explain Pollen pistil interaction
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Answer:
Pollen-Pistil Interaction:
Pollen grains are deposited on the stigma either due to closeness of the anthers to the stigma or by pollinating agents (biotic or abiotic). This unique feature brings about pollen-pistil interaction between the male gametophyte, the pollen grains, with the massive sporophytic tissue.
A successful pollination brings about sequential events in the pollen-pistil interaction that ultimately ends up by the discharge of the male gametes in the embryo sac (Fig 6.1).

All the events beginning from pollination to the release of gametes in the embryo sac form a part of the pollen-pistil interaction or the programic phase.
Pollen Attachment and Hydration:
The attachment of the pollen on the stigma depends upon its wall sculpture and stickiness. In wet stigma adhesion is mostly a mechanical process, whereas, in dry stigma it depends on the extent and composition of the pellicle, and the amount of surface-coat substances on the pollen.
Pollen hydration proceeds in a controlled manner characterized by distinct area of stability of increasing water content and can begin in the anther before pollen release. Its rapidity is dependent to a great extent on the nature of stigma, for instance in a dry stigma hydration is gradual and controlled by the water potential of the stigma and pollen.
This controlled hydration provides suitable conditions for the recovery of the membrane integrity of the vegetative cell.
A plausible pathway for hydration in dry stigma as proposed by J.Heslop-Harrison (1979) is given below:

In a stigma with aqueous exudates hydration is very rapid. For instance in Petunia the stigma is covered with a lipoidal exudates and a thin layer of water which establishes a moisture gradient through the lipoidal exudates. The pollen grain thus gradually gets hydrated.
Ultrastructural and physiological studies of pollen hydration in Brassica show two distinct phases of hydration. During the initial phase, putative signals are reciprocally exchanged between pollen and stigma. The second phase proceeds with an invagination of the intine in the colpial zone and formation of a ‘foot’ of pollen coating that contact the stigma papilla.
Freeze-etch preparation show microchannels at the papilla-pollen boundary through which water moves from stigma to pollen grain but not between grains. The area around the site of pollen tube emergence is rich in pectins, and one of the earliest visible alterations of macromolecules upon hydration is a loss of protein and pectic material from the length of the colpial slit.
Pollen Germination and Tube Growth:
The stigmatic surface provides the essential prerequisites for a successful germination that are absent in the pollen. In wet stigma, the role of the stigmatic exudates in pollen germination is highly variable.
In Amaryllis and Crinum, stigmatic exudates are essential for pollen germination, however, in Nicotiana and Petunia the exudates play no significant role during germination, since young stigmas free from exudates support satisfactory germination of pollen grains.
Answer:
Pollen grains are deposited on the stigma either due to closeness of the anthers to the stigma or by pollinating agents (biotic or abiotic). This unique feature brings about pollen-pistil interaction between the male gametophyte, the pollen grains, with the massive sporophytic tissue.
A successful pollination brings about sequential events in the pollen-pistil interaction that ultimately ends up by the discharge of the male gametes in the embryo sac (Fig 6.1).
Stepwise events in pollen-pistil interaction
All the events beginning from pollination to the release of gametes in the embryo sac form a part of the pollen-pistil interaction or the programic phase.
Pollen Attachment and Hydration:
The attachment of the pollen on the stigma depends upon its wall sculpture and stickiness. In wet stigma adhesion is mostly a mechanical process, whereas, in dry stigma it depends on the extent and composition of the pellicle, and the amount of surface-coat substances on the pollen.
Pollen hydration proceeds in a controlled manner characterized by distinct area of stability of increasing water content and can begin in the anther before pollen release. Its rapidity is dependent to a great extent on the nature of stigma, for instance in a dry stigma hydration is gradual and controlled by the water potential of the stigma and pollen.