A mechanism based on pollen growth-promoting substances has previously been proposed to explain the effects of mentor (killed compatible) pollen when it improves fertility of crosses between nearly incompatible parents. Similarly, pioneer pollination (two pollinations a day apart) stimulates growth of the last applied pollen and increases fruit-set. In the present experiments the effects of foreign-pollen pioneer treatments on fruit-set of apple, sweet cherry, and apricot were compared with the control, killed compatible pollen. All treatments were followed 24 h later with viable compatible pollen. The foreign pollen, Balsamorrhiza sagittata, did not inhibit fruit-set. When the foreign pollen was heat killed, fruit-set was at least as high as when untreated B. sagittata was used. Two other foreign pollens, Taraxacum officinale and Juniperus communis, resulted in lower fruit-set than B. sagittata but still more than the control. This effect was consistent with a previously hypothesized relationship for mentor pollen, relating size and degree of stimulation. The two nonpollen treatments examined, dried yeast and pectinase solution, slightly inhibited fruit-set. Mineral analysis of a composite of pollen and anther tissue indicated that boron and calcium were not deficient in the control pollen, and incompatibility mechanisms and heat-labile metabolites also were unlikely to have been the cause. No direct evidence was obtained, but pollen growth promotion by a diffusible growth regulator (such as gibberellin) is suggested as plausible and an area worthy of further study.
