Internal quality of seed potatoes

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The internal quality of potatoes is a broad concept, the exact meaning of which can vary depending on the intended use of the potato. The factors defining a good seed potato are therefore not necessarily the same as for fresh market or processing potatoes. There are, however, factors of overall interest, including the presence of physiological disorders, specific weight, as well as susceptibility to pathogenic infection.

Physiological disorders

This refers to non-pathogenic disorders that usually result from environmental conditions that negatively affect the normal functioning of the plant.

Brown core and hollow heart

Brown core and hollow heart are well-known physiological disorders that cause a reduction in quality and result in loss. There are no visible external symptoms, and cultivars differ in their susceptibility to such physiological disorders. Under certain conditions, brown core and hollow heart can be regarded as two different stages of the same disorder, and are most likely both caused by the same conditions.

Hollow heart can, however, occur without being preceded by brown core, and tends to affect large tubers rather than smaller ones.

Cavities can form along the length or width of the tuber and can also be irregular in shape. The tissue lining the cavities can be white or brown, and skin tissue sometimes forms. Cavities can occur in different places in the tuber, depending on the time of the season at which they originate. Where small tubers with internal brown spot are left to grow slowly and consistently, the dead brown cells are distributed among the normal living cells.

The causes of brown core and hollow heart are numerous and often confusing. Brown core develops when tubers are very small and the temperature is low (<15°C) for a few consecutive days, especially during tuber initiation, until the tuber reaches approximately 50 g. Cells die off, turn brown, and easily tear apart from one another. Where small tubers with brown core grow quickly and/or inconsistently, cells in the brown core tend to tear apart from one another and, as the tuber grows, hollow heart then develops. This condition is aggravated by over-irrigation and the application of large quantities of nitrogen, especially during tuber initiation.

Hollow heart not preceded by brown core is associated with a rapid growth rate in tubers, varying soil moisture levels, and in some cases also the application of large quantities of nitrogen.

Internal brown spot/fleck

Internal brown spot/fleck often occurs in large tubers, and not all the tubers on a plant are necessarily affected.

Cultivars also differ in terms of their susceptibility to internal brown spot, and the disorder can also develop during the storage of tubers. The tubers do not usually show external symptoms but irregular brown spots, found mainly inside the vascular ring, are characteristic.

Conditions leading to irregular tuber growth or moisture stress, including extreme or varying air and soil temperatures and soil moisture levels, are the primary causes of brown spot. It is more common in sandy soils, while improper fertilisation which can induce an internal calcium (Ca) deficiency, can also encourage the development of brown spot.

High soil temperatures, especially during the later stages of the tuber bulking period, are often associated with brown spot. Where soil temperature is extremely high, the roots are unable to function optimally, with the result that insufficient water and Ca are absorbed. Internal brown spot can also occur when the air temperature is high and the soil temperature low – transpiration occurs normally through the leaves, but the absorption of water and nutrients through the roots is delayed. As a result, the bulk of the water and nutrients, especially Ca, that is actually absorbed will be transported to the leaves and not the tubers, placing the tuber cells under stress. Under low soil moisture conditions, Ca intake is low as assimilable Ca is dissolved in the groundwater.

Plants cultivated in sandy soil are susceptible to brown spot, since the water retention capacity of sandy soil is low and the soil temperature tends to rise rapidly. On the other hand, an oxygen shortage under waterlogged conditions could prevent the absorption of water and Ca. Heavy soil tends to become waterlogged during the rain season and with excess irrigation.

Black heart

Black heart mainly affects large tubers. The affected tubers are grey-black to black inside, and the affected tissue is well defined. A cavity can form in the black heart, and the affected tissue becomes hard and leathery. Black heart occurs as a result of an oxygen shortage in the affected tissue, causing the tissue to die off, especially where a shortage of oxygen is accompanied by high temperatures. High temperatures lead to increased respiration, which in turn increases the need for oxygen.

Cold damage

Cold damage is a result of exposure to temperatures around 0 to 2°C.

Symptoms of cold damage are mainly internal in the vascular tissue and on the stem-end of the tuber and can cause the tissue to appear grey in colour. Tissue can appear dark grey or black, or grey or reddish in colour.

