Breeding for desired tuber traits: Assessing traits

By Team PotatoWorld, Oct 17, 2020 11:30:00 AM

There is great multiformity in tuber properties of fresh potatoes

Most breeding programs have their own procedures and standards.

Yet, they have a number of issues in common. Comparing the outcome of an assessment of a new genotype that emerges after a number of years of testing, is not done just prior to naming it as a variety but as soon as possible after the cross is made, in the early years of selection. Some assessments are done at first year’s clones such as tuber shape, whereas other ones such as fry quality are done at a much later stage.

A breeding program has a collection of existing varieties that are important in the market for which the new genotypes are intended. New genotypes are compared with such standard varieties to avoid sliding away from original assessment and to correctly rank new genotypes against those well established in the market. Moreover, most properties depend much on the growing environment and interact with it, so they vary among years. The length of the tuber dormancy period is an example thereof. his is one more particular reason to compare a new potential variety with a set of standard ones.

Undesired enzymatic browning, due to the presence of free phenols and polyphenol oxidase, occurs when a fresh tuber is cut accidentally or when handling on purpose during processing. It is subject of negative selection when creating new varieties. Blackspot formation following rough handling also is an enzymatic process involving polyphenol oxidase. In the same vein, attention is given to non-enzymatic after cooking darkening caused by forming of a complex of phenols and iron in genotypes with a low citric acid concentration. Greening of the skin when exposed to light (chlorophyll formation) is also very much determined by the genetic background. Also when exposed to light of high intensity due to a shallow position of the tubers in the field tubers turn green due to chlorophyll formation in the skin. This also is genotype dependent and varieties differ in this aspect.

Cold sweetening and senescence sweetening due to enzyme-induced sugar accumulation following breakdown of starch into sugars without subsequent respiration of these sugars is enhanced by low storage temperatures. The phenomenon is also genotype-dependent, so screening for low sweetening helps to reduce the problem.

When breeding new varieties, carbohydrate concentrations such as those of starch and sugars such as sucrose, glucose and fructose in the progeny of a cross range from low to high. The lowest possible levels of reducing sugars are desired, as reducing sugars in combination with high amino acid concentration are responsible for dark discoloration of chips and crisps in the Maillard reaction upon frying. Sugar or sucrose is considered bad tasting in table potato and caramelizes upon frying. Also undesired are high levels of glycoalkaloids: e.g. over 10 mg per 100 g fresh tuber weight is rejected because of bad taste and health risks. Desired levels of health-promoting substances are selected in dedicated breeding programs. Breeders then look for high concentrations of beneficial compounds such as protein, vitamin C and other vitamins, iron, carotenoids and anthocyanins. Different destinations of the harvested tubers have different specifications. Growers often receive higher payments the more all specs are met. Breeding programs delivering targeted varieties deliver added value for growers. Growers are willing to incur the higher costs of the seed as they pay for the use of its breeders’ right.

There are a few more specifications than tabled such as for baby potatoes, for pommes parisiennes, for early market ones and products like colorful tubers. Breeders look for opportunities in such niche markets. Specifically, varieties for the organic market and subsistence farming need to resist Potato Virus Y, late blight caused by Phytophthora infestans, early blight caused by Alternaria solani, black scurf caused by Rhizoctonia solani and silver scurf caused by Helminthosporium solani. Early maturity is sought after to escape late blight. In temperate climates, a long tuber dormancy period is needed as organic growers do not apply synthetic sprout inhibitors. In tropical environments where two growing seasons are possible per year a short dormancy is required as a few months after harvest the next crop is planted.

 

Various destinations of potato require different specifications leading to different breeding objectives and rigorous procedures to assess them in breeding programs.Various destinations of potato require different specifications leading to different breeding objectives and rigorous procedures to assess them in breeding programs.


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Topics: Breeding
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