The aim of this study was to check the effects of sugar type on the in vitro shoot multiplication of the tulip cultivar 'Heart of Warsaw' and the effects of paclobutrazol (PBZ) and 1-naphthylacetic acid (NAA) on the bulbing of previously multiplied shoots. In addition, the subsequent effects of previously used sugars on the in vitro bulb formation of this cultivar were checked. First, the optimum supplementation of Murashige and Skoog medium with plant growth regulators (PGRs) was selected for shoot multiplication. Of the six tested, the best results were obtained using a combination of 2iP 0.1 mg·L-1, NAA 0.1 mg·L-1, and mT 5.0 mg·L-1. The effects of different carbohydrates (sucrose, glucose, and fructose at 30 g·L-1 and a mixture of glucose and fructose at 15 g·L-1 each) on multiplication efficiency was then tested on this medium. The microbulb-forming experiment was carried out taking into consideration the effects of previously applied sugars, and at week 6, the agar medium was flooded with liquid medium containing NAA 2 mg·L-1, PBZ 1 mg·L-1, or medium without PGRs; in the first combination, the cultures were left on a single-phase medium, solidified with agar, as a control. After 2 months of treatment at 5 °C, the total number of microbulbs formed and the number and weights of mature microbulbs were assessed. The results obtained indicate the ability of using meta-topolin (mT) in tulip micropropagation and point to sucrose and glucose as the optimal carbohydrates for intensive shoot multiplication. The results lead to the conclusion that it is most advantageous to multiply tulip shoots on glucose medium and then to carry out cultures on a two-phase medium with PBZ, which results in a higher number of microbulbs and their faster maturation.
Keywords: meta-topolin; micropropagation; paclobutrazol; ‘Heart of Warsaw’; ‘Serce Warszawy’.