Effects of haulm killing and gibberellic acid on seed potato ( Solanum tuberosum L. ) and techniques for micro- and minituber production in northern latitudes

Seed potato is the starting point in the potato (Solanum tuberosum L.) production chain. In order to secure potato production in a variety of production conditions, plant diseases must be controlled and the yield characteristics of the used cultivars ensured. In addition, production must be costeffective. Characteristics particular to northern production conditions include long periods of daylight and a short growing season as well as a several months long seed potato storage period. The focus of the present study is on northern production conditions and methods, including haulm killing and sprout control, which are presumed to affect seed potato quality, as well as the initial stages of the seed potato production chain, i.e. microand minituber production, which could influence cost-effectiveness and propagation. Haulm killing is one of the methods used in seed potato production to regulate tuber size. It is often carried out on unsenesced plants. The present results, however, indicate that cultivar properties have a greater effect on the sprouting and crop yield of seed potatoes than productionphase haulm killing or temperature sum accumulation. Nevertheless, haulm killing carried out three weeks after flowering (75 DAP) accelerated emergence. When the effect of haulm killing methods on seed potatoes was compared with natural haulm senescence, haulm killing was shown to increase disease pressure. Black scurf (Rhizoctonia Solani) was present in seed tubers whose haulm had been destroyed by mechanical or mechanical-chemical haulm killing. Naturally senesced haulm had less black scurf, and crop quantity and starch content developed to a level typical of the cultivar. Storage periods lasting several months make controlling seed potato sprouting more challenging. Therefore, use of the plant hormone gibberellic acid (GA) in sprout control was investigated. GA treatments at lower concentration (100 mM) increased the number of tubers in the cultivar Fambo. Thus, the timing of haulm killing and in the case of Fambo, GA treatment influenced the characteristics of seed potatoes. Conventionally, the first tuber generation is produced using microplants to produce minitubers in greenhouses. This production method is, however, labour-intensive, and energy and investment costs are high. With the aim of increasing production efficiency in northern production conditions, the production of minitubers in the laboratory using a novel bioreactor technology and in growth rooms using the hydroponic technique was investigated. The Liquid LabTM Rocker bioreactor system was used in vitro, all the cultivars examined (Asterix, Timo, Van Gogh and Velox) produced microtubers. The quantity of tubers produced per dish varied between 30 (for the cultivar Asterix in eight weeks) and 75 (for the cultivar Velox in 11 weeks). The results showed hydroponic production of minitubers to be successful in indoor conditions: the cultivars Desiree and Van Gogh developed their first tuber three weeks faster than Asterix, and the minituber yield was 4.5 per plant for Desiree, 7.5 for Van Gogh and 4.0 for Asterix. When the results indicate that both the Liquid LabTM Rocker production method and the hydroponic production method are suitable for mass production of seed potatoes.