Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

58 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
way that ethylene cannot bind, but without inducing the conformational change as ethylene
would or (2) breeding plants that express a mutant receptor that, like etr1-1, does not bind
ethylene. Both of these means would cause at least part of the receptors to stay “active” in
the presence of ethylene, thereby blocking the signal transduction events leading to gene
expression. Unless the action in some way can be restricted to specific tissues, the result of
either method would be that all reactions to ethylene will be blocked. As discussed below,
this can have some unwanted effects.
4.4.1 Blocking ethylene perception by chemicals
Increased carbon dioxide and decreased oxygen concentrations are well-known antagonists
of ethylene action, and various packaging and storage methods (modified atmosphere,
controlled atmosphere) for fresh horticultural products partly rely on this principle. The
biochemical background of these effects, however, is still largely unknown. Ethylene physiologists
used CO 2 for years as an inhibitor of ethylene action in testing the ethylene
involvement in processes, which they examined and, till today, CO 2 is widely regarded
as a compound that interferes with ethylene sensitivity. The original hypothesis of Burg
and Burg (1967) that CO 2 and ethylene may compete for the same binding site, however,
has found not to be true. In ethylene binding experiments, it was shown that CO 2 could
not displace ethylene from the receptor, and by using CO 2 in combination with 1-MCP (a
proved inhibitor of ethylene perception as discussed below), it was clearly shown that CO 2
mainly acts independent of the ethylene receptor (de Wild et al., 2005). The most likely
site of action of CO 2 in inhibiting ethylene effects is at the activity of ACC synthase. An
increased ACC-synthase activity and, hence, an increased ethylene production is often a
major effect of ethylene treatment; the lack of increased ethylene production under elevated
CO 2 levels may seem to suggest that ethylene perception is altered. A similar reasoning
may hold true for decreased O 2 levels. Despite the general idea that oxygen is required
for ethylene perception, the observed decreased responses to ethylene under low-oxygen
conditions may be related to the effect of low oxygen on processes other than the binding
of ethylene to the receptor. For instance, low oxygen is known to suppress ACO activity,
thereby blocking the autocatalytic ethylene production. Although the effect of ethylene
certainly is less pronounced under elevated CO 2 or decreased O 2 levels, this effect may be
on processes beyond the actual perception of ethylene.
Lowering the temperature also dramatically reduces ethylene effects. For carnation
flowers it was calculated that for each 10 degrees drop in temperature, ethylene response
decreases over 10 times (Q 10 = 11.3), due to two different processes. At lower temperatures,
the time required for a given response (irreversible wilting) increased with a Q 10 of 2.7. In
addition to this, affinity of binding sites to ethylene was shown to decrease with a Q 10 of
4.2 (Woltering et al., 1994). This shows that at low temperatures, not only the response to
ethylene, but also the perception of ethylene at the receptor level are suppressed. Together
with a decreased ethylene production at lower temperatures (Q 10 = 2.7), this explains
the dramatic effect of temperature on ethylene effects and suggests that once stored at
low temperature, other ways to block ethylene effects may not be necessary. Although
controlled atmosphere storage is a common practice for many types of fruit, it is currently
not a common practice for storage of flowers or potted plants. Reid and Serek (1999)
recommend for carnations and roses an elevation of CO 2 to the level of 3% for inhibition