Agriculture Reference
In-Depth Information
12.2.4 Cytokinins
appears to be linked to the phase of rapid
cell division and growth (Fraser et al. ,
1995; Symons et al. , 2006; Zhang et al. ,
2007b). From their initial high levels, GA
levels decline to be low throughout
ripening (Fraser et al. , 1995; Symons et al. ,
2006; Zhang et al. , 2007b), and this
accumulation profi le makes them likely to
be involved in promoting early fruit
development and possibly inhibiting
ripening. Part of the role of GAs during the
early stages of development may involve
changes to the cell wall. Fruit fi rmness was
increased/maintained by the application of
GA to a wide range of fruit (Ju et al. , 1999;
Southwick et al. , 2000; Kappel and
MacDonald, 2002; Jiang et al. , 2004). In
sweet cherry, GA application caused a
delay in the conversion of pectins to the
water-soluble form (Kondo and Danjo,
2001), and in tomato this delay was
accompanied by reduced polygalacturon-
ase activity (Mignani et al. , 1995). Zhang et
al. (2007a) reported increased total pectins,
cellulose and hemicelluloses when pears
were treated with GA.
GA applications (sometimes even
applications quite early in development)
often lead to a delay in ripening, or at least
some aspects of it (Ju et al. , 1999;
Southwick et al. , 2000; Kappel and
MacDonald, 2002; Jiang et al. , 2004; Zhang
and Whiting, 2011). This delay may be an
indirect effect of these treatments, as GA
application is often associated with an
increase in fruit size (Ju et al. , 1999;
Kappel and MacDonald, 2002; Zhang and
Whiting, 2011), probably through the
ability of GAs to increase fruit sink
strength (Zhang et al. , 2012). Larger fruit
would be expected to require more
photosynthate to achieve the same con-
centrations of storage product (e.g. sugars)
compared with smaller fruit.
A microarray analysis of developing
grape berries indicated that the expression
of putative GA biosynthesis and response
genes was generally low with little
modulation (Grimplet et al. , 2007). The
decrease in levels prior to the initiation of
fruit ripening and their ability to delay
some aspects of ripening when applied
Cytokinins are involved in a range of plant
processes including fruit set (NeSmith,
2002) and early growth (Werner and
Schmülling, 2009). Higher levels are often
found in the fl esh of immature fruits,
which then decrease in maturing and
mature fruits (Desai and Chism, 1978;
Chen, 1983; Zhang et al. , 2003). This
pattern is consistent with a role in early
fruit development (cell division) and
perhaps a role in inhibiting ripening rather
than promoting it.
The application of a range of cytokinins
has been shown to increase fruit size,
especially when applied early in
development (Itai et al. , 1995; Famiani et
al. , 1999; NeSmith, 2002; Peppi and
Fidelibus, 2008). Effects on ripening have
also been reported and, like auxins, which
have a similar pattern of accumulation,
exogenous cytokinins tend to inhibit
ripening. Delays in ripening due to
cytokinin application have been reported
for a number of fruits, both climacteric and
non-climacteric (Itai et al. , 1995; NeSmith,
2002; Peppi and Fidelibus, 2008). The delay
observed in ripening may be related to the
increase in fruit size caused by cytokinin
application. However, as with other growth
regulators, the reported effects of the
application of cytokinins on fruit ripening
are varied and are diffi cult to generalize. For
example, in contrast to many reports, a
synthetic cytokinin applied to kiwifruit
increased fruit size but also increased sugar
levels, decreased acidity and decreased
fl esh fi rmness (Famiani et al. , 1999).
As with the other hormones discussed,
the data regarding cytokinins in fruit is far
from complete. Cytokinins appear to play a
role in fruit set and early fruit develop-
ment, and, although there may be
exceptions, it seems that cytokinins are
likely to inhibit ripening.
12.2.5 Gibberellins (GAs)
GA levels generally seem to be high in the
early stages of fruit development, and this
 
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