The hormonal control of callus induction in 'GM256' × 'M9' progeny of different stem rooting ability

The objective of this research was to find an efficient protocol to improve callus induction in apple rootstocks. Concentrations and ratios of auxin and cytokinin were investigated on 'GM256' × 'M9' progenies having either low or high rooting ability. Specifically, leaves of plants with different stem cutting rooting abilities were cultured for 21 days on Murashige and Skoog (MS) semisolid medium containing 2, 3 and 4 mg L^‑1 thidiazuron (TDZ) in combination with 0.2, 0.35 and 0.5 mg L^‑1 1-naphthaleneacetic acid (NAA) in the dark. Regeneration ability was positively correlated with stem cutting rooting ability. Easy-to-root progeny had a lower browning percentage and a higher callus formation percentage when compared with hard-to-root progeny. NAA was the main factor affecting hard-to-root progeny, while TDZ had a significant effect on easy-to-root progeny. Shoots were induced only in easy-to-root progeny. In conclusion, callus formation of progenies with both high and low rooting ability was promoted when cultured with 0.5 mg L^‑1 NAA + 2 mg L^‑1 TDZ.