Same variety, different crop: Why seed-tuber age deserves more attention

Physiological age can influence emergence, stem number and tuber size—yet growers still lack a quick, reliable way to measure it before planting

By Lukie Pieterse, Editor/Publisher of Potato News Today

Two seed tubers can be the same variety, harvested on the same date and certified to the same health standard—and still produce noticeably different crops.

One may emerge quickly and develop several strong stems. The other may remain dominated by a single sprout, establish slowly or lose vigour before it reaches the field. These differences can influence canopy development, tuber number, size distribution, maturity and ultimately the proportion of the crop that meets its intended market.

The reason lies partly in what potato physiologists call physiological age: the internal developmental condition of the seed tuber.

A major scientific review published in Potato Research on 17 July 2026 has brought renewed attention to this longstanding but incompletely solved production issue. Researchers Pramila Wagle, Nigel Swarts, Calum Wilson and Nathan Tivendale concluded that physiological age has important implications for seed quality and crop performance, but that most existing assessment methods remain too slow, labour-intensive or inconsistent for routine commercial use.

The industry can test seed for pathogens, grade it by size and verify its varietal identity. It still cannot place a tuber into a rapid test and receive a universally dependable reading of its planting readiness.

That is a significant gap. The first irrigation, fertilizer application or crop-protection decision may receive more attention than the biological condition of the material from which the entire crop must grow.

Chronological age tells only part of the story

The number of weeks since harvest is not the same as physiological age.

A seed tuber remains alive after it is separated from the mother plant. It respires, loses moisture, moves through dormancy and eventually begins activating its buds. The rate at which this internal clock advances depends on the variety, the conditions under which the seed crop was grown, maturity at haulm destruction, storage temperature, storage duration and subsequent handling.

Temperature is particularly influential. Warmer storage generally accelerates dormancy release, sprouting and physiological ageing, while colder conditions slow the process.

But the relationship is not a simple one in which every tuber gains the same amount of age for every degree of accumulated temperature.

Some cultivars are naturally slow to age and remain dormant for longer. Others progress rapidly from dormancy into multiple sprouting and eventually senescence. Seed produced under a stressful season may behave differently from seed of the same variety grown under cooler or more favourable conditions.

The history of the tuber therefore travels with it.

A buyer receiving seed does not receive only a cultivar and a grade. The buyer receives the accumulated effects of the seed field, season, harvest, storage facility and transport chain.

From one sprout to many

The visible sprouting pattern provides one of the most practical indications of physiological condition.

Following dormancy, a seed tuber commonly enters a period of apical dominance. One or a small number of buds at the apical end begin growing while other eyes remain suppressed.

Seed planted during this stage tends to produce fewer main stems. Depending on the variety and planting density, that can result in fewer tubers that have more opportunity to grow large.

As physiological age advances, apical dominance weakens and more eyes begin to grow. The tuber enters a multiple-sprout stage, typically producing more stems and more tubers. The average tuber size may consequently decline as the developing tubers compete for the plant’s resources.

If ageing progresses too far, the sprouts may become weak, excessively long or branched. The seed tuber loses weight and consumes more of the reserves intended to support establishment. Emergence may become uneven, crop vigour can decline and the plant may senesce before it has fully used the available growing season.

This means there is no universally “best” physiological age.

A seed grower seeking large numbers of smaller tubers may want a different stem and tuber population from a processing grower targeting large fries. An early fresh-market crop may benefit from rapid emergence and early bulking, while a long-season crop needs sufficient vigour and canopy duration to exploit the full season.

The correct question is not whether the seed is young or old. It is whether its physiological condition fits the variety, planting date, environment and market objective.

Recent research complicates the old rules

Traditional potato agronomy generally associates older seed with earlier emergence, more stems, earlier tuber initiation, more—but smaller—tubers and earlier maturity.

Recent research shows that these relationships are real but not equally strong in every production system.

A Wageningen-led study published in Potato Research evaluated Agria, Festien, Innovator and Lady Claire over three production cycles and eight field experiments in the Netherlands. Researchers created differences in physiological age by storing seed at several temperatures.

Storage at 17°C caused more pronounced ageing and some reductions in crop performance. However, seed stored between 4°C and 10°C generally performed well. Within that range, effects on final stem number, tuber number, yield and size distribution were often small or inconsistent.

The finding is useful because it challenges the idea that modest differences in storage temperature must inevitably produce large differences at harvest. Under favourable temperate growing conditions, good field management and a long enough season may allow the crop to compensate for some variation in seed condition.

But that should not be interpreted as proof that physiological age can be ignored.

The Dutch work involved four varieties, controlled seed production and temperate conditions. It did not cover every cultivar, warmer production region, delayed planting situation or poorly controlled supply chain.

Research under commercial conditions in Washington State, published in HortTechnology in 2025, found that controlled physiological ageing enhanced aspects of the performance of three potato cultivars. The broader scientific record likewise shows that planting physiologically unsuitable seed can affect emergence, stem number, tuber set and crop value.

The evidence therefore points away from universal prescriptions. Physiological age matters, but its expression depends heavily on the cultivar and environment.

Kenya demonstrates why origin matters

The international seed trade makes this complexity particularly important.

