Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

54 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
flowers, pollination often triggers an increase in ethylene production and subsequent rapid
senescence and it has been suggested that ethylene may have evolved as a mechanism to
terminate flower life after successful pollination as a way to benefit survival of the species
(van Doorn, 2001). In ephemeral and short-lived flower species, such a mechanism apparently
is not beneficial as the life of individual flowers is very short. Similarly, in Compositae
species, with numerous flowers in one flower head, continuous visits of pollinators are required
to fertilize all individual flowers and the senescence of pollinated flowers would not
be beneficial.
Although ethylene sensitivity is roughly fixed at the plant family level, still marked
differences may exist between species and cultivars within one family. Several carnation
cultivars (e.g., Chinera, Epomeo, and Ginevra) derived from crosses involving a long-life
noncommercial breeding line have been described with reduced ethylene sensitivity compared
to the cultivar White Sim. The vase life of these cultivars was negatively correlated
with their ethylene sensitivity (van Doorn et al., 1993). Basic properties of ethylene receptors
(number and affinity) were thought to be similar in cultivars Chinera and White
Sim (Woltering et al., 1993). The reduced response of cultivar Chinera to ethylene was
thought to be regulated at a point beyond the receptor. In other carnation cultivars (Sandra,
Sandrosa), the prolonged vase life was related to a decreased activity of the ethylene biosynthetic
pathway and not to reduced ethylene sensitivity (Mayak and Tirosh, 1993). Also, other
carnation cultivars with either a long vase life or decreased ethylene sensitivity have also
been described (Brandt and Woodson, 1992). Recently, another cultivar with a long vase
life (White Candle) was described (Nukui et al., 2004). This cultivar showed decreased
expression of ACC-synthase genes and repressed ethylene production in the gynoecium but
showed a normal response to exogenous ethylene. In another species within the Caryophyllaceae,
Dianthus barbatus, different genetic lines with greatly reduced ethylene sensitivity
were described (Friedman et al., 2001). Also in roses, marked differences in ethylene sensitivity
exist between varieties. In some cultivars, petals show marked growth variations
in response to ethylene (Reid et al., 1989), whereas in other cultivars this effect is absent.
In addition, in miniature potted roses, a range of ethylene sensitivities was found from
almost insensitive to highly sensitive (Muller et al., 1998). The above examples clearly
underline that the distinction between ethylene sensitive and insensitive senescence needs
to be handled with care. On the one hand, ethylene-insensitive flowers may show effects of
ethylene in processes other than petal senescence or abscission, while on the other hand,
considerable cultivar variation may exist so that cultivars of a given species may range from
insensitive to very sensitive. Controlling ethylene effects through interference with ethylene
perception may therefore be very beneficial in a variety of cases, both in ethylene-sensitive
and ethylene-insensitive flowers.
As said earlier, ethylene plays a crucial role in the senescence of “ethylene-sensitive”
flowers, coordinating senescence pathways and regulating floral abscission. Compatible pollination
triggers a series of postpollination events, including ovary growth, floral collapse
(wilting, abscission), and petal color change that are regulated by tissue-specific production
and sensitivity of ethylene (O’Neill, 1997). Antiethylene treatments are common place in
postharvest chains owing to the well-known damage and decay that free ethylene promotes
in horticultural produce. The biosynthetic pathway for ethylene has been fully elucidated in
higher plants, and plants with mutations that affect the perception or signal transduction of
ethylene (namely, Arabidopsis and tomato) have been used to define the ethylene signaling