2013 Annual Report
Translucent tissue defect (TTD) is an undesirable postharvest disorder of potato tubers characterized by the development of random pockets of semi-transparent tissue containing high concentrations of reducing sugars. Translucent areas turn dark during frying due to the Maillard reaction. The newly released cultivar, Premier Russet, is highly resistant to low temperature sweetening, but susceptible to TTD. The onset and rate of TTD development varied by storage temperature, year and growing region. The greatest incidence of TTD in ‘Premier Russet’ and its offspring A98345-1 occurred in tubers from southern areas of the Columbia Basin where soil temperatures are highest. Studies to evaluate the effects of heat stress on tuber quality using embedded soil-warming cables showed that increasing the temperature by 7oC during bulking (111-164 DAP) had no effect on the onset of TTD during storage (9oC, 95% RH), but the severity increased more rapidly, culminating in severe blackheart by 270 DAH. High temperatures during production increased tuber respiration, which accelerates tuber aging and thus the severity of TTD development in storage. In Premier Russet, TTD appears to be the result of an accelerated aging phenotype that culminates in loss of ability to modulate oxidative stress. Development of TTD was characterized by enhanced starch catabolism, increased respiration, lipid peroxidation, membrane permeability, protein catabolism, and apparent loss of ability control reactive oxygen species. The TTD disorder was genetically magnified in A98345-1, suggesting heritability and thus the opportunity to identify molecular markers for use in selecting against the disorder. Our work has identified the metabolic systems affected by TTD and revealed potential genes that should be investigated as candidates for marker assisted breeding against the disorder. Respiratory acclimation of tubers to temperature change reflected the extent of LTS later in storage. Premier Russet, GemStar Russet, and Defender differ substantially in their resistance to LTS and associated metabolism. Gemstar and Premier have moderate and high resistances, respectively, while Defender loses processing quality progressively during storage at 9oC and has virtually no resistance to LTS at 4.4 and 6.7oC. 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, glu and fru end products. The ratio of fru to glu during LTS was also an effective predictor of LTS resistance in these genotypes. Heat stress during tuber maturation in the field abolished the LTS resistant phenotype of Premier and compromised that of AO02183-2. This information is critical in selecting durable modes of LTS resistance that cannot be compromised by late season heat stress.
Besides factors that affect the processing quality of potatoes, we are also studying factors that influence nutritional quality and extensively analyzed the effect of development on potato phytonutrients, especially phenylpropanoids. We showed that numerous compounds from different branches in the phenylpropanoid pathway, including phenolic acids, flavonols and anthocyanins, are present in higher amounts in immature tubers. Gene expression and metabolite pools were measured and used to generate correlation matrices. Other than the gene encoding chlorogenic acid, most genes were more highly expressed in immature tubers. Besides development, environment was also shown to have a large effect on phenylpropanoids and carotenoids gene expression and metabolite pools. Carotenoid profiles from different environments differed markedly. In general, potatoes grown in cooler climates had higher amounts of antioxidants. Better knowledge of how environment influences tuber phytonutrients and processing quality will make it more feasible to optimize the crop’s phytonutrient content and processing value.