Assimilate Conversion in Potato Tubers in Relation to Starch Deposition and Cell Growth

Abstract

Abstract The activity of enzymes involved in the conversion of sucrose to starch together with the distribution of 14 C‐labelled photosynthate and 4 C‐sucrose was studied in potato tubers showing a range of growth rates and growth patterns. Within a particular tuber the uptake of 14 C from labelled photosynthate and the conversion to ethanol‐insoluble 14 C was greatest in the apical tissue where both the rate of production of new storage cells and starch synthesis were likely to be greatest. Uptake and conversion of 14 C was lowest in the older tissue of the tuber base. Pre‐treatment of tubers with gibberellic acid reduced the total input of 14 C from labelled photosynthate, reversed the gradient in 14 C uptake between apical and basal tuber tissue, increased the amount of 14 C per g fresh weight in the basal tissue and decreased the conversion of labelled sugars to starch. For tubers with different growth rates both the total uptake of 14 C from labelled photosynthate and the ratio ethanol‐insoluble 14 C/ethanol‐soluble 14 C appeared to be correlated with growth rate. In contrast when tubers were fed directly with 14 C‐sucrose via the tuber surface, total uptake was independent of growth rate but the correlation between growth rate and the ratio ethanol‐insoluble 14 C/ethanol‐soluble 14 C persisted. Within a particular tuber there was a decreasing gradient in sucrose synthetase activity between youngest tissue of the tuber apex and the older tissue at the tuber base but there was no clear correlation between mean enzyme activity and tuber growth rate. ADPG‐pyrophosphorylase and the ratio ADPG‐pyrophosphorylase/starch phosphorylase showed some correlation with tuber growth rate. Starch synthase, starch phosphorylase and UDPG‐pyro‐phosphorylase activities per g fresh weight of tuber tissue appeared to be relatively constant. The results suggest that the transport of sugar from the phloem sieve tubes to the tuber storage parenchyma cells, in particular the phloem unloading step, and the conversion of sugar into starch are subject to separate regulation in the potato tuber.