In-season heat stress compromises postharvest quality and low-temperature sweetening resistance in potato (Solanum tuberosum L.)

Assessing heat tolerance in potatoes: Responses to stressful Texas field locations and controlled contrasting greenhouse conditions

In recent years, heat stress has affected potato production more frequently, resulting in lower marketable yields and reduced tuber quality. In order to develop heat-tolerant potatoes, it is necessary to select under heat-stress conditions and consider traits affected by heat stress. The Texas A&M Potato Breeding Program has selected potatoes under high-temperature stress for several decades. Ten potato cultivars, representing heat tolerant and sensitive clones based on past performance in Texas, were included in field trials for three years at the two main locations used by the Texas Breeding Program (Dalhart and Springlake, TX) to assess if the Texas field locations are suitable for heat tolerance screening. Both locations were confirmed as appropriate for heat stress screening. However, Springlake was a more stressful location since it had significantly lower yields of marketable tubers and increased percentages of tuber defects. Planting time did not have a significant effect at the most stressful location. The same ten potato clones were included in greenhouse experiments with contrasting temperatures (normal versus heat stress). There was confirmation that heat stress conditions resulted in significantly lower marketable yields, specific gravity, dormancy, and significantly higher percentages of tuber defects; however, significant differences existed between potato clones. Under heat stress conditions, Russet Burbank had a high percent of tubers with external defects, whereas Atlantic showed the highest percentage of internal defects (mainly internal heat necrosis). Vanguard Russet produced the highest marketable yield while maintaining a low percentage of external and internal defects. Russet Burbank and Atlantic were heat-sensitive controls for external and internal tuber defects, respectively. In contrast, Vanguard Russet can be used as a reliable heat-tolerant control. Including appropriate controls in heat stress studies will help identify clones with heat tolerance.

Effects of appropriate low-temperature treatment on the yield and quality of pigmented potato (Solanum tuberosum L.) tubers

Temperature is one of the important environmental factors affecting plant growth, yield and quality. Moreover, appropriately low temperature is also beneficial for tuber coloration. The red potato variety Jianchuanhong, whose tuber color is susceptible to temperature, and the purple potato variety Huaxinyangyu, whose tuber color is stable, were used as experimental materials and subjected to 20 °C (control check), 15 °C and 10 °C treatments during the whole growth period. The effects of temperature treatment on the phenotype, the expression levels of structural genes related to anthocyanins and the correlations of each indicator were analyzed. The results showed that treatment at 10 °C significantly inhibited the potato plant height, and the chlorophyll content and photosynthetic parameters in the leaves were reduced, and the enzyme activities of SOD and POD were significantly increased, all indicating that the leaves were damaged. Treatment at 10 °C also affected the tuberization of Huaxinyangyu and reduced the tuberization and coloring of Jianchuanhong, while treatment at 15 °C significantly increased the stem diameter, root-to-shoot ratio, yield and content of secondary metabolites, especially anthocyanins. Similarly, the expression of structural genes were enhanced in two pigmented potatoes under low-temperature treatment conditions. In short, proper low temperature can not only increase yield but also enhance secondary metabolites production. Previous studies have not focused on the effects of appropriate low-temperature treatment during the whole growth period of potato on the changes in metabolites during tuber growth and development, these results can provide a theoretical basis and technical guidance for the selection of pigmented potatoes with better nutritional quality planting environment and the formulation of cultivation measures.

