What a drag – fading flowers
‘There is no Plan B’ we hear from committed
political, military or business leaders. However, more cautious strategists,
recognizing that even the best-laid plans may go wrong*, will formulate an
alternative. This happens in the plant world too, where natural selection has
sometimes led to development of a ‘back-up’ strategy. This is well illustrated
by the work of Qu et al. (Beijing
and Haifa, pp. 1155-1164) on pollination in Incarvillea
sinensis, an annual member of the family Bignoniaceae. In out-breeding mode, I.
sinensis is pollinated by bees, but as the
authors have shown, if out-breeding fails, there is a ‘Plan B’,
self-pollination, which depends on corolla abscission. Corolla
senescence/abscission is not triggered by fertilization, as occurs in many species,
but in these short-lived flowers occurs late in anthesis
whether or not fertilization has occurred. Field observations indicated that
abscission is wind assisted and that the movement of the corolla drags with it
the anthers (which are inserted at the base of the corolla). As they pass the
stigma, they brush against the stigmatic lobes (if the stigma is still
receptive) causing pollen to be deposited on the stigmatic surface. Thus, a
stigma that had not received non-self pollen via bees would now receive instead
self-pollen. How important is this for the reproductive success of the plant?
Firstly, as shown by the authors, self-fertilization is as effective as
non-self in leading to seed set: the species is completely self-fertile.
Secondly, as would be expected, growth of plants in an insect-proof cage did
not reduce seed set because of the effectiveness of selfing.
Thirdly, however, if the insect-proof cages were also wind proof, corolla
abscission was reduced very markedly while seed set was only approx. 40 % of
the value for plants that had undergone wind-assisted corolla abscission. This
is indeed a fascinating and previously undescribed
mechanism of delayed self-pollination, ensuring reproductive success when
pollinators are scarce.
* or, as the
Scottish poet, Robert Burns (1749-1796) put it: The best laid schemes o’
mice an’ men, Gang aft a-gley, An’ lea’e us nought but grief an’
pain, For promised joy.
