The conversion of starch to sugars and their subsequent metabolism occur in different compartments.
During the development of fruits, photosynthetically fixed carbon is utilized for
both respiration and biosynthesis. During this phase, the biosynthetic processes dominate.
As the fruit matures and begin to ripen, the pattern of sugar utilization changes. Ripening
is a highly energy-intensive process. And this is reflected in the burst in respiratory carbon
dioxide evolution during ripening. As mentioned earlier, the respiratory burst is characteristic
of some fruits that are designated as climacteric fruits. The postharvest shelf life of
fruits can depend on their intensity of respiration. Fruits such as mango and banana possess
high level of respiratory activity and are highly perishable. The application of controlled
atmosphere conditions having low oxygen levels and low temperature have thus become a
routine technology for the long-term preservation of fruits.
The sugars and sugar phosphates generated during the catabolism of starch are metabolized
through the glycolysis and citric acid cycle (Fig. 3.4). Sugar phosphates can also
be channeled through the pentose phosphate pathway, which is a major metabolic cycle
that provides reducing power for biosynthetic reactions in the form of NADPH, as well as
supplying carbon skeletons for the biosynthesis of several secondary plant products. The
organic acids stored in the vacuole are metabolized through the functional reversal of respiratory
pathway, which is termed as gluconeogenesis. Altogether, sugar metabolism is a
key biochemical characteristic of the fruits.