Raman spectroscopy detects chemical differences between potato tubers produced under normal and heat stress growing conditions

Potato is the most consumed vegetable worldwide. Potato tubers contain water, starch, proteins, minerals, and vitamins. The amounts of these chemicals depend on the cultivar and growing location. When potatoes are exposed to high temperatures during the growing period, tuber yield and quality are detrimentally affected; however, there is limited knowledge about the influence of high temperatures on tuber chemical composition. With temperatures rising around the globe, the reaction of potato cultivars to high temperatures is increasingly important, and heat-induced changes, including changes in the chemical composition of tubers, should be considered. The Texas A&M University Potato Breeding Program has been selecting potato clones under high-temperature conditions for many years. Several released cultivars are considered heat-tolerant based on high marketable yields and low internal and external tuber defects. In this study, we used Raman spectroscopy (RS), an analytical tool, to determine whether heat stress causes changes in the chemical composition of tubers of ten potato cultivars. RS is a non-invasive method that requires less time and labor than conventional chemical analysis. We found drastic changes in the intensities of vibrational bands that originate from carbohydrates in the spectra acquired from tubers of heat-stressed plants compared to tubers produced by potato plants grown under normal conditions. These results demonstrate that RS could be used as a replacement or complement to conventional chemical analysis to inspect the effect of heat stress on tuber chemical composition.

Development of an in vitro Microtuberization and Temporary Immersion Bioreactor System to Evaluate Heat Stress Tolerance in Potatoes (Solanum tuberosum L.)

High temperature (heat) stress reduces tuber yield and quality of potatoes. Screening potatoes for heat tolerance is increasingly important, considering the climate change scenario and expansion of potatoes to countries where heat stress is an issue. In vitro screening for tolerance to abiotic stresses offers several advantages, including quick evaluation of numerous genotypes (clones) in reduced space, controlled environmental conditions (temperature and photoperiod), and free from confounding variables inherent to greenhouse and field conditions. In this study, we explored the feasibility of using a temporary immersion bioreactor system for heat tolerance screening of potatoes. We determined the best hormone-free microtuberizing media for this system (MSG with 8% sucrose) to enhance microtuber number and size. Comparisons of microtubers produced at 30°C as heat treatment, with 16°C as normal condition, allowed to identify heat tolerant and susceptible potato clones. The use of bioreactors allowed distinguishing well-formed (non-deformed) from deformed microtubers. Heat stress increased the total biomass of plant tissues in all the clones. However, the effect of heat stress on microtuber number and weight varied among the clones. Incubation at 30°C decreased the weight and number of non-deformed microtubers in all the clones except for Reveille Russet in which the weight of non-deformed microtubers was significantly increased and the count of non-deformed microtubers was not affected. The potato variety Reveille Russet, which was selected under high-temperature field conditions in Texas, had many non-deformed microtubers per explant and the highest microtuber weight among four clones evaluated under heat stress. We described a faster and reliable in vitro microtuberization system for abiotic stress tolerance screening, identified Reveille Russet as a promising heat-tolerant potato variety, and confirmed Russet Burbank and Atlantic as susceptible heat-tolerant checks.

Improved resource allocation and stabilization of yield under abiotic stress

Sugars are the main building blocks for carbohydrate storage, but also serve as signaling molecules and protective compounds during abiotic stress responses. Accordingly, sugar transport proteins fulfill multiple roles as they mediate long distance sugar allocation, but also shape the subcellular and tissue-specific carbohydrate profiles by balancing the levels of these molecules in various compartments. Accordingly, transporter activity represents a target by classical or directed breeding approaches, to either, directly increase phloem loading or to increase sink strength in crop species. The relative subcellular distribution of sugars is critical for molecular signaling affecting yield-relevant processes like photosynthesis, onset of flowering and stress responses, while controlled long-distance sugar transport directly impacts development and productivity of plants. However, long-distance transport is prone to become unbalanced upon adverse environmental conditions. Therefore, we highlight the influence of stress stimuli on sucrose transport in the phloem and include the role of stress induced cellular carbohydrate sinks, like raffinose or fructans, which possess important roles to build up tolerance against challenging environmental conditions. In addition, we report on recent breeding approaches that resulted in altered source and sink capacities, leading to increased phloem sucrose shuttling in crops. Finally, we present strategies integrating the need of cellular stress-protection into the general picture of long-distance transport under abiotic stress, and point to possible approaches improving plant performance and resource allocation under adverse environmental conditions, leading to stabilized or even increased crop yield.

