It appears that tulip belongs to a small number of abscission
systems that do not respond to ethylene or ethylene
antagonists. This implies that, like fruit ripening and senescence,
there is a type of non-ethylene-dependent abscission
process. As tulip flowers do not develop elevated ethylene
production rates as they senesce, it is perhaps not surprising
that their abscission zones have not evolved the machinery
to respond to it. Preliminary experiments indicate that petal
fall in some dicotyledons such as Saxifraga species may also
be insensitive to the gas (W. G. van Doorn, unpublished
data), so the phenomenon might be more widespread. Just
how abscission is controlled in tulip is a matter of speculation,
but auxin and perhaps abscisic acid are also thought to
be involved in the primary regulation of abscission and so
would be strong candidates for future study (Sexton 1994,
1997, van Doorn and Stead 1997).