion mass spectrometry; TBS tris-buffered saline.

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In the course of a study on the early events of cambial derivative differentiation in Populus x euramericana, seasonal changes in the pattern of pectin methylesterase (PME, EC 3.1.1.11) isoforms were followed. During the resting season, cell wall extracts contained mainly alkaline isoforms with an M(r) around 55 kDa and optimal pH between 5.6 and 6.0. Neutral isoforms with an M(r) around 35 kDa and optimal pH between 6.0 and 6.6 predominated in the extracts during the period of high meristematic activity. In the active cambial initials and in their immediate derivatives, the enzymes were immunolocalized exclusively in the dictyosomes. In older cells, they were present both in dictyosomes and in wall junctions. These results indicate that exportation of neutral PMEs towards the walls might be considered as an early marker of differentiation in cambial derivatives.

The lifetime of many plant organs does not exceed a few weeks or a few months. These organs separate from the plant at the level of specialized abscission zones. The observation of xylem vasculature in abscission zones, a largely neglected subject, revealed original features when a vessel casting method was used. In all species of dicotyledons examined so far, flower and leaf abscission zones possessed heterogenous metaxylem vessels adjoining protoxylem and secondary xylem vessels with homogenous patterns of lignified thickenings. Heterogenous metaxylem vessel thickenings were helical, reticulate, or scalariform elements when in the abscission zone and pitted elements on the proximal and the distal sides. The origin and possible role of these vessels are considered. Data obtained on the flower abscission zone of tomato (Lycopersicon esculentum Mill.) suggest that formation of heterogenous vessels results from localized changes in the rhythm of cell differentiation and cell maturation inside the procambiumcambium continuum.Résumé : De nombreux organes végétaux ont une durée de vie limitée. Ils se détachent du reste de la plante au niveau de zones particulières, les zones d'abscission. La vascularisation de ces zones présente des caractères originaux qui ont pu être mis en évidence grâce à une technique de moulage vasculaire. Plusieurs exemples de zones d'abcission florales et foliaires ont été examinés chez les Dicotylédones. Dans tous les cas, nous avons observé que les vaisseaux de protoxylème et de xylème secondaire, ne présentent, chacun, qu'un seul type d'ornementation tandis que le métaxylème est formé de vaisseaux hétérogènes. Ces derniers sont caractérisés par la présence d'éléments spiralés, réticulés et (ou) scalariformes au niveau de la zone d'abscission et de type ponctué en amont et en aval. Les données obtenues sur la zone d'abscission florale de la tomate (Lycopersicon esculentum Mill.) suggèrent que l'hétérogénéité des vaisseaux du metaxylème pourrait résulter de différences locales dans le rythme de différenciation et de maturation cellulaire au sein du continuum procambium-cambium.

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