Sweet Cherry Fruit Firmness and Postharvest Quality of Late-maturing Cultivars Are Improved with Low-rate, Single Applications of Gibberellic Acid

Sweet cherry (Prunus avium L.) producers in the Pacific Northwest have devoted considerable acreage to late-maturing cultivars. By using these cultivars to extend the harvest window, producers avoid lower returns associated with cherries harvested during the peak period (i.e., midseason) when supplies are overly abundant. Over several years, we evaluated preharvest applications of gibberellic acid (GA(3)) between 10 and 100 ppm (a.i.) on the late-maturing sweet cherry cultivars Lapins, Skeena, Staccato, and Sweetheart. Individual trials examined the timing of GA(3) applications and/or rate on fruit quality attributes at harvest and after 4 weeks of cold storage at 0 degrees C. The influence of GA(3) timing and/or rate on sweet cherry skin color and harvest delay was also evaluated. Multiple applications split between the end of Stage II (pit hardening) and mid-Stage III (final fruit swell) of fruit development did not improve fruit quality attributes or delay skin color development of 'Skeena' and 'Sweetheart' compared with equivalent concentrations applied once at the end of Stage II. Low concentrations (between 10 and 25 ppm) consistently improved fruit firmness (FF) of all cultivars by 10% to 43%. No further improvements in FF were observed when rates exceeded 25 ppm. Skin color development was retarded by GA(3) but did not respond in a consistent manner to increasing rate. Fruit size was not uniformly increased by GA(3). In trials where GA(3) had a positive effect on fruit size, the effect was observed at low concentrations and was not further improved with increasing rate. A cultivar-dependent response to GA(3) was observed for return bloom. 'Skeena' reproductive buds per fruiting spur and flowers per floral bud in years after treatment were unaffected by GA(3) concentration. On the contrary, the number of flowers per bud of 'Lapins' was significantly reduced to 79% and 38% of control levels for 50 and 100 ppm GA(3), respectively. At 100 ppm, GA(3) additionally limited the number of reproductive buds returning on fruiting spurs of 'Lapins'. GA(3) reduced stem browning and surface pitting disorder of 'Sweetheart' and 'Lapins' after 4 weeks of cold storage at 0 degrees C; however, these effects were optimized at 25 ppm. Respiration rate and weight loss were unaffected by GA(3) at harvest or after 2 and 4 weeks of cold storage. Unidentified endogenous factors that regulate FF and are inducible by GA(3) appear to be largely responsible for improved resistance to pitting. Collectively, the results demonstrate high sensitivity of cherry FF and skin color to GA(3). Split applications did not provide further harvest delays or affect any of the attributes evaluated, possibly because low rates (20 ppm) applied at the first timing were sufficient to saturate the response. In general, fruit quality of late-maturing cultivars of sweet cherry was improved by low rates of GA(3) applied in a single application at the end of pit hardening.