Pollen and pollination experiments. VIII. The effect of successive pollinations with compatible and self-incompatible pollen in apple and pear

Summary

In compatible pollen combinations, viable but infertile irradiated pollen C_r appears to compete much more in a mixture (the mentor pollen situation: C+C_r), than when used one day in advance (the pioneer pollen situation: C_r/C). Seed set of C_r/C was usually also greatly superior to that of C/C_r and, using, self, incompatible pollen S instead of C_r, S/C was nearly always much better than C/S. This is in accordance with the promotion of the second by the first (pioneer) pollen. Hence, when C_r or S are applied second in C/C_r or C/S, more ovules are rendered ineffective (blocked or aborted) than in the reverse combination when C_r or S act as pioneer pollen. These observations are consistent with the trend that the combinations C/ C+S, C+S, C/S tended to produce more selfed seeds in that order. Although mentor or pioneer pollen assist in removing the style barrier for self pollen, embryo abortions appears to be a second obstruction towards seed formation. Indications are that high spring temperature during pollination promote the production of selfed seeds in the presence of compatible pollen. The surfeit of self pollen in orchards with a minority of pollinators is not necessarily an unfavourable factor, it may both directly and indirectly contribute to fruit and seed set.