Grapes - Crop Post-Harvest: Science and Technology - page 192

generally have small fruit unless treated artificially with
growth hormones. The seeds have high levels of
hormones such as abscisic acid and gibberellic acid-like
compounds. Girdling of certain cultivars such as the
parthenocarpic black Corinth is known to increase
gibberellic acids (GAs) in the fruit and increase the size
of the berries. GAs are sprayed on the developing
grape bunch to control bunch shape and berry size (Lynn
& Jensen 1966). Transcripts potentially involved in
seedlessness have been described recently (Costenaro-
da-Silva et al . 2010).

Abscisic acid and auxins
Abscisic acid (ABA) levels increase in the maturing fruit
and are thought to induce de novo synthesis of gluconeo-
genic enzymes (Palejwala et al . 1985). ABA-specific
binding sites have been located in the endomembranes of
grape berry mesocarp with maximum binding values
coinciding with development phase II and dropping off at
veraison (Zhang et al . 1999).
The abscisic acid is involved in the grapevine response
to partial root zone drying (Stoll et al . 2000), a method that
consists in boosting grape ripening by altering the irrigation
schedule, thus creating a stress beneficial to grape quality
when well managed.
On the contrary, auxins act primarily in the young berry
formation, thus the conjugation of auxin to amino acid
leading to low auxin levels in the berry may be part of the
ripening induction (Bottcher et al . 2010).

Ethylene
Grapes have been classified as nonclimacteric fruit as their
ripening phase was apparently not triggered by ethylene
and not associated with a respiratory burst. In fact ethylene
levels are very low in grapes, within the range of pmoles.
g −1 FW , however the use of a specific inhibitor of ethylene
receptors has shown that ethylene signalling is involved in
some ripening metabolisms such as the increase of the
berry volume during the second growth phase and the
anthocyanin accumulation (Chervin et al . 2004). Indeed
the ethylene signalling modulates the berry expansion, via
the transcription regulation of many genes, among which
xyloglucan endotransglucosylases and aquaporins seem
critical (Chervin et al . 2008) and changes occur in tran-
scripts related to ethylene signals (Chervin & Deluc 2010).
Wounding and water deficit have been shown to induce
ethylene emission by grapevines, but the influence on fruit
ripening was not assessed (Boschetti et al . 1997).
Goldschmidt (1998) has published a review about the
possible involvement of ethylene in the ripening of
nonclimacteric fruit.
It has been suggested that the levels of ethylene and
abscisic acid (ABA) act synergistically to promote
pre-harvest ripening (Coombe & Hale 1973). The grape
industry has already adopted the use of an ethylene
precursor (2-chloroethylphosphonic acid), also known as
ethephon, whose spray applications around veraison
enhance colour development, in pigmented cultivars, with
a concomitant fall in acid levels and sometimes
a rise in sugar levels (Weaver & Montgomery 1974;
Shulman et al . 1985). The impact on anthocyanin
accumulation may be due to ethylene effects on several
enzymes involved in anthocyanin synthesis, among which
the UDP glucose-flavonoid 3-O-gucoslyltransferase
(El-Kereamy et al . 2003). Ethephon also stimulates abscis-
sion and is used to chemically thin just after
full bloom and to improve berry removal during mechanical
harvesting for wine production (Szyjewicz 1984).

Jasmonates
There is an increasing interest in the study of jasmonate
roles in grape berry physiology. Their levels seem related
to the presence of seeds (i.e. more jasmonates in seeded
berries), and they seem to follow an accumulation kinetic
that resembles the ethylene one (Kondo & Fukuda 2001;
Chervin et al . 2004). They are also known to stimulate
stilbene accumulation, with a more pronounced effect on
leaves (Larronde et al ., 2003).

Brassinosteroids
A novel series of compounds has been determined as poten-
tial grape hormones, brassinosteroids are indeed produced at
relatively high level at the onset of berry ripening and stimu-
lated this berry developmental process (Symons et al . 2006).

Defence mechanisms
Immature fruit of all plant species contain preformed and/
or inducible defence systems (production of phytoalexins).
Usually however, these systems become less effective as
the fruit ripens. This appears to be true in grape berries for
the best studied grape phytoalexin, a stilbene called
resveratrol (Jeandet et al . 1991). Maximum levels of
resveratrol were shown to be induced by UV light from 1-5
weeks post-flowering, dramatically declining in maturing
berries sampled from 10-16 weeks post-flowering. It was
suggested that this might be a major factor in the increasing
susceptibility of ripening grape berries to Botrytis cinerea
infection (Bais et al . 2000). On the other hand, a study has
shown that levels of defence-related protein, basic chitinase
and a thaumatin-like protein (grape osmotin) rise propor-
tionally with fruit reducing sugar content (Derckel et al .