LONG-TERM MAINTENANCE OF BIRCH MICROPLANTS IN TISSUE CULTURE
The possibility of maintaining the cultures of downy birch (<i>Betula pubescens </i>Ehrh.), silver birch (<i>B. pendula </i>Roth.) and Karelian birch (<i>B. pendula </i>Roth. var. <i>carelica </i>Merckl.) on nutritional media supplemented with saccharose at higher concentrations, sorbitol, medical charcoal, abscistic acid, and BAP in different combinations during 12 months under in vitro conditions was revealed. Signifi cant differences between the birch species were determined according to their responses to such long-term maintenance. The addition of sorbitol to the nutrient medium at a concentration of 3 g l<sup>-1</sup> after 6 months of maintenance promoted the greater number of viable explants for all the the birch genotypes as compared to those in the control. The above substances did not inhibit the development of the birch explants subcultured on fresh hormone-free media after different periods of maintenance.