The pear tree (Pyrus spp.) is a temperate-climate fruit species, and its cultivation in subtropical regions was made possible by hybrid cultivars obtained from the cross Pyrus communis × Pyrus pyrifolia (Curi et al., 2017). This cross combines the quality of European pear trees (P. communis) with the low number of chill hours required by the Asian pear tree (P. pyrifolia) (Chagas et al., 2008).
Low fruit set is one of the limiting factors in the expansion of pear tree crops in subtropical regions (Bettiol Neto et al., 2014). The fact that no pollinizer cultivars have been reported for these hybrids in subtropical regions might be the reason for their low effective fruiting.
Most pear tree cultivars have gametophytic self-incompatibility, which causes a plant to reject its own pollen. Therefore, pear trees depend on cross-pollination for fruit production (Franklin-Tong and Franklin, 2003). In general, the use of two to three pear cultivars with a coincident flowering period is recommended for commercial crops. Thus, identifying compatibility between cultivars is of economic importance because partial compatibility may reduce yield (Goldway et al., 1999; Tatari et al., 2017).
Several methods can be applied to the study of the compatibility/incompatibility between cultivars and the determination of suitable pollinizers (Ortega and Dicenta, 2004). Field-controlled pollination allows the performance of several cultivars in the orchard to be estimated and is suggested for determining appropriate pollinizer groups (Mahmoudi et al., 2007). However, the use of this method alone in the determination of the incompatibility between cultivars may be misleading due to environmental and physiological effects that hinder differentiation between fully compatible and semicompatible crosses. Therefore, the use of another method to increase the reliability of the evaluation is crucial; such a method includes the analysis of genes encoding for S-RNase to identify the S-loci in cultivars, which can define the compatibility level between cultivars, complementing and corroborating the results observed in the field (Jacquemart, 2007).
Self-incompatibility based on S-RNase occurs in Rosaceae species. Compatibility is controlled by the polymorphic S-locus, which encodes at least two genes. S-RNases determine the specificity of pollen rejection in the pistil, and the F-locus and F-box proteins fulfill this function in pollen. S-RNase is believed to function as a specific S cytotoxin and as a recognition protein. Thus, the incompatibility is a consequence of the cytotoxic activity of S-RNase. Conversely, S-RNase cytotoxicity does not occur in compatible pollen tubes (McClure et al., 2011).
Pear breeding programs are carried out in cold regions and seek to develop cultivars adapted to these climatic conditions, with high-yield performance, which fit in the fresh fruit chain. However, pear tree exploitation in subtropical (Bettiol Neto et al., 2014) and tropical regions (Oliveira et al., 2015) has increased. In this sense, breeding programs for cultivars adapted to regions with high temperatures must be intensified.
This study aimed to characterize S-alleles in hybrid pear cultivars (P. communis × P. pyrifolia) adapted to subtropical climates and evaluate the effective fruiting from the cross between these cultivars.
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