In Vitro Apple Shoot Proliferation and Shoot Elongation Using Growth Regulators

To overcome the limitations of stunted shoot growth in micropropagation of apple rootstocks, we investigated the optimum PGR combination for shoot proliferation and shoot elongation of B.9, G.30 and G.41 apple rootstocks.  Benzyladenine (BA), thidiazuron (TDZ) and Zeatin (ZT) increased number of shoots per explant of G.30 compared to media with no cytokinin.  In G.30, shoot number increased as BA increased from 0.5 to 2.0 mg/L.  TDZ increased shoot number but with no difference due to increased concentration.  Shoot number decreased when ZT concentration increased from 0.5 to 2.0 mg/L.  Shoot number per explant varied from 1.0 in the control to 10.6 with BA at 2.0 mg/L.  TDZ had no effect on shoot length, but the higher concentrations of BA and the lower concentrations of ZT yielded longer shoots compared to 0 mg/L.  In B.9, only TDZ increased shoot number compared to no cytokinin.  TDZ produced the highest number of shoots at 1 to 4 mg/L.  ZT at 1 to 4 mg/L decreased shoot number per explant compared to 0 mg/L, and BA had no effect.  Similar to G.30, TDZ had no effect on shoot length and 1 to 2 mg/L ZT had taller shoots than no ZT and 4 mg/L.  BA at 1 mg/L increased shoot length but 2 to 4 mg/L had no effect compared to no BA.  GA₃ (0 to 1.0 mg/L) alone and in combination with BA had no effect on shoot growth.  In G.30, GA₃ at 0.5 to 4.0 mg/L combined with IBA from 0 to 0.5 mg/L had a small effect on shoot number and shoot length.  The combination of 1.0 mg/L GA₃ and 0.5 mg/L IBA increased shoot length.  G.30 microshoots were fewer and shorter with increasing brassinosteroid (BRs) concentration from 0 to 1 mg/L.  GA₃ concentration from 0 to 1 mg/L had no effect on G.41 shoot number per explant or shoot length, but these were increased in a second culture compared to the initial proliferation culture.  Cytokinins had a greater impact than GA₃ or BR on shoot growth during the initial proliferation in G.30 and B.9.