Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

22 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
sugar derivatives leads to various storage (starch) and structural components (cellulose,
pectin).
During photosynthesis, the glucose formed is converted to starch and stored as starch
granules. Glucose and its isomer fructose, along with phosphorylated forms (glucose-6-
phosphate, glucose-1,6-diphosphate, fructose-6-phosphate, and fructose-1,6-diphosphate),
can be considered to be the major metabolic hexose pool components that provide carbon
skeleton for the synthesis of carbohydrate polymers. Starch is the major storage carbohydrate
in fruits. There are two molecular forms of starch—amylose and amylopectin—and both
components are present in the starch grain. Starch is synthesized from glucose phosphate
by the activities of a number of enzymes designated as ADP-glucose pyrophosphorylase,
starch synthase and a starch-branching enzyme. ADP-glucose pyrophosphorylase catalyzes
the reaction between glucose-1-phosphate and ATP that generates ADP-glucose and pyrophosphate.
ADP-glucose is used by starch synthase to add glucose molecules to amylose
or amylopectin chain, thus increasing their degree of polymerization. In contrast to cellulose
that is made up of glucose units in β-1,4-glycosidic linkages, the starch molecule contains
glucose linked by α-1,4-glycosidic linkages. The starch-branching enzyme introduces glucose
molecules through α-1,6-linkages to a linear amylose molecule. These added glucose
branch points serve as sites for further elongation by starch synthase, thus resulting in a
branched starch molecule, also known as amylopectin.
Cell wall is a complex structure composed of cellulose and pectin, derived from hexoses
such as glucose, galactose, rhamnose and mannose, and pentoses such as xylose and
arabinose, as well as some of their derivatives such as glucuronic and galacturonic acids. A
model proposed by Keegstra et al. (1993) describes the cell wall as a polymeric structure
constituted by cellulose microfibrils and hemicellulose embedded in the apoplastic matrix in
association with pectic components and proteins. In combination, these components provide
the structural rigidity that is characteristic to the plant cell. Most of the pectin is localized
in the middle lamella. Cellulose is biosynthesized by the action of β-1,4-glucan synthase
enzyme complexes that are localized on the plasma membrane. The enzyme uses uridine
diphosphate glucose (UDPG) as a substrate and, by adding UDPG units to small cellulose
units, extends the length and polymerization of the cellulose chain. In addition to cellulose,
there are polymers made of different hexoses and pentoses known as hemicelluloses,
and based on their composition, they are categorized as xyloglucans, glucomannans, and
galactoglucomannans. The cellulose chains assemble into microfibrils through hydrogen
bonds to form crystalline structures. In a similar manner, pectin is biosynthesized from UDPgalacturonic
acid (galacturonic acid is derived from galactose, a six-carbon sugar) as well as
other sugars and derivatives and includes galacturonans and rhamnogalacturonans that form
the acidic fraction of pectin. As the name implies, rhamnogalacturonans are synthesized
primarily from galacturonic acid and rhamnose. The carboxylic acid groups complex with
calcium, which provide the rigidity to the cell wall and the fruit. The neutral fraction of
the pectin comprises polymers such as arabinans (polymers of arabinose), galactans (polymers
of galactose), or arabinogalactans (containing both arabinose and galactose). All these
polymeric components form a complex three-dimensional network stabilized by hydrogen
bonds, ionic interactions involving calcium, phenolic components such as diferulic acid and
hydroxyproline-rich glycoproteins (Fry, 1986). It is also important to visualize that these
structures are not static and the components of cell wall are constantly being turned over in
response to growth conditions.