CHEMICAL THINNERS REDUCE SWEET CHERRY NET CO₂ EXCHANGE, STOMATAL CONDUCTANCE AND CHLOROPHYLL FLUORESCENCE

Previous research in our lab has shown great potential for reducing crop load and improving fruit quality in productive sweet cherry (Prunus avium L.) orchard systems by chemically thinning blossoms. Although the mode of action of chemical thinning agents is unclear, one likely mechanism is the reduction in net carbon balance and supply of growth resources to developing fruit. In 2005, we applied 4% vegetable oil emulsion (VOE), 2% ammonium thiosulfate (ATS), 2% fish oil + 2.5% lime sulfur (FOLS) and 1% tergitol by air-blast sprayer to 12-year-old ‘Bing’/‘Gisela^® 5’ (Gi 5) sweet cherry trees at ~20% and 80% full bloom. In addition, FOLS was applied to ‘Bing’/Gi 5 trees 14 days after full bloom. Leaf gas exchange, stomatal conductance, and chlorophyll fluorescence were evaluated at two-day intervals for ~8 days. FOLS, tergitol, VOE and ATS suppressed leaf net CO₂ exchange rate (NCER) by 33%, 29%, 28% and 18%, respectively, over a 17-day period following application. The post-bloom thinner reduced NCER by 19% over the course of a week. These results suggest that the mechanism for reducing NCER is affected by thinning agent and timing of application.