Light Quality Affects Microshoot Growth of Apple Rootstocks: B.9 & G.30

Tissue culture using shoot cultures is a procedure to rapidly increase plant numbers of newly released apple rootstocks, especially when they are in short supply. However, slow growing microshoots limit the commercial availability of desired apple rootstocks.  Light quality can affect shoot elongation and shoot branching.  The effects of red, white, and blue light on microshoot growth in B.9 and G.30 apple rootstocks were investigated.  One-year-old greenhouse-grown shoots of both rootstocks were trimmed into 1.0–1.5 cm stem pieces with a single node and then were cultured in solid MS media without any hormone.  After 38 days, the sprouts were transferred to new solid MS media with GA₃ (0.5 mg/L), BA (1.0 mg/L), IBA (0.1 mg/L), and sugar (3%), and were then cultured under red, white, or blue light.  After 42 days culturing, the number of shoots, length of the longest shoot, and the number of elongated shoots (>1.5 cm) were collected.  Red light increased the number of new shoots, shoot length and the number of elongated shoots when compared with white or blue light, and there was no interaction with cultivar.  The number of new shoots increased from 2 under blue light and 4 under white light to 7 under red light.  Shoot length was 2.9 cm under red light compared to 1.2 cm under white light and 1.0 cm under blue light.  The number of elongated shoots in both cultivars was also increased by red light, which increased from 1 under blue light and 2 under white light to 6 under red light.  For the total number of shoots and the number of elongated shoots, there was no cultivar effect, but shoot length of G.30 was longer than that in B.9.  Thus, using red light to culture microshoots will be recommended to improve apple rootstock microshoot growth.  Moreover, survival and microbial contamination rate were also calculated and the results also presented that red light would be a better alternative to culture apple rootstock microshoots.