Government of New Brunswick

Physiological readiness to grow is an important factor in determining the production potential of potato tubers. This is usually referred to as physiological age of seed.

Physiological age can be compared to an internal clock and impacts all aspects of crop production. Physiological age increases with time and temperature; the higher the temperature the greater the increase in physiological age. The effects of planting seed of different physiological ages have been well researched and are widely known. Many factors must be taken into account when considering physiological aging of seed. These factors include;

  • variety
  • intended end use of the resulting crop
  • previous seed crop history including such
  • elements as fertility
  • seasonal rainfall
  • temperature

Temperature is the largest single factor that affects the physiological age of a seed tuber. The physiological age of a potato tuber is determined from the time of tuber initiation onwards. The aging process can be monitored by certain events, such as the timing and type of sprout development. The type of plant and yield produced will be affected by the physiological state of the seed tuber at time of planting. Changing temperature levels can either slow or speed the internal clock or aging process. Knowing the variety and the expected end use of the crop permits producers to decide the optimal seed age to plant. They should then attempt to attain this age through appropriate seed warming practices.

It is possible to evaluate the physiological age of a seed lot by taking samples from cold storage and observing sprouting activity following warming. Representative samples should be taken from the seed lot and immediately moved to a temperature of 15-18oC in the dark. The time to achieve optimal sprout development ie when sprouts just begin to emerge, (white point stage), should be noted. This will provide an estimation of the amount of time and warming that will be required in the storage for that particular variety and seed lot.

STAGES OF SPROUT GROWTH

The potato tuber passes through very definite stages which relate to the accumulation of physiological age. Tuber production is very much related to the various stresses to which the plant and seed tubers have been exposed. Seed grown during a season with extremes in weather conditions, which stress the crop, would be expected to be physiologically older at harvest than tubers from a more "normal" growing season.

DORMANCY

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With most potato varieties there is a period of time following plant death and harvest in which the tuber will not sprout regardless of temperature, light or moisture conditions. This time interval is called the dormant period. The length of this period is very dependent on the variety and storage conditions, especially temperature. Tubers may be treated chemically or in some cases induced to sprout earlier by high (20oC) and low temperature alterations.

APICAL DOMINANCE

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The apical or "bud" end of the tuber has the greatest number of eyes. Chemical processes within the tuber control the buds in the eyes so that the apical buds will normally be the first to produce sprouts. This process is called apical dominance. When this stage of sprouting is initiated, the other "buds" or eyes on the tuber are inhibited from sprouting. The resulting plant, from a whole seed tuber planted, at this stage of sprouting, will have a low number of main stems. Since the number of tubers is for the most part determined by the number of stems, a low set can be expected. This stage of sprouting is desired if the grower wishes to produce a crop of potatoes which sizes early in the season, however, a reduced set would be anticipated. The gradual warming of seed after dormancy has ceased will lead to apical sprouting. Greensprouting small tubers intended for whole planting to produce early tablestock represents an example of using apical dominance to advantage.

MULTI-SPROUT STAGE

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Plants with multiple stems, high tuber numbers and early plant maturity are highly desirable for the production of a seed potato crop. This type of plant arises from a seed tuber which has accumulated sufficient physiological age to have multiple sprouts. Given enough time, eventually all eyes will sprout. Exposing tubers to high temperatures before the apical bud sprouts, can promote multiple sprouting of all eyes.

Care must be taken to ensure that warmed tubers can be planted immediately, re-cooled or exposed to light, when sprouting has just begun. This is important to prevent excessive sprout lengthening. Long sprouts damaged or removed at planting may result in loss of plant vigour.

Proper seed preparation and early planting ensure maximum yield and set of tubers. In addition, the plant will acquire mature plant resistance to virus infection earlier in the season resulting in a higher quality crop.

Large seed tubers that are to be greensprouted should be precut and warmed in the manner described above so that each seed piece will produce multiple sprouts that will rapidly grow upon planting.

