Genetic diversity, reproductive compatibility and dormancy breaking in the pear tree production on subtropical region

Abstract

The pear tree (Pyrus spp.) is a fruit tree of temperate climate and its cultivation is possible in subtropical regions due to hybrid cultivars, obtained by crossing European (Pyrus communis) and Asian (Pyrus pyrifolia) species. However, obtaining adapted cultivars with high-yield performance is a breeding programs objective, making more information about the crop necessary. The diversity characterization among hybrid pear cultivars based on agronomic and morphological parameters, the selection of cultivars with less need for cold, combined with the use of sprouting inducers and compatibility evaluations between genotypes, are necessary to obtain cultivars with high productive performance in mild winter regions. Thus, in the first experiment, the objective was to evaluate the genetic diversity and phylogeny among pear cultivars cultivated in subtropical conditions, through the morphological characterization of 66 descriptors (43 qualitative and 23 quantitative multicategorical variables). The second experiment aimed to evaluate reproductive compatibility, pollen tube development, stigmatic receptivity and pollen grain viability of pear cultivars in subtropical regions. And, in the third experiment, the objective was to select doses of hydrogenated cyanamide (Dormex®) in breaking dormancy and bud sprouting in pear cultivars, under subtropical conditions, by brushing branches of 5 cultivars with Dormex® at 0, 1, 2, 3, 4, and 5% dosages. The multivariate techniques for the genetic diversity study showed great potential for use in the pear germplasm characterization, and the UPGMA grouping indicated the existence of genetic variability among cultivars, suggesting more dissimilar groups with potential to be used in future hybridizations. The evaluated cultivars showed stigmatic receptivity during all reproductive development stages and high pollen viability, especially the cultivar 'Shinseiki'. Cultivar ‘Seleta’ was the most adapted to mild winter conditions, with the cross ‘Seleta’ x ‘Shinseiki’ showing the highest fruiting percentage. After germination, the pollen tubes showed growth in the style, with a percentage gradual reduction until 72h, when most pollen tubes already showed growth towards the ovary. The highest percentages of pistils with pollen tube reaching the ovary were observed in the 'Triunfo' x 'Shinseiki' and 'Seleta' x 'Shinseiki' combinations, reinforcing the aptitude of the 'Shinseiki' cultivar as a pollen grain donor and the 'Seleta' cultivar ' as a recipient of pollen grains. European pear trees 'William's' and 'Packham's Triumph' had low sprouting rates compared to hybrid pears, which may be due to the greater requirement for cold hours, however, the higher doses of Dormex® (4 and 5%) promoted greater bud sprouting in a shorter period of time. Thus, the use of qualitative and quantitative variables allowed the efficient characterization of pear cultivars, aiming at sufficient variability for selection and use in future breeding programs. For the commercial exploitation of pear trees in a subtropical climate, the application of hydrogenated cyanamide and the recommendation of efficient pollinating cultivars are essential.