DORMANCY, SPROUTING

The potato plant has a lifecycle relatively hard to determine in Europe, some parts are usually connected, while others (flowering, tuber formation) are independent from the others. There are some artificial interactions, like the sprout destruction, which may have a big effect on the lifecycle and finally to the yield.

Dormancy:
The yield of the potato, the potato tubers spend the biggest part of their lives in the storage, first of all the earlier varieties. The ability of the tubers for storing depends on the dormancy of healthy tubers.
Only healthy, full ripe tubers have the potential for sufficient dormancy. Tubers infected by pathogens, unripe tubers, tubers which have been sprouted in the present vegetation period, crossbred tubers, tubers with too low dry matter content have a much shorter dormancy period than healthy tubers.Too young and too old tubers are not capable of utilising their potential dormancy endurance.The dormancy period has two major parts: obligate dormancy and enforced dormancy.

Obligate dormancy:

Potato tubers, when fully ripe, except for some extraordinarily rare situations, never develop sprouts in the present vegetation period.This situation is caused by an obligate plant-hormone-regulated vegetative period.
In this stage, tubers cannot develop sprouts when ambient parameters are good for sprouting. The duration of this vegetative life period depends on cultivated variety, dry matter content, health condition and temperature. The extra early and early varieties usually have a shorter obligate dormancy, the later varieties have a longer one. High temperatures cause shorter, lower ones cause longer dormancy, so the multiplication of the temperature and the time spent at this temperature usually result a constant value. This value is representative of the cultivated variety and sometimes published in the descriptive lists of varieties.
Higher starch or dry matter content usually results a little longer dormancy. Extraordinarily low dry matter content of tubers may result absolutely short dormancy, bacterial, or fungal infections, or tubers, which are unable to sprout. Attack by bacterial or fungal pathogens may result in a similar effect. The presence of pathogens usually results in much higher respiration and faster usage of stored assimilates. Finally, the pathogen attack usually results a dead or unsprouting tuber.

Enforced dormancy:
At the end of obligate dormancy under good external conditions for sprouting the potato tubers develop sprouts. The development of sprouts depends on external conditions. Under cold, not too wet conditions, when the tubers still contain enough stored dry matter, the sprouts do not develop. Warm wet storage conditions are optimal for sprouting.When the potato tuber has no more stored assimilates, it begins to develop sprouts, also under not so good external conditions. Under good storage conditions, the transition between obligate and enforced dormancy is not visible. The duration of enforced dormancy does not depend on cultivated variety, but the average dry matter content has some effect on this. Under optimal storage conditions, the time, when this stage ends, depends on the time, when the potato tuber gets exhausted. It depends first of all on the external storage conditions.

Sprouting
The development of sprouts is the first part of vegetative growth of the potato plant.
This stage begins with the appearance of small closed sprouts on the eyes of the potato tuber and ends with its surfacing.Sprouts may be light-sprout, storage-sprout and hair- sprout. A healthy stem develops from the first two.
The potato tuber needs sufficient temperature (higher than 8 °C) for sprout development. When soil temperature is lower, the sprouting process stops. The pauses in sprout development are disadvantageous. Growing sprouts have a thin fragile surface vulnerable to bacterial or fungal infection. In cool wet soils, the hazard of Erwinia attack (blackleg), Rhizoctonia or Fusarium attack is much higher than in dry warm soils.
Infected sprouts never develop stems, so yields are reduced. The infection may penetrate back to the tuber easily, it may cause missing hills. Missing hills caused by too early plantation may result a higher weed problem. At the holes in the foliage, caused by the missing hills, the soil surface is not covered by the crop and is open to weed attack.

The most advantageous is when the sprouting process is as fast as possible. For such optimal development, the potato plant needs warm soil and enough water. Nutrition in this stage is not so important, but too much nitrogen may cause fast growth with extraordinary loose plant tissues, which is vulnerable to attack by bacteria or fungi.