The work on Arabidopsis here may help our understanding of how Passiflora species manage to maintain their individual characteristics in the wild, with the stigma only allowing species-specific bonding. In captivity hybridization is easy if the species are closely related enough..so why does this seem to occur so rarely despite closely related species growing wild in the same areas?
Dr. John MacDougal comments, ''As do
all plants, Passiflora vines have the ability to distinguish their own
pollen from pollen of other individual plants of the same species and from
other species. This poorly understood ability to recognize 'self' from
'non self' prevents hybridization. In a sort of plant 'immune system'*,
'recognition' proteins interact between the stigma and style of the
receiving plant and the pollen and pollen tubes of the pollen-donor plant.
Additionally, recent studies have shown the importance of the very first
step in pollination, the first embrace. Pollen grains must first attach
and bond to the female receptive surface, the stigma."
Jen Sheen, Molecular Biology Department at Massachusetts General Hospital
(MGH) comments that ''plants have an effective and sophisticated immune
system'' See February 28 2002 issue of Nature.
adhesion hints at a small part of the story of how many species manage to
remain individual & distinct, responding only to their own pollen. At the
same time however many Passiflora species do not self pollinate, so the
stigma cells may be sensitised to respond preferentially (an immune
response?) to pollen from different clones of their own species. Perhaps
there is a lack of response to their own pollen. Now that we are beginning
to understand the mechanism, perhaps we may one day be able to induce
selfing in non-selfing rare species e.g. by treating the flower and its
own pollen in such a way as to go from initiating the binding process
right through to the sperm within the pollen grain making its way down the
tube and fertilizing eggs within the ovary.
pollen the flowers respond to is also complicated by the movements of the
styles & stigma from the time the flower first opens to when it shuts,
moving in & then out of range of the pollinators as they collect the
pollen from the anthers. The stigma may also only be responsive at certain
times. Perhaps when they are returning the the up position at the end of
the day it is too late for successful pollination.