Plant Cell, Tissue and Organ Culture 19:103-111, 1989. © 1989 Kluwer Academic Publishers. Printed in the Netherlands. Effects of media components and environmental factors on shoot formation from protoplast-derived calli of Solanum tuberosum CATHRINE LILLO Michigan Technological University, Department of Biological Sciences, Houghton, Michigan 49931, USA; (present address." Agricultural University of Norway, Department of Botany, Box 14, 1432 ,~S-NLH, Norway) Received 6 December 1988; accepted in revised form 27 April 1989 Key words: amino acids, ammonium, growth factors, light, mannitol, nitrate, organogenesis, polyamines, potato, protoplasts, Solanum tuberosum, sucrose, tissue culture Abstract. The influence of several media components and environmental factors on shoot formation in protoplast-derived calli of Solanum tuberosum (a Rosamunda cross) were stu- died. Low sucrose concentration (3-15mM) was beneficial for optimal shoot induction. Several concentrations of NOj- and NH; were suitable for shoot induction as long as the concentration of NO3 was about twice the concentration of NH2 or higher. No stimulatory effect of glutamine, proline, putrescine, spermidine, spermine or adenine sulphate at 0.5 and 2 mM were found. White light promoted shoot induction compared with red and blue light or darkness. The intensity of light was shown to be a critical factor for good shoot induction. Lower light intensity (30#E m -2 s -~) resulted in doubling of the number of calli producing shoots compared with higher (110/2E m-2 s- ~ ) light intensity. A temperature of 20 °C pro- moted shoot regeneration compared to 25 °C. Based on these results improved conditions for regeneration of S. tuberosum are suggested, and shown to enhance shoot formation in five other genotypes tested. Introduction Reliable regeneration of plants from protoplasts is essential for the appli- cation of protoplast techniques in plant breeding, and more systematic knowledge on regeneration from protoplast-derived cell cultures is needed. Identification of critical factors in the cultivation process for one cultivar may also suggest favourable conditions for other cultivars or related species. In the experiments presented here we studied shoot formation in protoplast-derived calli of a Solanum tuberosum breeding line (Beate x Maris Piper) x Rosamunda. The effects of growth-regulating substances