Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

446 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
provides energy (ATP) for metabolic processes. In general, starch and organic acids are the
main storage components in fruits. With the advancement of ripening, starch is metabolized
to sugars, which are also synthesized through gluconeogenesis giving rise to sweetness in
the fruits. The fruits develop attractive colors that are provided by phytochemical pigments.
Anthocyanins are the most common pigments in plants besides chlorophyll and carotenoids.
Anthocyanins are biosynthesized via the flavonoid biosynthetic pathway, which is linked to
sugar metabolism through the pentose phosphate pathway (PPP). Aroma, a sensory quality
of the fruits, is due to the evolution of several complex mixtures of volatile compounds. A
number of biosynthetic pathways are involved in production of aroma compounds (Seymour
et al., 1993).
21.4.1 Biosynthesis of sugars
In fruits, starch is the major carbohydrate reserve. It is synthesized from glucose-1-phosphate
by the action of AGPase (ADP-glucose pyrophosphorylase) enzyme. Starch-degrading enzymes
are found in the chloroplast, which convert starch to sugar with fruit ripening. Starch
is transformed to glucose-1-phosphate with the action of several enzymes. The glucose-
1-phosphate is mobilized into cytoplasm, where sucrose is synthesized by the enzymes
UDP-glucose pyrophosphorylase, sucrose phosphate synthase, and sucrose phosphate phosphatase
(Paliyath and Murr, 2006). Sucrose is the major sugar, which accumulates as the
fruit starts to ripen. With the advancement of ripening, sucrose is further converted to
glucose and fructose by the enzyme invertase. In general, glucose and fructose are predominant
sugars in ripe fruits of the most species. However, there are exceptions such as
mangoes, which show higher level of sucrose with fruit maturation (Selvaraj and Kumar,
1990).
Sugar and sugar phosphates formed during starch catabolism are metabolized through
glycolysis, as shown in Fig. 21.1. After a series of reactions, pyruvate is formed during
glycolysis and converted to acetyl-CoA in the presence of pyruvate dehydrogenase. Acetyl-
CoA serves as a precursor for synthesis of several organic acids, fatty acids, isoprenoids,
volatile esters, and phenylpropanoids (Seymour et al., 1993; Paliyath and Murr, 2006). Many
organic acids, including malate, citrate, and succinate, are synthesized through the citric
acid cycle that generates NADH (nicotinamide adenine dinucleotide) and FADH (flavin
adenine dinucleotide), reducing power used for the biosynthesis of ATP.
Sugar and sugar phosphates are also channeled through the PPP, which increases the
levels of pentose sugars during ripening. Pentose phosphate pathway provides carbon skeletons
for several secondary plant products such as anthocyanins and volatile compounds (Fig.
21.1). Throughout maturation of peaches, high activities of glucose-6-phosphate dehydrogenase
(G6PDH) and 6-phosphogluconate were found, which accumulated higher contents
of flavonoids and anthocyanins in fruits (Konga et al., 2007). Higher levels of phenylalanine
ammonia lyase (PAL) and chalcone synthase (CHS) were also observed with an increase in
G6PDH level (Logemann et al., 2000). Based on previous studies, it is clear that G6PDH
stimulates anthocyanin biosynthesis. PPP produces reducing power in the form of nicotinamide
adenine dinucleotide phosphate (NADPH) that is required during biosynthesis of
flavonoids, anthocyanins, isoprenoids, and amino acids (Fig. 21.1). NADPH also plays an
important role in the antioxidant enzyme system (Paliyath and Murr, 2006). Therefore,
carbon skeletons shift from glycolysis to PPP and subsequently to secondary products.