In vitro Shoot and Root Formation in the Apple Cultivar Åkerö

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Abstract
Regulation of in vitro shoot and root formation and the histological origin of newly formed shoots was studied in the apple cultivar Åkerö. Both composition of mineral elements and benzylaminopurine (BAP) concentration affected shoot multiplication. Similar numbers of shoots were obtained with Lepoivre and MS medium after two succeeding subcultures, but Murashige and Skoog medium was preferable due to production of longer shoots. The optimum BAP concentration wax around 8.8 μm. Higher concentrations caused vitrified shoots. The rooting ability increased with numbers of subcultures. Also the concentration of indol-3-yl butyric acid (IBA) affected rooting. A strong interaction between numbers of subcultures and IBA concentration was obtained. After insufficient numbers of subcultures, when shoots were still difficult to root, increasing IBA concentration exerted little effect on rooting. When shoots had reached an ‘easy-to-root condition’ root initiation depended on IBA concentration, showing an optimum at 2.5 μm. Supraoptimal IBA concentrations delayed root initiation. Dark treatment of shoots during the root-initiation phase increased rooting ability. The most positive effect was obtained at suboptimal IBA concentrations. Anatomical studies revealed both axillary and adventitious shoots. Two kinds of adventitious structures were demonstrated.