Solving the problem of shoot-tip necrosis in Harpagophytum procumbens by changing the cytokinin types, calcium and boron concentrations in the medium

Various experiments were conducted to characterize and control factors affecting shoot-tip necrosis (STN) in Harpagophytum procumbens. Higher cytokinin concentrations increased the incidence of STN and the problem was aggravated by the addition of auxin (IAA) to the multiplication medium. Optimum shoot multiplication was achieved by omitting auxin and using the cytokinin meta-Topolin riboside (mTR). In the presence of auxin, plantlets produced basal callus that interfered with rooting. The quantity of this basal callus was minimal when mTR was used. Increasing the concentration of either calcium or boron prevented the development of necrotic shoots. When the concentration of both elements was increased simultaneously, negative effects on both growth and STN were observed. Using 6 mM Ca in half-strength MS medium was optimum. Boron was toxic at higher (0.4 and 0.5 mM) concentrations. Plantlets rooted readily in half-strength cytokinin-free MS media supplemented with 2.5 µM IAA. Rooted plantlets produced, using the optimized protocol, were acclimatized successfully by direct transfer to the greenhouse in a 1:1 ratio of sand:soil mixture.