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Most pear (Pyrus communis L.) cultivars are impaired to set fruit under self-pollination, because self-fertilization is prevented by a gametophytic self-incompatibility system. However, accumulated information in this species shows that often for a same cultivar, after self-pollination, a variable response in fruit set can be obtained in different years or growing conditions. In this work, we characterize self-incompatibility and self-fruitfulness in ‘Agua de Aranjuez’, the main Spanish pear cultivar, which also shows a variable response to self-pollination. Two years with a different fruit setting response after self-pollination, one with no fruit set and the other with a moderate fruit set, were compared for parthenocarpic fruit development and for pollen tube performance. Results show that in both years, this cultivar behaves as self-incompatible with absence of parthenocarpy. In selfed flowers, most pollen tubes are arrested in the upper half of the style, although in a small proportion of the styles, a pollen tube can reach the base of the style and eventually effect fertilization. Self-fertilization, although occurring at a low level, can explain the fruit set levels obtained under self-pollination given that flowers with just one fertilized ovule are able to set fruit. This behavior could explain confusing results about self-fruitfulness in ‘Agua de Aranjuez’ and other pear cultivars.

Apples and pears are fruit crops particularly susceptible to cropping irregularities. A strong relationship has been observed between the effective pollination period (EPP) and the general cropping of the orchard. The EPP concept has also been proven to be a useful parameter to establish a relationship between the variation in the reproductive process and cropping behaviors. For apples and pears, a slow pollen tube growth has been shown to be the main limiting factor of the EPP in the traditional cooler temperate cultivation regions. However, while higher temperatures speed up the pollen tube growth, the expansion of these crops into warmer areas often results in failures of fruit set. Thus, with the aim to ascertain the main limiting factor responsible for fruit set failures in Mediterranean conditions we have evaluated the EPP for two consecutive years in `Agua de Aranjuez' pear, the main Spanish cultivar, by studying the stigmatic receptivity, pollen tube kinetics, and ovule development. Complete flower fertility was maintained for just 2 days after anthesis in both years. Pollen tube kinetics and ovule degeneration do not appear to limit flower receptivity. However, the stigmatic receptivity expressed as flowers with at least one receptive stigma, closely matches the duration of the EPP evaluated from fruit set experiments. This was consistent over the 2 years of experiments, in spite of the differences recorded in the EPP, suggesting that stigmatic receptivity is clearly the limiting factor of flower receptivity. This is the first report for stigmatic receptivity limiting the EPP in pears and suggests that stigmatic receptivity could be an important factor limiting pear flower receptivity and hence cropping performance under warmer conditions.

Pollen–pistil incompatibility in european pear (Pyrus communis L.) compromises adequate orchard pollination and fruit set and restricts cross-fertility between cultivars suitable as parents in breeding programs. Genetic control is simple, with a single locus expressed gametophytically in pollen controlling the rejection of the pollen tube in the style. Semicompatible pollination arises when only one allele of a pollen parent matches the pistil. Semicompatible test-crosses using partially S-genotyped european pear cultivars allowed the discrimination of 14 S-alleles (S₁ to S₁₄) at the phenotypic level and the assignment of 33 cultivars to 13 incompatibility groups. Partial genomic sequences of the S-RNase gene, spanning between the C1 and C5 conserved regions, were obtained for each new S-allele identified (S₆ to S₁₄). These sequences and those reported previously for the S₁ to S₅ RNases allowed a set of consensus primers amplifying all 14 S-RNase alleles to be designed. Allele-specific PCR allowed discrimination between those S-RNases giving amplification products of similar size with consensus primers. These two approaches provided a method for the molecular identification of all 14 S-alleles in european pear. With this methodology, we demonstrate that the S-RNase genotypes inferred from PCR exactly matches the S-phenotypes deduced from test-crosses. Comparison of the sequences obtained with those of S-RNases already published allowed us to relate S-alleles between studies. This will allow the prediction of cross-incompatibility among an even larger number of european pear cultivars.