concentrations of 0.5 to 1.0 L/L that are necessary to cause accelerated abscission if added exogenously
[72]. Because AZs are very tiny, it is difficult to extract enough gas to make these measurements. A strong
correlation exists between ethylene production rates and internal concentrations. Using this relationship,
it was estimated that ethylene production rates of 3 to 5 L of ethylene per kilogram per hour were nec-
essary to trigger abscission [83], and these were subsequently shown to be exceeded in many abscising
systems [72,82]. There have been some direct measurements of the gas concentration in AZs which
showed levels above the threshold [83,84]. Raspberry fruit are unusually well suited to these measure-
ments, having 70 to 100 abscission zones enclosed within the fruit. Ethylene levels around these zones
showed that concentrations were less than the threshold level of 0.5 L/L in green fruit but exceeded it
in ripening, abscising fruit [85] (Figure 7).
A clever alternative approach was adopted by Jackson et al. [86]. They measured the rate of ethylene
production in senescent bean leaves just prior to abscission and then applied (2-chlorethyl)phosphonic
acid (CEPA) to younger petioles to generate similar amounts. This treatment caused abscission.
Reducing the levels of internal ethylene in AZs has also been used to establish ethylene’s role. Early
experiments employed potassium permanganate or mercuric perchlorate to absorb the gas, and there are
several reports of delayed abscission as a result [87]. A more effective approach has been to use hypo-
baric or low pressures, which increase diffusive loss [72]. Aminoethoxyvinyl glycine (AVG), an inhibitor
of ethylene biosynthesis, has been shown to slow natural abscission [89–91]. Transgenic tomato plants
have been produced that synthesize very little ethylene, but unfortunately, their abscission behavior was
not recorded [92].
Inhibitors of ethylene action such as silver ions, which inhibit ethylene responses such as fruit ripen-
ing and floral senescence, are also very effective at preventing abscission [93–96]. The mechanism of the
Ageffect is not understood, although interaction with the ethylene receptor is assumed. Norbornadiene
214 SEXTONFigure 7 Correlation between the internal ethylene concentrations in raspberry fruit and the onset of abscis-
sion. The concentration of ethylene (in L/L) (pale columns) around the abscission zones within fruit at vari-
ous stages of ripening are shown. When green fruit progress to the mottled and ripe stages, the concentrations
exceed the 0.25 L/L threshold necessary to induce abscission experimentally in green fruit. The fruit removal
force (Newtons) required to break the abscission zones is also plotted (dark columns). Note that it starts to de-
cline in fruit that are mottled or riper, where ethylene levels exceed 0.25 L/L. (From Ref. 85.)