Physiological age is an important concept in potato production. It determines when the buds will sprout and how many shoots will develop from them. This affects the number of stems and leaf growth, tuber development, tuber yield and tuber size. Older seeds do not have one dominant sprout, but several.
Chronological age is the number of days since tuber formation. Physiological age refers to the internal age of the tuber as it is affected by biochemical changes.
Like chronological age, physiological age shifts over time. But it is also affected by genetics (like traits of a variety) and environmental stressors.
The same concepts are used to understand aging in humans. Some people appear much younger—or older—than they really are. Genetics is largely responsible for how we age, but lifestyle choices can also be important.
Plants respire to release the energy they need to keep their cells functioning - starches and sugars are consumed to produce energy.
When plants are under stress, they breathe more. Any conditions that increase the rate of respiration accelerate the aging process of tubers (extreme temperatures, lack of moisture, nutrient deficiencies, pest infestation and mechanical damage during harvest). Stress that occurs at any time in the life of a tuber can accelerate its aging; but many studies have shown that growing conditions have less effect on physiological age than post-harvest conditions.
Physiologically, a young tuber is characterized by the presence of one dominant bud, which suppresses the germination of other buds on the tuber.
This phenomenon in plants is called apical dominance, an adaptation that promotes upward growth rather than branching. In potato tubers, the result is a plant with fewer stems and smaller but larger tubers.
Older seed tubers are characterized by a loss of apical dominance. They give several sprouts that appear earlier. This means more stems and more tubers, but the size of the tubers is smaller.
Older tubers tend to produce less foliage and reach maturity earlier than younger tubers.
Planting older seeds can be very helpful in growing varieties that tend to produce oversized tubers, such as 'Yukon Gold' or 'Shepody'.
Younger seeds germinate more slowly, but produce a stronger plant with a longer tuber swelling period and later maturation.
But growers need to keep a close eye on the harvest towards the end of the season and remove haulm in time to limit the production of oversized tubers.
The addition of nitrogen fertilizers can partially offset the effects of aging and delay its onset. Nitrogen can help mimic the characteristics of young tubers because it has a strong effect on plant hormones that regulate the aging process. But there is still a danger of early senescence if the crop is stressed or nitrogen levels are reduced too early.
If your seeds are physiologically young and you are concerned that the number of stalks will be too low, you can raise the storage temperature to age the seeds or delay planting.
If your seeds are physiologically old and will produce too many stems, you should maintain ideal storage conditions to minimize further aging and plant seeds as soon as field conditions permit.
Some growers report successful stem reduction with Rejuvenate (Amvac), a seed treatment that contains a synthetic plant hormone to help restore apical dominance. Results may vary by variety and other conditions.
The most practical way to assess the physiological age of seed tubers in storage is to collect a sample and observe germination activity after heating.
The sample should be large enough to represent variability within a seed lot.
A few weeks before the expected planting date, cut the seed tubers into pieces (if you will be planting chopped seeds) and then place them in mesh bags or plant them in the soil. Note how quickly they germinate and how many sprouts are produced.