In several tulip cultivars propagated in vitro, a high decrease in propagation rate was observed after the 4th - 6th multiplication cycles. In vitro shoot cultures of the model tulip cultivars Blue Parrot and Prominence with relatively stable and high multiplication rate and shoots of new Polish cultivar Fringed Black which revealed a decreased regeneration capacity were used in the experiment. Shoots, multiplied at 8-week subculture period on MS modified medium supplemented with thidiazuron (TDZ) and a-naphthaleneacetic acid (NAA), were treated with low temperature (5°C) for 8, 10 or 12 weeks and with 1 mg·dm⁻³ GA₃ which was added to the medium prior to cooling at the 3rd week of multiplication subculture. The significant enhancement of shoot multiplication by cooling was found for the cultivar Fringed Black only. Low temperature treatment for 10 - 12 weeks markedly increased multiplication rate from 2.5 (non-cooled control) to 5.8 - 6.4 (in the 1st subculture). The effect of increased regeneration capacity was maintained during 8 months of cyclic multiplication. Application of GA₃ decreased affect shoot multiplication rate. It is suggested that the decrease in multiplication rate of cv. Fringed Black could result from dormancy development in shoots because the restoration of regeneration capacity occurred after low temperature treatment.
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