Q.What is meant by pollen-pistil interaction? Name the tissue responsible for recognising a compatible pollen grain.

Compatible pollen germinates and grows a tube; incompatible pollen is rejected.

Pollen-pistil interaction refers to the whole sequence of molecular and cellular events that begins the moment a pollen grain is deposited on a stigma, through which the pistil determines whether that pollen is compatible -- i.e. of the correct species and genetically acceptable -- or incompatible.

This recognition is carried out by the tissue of the stigma, specifically through proteins present on its receptive surface interacting with proteins exposed on the pollen exine. If the pollen is recognised as compatible, the stigma supports it: the pollen absorbs water and nutrients, germinates, and puts out a pollen tube that begins to grow down through the style toward the ovary. If the pollen is recognised as incompatible, the pistil actively prevents further progress, either by blocking germination outright or by halting pollen-tube growth partway through the style.

State what is being recognised (compatible vs incompatible pollen) and by which tissue (the stigma surface), then describe the two possible outcomes.

Assuming the whole pistil equally performs recognition -- the critical recognition step happens specifically at the stigma surface, at the very first point of pollen contact.