in cell cultures [35] and grapevine leaves [36]. However, when used as a preharvest
treatment, controversial results have been described [33, 37].
MEJA is the most active derivative of jasmonic acid. Both are endogenous plant
regulators that act as signaling molecules upon biotic stress and are involved in plant
defense mechanisms triggering the synthesis of secondary compounds [38].
Some studies have shown that the application of MEJA to grape bunches may
exert a profound effect on the phenolic content of both the grapes and wine,
particularly anthocyanins and stilbenes [39, 40].
In the case of stilbenes, MEJA treatment improved both the resveratrol and εviniferin content of Barbera berries in an accumulative manner until ripeness [41]. In
contrast, more recent results have described how resveratrol was induced a few days
after treatment but subsequently decreased throughout ripening until harvest [33]. In
fact, the final level of stilbenes after MEJA treatment depends on other factors, such
as variety, climate, and even viticultural conditions [42], which makes the treatment
difficult to apply from a technological point of view.
Additionally, apart from the stimulation of stilbene compounds, other
studies suggest that MEJA is also able to enhance wine quality, increasing the
content of both anthocyanins, and therefore chromatic parameters and aroma
compounds [43, 44].
2.2
Physical Elicitors
Many physical treatments have also been studied as elicitors in grapevines, the
following being particularly significant: ultrasonication (US), ozone (OZ), anoxic
treatments (AT), and UVC light [29].
US treatment has been proposed as a tool to produce a resveratrol-enriched
grape juice [45]. In all the grape varieties tested, a significantly higher elicited
amount of resveratrol was found in grape juice manufactured from fruit treated
with US for 5 min followed by 6 h of incubation. The accumulation of resveratrol
is transcriptionally controlled by the enzyme stilbene synthase and ultrasonication
treatment was found to elicit the activity of stilbene synthase, demonstrating the
underlying mechanism behind increases in resveratrol. However, in comparison
with grape skin or wine, the amount of resveratrol in grape juice was much lower.
Indeed, stilbenes are more soluble in alcoholic solutions such as wine than in
aqueous media [46].
OZ treatment is also known to stimulate the synthesis of resveratrol in grapes
under storage conditions. However, although OZ treatment (3.88 g h
À1 for 5 h)
increased the resveratrol concentration in grapes as much as UVC light did, the
treatment decreased the quality of the grapes enormously [47].
AT of grapes placed in a vacuum chamber with nitrogen gas enhanced the
resveratrol content [48]. After testing different times, 6 and 15 h were recommended
to both improve resveratrol concentration and grape quality.
UVC light with a wavelength range between 200 and 280 nm is a germicidal,
nonionizing radiation that has been used extensively to sterilize fresh fruit and
630
S. Cruz et al.
treatment, controversial results have been described [33, 37].
MEJA is the most active derivative of jasmonic acid. Both are endogenous plant
regulators that act as signaling molecules upon biotic stress and are involved in plant
defense mechanisms triggering the synthesis of secondary compounds [38].
Some studies have shown that the application of MEJA to grape bunches may
exert a profound effect on the phenolic content of both the grapes and wine,
particularly anthocyanins and stilbenes [39, 40].
In the case of stilbenes, MEJA treatment improved both the resveratrol and εviniferin content of Barbera berries in an accumulative manner until ripeness [41]. In
contrast, more recent results have described how resveratrol was induced a few days
after treatment but subsequently decreased throughout ripening until harvest [33]. In
fact, the final level of stilbenes after MEJA treatment depends on other factors, such
as variety, climate, and even viticultural conditions [42], which makes the treatment
difficult to apply from a technological point of view.
Additionally, apart from the stimulation of stilbene compounds, other
studies suggest that MEJA is also able to enhance wine quality, increasing the
content of both anthocyanins, and therefore chromatic parameters and aroma
compounds [43, 44].
2.2
Physical Elicitors
Many physical treatments have also been studied as elicitors in grapevines, the
following being particularly significant: ultrasonication (US), ozone (OZ), anoxic
treatments (AT), and UVC light [29].
US treatment has been proposed as a tool to produce a resveratrol-enriched
grape juice [45]. In all the grape varieties tested, a significantly higher elicited
amount of resveratrol was found in grape juice manufactured from fruit treated
with US for 5 min followed by 6 h of incubation. The accumulation of resveratrol
is transcriptionally controlled by the enzyme stilbene synthase and ultrasonication
treatment was found to elicit the activity of stilbene synthase, demonstrating the
underlying mechanism behind increases in resveratrol. However, in comparison
with grape skin or wine, the amount of resveratrol in grape juice was much lower.
Indeed, stilbenes are more soluble in alcoholic solutions such as wine than in
aqueous media [46].
OZ treatment is also known to stimulate the synthesis of resveratrol in grapes
under storage conditions. However, although OZ treatment (3.88 g h
À1 for 5 h)
increased the resveratrol concentration in grapes as much as UVC light did, the
treatment decreased the quality of the grapes enormously [47].
AT of grapes placed in a vacuum chamber with nitrogen gas enhanced the
resveratrol content [48]. After testing different times, 6 and 15 h were recommended
to both improve resveratrol concentration and grape quality.
UVC light with a wavelength range between 200 and 280 nm is a germicidal,
nonionizing radiation that has been used extensively to sterilize fresh fruit and
630
S. Cruz et al.