Improving Fruit Set in ‘Regina' Sweet Cherry with Aminoethoxyvinylglycine (ReTain) Is Facilitated by Delayed Stigmatic Senescence

Yields of sweet cherries, particularly of ‘Regina’, are very variable from season to season. Poor set is often due to unfavorable weather conditions for pollinator activity. Efforts have been made to extend the receptive period of the stigma by external control of ethylene evolution, and thus to improve pollination efficiency, and to ultimately increase fruit set. In this study, laboratory and field trials were set up to investigate the efficacy of 250 or 500 ppm aminoethoxyvinylglycine (AVG, ReTain®) applications on flowering phenology, floral biology and fruit set of ‘Regina’ sweet cherry. Both 250 and 500 ppm AVG treatments delayed whole flower and stigmatic senescence evidenced by a delay in the occurrence of flowering phenological stages of both detached and attached flowers, and by scanning electron microscopy imaging of the stigmatic surface. AVG treatments did not affect floral biological characteristics (intrafloral nectar production, nectar refraction, pollen production, and pollen viability) of cherry flowers, and therefore did not influence the attractiveness of flowers to insect pollinators. Ethylene evolution of detached cherry flowers and fruit drop were significantly decreased. While fruit set was significantly increased, significant differences in fruit quality (fruit size, color, soluble solids concentration) were not observed between control and AVG treatments. As it is proven in this study, increasing floral longevity by external application of aminoethoxyvinylglycine, a plant growth regulator that inhibits ethylene biosynthesis, can be a powerful tool to improve pollination efficiency and fruit set of sweet cherries, and possibly other crops, under unfavorable weather conditions.