THE ROLE OF PIONEER POLLEN IN COMPATIBLE AND INCOMPATIBLE POLLINATIONS OF APPLE AND PEAR

When pollinating twice with compatible pollen at an 1–2 day interval, it appeared that the first, so-called 'pioneer pollen' markedly stimulated the 'second pollen' in apple. To overcome (self) incompatibility, the pioneer pollen technique - compatible pollen first, incompatible second - was compared with the mentor pollen method in which both pollen types are applied simultaneously as a mixture. Inviable, methylated, mentor/pioneer pollen sometimes promoted selfing, sometimes it had no effect. Viable, irradiated, mentor or pioneer pollen promoted tube growth of self pollen in pear and stimulated fruit set in both apple and pear, though not seed set ('stimulative parthenocarpy'). The lack of seed formation is probably partly due to abortion of the selfed embryos and partly the result of competition by the irradiated pollen which enters and so appears to block the ovules. This competition is more severe in the mentor than in the pioneer pollen situation. As more fruits reach maturity, it is likely that untreated rather than irradiated mentor or pioneer pollen would be more effective in producing selfed seeds. The surfeit of self pollen in orchards with a minimum of pollinizers is not necessarely a disadvantage, it may both directly and indirectly contribute to fruit and seed set.