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 intacts plants

A method was developed for the purification of main intermediates and storage products of leaves and tubers of potato for analysis of their 13C content. The method was tested for recovery of metabolites and carbon isotope discrimination during the purification process. Leaf metabolite delta 13C values showed an enrichment of starch relative to sucrose and citrate. This result is in agreement with previous findings in other higher plants and indicates the existence of isotope discrimination steps during transport and metabolism of triose-phosphates in potato leaf mesophyll cells. Active anaplerotic replenishment of the tricarboxylic acid cycle in the leaves of the plants investigated was also deduced from the significant 13C enrichment of malate relative to citrate and asparagine/aspartate relative to glutamine/glutamate. Analysis of tuber metabolite delta 13C values showed no difference between starch and sucrose. However, tuber sucrose appeared significantly enriched compared with leaf sucrose and also relative to tuber citrate and malate. This finding suggests the existence of sites of isotopic discrimination during sucrose processing in developing tubers. It also confirms that metabolic cycles of sucrose synthesis and breakdown and of hexose-phosphate/triose-phosphate interconversion, which have been described in excised tuber tissue, also occur in intact organs. The delta 13C values were also used to estimate the metabolic rate of carbon oxidation in developing tubers on the assumption that pyruvate dehydrogenase is the main site of isotopic discrimination in the tuber cells. The result obtained was in agreement with the available literature suggesting that analyses of natural isotopic distribution in plant products may be a useful tool for the study of metabolic processes and sink-source relationships in intact plants.