With sufficient exposure to sub-zero temperatures, ice crystals can form in the tissue. Upon defrosting, cells die and turn into a watery mass.

Cold-damaged seed tubers’ ability to sprout is often negatively affected. Fried chips made from cold-damaged potatoes are dark in colour. Boiling such tubers leads to the tissue becoming grey or black in colour.

Physiological age and sprouting

The physiological age of seed potatoes, or stage of development, changes over time but can also be affected by the growth and storage conditions. External stressors can cause seed potatoes to age more rapidly, which can hamper sprouting after planting. Normal apical dominance of sprouts is more common in seed potatoes with a high Ca content.

Sprouts of seed potatoes with a low Ca content tend to form lateral branches, similar to physiologically old seed potatoes; such branches tend to form haulms with poor vigour.

Keeping quality

The keeping quality of tubers determines whether they can be transported by road over long distances and is a function of various internal tuber properties, including moisture loss and resistance to mechanical damage during handling and transportation. The keeping quality also depends on the cultivar, meaning that certain cultivars are more suited to long-distance transportation.

Specific gravity

Specific gravity (SG) is a characteristic of potatoes that is often used as a measure of internal quality. Processing companies, for example, demand potatoes with a specific gravity of at least 1.075. SG is an indication of the density (percentage of solid matter), including starch. In the case of potatoes with a low SG, more oil is required for frying, which raises the cost of processing. Low SG also has a detrimental effect on the texture of processed chips, as required by a large sector of the processing industry.

High temperatures during the growing season are particularly detrimental to SG. At temperatures of >28°C, photosynthesis is reduced, meaning that less starch and other solids can be formed.

Reducing sugars

High concentrations of reducing sugars, specifically glucose and fructose, result in undesirable browning of the tissue during frying.

Known as the Maillard reaction, this is caused by a chemical interaction between the sugars and amino acids at high temperatures. Cultivars differ significantly in terms of the percentage of reducing sugars in the tuber tissue, but conditions during cultivation as well as during storage are of the utmost importance. Both low and high temperatures during cultivation can lead to an increase in the concentration of reducing sugars.

In order to limit the formation of reducing sugars, the optimal temperature during tuber growth is between 15 and 25°C. High levels of nitrogen during tuber growth are also associated with a high concentration of reducing sugars. Increasing the level of fertilisation with potassium, however, can reverse this effect, with sufficient potassium being deemed essential in optimising the percentage of dry material as well as the reducing sugar concentration, in processing cultivars.

Role of Ca in internal quality

Of all the nutrients, Ca appears to play the most important role in determining a tuber’s susceptibility to cell damage. Physiological disorders in potatoes can be associated with a localised shortage of Ca in the tuber tissue. Calcium is important in the cell to maintain the integrity of the membrane and cell wall and thus improve resistance to damage from biotic or abiotic stress. A shortage of Ca causes the cell membranes and cell walls to lose integrity under conditions of stress, with the result that the contents of the cytoplasm and the vacuole mix together and the cells die off.

The highest percentage of Ca in the plant occurs in the foliage (up to 1.5%), while a very low percentage is found in the tuber (0.01 to 0.15%). Calcium is also not uniformly distributed throughout the tuber. There are higher concentrations of Ca in the periderm or skin (0.05 to 0.25% dry mass) than in the medulla or inner cells (0.01 to 0.06% dry mass).

The Ca concentration in the medulla correlates with the tubers’ susceptibility to physiological disorders, while the Ca content of the periderm helps to strengthen the cells’ resistance to pathogens.

In terms of resistance to pathogens such as soft rot bacteria, the Ca content of the medulla and periderm is important. Such bacteria penetrate the host cells when the enzymes that they release break down the pectin between cells. This leads to a loss in cell structure, which destroys the cells. The more Ca present in the cells, or more specifically in the pectin layer, the better the resistance to infiltration by the enzymes and the spread of the bacteria in the tissue.

Choice of cultivar

Cultivars differ in respect of their propensity to poor internal quality.

This is related to a cultivar’s tendency towards lush foliage growth, the development of only a few tubers per haulm, its susceptibility to soilborne pathogens, leaf diseases and insect damage, its level of heat and drought tolerance, or the number of root hairs on the root tips. It is because of all these factors, that certain cultivars tend to exhibit one or more characteristics of either poor or good internal quality.

