Efficacy and mode of action of blossom thinners on ‘Fuji More’ apple trees

The objective of this study was to investigate the mechanism of action of the treatments: Ammonium thiosulfate (ATS), benzyladenine (BA), GA4+7+BA, foliar fertilizer, lime sulphur (LS), LS + mineral oil, mineral oil, NAA, urea, and vegetal oil in the interaction of these treatments with: 1) pollen grain germination, 2) stigma/style pollen tube (PT) development, and 3) the effectiveness of the treatments in field conditions on decreasing crop load and enhancing fruit quality of ‘Fuji More’ apple trees. In experiment 1 it was evaluated the effect of the treatments in inhibiting the in vitro pollen germination of ‘Maxi Gala’ apples. In Experiment 2 it was evaluated in vivo ‘Maxi Gala’ pollen germination in ‘Fuji’ pistils to assess the effect of the treatments applied 24 h before and after the pollination, in three portions of the styles. In experiment 3 it was evaluated the effect of the treatments sprayed at 90% full bloom in a commercial ‘Fuji More’ apple orchard on the variables: fruit set, crop load, yield efficiency, mean fruit weight, cluster size, fruit height and diameter, total soluble solids, flesh firmne, mean seed number, skin russet and red color. ATS reduced in vitro pollen germination and caused the greatest damage to the stigmas/styles, while in the field, it was effective to reduce crop load. Treatments containing LS were the most effective to reduce in vitro pollen germination, but the in vivo essay, mode of action seemed to be related to arresting pollen tube growth, and in the field these treatments caused severe russeting. Mineral and vegetal oil do not have direct effect on pollen germination, but arrest pollen tube growth, and in field conditions mineral oil and ATS reduced crop load. All growth regulators (BA, GA4+7 + BA, and NAA decreased greatly crop load and increased fruit weight and quality.