Agriculture Reference
In-Depth Information
Fig. 4.9
Ethylene biosyn-
thesis in climacteric and
non climacteric fruits, with
respect to respiration and
growth rates. The different
phases of fruit development
are also indicated
1-MCP, an inhibitor of ethylene action. Research pointed out that ethylene may
be involved in the stimulation of ABA biosynthesis occurring at veraison and the
subsequent sugar accumulation. Recently, it has been shown that an increase of
brassinosteroids precedes the onset of ABA pointing out that other hormones may
be involved in the regulation of the ripening syndrome.
The main biochemical and structural changes occurring during fruit ripening,
are the depolymerization of cell wall components, the loss of cell turgor, the in-
crease of mono and disaccharides, the biosynthesis of volatile compounds, deg-
radation of organic acids and color development. Softening is related to juici-
ness and crunchiness. The fruits can be divided into two categories depending
on the degree of softening: in the first group peaches, pears, plums and strawber-
ries are characterized by an intense softening rate, while apple, quince and some
pear varieties have a moderate softening rate and achieve a crunchy texture type.
This different physiological behaviour affects storage and shelf life. The loss of
cellular turgor is mainly due to the accumulation of solutes (carbohydrates, or-
ganic acids, ions, etc.) and by the loss of water by the fruit. In this sense, the
structural characteristics and composition of the fruit cuticle play a fundamental
role in regulating the loss of water through transpiration and, therefore, may af-
fect the softening rate. The enzymes responsible for these modifications are cell
wall hydrolases, among which are the endo-β-(1,4)-glucanases (EG), xyloglucan
endotransglicosidases (Xet), pectin methyl-esterases (PME), pectate lyases (PL),
polygalacturonases (PG) and β-galactosidases (ß-GAL). In addition the expansins
(EXP) induce cell wall relaxation necessary to expose cell wall components to the
action of cell wall hydrolases such as EG, ß-GAL, PME, PL, and PG, acting in a
sequential manner. In the case of peach, it has been observed that an increase of
PG activity is detectable only after the fruit has reached a firmness value of 20N,
concurrently with the ethylene climacteric.
