Self-incompatibility, parthenocarpy and reduction of seed presence in ‘Afourer’ mandarin

Seedlessness is one of the main requirements for the Citrus fresh market. ‘Afourer’ mandarin has been registered as self-incompatible, producing seedless fruit in the absence of cross-pollination. Under field conditions, a high number of seeded fruits is produced, which reduces its quality and commercial value. The purpose of this work was to characterize self-incompatibility type of ‘Afourer’ mandarin, to determine its parthenocarpic capacity, and to reduce the presence of seeds under field conditions. Four experiments were performed, including net-covered trees during flowering period (experiments 1 and 2), single flower emasculation and self-pollination (experiment 3), in vivo pollen germination, pollen tube development and ovule viability (experiment 1), and field application of copper sulfate (CuSO4) and gibberellic acid (GA3) during flowering period (experiment 4); open pollination was the control in all situations. Under open pollination, relatively low seedless fruit percentage was found (7% and 34%), whereas with anti-bee nets this percentage reached 98–99% of the fruits. Relatively high fruit set percentage and fruit number per tree was found in net-covered trees and in bagged or emasculated flowers, indicating that ‘Afourer’ mandarin presents facultative parthenocarpy. Emasculated or self-pollinated flowers resulted in similar seedless fruit number, indicating an autonomous parthenocarpy. In flowers from open or net-covered trees, similar pollen tube growth was registered until 6 days after anthesis, but, thereafter, growth was interrupted in self-pollination condition, reaching only 40% of the style length, which suggests a gametophytic self-incompatibility system. GA3 applied during flowering period reduced the percentage of seeded fruits and seed number per fruit; the most efficient treatment was three applications of GA3 (50 mg l−1) combined with CuSO4 (25 mg l−1), that increased seedless fruit from 19% to 31% and reduced from 3.7 to 2.3 the number of seeds per fruit.