— La culture in vitro de cals et la régénération de plantes entières a été réussie chez une vingtaine de clones de peupliers représentant la plupart des espèces et hybrides de la section Leuce. Le milieu de Linsmaïer et Skoog modifié par Chalupa assure généralement la meilleure croissance des cals. La benzylaminopurine n'induit que rarement la néoformation de bourgeons et toujours à de faibles taux. À l'inverse, le thidiazuron assure la régénération de bourgeons chez tous les clones testés et

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... ul un clone de Populus alba x P tremuloïdes requiert l'association de cette hormone avec la BAP. Les bourgeons néoformés s'enracinent aisément après repiquage sur le milieu de Murashige et Skoog modifié ou le Woody Plant Medium dépourvus d'hormone. La culture in vivo des plants obtenus ne pose aucun problème. Après un nombre important de subcultures des cals, on note une perte de l'aptitude organogénétique chez certains clones. peuplier / Leuce / in vitro / cals / régénération Summary — Callus culture of Leuce poplar and plant regeneration. Successful callus culture and regeneration of whole plants were achieved for 20 clones representing most of the poplar species and hybrids from the Leuce section (table I) . During the first subculture of calli from 8 clones, the frequency of necrosis was influenced by the nature of the media and its growth regulators. Generally, Linsmaïer and Skoog media modified by Chalupa (and containing 10 μM NAA and 4.5 μM BAP) was suitable, but Wolter and Skoog media modified by Winton (0.18 μM 2,4-D and 4.5 μM BAP) was more suitable for aspens (table II). The woody plant medium (WPM) was the least interesting medium for callus culture. Callus necrosis was more frequent on medium containing only BAP than on medium containing the hormones proposed by the above-mentioned authors. There was a positive correlation between thidiazuron concentrations (between 0-1 μM) and callus growth. When BAP was mixed with thidiazuron, the positive effect of the latter hormone was significantly greater than the negative influence due to BAP (table V) . Whatever the growth regulator used for the 18 clones, the following subcultures were free from necrosis (table III) . BAP (2.5 μM) induced bud formation on callus of only 1 clone (Astria). However, thidiazuron promoted shoot formation on all clones except P alba x P tremuloides cv 802-36-7. For most clones, optimal thidiazuron concentrations were 0.01 and 0.1 μM (table IV). Regeneration of 802-36-7 was achieved by combining BAP and thidiazuron. Finally, a comparison conducted on 4 other clones showed that using thidiazuron alone showed better results in 2 of them while the 2 others produced more buds when BAP was associated with thidiazuron (table VI) . After 2 yr (≈ 20 subcultures), 7 out of 14 clones had lost their ability to regenerate, at least using the hormone concentrations initially found to be the most suitable (table VII) . Neoformed shoots rooted easily when transferred onto hormone-free modified Murashige and Skoog medium (mainly P alba and its hybrids) or onto WPM (mainly for aspens). Regenerated plants were transferred and cultured in the nursery.