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, Linsmaier and Skoog media modified by Chalupa (and containing 10 muM NAA and 4.5 muM BAP) was suitable, but Wolter and Skoog media modified by Winton (0. 18 muM 2,4-D and 4.5 muM 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 muM) 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 muM) 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 muM (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 (almost-equal-to 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.