Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

450 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Different anthocyanidins give different colors to fruits (Jaakola et al., 2002; Paliyath and
Murr, 2006).
In red and nonred apples, differential expression of anthocyanin biosynthetic genes
was observed. The expression of CHS, F3H, DFR, and ANS genes was found in red
apples when anthocyanin was not detected (before ripening stages). However, UFGT gene
expression was observed only during the ripening stages with the formation of anthocyanins.
This suggests that UFGT gene expression regulates the production of anthocyanins during
ripening in apples (Kondo and Hiraoka, 2002). Environmental factors such as light and
temperature affect anthocyanin accumulation in fruits. No anthocyanin was found in apples
in the absence of light (Proctor, 1974).
21.4.5 Interrelationships of metabolic pathways
In fruits, quality-attributing pathways are interrelated. During starch degradation, glucose-1-
phosphate is generated, which enters into several metabolic pathways such as glycolysis and
PPP. Glycolysis converts glucose to acetyl CoA after a series of reactions. The acetyl CoA
is a precursor for the synthesis of fatty acids, volatile esters, isoprenoids, and organic acids
as show in Fig. 21.3. The PPP is an important pathway, which provides carbon skeletons to
several biosynthetic pathways including amino acids, secondary metabolites, and nucleic
acids. Besides providing carbon skeleton, it also gives reducing energy (NADPH) to various
pathways (Fig. 21.3). The carbon skeletons for the synthesis of anthocyanins come from
p-coumaroyl-CoA and malonyl-CoA. The phenylpropanoid pathway donates p-coumaroyl-
CoA, which derives from erythrose-4-phosphate and pyruvate that are generated during PPP.
Therefore, PPP plays a significant role in the fruit quality regulation.
In flavonoid biosynthesis, the hydroxylation of flavonoids by F3H requires NADPH,
which comes from PPP. Isoprenoid synthesis also needs NADPH from PPP (Fig. 21.3).
Moreover, it is an important component for the antioxidant enzyme system. The enzymes
of ascorbate–glutathione cycle, glutathione reductase (GR), and monodehydroascorbate reductase
(MDHAR), require NADPH to inactivate reactive oxygen species (ROS) generated
during stress and senescence (Fig. 21.3). In order to maintain fruit quality and shelf life, the
precise functioning of the antioxidant enzyme system is necessary.
During membrane lipid catabolism, several fatty acid intermediates are formed that are
required for the synthesis of aroma volatile components. The precursors of volatile esters,
including hexanal, hexanol, and short-chain alcohols, are also derived from metabolism
of lipids. The effect of hexanal has been investigated on PLD activity of membrane, and
soluble fractions isolated from corn kernel (Paliyath et al., 1999). Inclusion of hexanal in
assay mixture resulted in over 75% inhibition of PLD activity. Other technologies have
also been used for the inhibition of PLD activity. A naturally occurring lipid, lysophosphatidylethanolamine
(LPE), showed potent inhibition of PLD activity in leaves, flowers,
and postharvest fruits. Ryu et al. (1997) noticed a reduction in PLD activity by increasing
the length and unsaturation of LPE acyl chain. In addition, decreased ethylene production
was found in LPE-treated fruits. Antisense PLD tomatoes showed PLD inhibition. Furthermore,
they also possessed higher levels of lycopene, firmness, and soluble solids in
the fruits (Pinhero et al., 2003), suggesting that by inhibiting certain metabolic pathways,
quality components and shelf life of fruits can be increased.