WEAK-BRANCHED (HAIRY) SPROUT STAGE

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If seed physiological aging progresses too far, the sprouts will become progressively weaker and form branches. The resulting plant will be weak and will mature before maximum yield is attained. This results in a lot of small sized tubers with low harvest weight.

It is possible to age tubers so excessively that no plant will emerge when the seed is planted. Instead small tubers will form directly. This condition should be avoided.

SEED CUTTING AND PHYSIOLOGICAL AGE

Seed cutting sufficiently shocks the seed tuber to promote sprouting of buds (eyes) if the tuber is beyond the dormant period. In this way the practice of seed cutting compensates for improperly aged seed by promoting the premature growth and development of some eyes. Planting cut seed which has not been properly warmed will contribute to the development of poor or uneven plant stands. It is therefore important to properly warm and size seed for the intended purpose prior to seed cutting.

WARMING IN STORAGE (WITHOUT LIGHT)

Do not plant directly from a cold storage (4o-5oC). Warming the seed to 10oC - 15oC increases the physiological age and enhances sprout formation. By careful control of the warming period, seed with sprouts just emerging (white points) can be produced at planting. Longer sprouts are tender and susceptible to mechanical damage. If the rate of sprout formation is too rapid, or a delay in planting is anticipated, cooling the storage slows sprout development. Through-the-pile-ventilation with a large volume of high humidity air also helps to slow sprout growth.

REENSPROUTING (CHITTING) OF SEED

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Tubers warmed and sprouted in the presence of light form short, compact, tough, green sprouts. These short sprouts are less subject to damage than are the longer white sprouts formed in the dark.

Greensprouting will give earlier emergence, tuberization, sizing and maturity. The greatest advantage of greensprouting occurs with early harvest particularly in early frost-prone areas and diminishes as the crop is allowed to grow to maturity. Early market tablestock and early topkilled seed will benefit most from greensprouting as it may advance the maturity by about two weeks. In order to obtain greensprouted seed tubers, tubers are warmed at 15oC - 20oC until the sprouts just emerge (white point stage). These tubers are then exposed to light. This is best accomplished in special stacking trays. No more than 2 layers of tubers should be placed in these trays in order that light can reach every tuber.

The light requirement is not high. One 40 watt fluorescent tube per 4 square meters hung vertically between rows of stacked trays is sufficient. Trays could also be stacked on a wagon which can be moved outside each day, or stacked in a plastic type greenhouse shelter. The recommended temperature is 5o- 10oC. Tubers must be kept from getting wet or frosted.

Use care in handling and planting greensprouted tubers. Cup-type planters are the most suitable for greensprouted seed. The pick-type planters presently used are not designed to handle greensprouted seed without sprout damage. Some belt-type planters with additional modifications to reduce sprout damage may be suitable. Small hectarages can be planted with hand-assisted feed planters with hardly any sprout damage although their planting rate is slow.

SEED TREATMENT

Proper chemical treatment of seed controls some of the diseases which weaken or kill the sprout before or during emergence. It is an inexpensive insurance that partially protects against invasion by microorganisms in the soil and those on the tuber surface; however, chemical treatment usually only controls those microorganisms attacking the seed surface.

Cut seed tubers require lots of attention. While potatoes are being cut, periodic disinfection of cutting and handling equipment is recommended, especially between different seed lots. Where cut seed is not planted immediately, very specific environmental conditions for storage must be provided:

  1. Use a dust type seed treatment.
  2. Use forced air circulation through the pile of tubers (the pile should not be more than one meter deep).
  3. Hold at a temperature of about 12-15oC and a relative humidity of 85 95% for at least three days in order to suberize or cure the cut sets.

Ventilate intermittently only to provide enough air to dissipate carbon dioxide and heat from respiration and uniformly distribute temperature in the pile. After the curing period, maintain high humidity and reduce temperature to 5oC. Seed must be rewarmed before planting to ensure vigorous sprouting.

