To elucidate the overall metabolism of sugars in coffee fruits, in situ metabolism of exogenously supplied 14C-glucose were investigated in
segments of pericarps and seeds of two cultivars of Coffea arabica and C. canephora fruits obtained from different growth and ripening
stages. The coffee fruits were categorized into five stages 1 to 5 which were corresponding to young, developing, mature (green), ripening
(pink) and fully-ripened (red) fruits, respectively. The rates of uptake and metabolism of [U14C]glucose in fruits of the stages 1–3 was
higher than in fruits of stages 4–5. Release of 14CO2 which represented cellular respiration was high in both pericarp and seeds of C.
arabica and C. canephora up to the stage 3, but it gradually reduced and the minimum was found in the fully ripened stage 5 fruits. The
highest incorporation of radioactivity into the methanol-soluble metabolites was found in the stage 2 fruits. In pericarp, incorporation of
radioactivity into the acidic component (mainly consisted of organic acids) was much higher than into basic component (mainly consisted
free amino acids). In seeds, significant amounts of the radioactivity were found in the basic fractions especially at the stages 4 and 5. [U14C]glucose was converted to fructose and sucrose in both pericarp and seeds. Only small rate of sucrose synthesis from [14C]glucose was
detected 18 h after administration, however, its relative rate is slightly increased at stages 4-5 fruits. Incorporation of radioactivity into the
methanol-insoluble metabolites (mainly consisted protein starch and cell wall constituents) was found in any stages of fruits. The maximum
rates were found in pericarp and seeds of developing (stage 2) fruits of C. arabica and in those at early three stages 1–3 of C. canephora.
Possible metabolic routes and relative participation of carbon in different metabolite biosynthesis in coffee fruits are discussed.
Key words: carbohydrate, biosynthesis, metabolism, Coffea arabica, Coffea canephora.
|