Seed potatoes may be produced at high altitude, stored at lower elevation and planted in a different season hundreds or thousands of kilometres away. The name on the bag remains the same; the physiological behaviour may not.

A study involving researchers from Kenya, Ireland and Wageningen examined 47 potato genotypes grown at three altitudes in north-western Kenya during two contrasting seasons.

Dormancy release varied widely—from 54 to 136 days in the principal analysis. The researchers found significant differences among genotypes, locations and seasons, as well as interactions between them.

Seed produced at the highest-altitude site generally had shorter dormancy than seed grown at lower altitude. However, the effect of genotype was larger than the effects of altitude and season for most characteristics.

This has immediate practical implications in regions where two or more potato crops may be grown each year.

A long-dormancy variety can be useful for ware storage but may still be dormant when it is needed as seed for the next planting window. A short-dormancy variety may be ready to plant sooner but deteriorate rapidly when storage or transport is delayed.

Smallholders with limited access to refrigerated storage have especially little room for error. They may receive seed that is still dormant, or seed that has already developed fragile sprouts during transport. In either case, the farmer carries the production risk created elsewhere in the chain.

A variety recommendation that ignores dormancy and physiological age may therefore be agronomically incomplete, even when the variety has excellent yield potential and disease resistance.

The planting delay that changes the seed

Weather adds another layer of uncertainty.

A seed lot may be in suitable condition on the expected planting date but excessively aged three weeks later. Wet soil, unavailable machinery, delayed delivery or a missing planting window can turn properly prepared seed into a problem.

The industry often discusses planting delays mainly in terms of a shortened growing season. The effect on the waiting seed deserves equal attention.

Once warmed and actively sprouting, a seed lot cannot simply be placed on pause. Re-cooling can slow further development, but repeated temperature changes and extended holding periods may affect sprout strength and uniformity.

Long sprouts are vulnerable during handling and planting. If they break, the seed tuber must draw again on its stored reserves to activate replacement buds. The result may be later or less uniform emergence.

This is not solely an agronomic failure. It is often a logistical one. Seed delivery dates, store capacity, planter availability, contractor schedules and field readiness all have to be coordinated around living material.

What growers can assess now

The 2026 review concludes that the potato industry still lacks a rapid, scalable and universally reliable test for physiological age. That does not leave growers entirely without options.

A representative seed sample can be removed from storage, warmed under controlled conditions and observed for dormancy release, sprout number, sprout position and vigour. Government of New Brunswick guidance recommends warming representative samples at 15°C to 18°C in darkness and recording the time required to reach the early “white point” stage of sprouting.

Such a test is not a laboratory diagnosis. It is a practical comparison among lots.

Growers can also examine whether sprouting is dominated by one apical bud, whether multiple eyes are active, whether sprouts are short and vigorous or long and fragile, and whether tubers show excessive shrivelling or weight loss.

These observations should be interpreted by variety and intended market. A single visual standard applied to every seed lot can be almost as misleading as ignoring physiological age altogether.

Better records would help. Seed suppliers could routinely provide the seed-crop location, haulm-destruction and harvest dates, storage-temperature history and any warming or sprout-management treatment already applied.

Growers, in return, can record arrival condition, warming regime, planting date, emergence, stem number and harvested size profile. Over several seasons, those records can turn a vague impression about “strong” or “tired” seed into locally useful evidence.

Can biomarkers read the internal clock?

The 2026 review highlights proteomics and metabolomics as possible routes towards a more objective test.

These technologies examine proteins, metabolites and other biological signals that change as the tuber moves through dormancy, sprouting and senescence. If researchers can identify biomarkers that consistently correspond with planting performance, a future commercial assay might estimate physiological condition more directly.

The opportunity is real, but the work remains at an early stage.

The review found remarkably little dedicated proteomic research focused specifically on physiological ageing in potato. Researchers must first separate reliable age-related signals from changes caused by disease, cultivar, field stress and storage conditions.

A molecular test also has to become affordable, rapid and interpretable. A technically sophisticated result that arrives after planting, or costs more than the decision is worth, will not solve the commercial problem.

The realistic near-term opportunity may be a combination of biological observation, temperature history and cultivar-specific modelling, with biomarker tests eventually adding greater precision.

Seed quality is more than disease status

Certified health, varietal purity and physical condition remain fundamental to seed quality. Physiological readiness should not replace any of them.

It should sit alongside them.

The significance extends across production systems. A highly mechanised processor may lose value when physiological age shifts the crop away from the contracted size profile. A seed grower may fail to achieve the desired tuber number. A smallholder may miss a short rainy-season planting window because the purchased seed has not broken dormancy.

The science also reminds us that seed management cannot end when the crop leaves the seed producer’s store. Transporters, merchants, agronomists and buyers all influence whether the tuber reaches the planter in suitable condition.

What appears to be one input is in fact a living production system.

The first signs of the next potato crop are already developing before the field is opened, the planter is calibrated or the fertilizer is applied. They are present in the quiet movement from dormancy to sprouting—and in an internal clock that the industry can influence but still cannot read with complete confidence.

Until that changes, the most sensible approach is to treat every seed lot as biologically specific, ask more questions about its history and match its condition to the crop the grower actually intends to harvest.