Summary
      Carotenoids are the isoprenoid compounds responsible for the colour of Citrus fruits. In addition, carotenoides have a mayor nutritional role due to their high antioxidant activity and provitamin A value. The main objective of this Doctoral Thesis has been to study the effects of different postharvest storage conditions on the biosynthesis and accumulation of carotenoids in the peel and pulp of different Citrus cultivars. In addition, changes in carotenoides content and composition in different postharvest conditions have been correlated with changes in the expression of key genes of the carotenoides biosynthesis pathway, in order to characterize not only alterations these compounds, but also to deep insight the regulatory mechanisms involved.
      The storage of 'Navelina' and 'Navelate' oranges and 'Clemenules' mandarin at 12 °C (non-chilling temperature) promotes, in general, fruit colouration both in the skin and pulp. This increase in colour, at certain stages of maturation, was associated to an increment of total carotenoides by 2-3 fold. These effects seem to be related to the stage of fruit ripening, being the colour increase lower in fruits at green or breaker stage. The analysis of individual carotenoides revealed an increase of the linear carotenes (phytoene and phytofluene) and, especially, in the concentration of ß-citraurine at 12 ºC, which was well correlated with the increase in fruit colour in both cultivars. In addition, in 'Clemenules' was also observed an increased in the concentration of ß-cryptoxanthin. These changes in the content and composition of carotenoids were associated with an increase in the expression of carotenoid biosynthetic genes phytoene synthase (PSY), lycopene ß-synthase 1 and 2 (ßLCY1 and ßLCY2) and ß-carotene hydroxylase (ßCHX), while at 2 °C the expression of these genes remained almost unaltered. The effect of storage at 12 °C on the biogenesis of carotenoids seems to be mediated, at least partially, by the action of ethylene.
      In this work it has been also characterized the effect of the conditioning or curing (3 days at 37 ºC) on fruit colouration, content and composition of carotenoids, and expression of carotenoid biosynthesis genes in fruits of 'Fortune' mandarin and 'Navelina' orange. At certain stages of maturation, it was observed a decrease in fruit external colouration and total carotenoid content. The effect of curing on fruit colouration was clearly associated with a decrease in the concentration of ß-citraurina. Another remarkable effect of curing was the inhibition of the carotenoid biosynthesis genes expression. Most of these effects were reversible since ethylene application upon curing restored the colour and expression of carotenoid biosynthesis genes.
      Finally, the mode of action of ethylene in the carotenogenesis of citrus fruits has been studied by using of the inhibitor of ethylene action 1-MCP (1-methylcyclopropene). 1-MCP has been applied to fruits of 'Fortune' mandarins and 'Clemenules' which have a slow and fast rate of degreening, respectively, at different stages of maturation, both on fruits attached to the tree and harvested. The results showed that inhibition of ethylene action delays the fruit colouration during the postharvest storage, but no significant effects were observed in fruits on attached on the tree. Treatment with 1-MCP, however, significantly inhibited the expression of carotenoid biosynthetic genes, especially, PSY, ßCHX and ßLCY2, but this effect was not reflected in changes in levels of carotenoids. Therefore, it seems that in citrus fruits ethylene perception is required to maintain the expression of carotenoid biosynthetic genes, but other determinant mechanisms/factors, in addition to transcriptional control, may be considered to regulate carotenoids synthesis and accumulation in citrus fruits.