Stable isotope distribution in the major metabolites of source and sink organs of Solanum tuberosum L.: a powerful tool in the study of metabolic partitioning in intact plants

A method was developed for the purification of main intermediates and stora ge products of leaves and tubers of potato for analysis of their C-13 conte nt. The method was tested for recovery of metabolites and carbon isotope di scrimination during the purification process. Leaf metabolite delta(13)C va lues showed an enrichment of starch relative to sucrose and citrate. This r esult is in agreement with previous findings in other higher plants and ind icates the existence of isotope discrimination steps during transport and m etabolism of triose-phosphates in potato leaf mesophyll cells. Active anapl erotic replenishment of the tricarboxylic acid cycle in the leaves of the p lants investigated was also deduced from the significant C-13 enrichment of malate relative to citrate and asparagine/aspartate relative to glutamine/ glutamate. Analysis of tuber metabolite delta(13)C values showed no differe nce between starch and sucrose. However, tuber sucrose appeared significant ly enriched compared with leaf sucrose and also relative to tuber citrate a nd malate. This finding suggests the existence of sites of isotopic discrim ination during sucrose processing in developing tubers. It also confirms th at metabolic cycles of sucrose synthesis and breakdown and of hexose-phosph ate/triose-phosphate interconversion, which have been described in excised tuber tissue, also occur in intact organs. The delta(13)C values were also used to estimate the metabolic rate of carbon oxidation in developing tuber s on the assumption that pyruvate dehydrogenase is the main site of isotopi c discrimination in the tuber cells. The result obtained was in agreement w ith the available literature, suggesting that analyses of natural isotopic distribution in plant products may be a useful tool for the study of metabo lic processes and sink-source relationships in intact plants.