Summary

Potatoes destined for making chips, French fries and other fried products, need to have low sugar content to avoid browning of the finished product. The sugar content of potatoes is determined by the genotype and several pre- and post-harvest factors. The major pre-harvest factors affecting sugar content are crop maturity, temperature during growth, mineral nutrition and irrigation, while important post-harvest factors are mechanical stresses and storage conditions. Each genotype requires an ideal pre-and post-harvest treatment to maintain low sugar levels; any kind of stress results in sugar accumulations.

An overview of the factors affecting sugar content of potatoes

Starch-sugar interconversion in Solanum tuberosum

The tissue of resting plant storage organs such as carrots, red beets, sugar beets or potato tubers can be activated by slicing into thin disks and incubation of these fragments in a moist atmosphere for different periods of time (“aging”). Activation comprises the turning-on of various genes with subsequent synthesis of transfer-, ribosomal and messenger RNAs and their transport into the cytoplasm. The immediate consequence of all these primary reactions is a vigorous synthesis of various enzymes and structural proteins which enable the cell to greatly enhanced metabolic activities. Thus, degradation of storage polymers and the procession of the resulting products through glycolysis, the pentose phosphate shunt and the shikimateprephenate-pathway and cellulose biosynthesis occur. Deinhibition of the tricarboxylic acid cycle opens the flow of metabolites into fatty acid, phospholipid and steroid biosynthesis, simultaneously providing the respiratory chain with electrons. In spite of functional modifications within the electron transport chain, the enhanced respiration of tissue slices serves as an energy source for the various energy-dependent reactions of the cell such as syntheses and the uptake of solutes. All of these activities accompany a concomitant dedifferentiation process and ultimately lead to renewed redifferentiation of the tissue slice cell.

Metabolism in plant storage tissue slices

Mechanism of glucose transfer from sucrose into the starch granule of sweet corn.

The seasonal variations in the carbohydrates of the perennial tissues of trees have been often investigated in the attempt to find some clue to their frost hardiness. The products of photosynthesis are stored as soluble sugars and starch in various parts of trees, reaching a maximum value just before leaf drop. When cold weather comes, the starch disappears from the more exposed tissues and often from the entire tree and is not to be found again until warm weather arrives in spring. The relationships between temperature and the form of carbohydrate present vary from species to species and also with different climates (6,16,19, 22, 30, 33). Levitt (21) has reviewed a large number of papers dealing with this subject and has attempted to correlate the carbohydrate variations with frost hardiness. The present investigation grew out of a similar attempt to assess the relationship of variations in the carbohydrate constituents of the living bark of the black locust tree (Robina pseudo-acacia L.) to the variations in frost hardiness. The variations and transformation in the carbohydrate constituents of the bark, including reducing sugars, sucrose and starch, were followed seasonally in normal trees for a year and a half. In addition, to determine whether, and to what extent, transformations and variations in the carbohydrates of the bark were independent of the normal seasonal conditions, samples were taken for analysis at various times throughout the year from sections of the bark of trees in which translocation had been intercepted by previous ringing above and below the sampling area. Such unseasonal and independent transformations were also produced and examined in logs cut from trees with the bark intact and stored at different temperatures in the laboratory.

Studies on the Chemistry of the Living Bark of the Black Locust in Relation to Its Frost Hardiness. V. Seasonal Transformations and Variations in the Carbohydrates: Starch-Sucrose Interconversions.

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Barbarin Arreguin-Lozano

Papers

Experiments on sucrose formation by potato tubers as influenced by temperature.