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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable and Forage Crops Production Research

2012 Annual Report

1a.Objectives (from AD-416):
Characterize factors that affect the quality or nutritional value of potatoes.

1b.Approach (from AD-416):
Tubers from multiple genotypes will be analyzed for quality or nutritional value. Genotypes with traits useful to the potato industry such as tuber size and set or superior phytonutrients with be characterized. Factors influencing or controlling trait expression, such as molecular, developmental and environmental control will be characterized.

3.Progress Report:

This is the final report for this project. Studies to characterize the physiological basis of mottling disorder in long russet cultivars for frozen processing were completed. This research adresses objective 1.A. of the related in-house project, "Identify germplasm with a range of expression of phytonutrients, study inheritance, identify associated markers, and produce superior parents". Mottling is an undesirable postharvest disorder of potato tubers characterized by the development of random pockets of translucent tissue containing high concentrations of reducing sugars. Mottled areas turn dark during frying due to the reaction of reducing sugars with free amino acids (Maillard reaction). The newly released cultivar, Premier Russet, is susceptible to mottling; symptoms appear as early as 180 days and get progressively worse with time in storage (4-9oC, 95% RH). The mechanism of mottling is unknown; however, the disorder has similarities with the irreversible senescent sweetening that normally occurs in tubers during long-term (>12 months) storage. Much of the decline in tuber quality during prolonged storage is a consequence of progressive increases in oxidative metabolism. We determined that mottling is a phenotypic manifestation of oxidative stress associated with an accelerated aging genotype. Tissue from mottled and control tubers were compared for their respiration rates and various markers of oxidative metabolism. The respiration rate of mottled tissue was 1.8-fold greater than control tissue. In addition to higher concentrations of glucose, fructose, and sucrose, mottling resulted in lower dry matter, higher specific activities of starch phosphorylase and glucose-6-phosphate dehydrogenase, higher protease activity, and loss of protein. Moreover, membrane integrity declined during mottling, likely due to increased peroxidation of membrane lipids. Superoxide dismutase activity and the ratio of oxidized to reduced glutathione were substantially higher in mottled tissue. Mottling thus appears to be the consequence of oxidative stress associated with accelerated aging of Premier Russet tubers. The role of temperature during bulking and maturation on susceptibility to mottling, premature sweetening, and sugar end disorders is currently under investigation.

We also made considerable progress in understanding how temperature affects tuber respiration during low temperature-induced sweetening (LTS) in resistant and susceptible genotypes. Premier Russet, GemStar Russet, and Defender differ substantially in their resistance to low temperature sweetening (LTS) and associated metabolism. Gemstar and Premier have moderate and high resistances, respectively, while Defender sweetens and loses processing quality progressively during storage at 9oC and has virtually no resistance to LTS at 4.4 and 6.7oC. At 4oC, Premier maintained low levels of sucrose (suc), glucose (glc) and fructose (fru), GemStar accumulated suc with little inversion to glc and fru, and Defender accumulated glc and fru but comparatively little suc. Respiratory acclimation responses to temperature change (9 to 4oC) reflected the varying degrees of sweetening of these cultivars and were thus diagnostic of the LTS phenotype. In response to an immediate drop from 9oC to 4oC, tuber respiration decreased 42%, reaching a minimum within 45 h for all cultivars, and then increased to a new maximum over the next 4.5 d before decreasing to establish a constant basal rate at 4oC over the next 14 days (acclimation response). The acclimation maxima were 56, 28, and 14% greater than the final resting basal respiration rates at 4oC for Defender, Gemstar, and Premier, respectively. The acclimation responses thus correlated with the extent of LTS from these genotypes and likely reflected the level of metabolic energy required to catabolize starch to suc, glc and fru end products. The ratio of fru to glc during LTS was an effective predictor of LTS resistance in these genotypes. Heat stress during tuber maturation abolished the LTS resistant phenotype of Premier. This information is critical in selecting durable modes of LTS resistance that cannot be compromised by late season heat stress. Studies to determine how in-season nitrogen management affects the ability of tubers to retain processing quality after harvest were also completed. Changes in foliar and tuber fresh weight, sucrose, reducing sugars, and specific gravity over time and in response to four rates of in-season N were profiled over three growing seasons for cvs Alturas and Premier Russet. On average, increasing N rate delayed the attainment of 50% harvest index (HI), increased foliar and tuber biomass at 50% HI, shifted maximum foliar growth later, increased foliar biomass, reduced the HI at maximum foliar growth, delayed vine senescence, and increased tuber yield. Vine growth was more sensitive to in-season N than tuber yield. Source/sink imbalances occurred when tuber growth dominated plant growth at maximum foliar development (HI>50%), resulting in significantly lower final yields. Physiological maturity (PM) was estimated as the average of days after planting to reach maximum yield, maximum specific gravity, minimum sucrose, and minimum reducing sugars in the stem ends of tubers. PM ranged from 143 to 158 DAP and occurred later in the season with increasing level of N. Hence, tubers grown with high N were less mature (physiologically younger) at harvest than tubers grown with less N. Minimizing the interval between PM and harvest resulted in longer retention of fry processing quality (low reducing sugars) in storage. Nitrogen rate therefore affects the retention of processing quality indirectly by influencing the timing of attainment of tuber PM in relation to harvest.

Last Modified: 4/24/2014
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