For chemical seed piece treatment recommendations, see Publication 1300A.

Planting for Better Plant Stand, Yield & Quality

The establishment of a uniform crop, which will develop and mature evenly and produce a regularly sized run of tubers, largely depends upon optimization of the preplanting activities and the planting operation itself. These activities include: seed preparation and sizing, seed treatment and cutting, equipment adjustment and proper operation of planting equipment.

When evenly sized, prepared, and cut seed pieces are appropriately placed in the seed furrow an effective plant stand will be established.

The potato planting season extends from late April until early June, depending on location and climatic conditions, April planting is very limited by frost potential and wet soils in most locations of the region.

Potatoes should be planted as early as conditions permit but soil temperature should be at least 7oC. Planting in cold or wet soils will delay emergence, increase the chance of seed piece decay and result in poor plant stands. Properly conditioned whole seed or well suberized cut seed should always be used. This is especially important when planting conditions are less than ideal; i.e., cold, wet soil.

A between-row spacing of 91 cm is common, with seed spaced anywhere from 15 to 40 cm apart within the row. The closer spacings are used for seed crops to keep tuber size small, whereas the wider spacings result in production of larger tubers required for the fresh market and for processing. The variety, soil fertility, soil moisture and length of the season will also influence choice of spacing. Seed of a variety prone to produce tubers with hollow heart should be planted at a closer within-row spacing.

The planters currently used include the automatic pick-type, the assisted feed, cup and tuber unit types. The pick-type is fast and operates most efficiently for uniform blocky seed pieces. However, punctures made by the pickers may spread tuber borne diseases from one seed piece to another. The assisted feed type, while slower and requiring more labour, allows greater placement precision ensuring better stands. The cup type, like the assisted feed type, reduces the spread of any disease which may be present. Cup type planters, though automatic, are sensitive to seed size and function best with uniform sized whole or cut seed. Tuber unit planters are primarily used by producers of Elite seed classes to meet planting requirements for certification.

It is important that planters be adequately cleaned and disinfected prior to use and in between fields. Proper mechanical operation contributes significantly to a good plant stand. Planters should be maintained in good working condition, operated at rates according to manufacturers' recommendations and run by competent persons.

No specific depth of planting will give equally good results under all conditions. Planting depths of 8 to 13 cm are recommended, but it is very important for good germination to place seed in warm, moist soil. Shallow planting can result in uneven emergence and contribute to some tuber greening. Excessive depth of planting may delay emergence, increase chances of disease and seed decay, reduce vigour and result in poor stand.

STEM NUMBERS

The numbers and size of tubers produced in the field is largely affected by the number of stems per unit of area. The greater the stem density, the greater in number and the smaller in average size are the tubers produced.

Small whole seed production requires a high stem density (over 30 stems/square meter). Tablestock and especially processing production require reduced stem densities.

Stem density is controlled by set spacing and the number of stems produced per set. The number of stems per set is influenced by the seed pre-treatment and seed size and variety.

For early market tablestock and processing potatoes, low stem densities are usually obtained by planting cut sets from larger tubers. In this manner, sets of 40 to 55 g having only a few eyes can be planted relatively far apart.

The close planting of whole tubers results in the multiple stems per set and high stem densities as is required for seed production.

CUTTING OF SEED

The shock of cutting seed reduces apical dominance, allowing additional sprouts to develop. Cut seed pieces have less eyes than whole seed of the same seed piece weight. Due to the shock of cutting, cut seed emerges earlier than whole seed which has not been subjected to a heat shock or presprouting.

Cleanliness and disinfection are most important in cutting seed to reduce the spread of any disease by the cutting knives.

The ideal situation is to cut seed ahead of time, and allow it to suberize under conditions of good ventilation, high humidity and a temperature of 12o-20oC. This is most important as the seed can spoil very quickly if these conditions are not met.