Brevis thinning efficacy at different fruit size and fluorescence on 'Gala' and 'Fuji' apples

Brevis thinning efficacy depends on climatic and cultivar conditions. The objective of this work was to evaluate the efficacy of one application of Brevis in 'Gala' and 'Fuji' apple applied at different fruit sizes (fruit king diameter ranging between 6.5 and 21.5 mm) and to determine which fruit diameters were most sensitive to Brevis application. Trials were conducted over two seasons from 2015 to 2016 in apple orchards in Lleida (Spain). Photosynthesis inhibition caused by Brevis was also analysed and measured, using chlorophyll fluorescence and biexponential pharmacokinetic models. In 2016, for all Brevis treatments and an untreated control, quantum yield (Qy) was measured in all leaves in different shoots, with photosynthesis inhibition and its evolution analysed in three sections (closest to branch, mid-shoot and vegetative section). Under the trial conditions, Brevis thinning effect was observed at king fruit diameters from 9 to 19 mm, with maximum efficacy observed in the 11.5-14 mm range. However, susceptibility to Brevis differed between varieties and years. The fluorescence analysis using a biexponential equation showed adequate fits and the calculated values correlated well with the measured Qy(%) values. The area under curve per day analysis showed that, at the same application dose, fluorescence inhibition decreased with increasing fruit diameter. The fluorescence analysis of shoot sections four days after Brevis application showed differences between varieties, with the inhibition caused by Brevis higher in 'Gala' than in 'Fuji'. However, this analysis showed no significant differences in 'Gala', with all sections showing similar inhibition (27%-35%). By contrast, 'Fuji' showed different inhibition values in the different sections. The vegetative section showed the significantly highest inhibition, and the zone nearest the branch the lowest.