Effect of Sucrose Level in the Nutrient Medium at the Elongation Stage on the Rooting Ability of In vitro Apple Rootstock M 26 Malus domestica Borkh | ВЛИЯНИЕ НА КОНЦЕНТРАЦИЯТА НА ЗАХАРОЗА В ХРАНИТЕЛНАТА СРЕДА ВЪВ ФАЗА УДЪЛЖАВАНЕ ВЪРХУ ВКОРЕНЯВАНЕТО НА IN VITRO РАСТЕНИЯ ОТ ЯБЪЛКОВАТА ПОДЛОЖКА М 26 Malus domestica Borkh

The aim of the present research is to observe the effect of sucrose level 0 – 6% in the nutrient medium at the elongation stage on the rooting ability of in vitro cultivated apple rootstock M 26 Malus domestica Borkh. The source microplants are maintained on a modified MS multiplication medium 5 мM BAP, 30 g/l sucrose and 5.8 g/l agar. For the present experiment the plantlets are transferred for 14 days on an elongation hormone-free medium with the same mineral content with different sucrose concentrations 0, 1, 2, 4 and 6% w/v. They are cultivated in polypropylene vessels with gas-permeable closure Combiness, Belgium, gas exchange rate – 10 GE/day for 14 days. After that shoots 20 mm high are set for rooting in the same vessels on two types carbohydrate free medium – solidified with agar or liquid with perlite. Rooting is evaluated after 20 days. The content of glucose, fructose, sucrose, sorbitol and starch in the plants elongated on media with different sucrose concentrations is determined before transferring on rooting media. The increase in the sucrose concentration in the elongation medium 0 – 6% leads to different content of soluble carbohydrates and starch, especially at the extreme concentrations of 0 and 6%. On a sugar-free nutritional medium (photoautotrophic variant) sorbitol predominates and this confirm the photosynthetic competence of the apple plants in vitro. At the highest sucrose concentration 6% in the elongation medium accumulation of glucose, sucrose and starch in plantlets is observed, but these do not improve the rooting on carbohydrate – free medium. Elongation on the medium with 4% sucrose stimulates rooting on the agar based sucrose-free medium. When rooting is carried out in perlite based liquid medium, rooting percent is in the range of 50 – 65% and the sucrose concentration at elongation stage has no significant effect. The obtained results show some technological approaches for improving the rooting of micropropagated apple rootstock M 26 Malus domestica Borkh.