Plant factors

• The age and internal quality of seed potatoes determine the number of haulms that form, as well as the vigour of those haulms. Seed potatoes that are physiologically old, or which have a low Ca content, form branched sprouts with reduced vigour.
• Rapid tuber growth. When tubers bulk rapidly, cells under certain conditions can tear apart from one another, leaving cavities in the tuber (hollow heart).
• Leaf surface. Sufficient assimilates must be produced through photosynthesis before tubers will initiate. Foliage damage during tuber formation leads to fewer tubers forming per plant than normal. This can increase the growth rate of these few tubers at a later stage, when the foliage recovers. Foliage can be damaged by wind, hail, frost, leaf diseases, and chewing insects. Overly lush foliage could be the result of over-fertilisation, high levels of organic material in the soil, or a nutrient imbalance in the soil.
• A low number of haulms per seed potato can result in only a few tubers forming per plant, which, under conditions of high nitrogen (N) and high temperature in particular, can bulk too quickly and lead to hollow heart. A low number of haulms per plant usually results when physiologically young seed potatoes are planted – a condition influenced by the conditions and period of storage.
• Number of root hairs. Calcium is absorbed through the unthickened cell walls of the root hairs. When the soil dries out, so too do the root hairs and they subsequently die. Once moisture conditions have been restored, it takes approximately four days before new root hairs are formed. It is therefore important that soil moisture does not drop too low, especially during tuber formation.

Interaction with climate

• Excess transport of water to the foliage can result from a high rate of transpiration due to high temperature (>25°C), low air moisture (<60% RH), and/or windy conditions. The rate of transpiration is especially high when the latter factors prevail simultaneously. This condition is aggravated when a low potassium (K) content in leaves hinders the stomata from closing properly and thus preventing moisture loss.

Interaction with soil conditions

• Conditions in the soil environment, such as high (>25°C) or low (<10°C) soil temperature, availability of oxygen, soil moisture and salt quality of the soil, have an effect on the growth rate of tubers, the absorption of Ca, the functioning of the roots, and the withdrawal of water and nutrients from the tubers.
• Soil conditions, such as waterlogging and crust formation, lower the O2 concentration in the soil, leading to poor root function. Every effort must therefore be made to prevent ploughsole formation, soil compaction, over-irrigation and poor drainage. Poor root function inhibits the absorption of nutrients, especially Ca, which is absorbed only through the young root tips.
• Low transpiration rate. Where there are high levels of moisture in the air (>90% RH) and low temperatures (<15°C), transpiration from the leaves occurs slowly, particularly where these two conditions occur simultaneously. The transpiration stream is consequently weak, with poor absorption of water and nutrients, including Ca. Under these conditions, the absorption of Ca through the stolon roots is too slow to ensure sufficient Ca in the tuber tissue.
• Soil-borne diseases and pests are responsible for causing physical damage to roots, which has an impact on root function. The presence and functioning of pathogens and pests, such as nematodes, can also be affected by soil health.

Interaction with fertiliser

• Plant nutrients (shortage, excess and imbalance, as well as plant availability) have an effect on root function, Ca intake and vigour. An excess of nutrients can lead to overly lush foliage and subsequent excess transpiration, leading to a shortage of Ca in the tubers since Ca is transported towards the foliage instead.
• Time and placement of Ca application. Research at Stellenbosch University found that Ca must be available for absorption through the stolon roots at the stage of tuber initiation and for about four weeks thereafter. Calcium that is applied too late has no effect on tuber quality.
• Reduced allocation of nutrients and water to tubers. Under conditions of wind, high temperature and low air moisture, water and nutrients (especially Ca) are primarily transported to the foliage and not the tubers. This causes water gradient pressure or stress, which can ultimately result in tissue damage. Where very young tubers are subjected to such conditions, it could result in brown spot, internal brown spot and hollow heart.
• Poor absorption of Ca through the roots can also result from a low (Ca) concentration in the soil solution, which in turn is affected by the Ca source, as well as a high NH₄ concentration and low Ca:Mg (magnesium) ratio in the soil solution. – Condensed Potatoes SA Fact Sheet, 2016

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