The Press Release below from The University of Chicago Hospital gives a remarkable insight into the nature of the the bonding between the pollen and the stigma, the plant studied being Arabidopsis. It is very probable that the same bonding occurs in Passiflora.

  November 9, 1999

A collaborative effort between plant biologists and physicists at the University of Chicago has measured the binding force between pollen grains and their female counterparts, the stigma cells. The unexpected strength and specificity of pollen binding could form the basis of an entire family of superglues with wide ranging applications in medicine and technology.

"You could conceivably design adhesion systems where you want two things to stick to each other but not to themselves," says David Grier, Ph.D., associate professor of physics in the James Franck Institute at the University of Chicago, and one of the authors of the paper to appear in the November 9 issue of Development. "And because pollen-stigma binding is so species specific, you could make as many different such glues as there are plants."

"The force with which pollen grains bind to the female parts of the plant is strong, very species-specific, and happens very fast," says Greg Zinkl, Ph.D. a post doctorate student in the lab of Daphne Preuss, Ph.D., assistant professor of molecular genetics & cell biology at the University of Chicago.

When a pollen grain of the right species lands on the pistil and comes into contact with a female stigma cell, a tight binding between the outer surface of the pollen grain-the exine-and the stigma cell occurs. After binding, the stigma cell causes the pollen grain to absorb water, enabling it to send a pollen tube down through the female tissue to the flower's ovary. Sperm carried inside the pollen grain makes its way down the tube and fertilizes eggs within the ovary.

Until now, scientists knew very little about how the right pollen stuck to the right stigma while pollen from other species falls off.

To test the force of adhesion between pollen and stigma cells, Zinkl and Preuss teamed up with Greir and undergraduate student Ben Zwiebel created a novel tool to measure the strength of the pollen-stigma bond designed using the same principles as an atomic force microscope.

The researchers attached a pollen grain to the end of a fine glass fibre that acted like a simple spring. A stigma mounted on a retractable stage was brought into contact with the pollen grain. After binding occurred, the stage was retracted until the adhesion between the grain and the stigma was broken. The distance the stage travelled is proportional to the strength of the interaction.

The researchers found that the adhesion between pollen and stigma of the same species was unexpectedly strong, perhaps helping pollen borne by high winds to tightly bind to flowers of their species. "The force was strong enough that you could use it to suspend a 100 kg object from an area the size of a dinner plate coated with this adhesive," says Preuss.

Zinkl and Preuss also showed that adhesion between pollen and stigma does not rely on the outer pollen coating, but instead on the exine, a stable polymer made up of fatty acids and phenolics. Using pollen grains lacking a pollen coating due to genetic mutation, Preuss and Zinkl showed that these grains retained the ability to bind to same-species stigmas.

"This indicates that the binding factors for the pollen grain reside within the exine itself, or even the cell wall of the pollen grain, which was once thought to be inactive in the binding process," says Preuss.

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