[en] Full text: Kohleria hybrids (Gesneriaceae) are used as indoor ornamentals. As in other pot plants, compact plant habit and low energy requirement are major breeding objectives. A trihybrid of the following composition was used in the experiments: (K. amabilis x K. bogotensis) x K. eriantha. This hybrid has attractive flowers but long internodes, large leaves and is late flowering under low light conditions. Adventitious buds were induced in high numbers by cultivating internode segments aseptically on agar medium supplemented with 1 mg/l kinetin and 0.5 mg/l IAA. For inducing mutations, internode explants from in-vitro grown shoots were soaked for 1 hour in a filter-sterilized aqueous solution of N-nitroso-N-methylurea (NMH; 500 mg/l) at 20 deg. C; the solution was unbuffered and immediately used after preparation. Two-node micro-cuttings of regenerated shoots were rooted ex vitro in a mixture of peat and sand (1:1 by volume) and grown into mature plants without losses. Whereas control plants regenerated in-vitro through adventitious buds showed as little variation as plants raised from conventional tip cuttings, conspicuous phenotypic changes were observed in plants originating from NMH-treated explants. In a first experiment, 21% of the plants differed from the controls in one or more characters. While the majority of these variants were inferior to the control, one designated as ''II-2-0'' showed early flowering under the natural low light conditions during winter. This character was retained after cloning. A second mutagenesis experiment was performed using aseptic stock cultures of this mutant ''II-20-0'', in order to obtain a more desirable plant habit. In this case, plants bearing phenotypic changes occurred about twice as frequently as in the first experiment, although the mutagenic treatment was the same. Among the selected variants there was one, designated ''II-2-32'', in which early flowering under low light conditions was coupled with significantly shorter internodes as well as smaller and more attractive leaves. The mutant ''II-2-32'', its ancestor ''II-2-0'', and the control were compared in three successive generations of vegetative propagation under different light conditions. It turned out that flowering was much less controlled by daylength than by light intensity. Under conditions of high natural light intensity, there was no difference in the time required for flowering under long-day (LD) and short-day (SD) treatments among the three genotypes. In contrast, under low natural light intensity, the mutant types came into bloom up to 55 days earlier than the control; again the daylength had no effect on the control and mutant ''II-2-32'', whereas type ''II-2-0'' flowered earlier in LD than in SD conditions. The differences in internode length and leaf characters were consistent through all generations and under all light conditions. Clones of the mutant type ''II-2-32'' are now being evaluated for commercial cultivation on a larger scale. (author)