Assuring Potato Tuber Quality during Storage: A Future Perspective

Courtesy by phhere

Frontiers in Plant Science published in 2017 the article Assuring Potato Tuber Quality during Storage: A Future Perspective. Due to the importance of its content, I offer a more accessible version to the general public.

Introduction

Potatoes represent a vital staple food crop across the planet. To maintain their quality and extend their availability, farmers must store them for long periods. And they are often using industrial-scale facilities. Therefore, preserving potato quality is critical for the seed, fresh produce and processing sectors. Today potato industry is innovating and investing in improving post-harvest storage, especially post-harvest treatments. But because of strict legislation and changes in consumer habits, there is a tendency to find alternative or complementary approaches, none of them including chemical substances. More industry-targeted research will aid the preservation, enhancement, and viability of future tuber quality.

Current challenges in the potato industry include:

• Preservation of tuber quality throughout storage
• Restriction of chlorpropham or CIPC residues (above all for ware potatoes destined for processing)
• Control of sweetening processes
• Strengthening tuber marketability (visual appearance is the main factor driving consumers’ buy of fresh potatoes).

Biochemical factors governing dormancy

People use potatoes in different ways, but most involve using fresh tubers. Growers cannot produce fresh tubers throughout the year, so long-time storage is essential. Since this action does not dehydrate the tubers, an active metabolism occurs inside them, and, as a consequence, the sprout growth starts following a period of dormancy. The rupture of this latency is a physiological phenomenon regulated by exogenous (environmental factors) and endogenous signals.

Exogenous signals (Environmental factors)

Genetics influences latency duration, but pre-and post-harvest environmental factors do it too. Since dormancy occurs during tuber formation, factors that affect sprouting also affect dormancy. These factors are day length, temperature and nutrients.

Day length

Potatoes cultivated under short days have a shorter dormancy period than those grown on long days.

Temperature

The temperature is one of the critical factors affecting dormancy. The higher the temperature, the shorter the dormancy length. Likewise, high temperatures affect tuber formation and tuber dry-matter accumulation. Besides, they cause tuber chain formation, secondary growth, and premature sprouting. During tuber maturation, heat stress interferes with the onset of dormancy; the stolon tips resume growth and form a second tuber under favourable conditions.

Nutrients

Repeated cycles of high and low nitrogen levels can also result in the formation of chain tubers.

Other post-harvest conditions impair dormancy, such as humidity, the composition of the atmosphere, and water supply.

Endogenous signals

The concentration of many biochemical compounds actives the onset and further dormancy break. Those biochemical compounds are ethylene, abscisic acid (ABA), auxins, cytokinins (CKs), gibberellins (GAs) and strigolactones (SLs).

Ethylene

The earliest stage of dormancy initiation (endodormancy induction) requires endogenous ethylene, but its role during dormancy and sprouting is still unclear. Foukaraki has reported exogenous ethylene to break endodormancy following short-term treatments. The same author has seen that ethylene inhibits sprout growth and promotes endodormancy when supplied — either starting immediately after harvest or at the first sign of sprouting. But a study with russet Burbank minitubers showed that ethylene was not involved in hormone-induced dormancy break. These findings support the suggestion that the effect of ethylene depends on the physiological state of potato tubers.

Abscisic acid (ABA)

The role of ABA is better understood. Scientists now know that dormancy induction and maintenance need ABA’s sustained synthesis and action. And although ABA levels decrease as endodormancy weakens, there is no evidence of an initial concentration for dormancy release. They also know that there is cross-talk between ABA and other phytohormones and sugar metabolic pathways, facilitating the onset of dormancy break and further sprouting. Some authors have suggested that the increase in ABA is due to exogenous ethylene application to delay dormancy breaks. And that concurrent to the ABA decline, there is an increase in sucrose contents, a prerequisite for bud outgrowth.

Auxins

In the above context, auxins are essential for vascular development. Auxins favour the symplastic reconnection of the apical bud region — a discrete cell domain that remains isolated throughout tuberisation. This reconnection is vital for sucrose to reach the meristematic apical bud. High sucrose levels promote trehalose-6-phosphate accumulation (T6P) and support sprouting, decreasing ABA sensitivity.

