Chilling Duration Affects the Microshoot Elongation of ‘G.30' Apple Rootstock

Micropropagation is an alternative to regenerate some highly desired and difficult to propagate rootstocks.  The multiplication phase requires rapid shoot growth and lateral branching. During the initial phase of nodal culture, shoot elongation can be inhibited which may be caused by endodormancy when cultures are initiated from axillary buds collected in summer. The objective of this study was to overcome the repressed shoot elongation by chilling cultures, which were about 1.5 cm length stem with a single node cultured in MS media. The cultures were put in an incubator at 4 °C for 0, 0.3 (2 days), 4, 6, or 8 weeks. Then cultures were moved to the regular culture room to let shoots sprout. The shoots were transplanted to MS media with 0.5 mg/L GA, 1.0 mg/L BA, 0.1 mg/L IBA, and media pH at 5.6 to 5.8.  After 42 days culturing, number of lateral shoots, length of the longest shoot, and number of elongated shoots were measured. As chilling duration increased, the microshoot growth increased. Although the highest number of elongated shoots (0.7) occurred in the cultures chilled for 8 weeks, they had a similar number of lateral shoots (0.9) and lateral shoot length (1.6) as 6 weeks chilling. The highest number of lateral shoots (1.6) and the longest shoot length (1.9) were obtained by chilling the initial cultures for 6 weeks, which was significantly higher than that for chilling 0 (0.4 and 1.3) and 0.3 (0.1 and 0.5) weeks, but not significantly higher than chilling 4 (1.4 and 1.7) and 8 weeks.  High concentration of GA, 4 mg/L, increased microshoot growth compared to 0.5 mg/L, but not as much as 6 weeks chilling. Chilling nodal cultures was the most effective method of promoting microshoot growth in G.30 apple rootstock.