Flowering in the wild olive (Olea europaea L.) tree (oleaster): Phenology, flower abnormalities and fruit set traits for breeding the olive

Abstract

Although, the olive trees produce hermaphrodite flowers, abnormal flowers (flowers with absence or reduced stamens and flowers with absence of pistil) are frequently observed and may reduce fruit set.
This study investigates the phenology evolution and the male and female abortion of the oleaster tree (or the wild olive tree) flowers (Olea europaea L. subsp. europaea var. sylvestris) from natural ecosystems represented in two Tunisian Parks. The female abortion was evaluated by the percentage of flowers lacking pistil, and male abortion by differential staining for the cytoplasm of pollen grain. Flower abortion was examined and compared for eight oleaster trees in two different natural sites. At the beginning of flowering (the last two weeks of April), the flower numbers ranged from 15 to 26 flowers per inflorescence. However, one month after blossoming, the fruit set ranged from 1 to 3 drupes
per inflorescence. Thus, a significant decrease of flowers per inflorescence was observed on all trees. The percentage of flowers without stamens and flowers without pistil per inflorescence ranged from 5.7 to 38.8% and from 4.9 to 88.1%, respectively depending on the oleaster tree. Moreover, abnormal flowers had effect on fruits number per inflorescence. Indeed, the r² linear regression values were 0.89
and 0.83, respectively. Therefore, due to the similar flower abnormalities occurring for the olive trees, the transfer of a low rate of abnormal flowers to cultivated olive may improve the crop.

Keywords: Olea europaea, oleaster, wild olive trees, stamen abortion, flower without pistil, pollen grain, natural site, drupe abscission