Stages of Tuber Aging
Pages in tuber aging section:
There are five stages of tuber physiological aging based on sprouting (Iritani et al., 1983; Schrage, 1999).
This is sometimes considered a pre-aging stage. When tubers are harvested, under normal circumstances, i.e., no heat sprouting, the eyes (or buds) are in a state of dormancy. Dormancy is a physiological state induced primarily by a hormone called abscisic acid (ABA) that enters tubers from the vine during the season during tuber growth. Dormancy is characterized by a period during which sprouting will not occur even under optimal conditions. Dormancy is broken when the amount of ABA in the eyes decreases through metabolic breakdown to a level that allows the eyes to respond to conditions favoring sprouting. (This is the same hormonal mechanism involved in seed and bud dormancy.) In physiological aging models, tuber aging does not commence until dormancy is broken.
2. Apical Dominance
Young seed is characterized by the exhibition of a dominant eye over the other eyes, that is, suppressing the sprouting of other eyes. The suppressing eye is at the apical end of the tuber or “bud” end which is the furthest eye from where the tuber was attached to the vine. This physiological phenomena is termed apical dominance and is common in the plant kingdom, e.g. suppression of growth of lower branches by the uppermost branch. It is mediated primarily by the hormone, indole acetic acid (IAA), an auxin (Kumar and Knowles, 1993). In the case of potato tubers, the eye and young sprout produce IAA which travels down the tuber suppressing the other eyes from sprouting. This suppression is dependent on IAA concentration as dormancy is dependent on ABA concentration. In both these phenomena, there are hormones, cytokinins and gibberellins, which can build up and counteract ABA and IAA. The result is that physiologically young tubers produce one or two main stems or low stem number per acre. There is initially one sprout at the apical end and later a second sprout may appear from the eye furthest from the dominant eye as the IAA level is lower. It is important to note here that the genetic makeup of a cultivar is the basis of how much ABA and IAA are produced, how sensitive are tissues to them, how fast they are broken down, and how much and when counteracting hormones are produced. Environment influences all of these but the basis is genetics.
3. Multiple Sprouting
Older seed tubers (“middle-aged”) are characterized by the loss of apical dominance between eyes, i.e., effective IAA levels in tubers are decreased. This develops gradually in time but the time can be shortened by heat. Or, the dominance can be disrupted by cutting the tuber and thereby breaking the translocation path of IAA in the tuber. Removal of the apical sprout will also disrupt apical dormancy although in young tubers, it may be reinstated by the next eye closest to the apical end. Therefore, this stage is characterized by the appearance of several eyes sprouting along the tuber resulting in several main stems per plant. Main stem number correlates with yields and will be discussed later (Iritani et al., 1983).
As seed tubers age further, there is a breakdown of apical dominance within the sprout, that is, the dominance effect of the tip of the sprout, where IAA is produced, on branch buds below has broken down. The result is that sprouts of physiologically old seed tubers are branched. These branches may be weak, referred to as “hairy.” Or, there may be a proliferation of small stolons and a large tuber set that will not be poorly maintained.
5. Little Tuber Disorder
The IAA mechanism is now so broken down that not only do sprouts proliferate their branches into underground stolons but these do not grow much and form tiny tubers quickly. Some cultivars like Russet Norkotah are prone to this (Picture).
Iritani, W.M., Weller, L.D. and Knowles, N.R. 1983. Relationship between stem number, tuber set and yield of Russet Burbank potatoes. Amer Potato J 60:423-431.
Kumar, G.N.M. and Knowles, N.R. 1993. Involvement of auxin in the loss of apical dominance and plant growth potential accompanying aging of potato seed tubers. Can J Bot 71:541-550.
Schrage, W. 1999. The influence of physiological age on the yield potential of seed potatoes. In Seed Potato Management. Univ Minnesota. (www.mnseedpotato.org/extension/article).