Carbon Disulphide Promotes Sprouting of Potato Minitubers (Report‪)‬

Introduction One of the most popular ways to produce pre-basic seed of potato (Solanum tuberosum L.) is to grow minitubers in the greenhouse from in vitro plantlets produced from nodal cuttings. Minituber production has found its place in the seed production systems all over the world as it creates a bridge between the in vitro rapid multiplication based on nodal cuttings and the field multiplication of seed tubers. Minitubers are more flexible, can be stored and mechanically planted, and show a larger vigour than either microtubers or in vitro plantlets (Struik, 2007a). Similar to normal seed tubers, minitubers go through different stages of development after harvest, including dormancy (no sprout), apical dominance (only one sprout), normal sprouting (a few, normal sprouts per seed tuber), advanced sprouting (many sprouts per seed tuber which are often branched), senility (excessive sprouting with very weak sprouts), and incubation (little tuber formation) (Struik, 2007b). The physiological status of seed potatoes has a great impact on the emergence, number of stems per plant, number of tubers per stem, tuber-size distribution, and tuber yield of the progeny crop (Struik et al., 2006). This is also true when minitubers are used (Struik and Lommen, 1999; Struik and Wiersema, 1999). After harvest, normal seed tubers show dormancy for about 1-15 weeks, depending on cultivar, tuber size, conditions before harvest and storage conditions. Small tubers, such as minitubers, even have longer periods of dormancy (Lommen, 1993) and are more sensitive to adverse conditions during storage (Struik and Wiersema, 1999). The progress of the physiological ageing can be accelerated by conditions during storage, especially by storage temperature. Cold shocks, heat shocks and warm temperatures all advance breaking of dormancy (Struik and Wiersema, 1999). But, when the time between harvesting and planting is very short, these methods might not be effective (enough). Chemical dormancy breaking is then an option to achieve rapid and uniform crop emergence as well as a high number of stems per plant. In Iran, a large amount of minitubers is harvested between March and April. Normal planting time of these minitubers in the field or in the greenhouse is in the month of May or beginning of June. So there is not enough time between harvest and planting to break dormancy naturally. This calls for a reliable technique to break the dormancy of these small tubers chemically. At commercial scale, Rindite (Rehman et al., 2001), bromoethane (Coleman, 1983), C[S.sub.2] (Meijers, 1972) and G[A.sub.3] (Alexopoulos et al., 2008) have been used to break the dormancy of potato seed tubers. There are only a few records about the effect of C[S.sub.2] on potato dormancy, because the dormancy breaking effects of Rindite and bromoethane are considered stronger than the effect of C[S.sub.2]. However, C[S.sub.2] might have some important advantages compared with common commercial treatments. For normal seed tubers, is has been shown that the efficacy of C[S.sub.2] at low concentration (12 - 25 ml [m.sup.-3]; Meijers, 1972) offers a more economical method to break potato dormancy in comparison with bromoethane (100 - 200 ml [m.sup.-3], Coleman, 1983) and Rindite (200 - 400 ml [m.sup.-3], Kim et al., 1999). This economic advantage is relevant, especially in developing countries. C[S.sub.2] might also be safer than some of the other commercial compounds as minitubers may be too delicate to withstand coarse chemical treatment. Moreover, some potato cultivars hardly respond to G[A.sub.3] treatment. Also, treating minitubers with G[A.sub.3] induces excessive sprout elongation resulting in sprouts that are thin, fragile, and prone to breakage during handling (Suttle, 2008; Salimi et al., 2010). In contrast, treatment with C[S.sub.2] effectively terminates dormancy and the resulting sprouts are short, thick, robust, and resistant to breakage (Salimi et al., 2010). Breaking the pot