The metabolic fate of citric acid as affected by cold storage in blood oranges

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Citrus fruit are natural source of several phytochemicals useful in the maintenance of a satisfactory status of the human health. As citric acid is one of the main determinant of citrus fruit quality, researches focusing the elucidation of citrate metabolism have been constantly carried out, often leading to a deeper understanding of the biochemical and physiological changes occurring during fruit development and ripening. Lower interest has been directed to the knowledge of the metabolic fate of citrate during fruit storage, although the temperature controlled conservation is a spread and, in many cases, mandatory practise. In this work, we evaluated the expression of the main enzymes involved in the citrate metabolism, such citrate synthase, citrate lyase and both the cytosolic and mitochondrial isoforms of aconitase, in blood oranges subjected to cold storage (4 °C × 15 days). Total acidity (TA) and total soluble solid (TSS) were also determined to correlate such fruit quality parameters with gene expression. The results suggest that cold storage influences both the analytical parameters and gene expression, in particular, a strong induction of the transcription of all the selected gene was observed simultaneously with the sudden reduction in acidity. These findings suggest that aconitase gene products localized in the cytosolic compartment are strongly implicated in the consumption of the citrate released from vacuoles and meanwhile mitochondrial aconitase is involved in the catabolism of organelle-localized citrate. The expression of citrate lyase turned out to be also induced during cold storage of blood oranges. The alternative breakdown of citrate through the citrate lyase, which is ruled out during fruit development and ripening, might be a pathway exclusively activated in response to cold storage and therefore correlated with the sharp cold induction of the flavonoid biosynthesis which is supplied by the citrate lyase reaction products.