Metabolism in slices from growing potato tubers responds differently to addition of sucrose and glucose

The short-term changes in metabolism that occurred after adding glucose or sucrose to freshly cut discs from growing potato (Solanum tuberosum L.) tubers were investigated. (i) When glucose was supplied, there was a marked increase in glycolytic metabolites, and respiration was stimulated. When sucrose was supplied, amounts of glycolytic metabolites including hexose phosphates and 3-phosphoglycerate (3PGA) were similar to or lower than in control discs incubated without sugars, and respiration did not rise initially above that in control discs. This different response to sucrose and glucose was found across the concentration range 5-200 mM. A larger proportion of the metabolised C-14 was converted to starch when [C-14] sucrose was supplied than when [C-14] glucose was supplied. The different effect on metabolite levels, respiration and starch synthesis was largest after 20-30 min, and decreased in longer incubations. (ii) When 5 or 25 mM sucrose was added in the presence of [C-14] glucose, it led to a decrease in hexose phosphates and 3PGA, and a small increase in the rate of starch synthesis compared to discs incubated with glucose in the absence of sucrose. These differences were seen in a 30-min pulse and a 2-h pulse, Whereas ADP-glucose levels after adding sucrose resembled those in control discs, glucose led to a decrease in ADP-glucose. This decrease did not occur when 5 or 25 mM sucrose was added with the glucose. (iii) To check the relevance of these experiments for intact tubers, water or 100 mM mannitol, sucrose or glucose were supplied through the stolen to intact tubers for 24 h. A 0.2 mM solution of [C-14] glucose was then introduced into the tubers, and its metabolism investigated during the next 30 min. Labelling of starch was increased after preincubation with, sucrose, and significantly inhibited after preincubation with glucose. (iv) II is concluded that glucose and sucrose have different effects on tuber metabolism Whereas glucose leads to a preferential stimulation of respiration, sucrose preferentially stimulates starch synthesis via a novel mechanism that allows stimulation of ADP-glucose pyrophosphorylase even though the levels of hexose phosphates and the allosteric activator 3PGA decrease.