Cytokinins (CKs), gibberellins (GAs) and strigolactones (SLs)

Scientists know the reactivation of meristematic activity and sprout growth need for CKs and GAs. They also know that increased cytokinin concentration and sensitivity are vital factors for meristematic reactivation. And that CKs coordinated with auxins stimulate sprout elongation.

But SLs affect the sensitivity to GAs, which increase throughout post-harvest storage and are responsible for sprout vigour. SLs may be related to paradormancy establishment instead of eco- and endodormancy since they are vital regulators of lateral bud development.

In brief, the optimum length of dormancy differs depending on cultivars and the final usage of potato tubers. Thus, more extended dormancy and delayed sprouting (at the desired time) will be best for ware potatoes storage, while speed-up sprouting will be preferable for seed potatoes.

Use of CIPC During Potato Storage

Potato News Today already analyzed this topic in the article CIPC in the crossfire: Challenges and possible alternatives.

Effect of Pre-Harvest Factors and Storage Conditions on Tuber Quality

Pre-Harvest Factors

Cultivar and seasonal variability

As a rule, growers establish the potato quality in the field and try to preserve it post-harvest. Abiotic factors, like high temperature, drought, soil salinity and nutrient, influence tuber maturity. Yet, the cultivar and seasonal variability are the factors that impact its final quality.

Sustainable farming practices

High soil nutrient demand for good tuber quality requires a high organic matter and nitrogen input. In this regard, sustainable farming practices, such as balanced fertilization regimes, improve tuber yield and potato marketability quality (e.g., tuber size).

Vine killing

Vine killing is another factor that impacts quality — eliminating the potato vines before harvest induces tuber maturation. This practice triggers both tuber periderm maturation and stolon release. And in seed potato production, it can also control tuber size. A multifactorial approach will manage all these variables and mitigate quality’s side effects.

After harvest, tuber quality management aims to delay dormancy breakdown and limit potato weight loss and sweetening. Senescent sweetening is a natural process due to tuber ageing; it is irreversible and involves cell breakdown.

Storage conditions

Following cell breakdown, hydrolytic enzymes depolymerize the structural and non-structural carbohydrates. So the correct storage conditions are crucial to delay this process.

Cold Storage

Cold storage controls sprouting, but temperature management depends on the intended market:

• growers must store tubers for the fresh market at temperatures below 7°C
• growers must store tubers for the processing market at a higher temperature (8–13°C) to preserve frying quality.

Ethylene supplementation

As mentioned above, growers use continuous ethylene supplementation as a sprout suppressant during storage; yet, it can induce sucrose hydrolysis (ethylene-induced sweetening). A recent study has shown that they can avoid this sweetening with a single application (24 h) of 1-methylcyclopropene (1-MCP) before early and late ethylene supplementation. The impact of CO2, another storage extension gas, on frying quality is less clear. Studies on processing potato varieties showed adverse fry colour effects when the authors applied ethylene and CO2. Despite this, cultivar, gas concentration and timing, and seasonality affect tuber responses to CO2 treatment.

Cold-induced sweetening

Quality loss is also caused by “cold-induced sweetening.” This happens when sucrose hydrolysis leads to the accumulation of reducing sugars (although temperature reconditioning can reverse it in part). But cold-induced sweetening depends not only on post-harvest storage conditions but also on potato variety and crop location.

The Maillard reaction is an array of non-enzymatic, consecutive, parallel chemical reactions that supervise food quality and safety. Since this reaction may occur when the ovens cook the tubers at high temperatures (>120 °C), processing industries prefer reducing sugar levels in potatoes. During the Maillard reaction, reducing sugars are responsible for the product’s browning (French fries, crips). And, as a side effect, acrylamide, a probable human carcinogen, may also accumulate. So the main pathway of acrylamide formation is the deamination and decarboxylation of free asparagine at high temperatures. And its reaction with reducing sugars.

