Lasting Freshness - Inner health, outer beauty

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 soil-borne 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

1. 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.
2. A low number of haulms per seed potato can result in only a few tubers forming per plant, which, under conditions of high 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.
3. Rapid tuber growth. When tubers bulk rapidly, cells under certain conditions can tear apart from one another, leaving cavities in the tuber (hollow heart). Balanced nutrition could help overcome this tissue breakdown.
4. 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. Given that transpiration and the associated absorption of water and Ca are limited during tuber formation (the critical stage for Ca absorption). it is evident that calcium-related internal defects are possible. Foliage can be damaged by wind, hail, frost, leaf diseases and chewing insects.

Overly lush foliage, on the other hand, will lead to significant moisture loss, while the transpiration stream will channel Ca to the leaves instead of the tubers. Lush foliage could be the result of over-fertilisation, high levels of organic material in the soil, or a nutrient imbalance in the soil.

5. Number of root hairs. Ca is absorbed through the unthicken-ed cell walls of the root hairs. Root hairs are delicate extensions of the epidermal cells on the tips of the roots. 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. Relative to other crops, potatoes form few root hairs and this is a cultivar related characteristic. It is therefore important that soil moisture does not drop too low, especially during tuber formation.

Interaction with climate

6. 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 K content in leaves hinders the stomata from closing properly and thus preventing moisture loss.
7. 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.

Interaction with soil conditions

8. 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. 

9. Soil conditions, such as waterlogging and crust 14. formation, lower the 02 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 calcium, which is absorbed only through the young root

10. Soil-borne diseases and pests are responsible for causing physical damage to roots, which has an impact on roots function. The presence and functioning of pathogens and pests, such as nematodes, can also be affected by soil health. In general, it can be said that conditions that negatively affect root function can lead to stress in the roots, which lowers their resistance to infection. Healthy and balanced populations of soil microbes surrounding the roots, on the other hand, not only create an unfavourable environment for such pathogens, but are also responsible for providing the nutrients to be absorbed by the roots. 

Interaction with fertiliser

11. Plant nutrients (shortage, excess and imbalance, as well as plant availability) have an effect on root function, calcium intake and vigour. An excess of plant nutrients can lead to overly lush foliage and subsequent excess transpiration, leading to a shortage of calcium in the tubers since Ca is transported towards the foliage instead.
12. 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 NH• concentration and low Ca:Mg ratio in the soil solution.

13. Time and placement of calcium application. Research done locally at Stellenbosch University, found that Ca must be available for absorption through the stolon roots
at the stage of tuber initiation and for approximately four weeks thereafter. Calcium that is applied too late has no effect on internal tuber quality.

14. 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. 

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