Effect of excessive nitrogen on levels of amino acids and sugars, and differential response to post-harvest cold storage in potato (Solanum tuberosum L.) tubers

Nitrogen (N) is an important nutrient for increased potato tuber yield. However, excessive N can decrease tuber quality. Furthermore, the impact of optimal and higher N levels of potato tuber metabolic profile at harvest and cold storage remains unclear. This study aimed to investigate the metabolic profiling of free amino acids and sugars in potato tubers affected by different nitrogen levels (optimal, ON; and excessive, EN) at harvest (AH) and cold storage (CS) (~4 °C, 4 weeks) through untargeted GC-TOF-MS, and targeted UHPLC-QqQ-MS. Carbohydrate content and vacuolar invertase activity (IV) were determined. Principal component analysis of metabolite data indicated a distinct separation between ON and EN treatments at harvest and cold storage. Multivariate data analysis revealed that sucrose, reducing sugars, and free asparagine were the most altered metabolites (VIP > 1 and P < 0.05), which were involved in starch and sucrose metabolism, and alanine, aspartate and glutamate metabolism. At harvest, the absolute contents of various free amino acids including asparagine were higher (by 1.3-1.5 fold) in the EN treatment than ON treatment, and this difference was maintained at 4-week cold storage. Under the EN treatment, tuber maturity was reduced, and sucrose accumulation was increased at harvest, while IV was increased after cold storage, reducing sugar also accumulated. These results highlighted the negative effects of EN on free amino acid and sugars metabolism in the post-harvest tubers and provided useful information for understanding the underpinning physiological mechanisms.

Plastic Film Mulch Performed Better in Improving Heat Conditions and Drip Irrigated Potato Growth in Northwest China than in Eastern China

Plastic film (i.e., transparent and black films) is widely applied in mulching to improve thermal environments and plant growth in China. However, choosing suitable film for potato cultivation in different areas is still a problem. A four-year field experiment was conducted to explore the influences of plastic mulch on heat transfer (soil heat flux, G; soil temperature; and canopy net radiation, Rn) and potato growth in two typical areas of China: Experiment 1 in Northwest China (2014 and 2015) and Experiment 2 in Eastern China (2016 and 2017). Three mulch treatments were designed as follows: (1) transparent film (TF), (2) black film (BF), and (3) no film (NF). Results showed that the Rn and G differences between TF, BF, and NF varied at different areas. However, the Rn in TF was always smaller (average 12.4 W/m2 in 2014–2017) than in BF in the two study areas. The differences of soil temperature between TF, BF, and NF in Experiment 1 were greater than in Experiment 2. The average soil temperature difference between TF and NF was 1.4 °C higher in Experiment 1 than in Experiment 2. TF had 23% and 19% more jumbo tubers than BF in weight and number on average with no significant difference in 2015–2017. TF and BF significantly had greater potato yield and irrigation water use efficiency than NF on average in Experiment 1, while no significant differences were found in Experiment 2. The transparent film was more beneficial to increase soil temperature and jumbo tubers than the black film. Film mulch functioned better for improving heat conditions and drip-irrigated potato growth in Northwest China than in Eastern China.

Transient heat stress during tuber development alters post-harvest carbohydrate composition and decreases processing quality of chipping potatoes

BACKGROUND

Adverse air and soil temperatures are abiotic stresses that occur frequently and vary widely in duration and magnitude. Heat stress limits productivity of cool-weather crops such as potato (Solanum tuberosum) and may degrade crop quality. Stem-end chip defect is a localized discoloration of potato chips that adversely affects finished chip quality. The causes of stem-end chip defects are poorly understood.

RESULTS

Chipping potatoes were grown under controlled environmental conditions to test the hypothesis that stem-end chip defect is caused by transient heat stress during the growing season. Heat stress periods with 35 °C days and 29 °C nights were imposed approximately 3 months after planting and lasted for 3, 7 or 14 days. At harvest and after 1, 2 and 3 months of storage at 13 °C, potato tubers were evaluated for glucose, fructose, sucrose and dry matter contents at the basal and apical ends. Chips were fried and rated for defects at the same sampling times. Differences in responses to heat stress were observed among four varieties of chipping potatoes. Heat stress periods of 7 and 14 days increased reducing sugar content in the tuber basal and apical ends, decreased dry matter content, and increased the severity of stem-end chip defects.

