The physiological changes of the tuber (incubation or physiological ageing) is an irreversible phenomenon which takes place in two stages.
The first stage, which begins immediately after the harvest, is a rest period during which the tuber is unable to sprout, even under ideal conditions. It is in a state of vegetative rest or deep (innate) dormancy.
The length of dormancy can vary according to the cultivar and conditions in the year of production. It may range from 17 to 40 weeks starting from tuber formation until the appearance of the first visible sprout (around 1 mm in length) on tubers stored in darkness immediately after the harvest, at 15°C with a relative humidity above 85%.
The second stage, sprouting, consists of three successive phases reflecting the degree of incubation of the seed tuber:
- phase I is characterised by slow growth and the emergence of a single sprout at the tip of the rose-end (apical dominance, photos 1 and 2); tubers showing this sort of sprouting are said to be low or under-incubated;
- phase II is characterised by the rapid growth of several sprouts (photos 3 and 4).
This is the physiological age at which the seed tuber’s growth is at its most vigorous;
- phase III during which sprouting slows, is characterised by the formation of numerous very elongated sprouts (spindly sprouts, photos 5 to 7). At the end of this phase (fully incubated seed tuber), during which the development of numerous small sprouts per eye may also be observed (photo 8), premature daughter tuber formation can occur. This characterises the final stage of the physiological ageing of seed potatoes (photos 9 to 12).
The growth vigour of the plant (photo 13) is closely related to the physiological state of the mother tuber at the time of planting.
Symptoms on foliage
Despite all the care given to the crop, over-incubated seed potatoes (phase III) have a slow, staggered, heterogeneous emergence. The developing aerial parts remain stunted and the root system undeveloped, rendering the plants vulnerable to drought. The slow growth of the shoots makes them extremely susceptible to attacks by soil pathogens (especially black scurf). As the tubers are formed early, the period of vegetation is shortened (which can be beneficial in the case of early potatoes). In extreme cases, notably when the temperatures immediately following planting are low, or when the tubers are desprouted, the plant tuberises in the soil with no preliminary formation of aerial stems.
Conversely, under-incubated seed potatoes (in phase I) result in late emergence and a limited number of stems (numerous single-stem plants) but subsequently have normal vigour. This phenomenon is observed mainly with early export and prompt planting in autumn.
- Store the seed potatoes in conditions that will ensure optimum physiological age and growth vigour. In practice, in Western Europe, the cultivars which are the most susceptible to incubation (rating less than 5) should be placed in cold storage early (before mid- October), and cooled down to 2-3°C , bearing in mind the susceptibility of the cultivar to low temperatures. Furthermore, if their vegetative rest period is short, presprouting, if practised, should be restricted to a maximum of one and a half months. Less susceptible cultivars (rating 5 to 7) may be placed in the cold room a little later (up to mid-November) and kept at 4°C. With slow-incubation cultivars (rating above 7), cooling should be regulated to avoid sprouting that is excessive or too early, and eliminate the phenomenon of apical dominance.
- Use certified seed potatoes, and adapt the preplanting conditions according to the cultivar, the duration of the cycle of the crop and the physiological age of the seed tubers. Presprouting seed tubers to at least the “white dot” stage results in advanced emergence and usually good growth vigour.
- Planting should be carried out in a warm, dry soil to ensure a regular emergence and growth.
Facteurs de risque
The tuber’s physiological ageing speed, or incubation speed, differs with each cultivar. It is rated in the French catalogue of potato cultivars from 1: very rapid (highly susceptible to incubation) to 9: very slow.
The age of a tuber of a given cultivar at a given time increases in proportion to the length of time since its initiation (chronological age), and to the temperature at which it was stored after being harvested. Cultivars characterised by slow physiological development tolerate basic storage conditions.
The knowledge acquired concerning the physiological age of seed potatoes has found a practical application in low-temperature storage and the use of cold rooms (2 to 4°C). This provides high-quality seed potato tubers that are physiologically suitable for conventional planting.
Apart from physiological ageing, seed potatoes of some cultivars, stored at too low a temperature or air moisture, may suffer dehydration of the eyes which may totally or partially inhibit sprouting.