Abscisic Acid Reduces Photosynthesis But Does Not Consistently Thin 'Bartlett' Pears in Oregon

Flower and/or fruitlet thinning is required to achieve commercially-acceptable fruit size of ‘Bartlett’ pear (Pyrus communis) in Oregon.  Suboptimal temperatures during sensitive stages of fruit development, however, often limit the efficacy of commercially-available thinning agents.  As a consequence, hand-thinning is required.  Efficacy of the plant hormone, abscisic acid (ABA) was evaluated to thin ‘Bartlett’ pears between 2012 and 2014.  ABA reduced fruit set with increasing rate (0 to 500 ppm) in three of five field trials, despite producing similar reductions in net photosynthesis (Pn). Depending on ABA rate, Pn was reduced 75% to 90% within 1 day of ABA application, but returned to ~80% of control levels within 4 to 6 days and fully recovered by 10 to 14 days.  When effective, ABA thinned well at ~100 ppm. Rates exceeding 400 ppm were phytotoxic.  ABA did not negatively affect fruit growth, fruit weight, or quality at harvest. Return bloom was positively related to ABA rate.  In general, pear trees likely have ample carbohydrate reserves to ameliorate short-term limitations to Pn, such as those induced by ABA.  The best thinning occurred in combination with low, daily solar radiation (~30% of clear sky conditions) within the first several days from application. To assess the additive effects of shade and ABA on fruit set and gas exchange, a separate, two-way factorial design comprising two levels of ABA (0 and 125 ppm) and three levels of whole-canopy shade (0%, 44% and 77%) was administered.  ‘Bartlett’ trees were shaded immediately following ABA application for a period of 15 days.  ABA reduced Pn by 10-fold (i.e. from 10 to ~0 µmol∙m^-2∙s^-1) within several hours of application, relative to controls.  Pn recovered to ~75% to 80% of control levels by day 3 as similarly observed in field trials.   Fruit set of ABA-treated trees was reduced by ~35%, compared to untreated controls, although this effect may have been influenced by non-forecasted, cloudy conditions during day 3 to 5 after application. Moderate shade (44%) produced only a minor reduction of Pn (i.e., ~90% of control) and similar fruit set compared to controls.  Increasing shade to 77%, however, had a markedly stronger effect on thinning and Pn. An interaction between shade and ABA was not significant for fruit set.