Balance between pollination and parthenocarpy in the pear (Pyrus communis) variety Conference in Belgium

Pear (Pyrus communis) is the first fruit growing in Belgium and the variety Conference represents 90% of the pear production. Our research project aims to better highlight the Conference pear tree reproduction on three complementary aspects: 1) S-alleles identification for the main Belgian varieties; 2) pollination by insects; 3) parthenocarpy induced by phytohormones. As a self-incompatible species, pear tree requires inter-variety cross-pollination to develop fertilized fruits. However, blooming occurs early in the spring and climatically unfavorable conditions (frost, low temperatures, rain, etc) could prevent the pollination by insects. In this case, parthenocarpy induction by spraying phytohormones, mainly gibberellins, is a common practice in Conference orchards. However, data are still needed to determine the most efficient phytohormones, the moment of their application and the required amounts to apply. In addition to investigate the attractiveness of Conference trees for insects and insect efficiency, our research focuses on parthenocarpy processes. Field trials were realized during 3 years to compare fruits resulting from 1) spontaneous intrinsic parthenocarpy with emasculated flowers, 2) induced intrinsic parthenocarpy with flowers pollinated with intra-variety incompatible pollen, 3) induced extrinsic parthenocarpy with phytohormones (gibberellins, cytokinins or auxins) at different doses and different time of application during the flowering period, 4) hand compatible inter-variety pollination, 5) open pollination by insects. We observed that spontaneous intrinsic parthenocarpy due to endogenous phytohormones occurred in Conference. However, pollen deposit (even if incompatible) increased the number of fruits initiated by 10 to 40% depending on the climatic conditions. Trees treated with gibberellins and/or cytokinins produced fruits up to 10 percent larger than trees treated with auxins. According to the year, parthenocarpic fruits remained, nevertheless, up to 12 percent smaller than fertilized ones. Our results also suggest that endogenous gibberellins do not act alone to induce parthenocarpy since gibberellin inhibitor treatments did not completely prevent parthenocarpic fruit initiation.