Carbohydrate metabolism during potato tuber dormancy and sprouting

Potato tuber dormancy is part of the vegetative life cycle of potato. It refers to a period, in which no growth is occurring, although the tuber is stored under conditions that are favourable for growth. Factors in the tuber are responsible for this growth arrest; soon after dormancy is broken, sprouting occurs. Dormancy is already initiated during tuber formation.

Potato tubers are often placed in short- to long-term storage for a stable year round supply. The occurrence of sprouting can be detrimental for further (industrial) processing of the tubers. On the other hand, sometimes sprouting of the tubers is already necessary soon after harvest, e.g. when tubers need to be planted for the next growing season. Therefore, it is of major interest to investigate the physiological regulation of potato tuber dormancy, dormancy breaking and sprouting. In this research, microtubers obtained with an in vitro system were used; it was concluded (chapter 6) that microtubers were a good model for soil-grown tubers.

Some physiological processes during tuber dormancy initiation (tuber formation) and dormancy breakage (just before visible sprouting), seemed to be reversed, e.g. increasing levels of GA were necessary to induce sprouting during dormancy initiation, while at the end of dormancy lower doses were sufficient. Also the endogenous levels of ABA, known as a dormancy hormone, are reported to increase during the initiation of dormancy and to decrease again at the end of dormancy, thus initiating sprouting. Starch, which is the major carbohydrate reserve in potato, is synthesized during tuber formation and degraded again during sprouting to supply carbohydrates for growth of the new plant. The hypothesis that the developmental processes of initiation leading to dormancy (tuber formation) and dormancy breakage / sprouting were more or less the opposite of each other in terms of levels of endogenous hormones, carbohydrates and intermediates and with respect to the involved enzymatic conversions, is reviewed in chapter 2. Especially the endogenous hormone levels during tuber formation and dormancy breakage / sprouting are extensively discussed (summarised in figure 2, chapter 2) and it was concluded that these hormone levels were partly the reverse in the two different developmental stages.

Carbohydrates and enzymes involved in sugar and starch metabolism are also discussed in chapter 2, although litterature about data during dormancy breaking was relatively scarce. It was concluded that indeed carbohydrate metabolism related processes might be reversed when dormancy initiation and dormancy breaking are compared.

The effect of GA application on dormancy breaking is undisputed; however, the observed changes in carbohydrate metabolism after GA-application could be the result of the developmental process of dormancy breaking, or a direct result of the hormone and not related to sprout induction. Therefore, another way of inducing sprouting, by the use of ethanol, is investigated and discussed in chapter 3. In this chapter, the role of ADH in ethanol-induced dormancy breaking was investigated. It was concluded that ADH seemed to be involved in dormancy breaking, but a role for the (acidic) products could not be shown. Therefore, a role for ADH in the NAD ⁺to NADH conversion was suggested. Applying ethanol with 1% sucrose in the medium resulted in a sprout while ethanol with 8% sucrose in the medium resulted in a secondary tuber. We hypothesized, based on a theory developed for tuber formation, that sucrose could affect endogenous GA levels in a negative way. This would mean that high sucrose would result in low endogenous GA, which in its turn would result in a secondary tuber and vice versa. Determinations of endogenous GA levels did not confirm this hypothesis, low sucrose levels even lead to a decrease of GA ₁ levels. Apparently, the role of sucrose and GA in the regulation of sprouting differed from their role in the regulation of tuber formation. Also ethanol, by no means, increased the level of GA ₁ .

As already mentioned, data on sugar and starch metabolism during dormancy and sprouting were rare. In chapter 4, starch cycling during tuber formation, dormancy and sprouting was investigated by determining enzyme activities involved in starch synthesis and degradation. AGPase activity appeared to increase before visible sprouting, but this was only shown in histochemical staining and not in activity measurements. Also AGPase S- luc tranformants showed an elevated expression in sprouting tubers. Amylase activity was also high during tuber formation and decreased during dormancy. It was only after sprouting that the activity of β-amylase increased. STP was high during tuber formation, decreased during dormancy and remained constant during sprouting. STP was suggested to be involvedin starch degradation and starch cycling was suggested to be active during all developmental stages of the tuber.

The fact that a switch in sucrose breakdown from an invertase to a susy-regulated one occurs during tuber formation made us investigate sucrose breakdown during dormancy and sprouting. Also enzymes involved in the conversions of the sucrose degradation products were investigated, as well as the levels of sugars and intermediates. The results are discussed in chapter 5. Both invertase and susy were very low in the tuber both during dormancy and sprouting. As only susy showed some activity in histochemical staining, sucrose degradation in the tuber was suggested to be supported by susy. At the end of dormancy, glucose-1-phosphate, derived from starch breakdown by STP, is supposed to be converted by UGPase, SPS and SPP into sucrose, which is being exported from the sink tuber to the growing sprout. Invertase showed a huge activity in the sprout and hexokinase showed almost no activity. As a result, high levels of glucose were found in the sprout. The results of chapter 4 on starch cycling are implemented in the results on sucrose metabolism in chapter 5. A schematic model shows the roles of starch and sucrose cycling in both the tuber and the sprout, during dormancy breakage.

In the general discussion (chapter 6), starch and sucrose metabolism during tuber formation and sprouting are compared and it could be concluded that both developmental processes use the same pathways (sucrose and starch cycling), with net fluxes in opposite directions, in the tuber. The conclusion of chapter 2, could be extended with the above conclusion. Carbohydrate metabolism in stolon and sprout are largely similar, also with respect to the flux direction.

Furthermore, carbohydrate metabolism during induced sprouting (by GA or ethanol) is compared to the one occurring during spontaneous sprouting (chapter 4 and 5). It could be concluded that both methods for induced sprouting showed too many differences compared to spontaneous sprouting to use them as a model for the processes occurring during spontaneous sprouting (chapter 6).