Agriculture Reference
In-Depth Information
The most common change that occurs at fruit ripening is the loss of green color.
This process also occurs in several non-climacteric fruits (grapes, citrus) at the ac-
quisition of the best edible properties. The loss of the green color is caused by the
degradation of chlorophyll due to both non-enzymatic processes (changes of pH)
and to the action of specific enzymes which oxidizes the molecules of chlorophyll.
The other major color change is related to the synthesis of carotenoids and antho-
cyanins. Carotenoids, responsible for the yellow-orange, are isoprenoids synthe-
sized in chromoplasts from isopentenil diphosphate (IPP), the common precursor
of all isoprenoids. The first synthesized molecule is the phytoene (C40) which is
first converted to pro-lycopene and, subsequently, to lycopene. The introduction
of oxygen molecules in the cyclic structure of the alpha-carotene determines the
synthesis of xanthophylls (neoxantin, violaxanthin, cryptoxanthin), the pigments
mostly responsible for yellow color in peaches and apricots. The accumulation of
different carotenoids (lycopene, alpha-carotene, xanthophylls) in the mature fruit
is genetically controlled and is the result of the selective adjustment of specific
biosynthetic steps. In this context, the role played by ethylene in climacteric fruit
is crucial. The biosynthesis of anthocyanins, responsible for the color ranging from
pale-red to deep-blue/purple is genetically regulated, tissue specific and depends
on the stage of development and environmental factors. Anthocyanins are water
soluble phenolic compounds that accumulate in vacuoles and belong to the cat-
egory of flavonoids, synthesized from phenylalanine by the action of phenylalanine
ammonium lyase (PAL). The biosynthetic pathway, studied in details in grapes,
proceeds through several steps leading sequentially to the formation of chalcones,
dihydroflavonols, anthocyanidins and, finally, anthocyanins, responsible for the red
color. This reaction is catalyzed by a structural gene (
UFGT
), whose expression is
highly regulated both by endogenous factors (development) and exogenous ones
(cultivation environment) (Castellarin et al.
2011
). A genomic approach applied in
grape and apple, established that anthocyanins biosynthesis is positively regulated
by a MYB-type transcription factor.
The fruit taste and flavor are important sensory qualities defined primarily from
the set of processes that regulate the metabolism of sugars and organic acids. Acid-
ity is a key component of the flavor and develops during early fruit growth, follow-
ing metabolic pathways shared by malate and citrate, the main organic acids in most
of the fruits. The acidity decreases during ripening, mainly because of the use of
organic acids as substrates for respiration (Cercós et al.
2006
). The respiration rate,
expressed as mole of CO
2
produced, related to O
2
consumption, tends to increase
during ripening in both climacteric and non-climacteric fruits, and reflects the res-
piration substrate. The organic acids degradation is dependent upon both genetic
factors and environmental conditions. As far as genetics, the low-acidity character
is recessive in apple and citrus, while it is dominant in peach. The rate of respira-
tion reflects the substrates type and is dependent upon environmental conditions
(temperature) that may affect the activity of the malic enzyme and the availability of
malate. Sugars, together with the organic acids, are among the most important com-
pounds influencing fruit flavour. The main soluble solids are sucrose, glucose and
fructose; the latter being present in all fruit, often in similar concentrations, while
