AVG Reduced Ethylene Production Rates of Pear Flowers and Fruitlets and Increased Fruit Set When Applied One to Two Weeks after Bloom

The European pear (Pyrus communis L.) cultivars ‘d’Anjou’ and ‘Comice’ have low fruit set, particularly during the first ten years from planting. Establishing these cultivars in high-density orchards, therefore, is impractical given insufficient crop loads to control vegetative growth. Over several years, we tested the hypothesis that applications of amino-ethoxy-vinyl-glycine (AVG) would improve fruit set of Comice and ‘d’Anjou’. We had two main objectives: To characterize the ethylene production rate of untreated and AVG-treated flowers and fruitlets during development, and to determine the most efficacious rates and timings of AVG. The pattern of ethylene production rate was consistent in all years, beginning around 0.5 µl ∙ kg^-1 ∙ h^-1 at anthesis, peaking ~14 days after anthesis, and rapidly declining to undetectable levels within a few days. Ethylene production rates of ‘Comice’ were generally higher than ‘d’Anjou’ but peak amplitude varied among years, irrespective of cultivar. AVG practically eliminated ethylene production within hours of application. Respiration rates of flowers and fruitlets, on the other hand, were unaffected by AVG. In two of three years, ethylene production was reduced between 50% and 90% for ~10 days after application; however, in one year the effect persisted for only two days. Generally, maximum effects were observed at rates of 60 ppm AVG. AVG applications at anthesis did not improve fruit set, irrespective of rate (30, 60, 90, 120 ppm) compared to untreated controls. Applications between one and two weeks after anthesis, however, led to a marked increase in fruit set in 70% of trials. Harvested fruits from AVG-treated trees had an equivalent number of seeds as untreated fruits, indicating that the usual poor fruit set of ‘d’Anjou’ and ‘Comice’ was not associated with pollination or fertilization issues. Fruit size was only significantly reduced by AVG when crop loads were excessive. Return bloom was unaffected by AVG, irrespective of rate, timing or cultivar.