CONCLUSION

Transient heat stress during the growing season decreased post-harvest chipping potato quality. Tuber reducing sugars and stem-end chip defects increased while dry matter content decreased. Planting varieties with tolerance to transient heat stress may be an effective way to mitigate these detrimental effects on chipping potato quality. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Heat stress affects carbohydrate metabolism during cold-induced sweetening of potato (Solanum tuberosum L.)

Tolerance to heat stress for retention of low-temperature sweetening-resistant phenotype in potato is conferred by insensitivity of acid invertase activity to cold induction. Heat stress exacerbated cold sweetening (buildup of reducing sugars) of the LTS (low-temperature sweetening)-susceptible potato (Solanum tuberosum L.) cultivars, Ranger Russet and Russet Burbank, and completely abolished the resistance to cold sweetening in the LTS-resistant cultivars/clones, Sage Russet, GemStar Russet, POR06V12-3 and A02138-2. Payette Russet and EGA09702-2, however, demonstrated considerable tolerance to heat stress for retention of their LTS-resistant phenotype. Heat-primed Payette Russet and EGA09702-2 tubers accumulated fourfold more sucrose when subsequently stored at 4 °C, while reducing sugar concentrations also increased marginally but remained low relative to the non-heat-tolerant LTS-resistant clones, resulting in light-colored fries. By contrast, sucrose concentrations in heat-primed tubers of the non-heat-tolerant clones remained unchanged during LTS, but reducing sugars increased fivefold, resulting in darkening of processed fries. Acid invertase activity increased in the LTS-susceptible and non-heat-tolerant LTS-resistant cultivars/clones during cold storage. However, Payette Russet tubers maintained very low invertase activity regardless of heat stress and cold storage treatments, as was the case for Innate^® Russet Burbank (W8) tubers, where silenced invertase conferred robust tolerance to heat stress for retention of LTS-resistant phenotype. Importantly, heat-stressed tubers of Payette Russet, EGA09702-2 and Innate^® Russet Burbank (W8) demonstrated similar low reducing sugar and high sucrose-accumulating phenotypes when stored at 4 °C. Tolerance to heat stress for retention of LTS-resistant phenotype in Payette Russet and likely its maternal parent, EGA09702-2, is, therefore, conferred by the ability to maintain low invertase activity during cold storage of heat-stressed tubers.

Gene expression profiles predictive of cold-induced sweetening in potato

Cold storage (2-4 °C) used in potato production to suppress diseases and sprouting during storage can result in cold-induced sweetening (CIS), where reducing sugars accumulate in tuber tissue leading to undesirable browning, production of bitter flavors, and increased levels of acrylamide with frying. Potato exhibits genetic and environmental variation in resistance to CIS. The current study profiles gene expression in post-harvest tubers before cold storage using transcriptome sequencing and identifies genes whose expression is predictive for CIS. A distance matrix for potato clones based on glucose levels after cold storage was constructed and compared to distance matrices constructed using RNA-seq gene expression data. Congruence between glucose and gene expression distance matrices was tested for each gene. Correlation between glucose and gene expression was also tested. Seventy-three genes were found that had significant p values in the congruence and correlation tests. Twelve genes from the list of 73 genes also had a high correlation between glucose and gene expression as measured by Nanostring nCounter. The gene annotations indicated functions in protein degradation, nematode resistance, auxin transport, and gibberellin response. These 12 genes were used to build models for prediction of CIS using multiple linear regression. Nine linear models were constructed that used different combinations of the 12 genes. An F-box protein, cellulose synthase, and a putative Lax auxin transporter gene were most frequently used. The findings of this study demonstrate the utility of gene expression profiles in predictive diagnostics for severity of CIS.