Potato is a major contributor to dietary acrylamide in the European Union. The European Commission issued “indicative” levels (not regulatory or safety thresholds) for acrylamide in food in 2011. This branch of the European Union revised downward for many products in 2013 (e.g., crisps = 100 μg kg-1 and French fries = 600 μg kg-1). In this context, FoodDrinkEurope created a “toolbox” compiling different food industry strategies to reduce acrylamide formation by modifying food processing.

The Role of Genomics in Potato Quality Improvement

Breeding programs aim to develop new cultivars with improved features (productivity, resistance to pathogens and stress). But conventional breeding takes a significant time (often more than ten years) to produce improved germplasm. So today, more than ever, there is a pressing need to speed up genetic improvement in potatoes.

Gaining knowledge about gene-phenotypic relationships and the availability of new technologies have enabled the development of “precision breeding.” This set of techniques:

• Reduces breeding costs.
• Diminishes field-related expenses.
• Simplifies the selection of interesting relatives.
• Makes the rapid screening of large populations more accessible.
• Increases the efficiency of selecting specific traits using genetic techniques (e.g., marker-assisted selection, MAS)
• Has got short the selection cycle.
• Identifies disease resistance genes in wild relatives and crosses them to commercial potatoes.
• Improves the identification of quality candidate traits.

But despite these advances, many traits relevant for the selection of commercially viable varieties are complex. And many genetic and environmental factors regulate post-harvest characteristics, such as tuber yield, starch content, crisp colour, or susceptibility to bruising. The complexity of these traits requires a deeper understanding of geno-phenotypic interactions and more powerful technologies.

The publication of the potato genome improved genetic annotations, and linkage maps led to new genetic resources. Researchers have used them to analyze the regulation of complex traits and QTL (quantitative trait loci).

Among those new resources are:

Single nucleotide polymorphism array (SNPs Arrays).

The Infinium 8303 Potato Array provides a standard SNP marker that researchers use for mapping, germplasm assessment, and fingerprinting. This array is a valuable tool to advance their understanding of the potato genome and the genetic control of several complex traits such as tuber dormancy and starch metabolism.

Genome-wide association studies (GWAS).

GWAS proved beneficial in elucidating other quality traits of potato tubers, i.e., maturity and “browning after cooking.

On the other hand, gene-editing technologies are increasing due to non-transgenic genetically modified organisms. These technologies allow targeted mutation with high specificity and precision at selected loci. And they achieve this goal by inducing breaks in the genome and DNA repair pathways to change target genes.

DNA double-strand breaks (DBSs) are forms of DNA damage, and their repair is critical for cell survival. Endonuclease enzymes that induce dBSs are transcription activator-like effector nucleases (TALEN) and CRISPR-associated (Cas) endonucleases.

Following DBSs, the DNA repair pathways can inactivate a gene (knockout) through a non-homologous end joining (NHEJ) pathway or replace-insert a gene through the homologous recombination (HR) pathway, a DNA metabolic process found in all forms of life.

Researchers achieve gene editing with insertion by combining the action of Agrobacterium tumefaciens with plant viruses. This combination shows promising results when Agrobacterium couples to DNA virus Geminivirus replicon (GVR). The joint allows a higher loading capacity than RNA viruses, and potato plants modified with this technique show reduced susceptibility to herbicides.

Conclusion

To ensure the future quality of potato tubers, the industry and academic communities must collaborate with consumer preferences in mind.

Deploying molecular and phenotyping techniques will improve tuber quality. Especially those techniques that increase understanding of the mechanisms that mediate physiological responses during (1) pre-harvest production, (2) post-harvest storage, and (3) processing (e.g., acrylamide formation).

These combined efforts will benefit the development of new cultivars with improved characteristics and provide guidelines for more sustainable farming techniques and storage strategies.

At the same time, the potato industry must adopt and apply alternative pre- and post-harvest technologies. Through more industry-oriented research, the combination of genomics and pre- and post-harvest technologies will contribute to the conservation, improvement, and viability of future tuber quality.

About the author

Jorge Luis Alonso G. is a writer specializing in potato cultivation who writes marketing content for ag-tech companies. He has lived with his family in Canada since 2018.

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