The third chapter of the Potato Handbook discusses the propagation material of the potato plant. This consists of the genetic component - its genotype (species and variety) and the physical component - its seed tuber. The seed tuber as propagation material is determined, besides genotype, by its size, age, appearance, sprouting condition and health.The size of the seed potato is usually smaller than that of table and processing potatoes. This because the number of sprouts and stems per unit planted weight diminishes with increased seed tuber size. Therefore, planting smaller seed means planting more eyes and potentially more sprouts and stems per unit weight. When seed tubers are too large in size, some growers cut them into pieces. In a few niches, growers transplant rooted cuttings to the field to grow seed for the next one or two generations.
Dormancy and sprouting are associated with the physiological age of the seed tuber. Age is often expressed as the temperature sum in day degrees after harvest. For some time after harvest, tubers do not sprout even when placed under favorable conditions, this period is known as absolute dormancy. In nature as well as for growers, this is a desirable property of the plant as it allows it to overcome unfavorable conditions after harvest. New plants are only formed when the cold or dry period after crop maturity has passed. The dormancy period and sprouting stages are manipulated by growers through the choice of the variety and storage methods. In storage they manipulate the temperature, humidity and light to which the seed piece is subjected during the relative dormancy period. Dormancy is lengthened by storing at low temperatures close to 2 °C, while it is shortened by high temperatures and by temperature fluctuations, as well as chemically by applying e.g. ethylene and essential oils.
Vegetative propagation material consists of whole (1) or cut (2) seed tubers, mini and microtubers (3) and rooted cuttings (4).
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