Sprouting and plant establishment
According to Smith (1977), morphologically the tuber is a fleshy stem, bearing buds or eyes in the axil of scale-like leaves which soon shed, leaving a ridge or leaf scar subtending the bud. Bud primordia are present even in young tubers but the sprout does not begin to form until the later stages of tuber development. Potato growers in the Andean region in Latin America have traditionally been using naturally occurring compounds to control sprout growth (Oosterhaven 1995). Sprouting strongly responds to the physiological age of the seed tuber, to inter-and intra-plant competition, nitrogen and water supply and air temperature (Struik and Wiersema 1999). Irrigation during stolon formation is crucial to the manipulation of stolon number per stem (Lugt et al. 1964). The size of seed tuber has an effect on number of eyes which is directly related to the physiological status of the tuber, smaller tubers are generally physiologically younger. The number of potential sprouts and stems per seed and the plant growth vigour are thus influenced (Struik and Wiersema 1999). According to Steyn et al. (1998), small tubers such as microtubers or minitubers already bear a surprisingly high number of potentially active eyes, especially at the apical end. There is significant unevenness in sprout emergence depending on whether the meristem is apical or lateral on the tuber, with the apical buds sprouting first and displaying an apical dominance over the lateral buds (Burton 1989). Fluctuating the humidity of the storage atmosphere, mechanical or chemical (carbon, disulphide or GA) damage, can encourage sprouting (Claassens and Vreugdenhill 2000). The number of leaf primordia formed on each stem is determined as well by storage conditions of the seed and conditions after planting (Struik and Wiersema 1999).
At the onset of sprouting, mobilisation of reserves, which consist mainly of starch and proteins, is of great importance. Potato tubers turn into a source organ before phytohormones can exert their growth control. During this process starch and protein will be degraded into soluble sugars and amino acids providing energy for the developing shoot. Starch breakdown can occur via phosphorolytic and hydrolytic reactions (Biemelt et al. 2000). High temperatures during growth also resulted in more sprouts per tuber after dormancy had ended for certain cultivars. According to Kempen (2012) for optimal tuber formation and high yields a productive canopy is required. If crop growth rate is assumed to be proportional to the rate of photosynthesis and thus net assimilation, then