Effects of concentration and propagule age on the efficacy of gibberellic acid for breaking dormancy in potato minitubers By: KH. SALIMI 1 * , M. B. HOSSEINI 1 , R. TAVAKKOL-AFSHARI 1 and P. C. STRUIK 2 1 Department of Agronomy & Plant Breeding, University of Tehran, Karaj, 31587-77871, Iran 2 Centre for Crop Systems Analysis, Plant Sciences Group,Wageningen University, Droevendaalsesteeg, 6708 PB Wageningen, The Netherlands (e-mail: salimikh@ut.ac.ir) (Accepted 21 May 2010) SUMMARY The effects of a post-harvest application of gibberellic acid (GA 3 ) at various concentrations (0, 10, 25, 50, or 100 mg l –1 ) and for different periods of incubation (1, 2, or 3 h) on the breaking of dormancy and on sprouting in potato (Solanum tuberosum L., cv. Marfona) minitubers of two ages (freshly-harvested or 1 week after harvest) were investigated. Compared with untreated, control minitubers, GA 3 -treated minitubers showed a significantly shorter period of dormancy (46 d), especially when the minitubers were treated at all concentrations immediately after harvest (41 d). GA 3 treatment had a greater effect on dormancy in freshly-harvested minitubers than in 1 week-old minitubers, probably because of the reduced uptake of GA 3 solution after skin-set. At lower concentrations of GA 3 , the period of dormancy tended to decrease the longer the minitubers were immersed in the GA 3 solution. In contrast, at higher concentrations of GA 3 , the duration of immersion only had an effect up to 2 h and any longer immersion was ineffective. The average number of sprouts was not affected by GA 3 concentration, or by the time of incubation, but was significantly higher for freshly-harvested minitubers (1.4) than for 1 week-old minitubers (1.2). The lengths of the sprouts increased with an increase in GA 3 concentration, especially in freshly-harvested minitubers. Consequently, immersing freshly-harvested potato minitubers in a low concentration of GA 3 (up to 25 mg l –1 ) for up to 3 h effectively shortened the period of dormancy and led to the formation of sprouts of optimum length. P otato (Solanum tuberosum L.) minitubers can be used for seed tuber production under field conditions. However, early attempts to grow a crop from minitubers were often unsuccessful because of reduced or delayed emergence, probably due mainly to the prolonged period of dormancy associated with small-sized tubers (Lommen, 1994). The dormant period of minitubers is longer with smaller tubers (Lommen, 1993). Potato tuber dormancy is defined as the physiological state in which autonomous sprout growth will not occur, even when the tuber is placed under ideal conditions for sprout growth (Reust, 1986). Potato seed tubers should be allowed to pass through their normal period of dormancy and to sprout naturally. However, excessive dormancy might limit seed usage and, in such a case, inducing sprout growth may be necessary. Rapid and uniform sprouting of seed tubers (or tuber pieces) is a pre-requisite for establishing a healthy crop (Suttle, 2008). Dormancy can be broken by a range of chemical and/or physical treatments (Suttle, 2008).Treatment with bromoethane (BE; Coleman, 1984) or gibberellic acid (GA 3 ; Rappaport et al., 1957) is the most common practice in commercial seed tuber use. The main disadvantage of BE is its toxicity to humans, while exogenous application of GA 3 offers an economical and safe method by which to break potato dormancy. Breaking of dormancy may be achieved by applying GA 3 to the mother plant during tuber growth (van Ittersum and Scholte, 1993), but the efficacy of this method depends on the growth stage of the crop and on tuber development at the time of GA 3 application (Alexopoulos et al., 2006). Breaking dormancy can also be promoted by the immersion of tubers in a solution of GA 3 after harvest (Rehman et al., 2001). The correct concentration of GA 3 required to break dormancy in potato tubers is contentious. Some authors (e.g., Rehman et al., 2001) recommend extremely high concentrations of GA 3 (e.g., 1,000 mg l –1 ). Such concentrations may be too expensive to be used in practice. Moreover, high concentrations of GA 3 induce abnormalities in the growth and development of both the above-ground parts of the plant and in the tubers (Slomnicki and Rylski, 1964). Alexopoulos et al. (2008) suggested that, to break dormancy, the duration of incubation in a GA 3 solution was more important than the concentration of GA 3 . In contrast, Rappaport et al. (1957) reported that 5 min and 90 min dip treatments in 500 mg l –1 GA 3 were equally effective. In the case of normal seed potato tubers, a combination of cutting each seed tuber (150 g) into two pieces and dipping them in a 50 mg l –1 GA 3 solution was effective in breaking dormancy (Slomnicki and Rylski, 1964), presumably because the GA 3 could penetrate easily through the wounded surfaces. However, cutting seed tubers may also facilitate the spread of bacterial infections and could reduce the vigour of the seed, especially when the tubers were small (Struik and Wiersema, 1999). As minitubers are now part of the commercial seed potato production scheme aimed at the *Author for correspondence. Journal of Horticultural Science & Biotechnology (2010) 85 (6) 461–464