Potato plants (Solanum tuberosum L.) were grown in water culture in a controlled environment. Cooling (+8°C) of individual tubers decreased their growth rates and increased the growth rates of non-cooled tubers of the same plant. The carbohydrate concentration in non-cooled and cooled tubers did not differ significantly, but 14C-import from labelled photosynthate was lower in cooled than in non-cooled tubers. The markedly lower conversion rate of ethanol-soluble 14C to starch in cooled, in comparison to non-cooled tubers, was not associated with significant differences in the in vitro activities of starch synthase, ADPG-pyrophosphorylase and starch phosphorylase under standard assay conditions (+30°C). However, the Q10-values of the enzymes differed in vitro in the temperature range between 30°C and 8°C, leading to a marked decrease in the activity ratio of ADPG-pyrophosphorylase/starch phosphorylase in cooled tubers. In tubers differing in growth rates without manipulation, 14 d after tuber initiation significant positive correlations were found between 14C-concentration of tuber tissue and the in vitro activities of starch synthase and ADPG-pyrophosphorylase and a significant negative correlation between 14C-concentration and starch phosphorylase. In contrast, in tubers which were analysed 5 d after initiation, there were only small differences between tubers in growth rate, 14C import and the activity ratio ADPG-pyrophosphorylase/starch phosphorylase. From various direct and indirect evidence it is concluded that the growth rate of individual tubers, and thus the sink strength, is at least in part controlled by the activity of starch synthesizing enzymes.
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Engels et al. (1986) studied this question.