Physiological Aging of Seed Tubers
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For many decades, almost since the discovery of hormones in the late 19th century by Fritz Went and Francis Darwin (son of Charles), biologists realized that organisms, plant and animal, age internally at different rates that is not accounted for by time alone. Physiological aging is affected by two factors influencing internal biochemistry especially hormones: genetic predisposition and environmental stress. Since potato is grown from cloned stem tissue (tubers), genetic predisposition is at the level of cultivars (van der Zaag and van Loon, 1987). Environmental stresses in the field are primarily moisture, temperature, nutrients, pest injury, and mechanical damage. In storage, stresses are temperature, moisture, aeration, bruising, and disease.
Physiological aging in potato encompasses two types or models, vine during the growing season and tuber during the storage season. Physiological aging in vines is calculated based on daily air temperature fluctuations and is used to predict when plants are susceptible to infection by early blight (Alternaria solani), an opportunistic disease that attacks senescing plants. The other type of physiological aging concerns the viability of tubers used for seed. This is broadly defined as “... physiological status of the tuber as it affects productivity.” (Bohl et al., 2003) or “... internal age of the seed (tuber) resulting from biochemical changes...”
Although physiological aging of tubers may occur during the growing season due to stress, it is poorly understood and not quantifiable. However, tubers from plants that died prematurely tend to be physiologically older. Soil temperature at the end of the season clearly can play a major role; high soil temperatures under dry conditions in sandy soils can stimulate sprouting before harvest. The only way to measure season-stimulated physiological aging of tubers is to conduct a bioassay determining dormancy and sprouting characteristic. The best general indication is to look at the field history of the seed lot in comparison to previous years’ seed lots’ performance. However, the major aging of seed tubers occurs during storage.
During tuber storage, the primary influence on physiological aging is temperature. Higher storage temperatures are associated with greater physiological aging. The exact relationship is not yet established but a correlation exists; however, there is no predictor of aging. There is no direct measure of aging although a heat accumulation model is sometimes used. So to determine the age of a seed lot, samples are removed and bioassayed for morphological stages of aging.
References:
Bohl, W.H., Olsen N., Love, S.L., and Nolte, P. 2003. Seed and planting management. pp. 91-114. Chap. 7. In Potato Production Systems. Stark, J.C. and Love, S.L (Eds.). Publ. Univ. Idaho Extension.
van der Zaag, D.E., and van Loon, C.D. 1987. Effect of physiological age on growth vigour of seed potatoes of two cultivars. 5. Review of literature and integration of some experimental results. Potato Res 30:451-472.