1a. Objectives (from AD-416):
This collaboration will assess phytonutrient levels in diverse potato germplasm and characterize mechanisms and factors that control phytonutrient content or processing quality.
1b. Approach (from AD-416):
Potato can be made even more nutritious by developing varieties that have higher levels of antioxidants such as phenolics, vitamin C, anthocyanins and carotenoids. A better understanding of the mechanisms that influence the amounts of phytonutrients in tubers, including both in planta and external environmental influences including post-harvest factors or crop management will facilitate development of high-phytonutrient potatoes. A combination of physiological, biochemical and molecular techniques, including metabolic profiling, gene expression, metabolic engineering and enzyme analysis will be used to address these factors.
3. Progress Report:
Research to define the effects of soil and thus tuber temperature (heat stress) during crop development on tuber physiology, retention of processing quality, and resistance to low temperature sweetening (LTS) during storage began in 2011. This research directly relates to sub-objective 3.A of the in-house project, "Elucidate genetic, molecular and biochemical processes governing accumulation of select phytochemicals and vitamins with respect to improving potato as a food". Soil warming cables were installed in-furrow to control temperature during tuber development. ‘Premier Russet’ (LTS resistant) and ‘Ranger Russet’ (LTS susceptible) potatoes were grown at 16 (ambient), 23, and 29oC soil temperatures during bulking (111-164 days after planting) and maturation (151-180 days after planting). Total yields were 17% higher when bulking occurred at 23oC compared with 16oC; however, bulking at 29oC virtually eliminated yield despite vigorous vine growth. Tuber specific gravity decreased as soil temperature increased during bulking, but was not affected during maturation. Tubers were stored for 24 days at 4oC (98% RH) to evaluate the effects of growing temperature on LTS and retention of process quality. Bulking of Premier tubers at 23oC induced severe sugar ends and stimulated subsequent LTS. Maturation of tubers at 23oC enhanced LTS without inducing sugar ends. Maturation of tubers at 29oC completely abolished Premiere’s LTS-resistant phenotype. Higher bulking and maturation temperatures also accelerated postharvest loss of process quality and LTS of Ranger tubers. The initial respiratory acclimation responses of tubers to storage at 4oC reflected the varying degrees of sweetening induced by 16, 23, and 29oC during bulking and maturation. While morphologically indistinguishable from control tubers, tubers grown at elevated temperature had higher basal respiration rates, reduced dormancy, and increased weight loss during storage. Breeding strategies for more robust retention of postharvest quality and LTS resistance should consider tolerance to in-season heat stress. Studies to understand the mechanisms regulating ascorbate (AsA) content of potatoes during development and loss during storage are prerequisite to developing high vitamin C genotypes. Transcript levels of genes in the AsA biosynthetic pathway (Smirnoff-Wheeler) increased as tubers developed to 10 g, suggesting that de novo synthesis by tubers contributes to AsA content early in development. Transcript of GGPP (GDP-L-galactose-1-phosphate phosphorylase), a potential rate limiting step in biosynthesis, increased as tubers developed from non-tuberized stolons to the 0.6- to 1.5-g tuber stage, in parallel with an increase in AsA concentration of tubers. High levels of GGPP expression continued through 84 DAP (~54-g tubers) when 75% of the final AsA concentration of fully mature (240-g) tubers had been established. Expression of other key genes in the AsA pathway was also temporally correlated with AsA accumulation during tuberization and early bulking. Tuber AsA levels began to fall during vine senescence and declined progressively through maturation and storage, with losses reaching 65% over an 8-month storage period. The rate of loss was genotype-dependent. Wounding of tubers induced gene expression for AsA biosynthesis, recycling, and accumulation, indicating metabolic competence for AsA synthesis in the detached organ. Storage of tubers under reduced O2 attenuated AsA loss, suggesting a regulatory role for oxidative metabolism in AsA loss/retention. Attenuating postharvest loss of AsA in tubers could greatly increase the contribution of potatoes to dietary vitamin C. Crop breeding and postharvest handling strategies for enhancing tuber AsA content will evolve from a better understanding of the regulation of these biochemical processes.