Abstract:
The effect of cytokinin and gibberellin, and in particular a combination of the two, on termination of dormant potato tubers was investigated. The objective was to effectively terminate dormancy through the external application of a combination of cytokinin and gibberellin. Freshly harvested tubers were treated and either cut at the stolon end with the apical portions placed on moist cotton wool, or left intact and dry. Tuber segments treated with a high concentration of cytokinin (0.1g.Lˉ¹) or a combination of cytokinin and gibberellin sprouted within 5 days, whereas high gibberellin concentrations (0.1g.Lˉ¹) stimulated sprouting within 9 days. Untreated tuber segments supplied only with moisture terminated dormancy later than hormonal treated tubers, but much earlier than segments that were kept dry. Tuber segments treated with a combination of cytokinin and gibberellin, or a high concentration of gibberellin (0.1g.Lˉ¹), produced more and longer sprouts than tubers treated with only cytokinin (0.1g.Lˉ¹) or a low concentration of gibberellin (0.005g.Lˉ¹). Sprouts on tuber segments treated with a combination of cytokinin and gibberellin attained maximum sprout growth rate nine days after treatment, but thereafter the growth rate decreased. This decrease may be a consequence of closed plasmodesmata although membrane permeability and its affect on assimilate availability may play a role. This phenomenon deserves further research attention. Removal of wound periderm did not reactivate sprout growth. The wounding of tubers by removing a portion at the stolon end and supplying moisture greatly enhanced the termination of dormancy and subsequent sprout growth, indicating that the availability of water may be a factor in initiation of sprouts. The results are compatible with the hypothesis that cells in dormant buds are arrested in the G1 phase of the cell cycle. Cytokinin is needed to initiate cell cycling, but gibberellin is also needed to initiate and maintain cell growth. These two growth regulators are also involved in the opening of the plasmodesmata as well as the creation of new plasmodesmata witch would establish communication between the apical meristem and the rest of the tuber.
