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 (GA3) 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 GA3 applications and/or rate on fruit quality attributes at harvest and after 4 weeks of cold storage at 0 °C. The influence of GA3 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 GA3 but did not respond in a consistent manner to increasing rate. Fruit size was not uniformly increased by GA3. In trials where GA3 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 GA3 was observed for return bloom. ‘Skeena’ reproductive buds per fruiting spur and flowers per floral bud in years after treatment were unaffected by GA3 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 GA3, respectively. At 100 ppm, GA3 additionally limited the number of reproductive buds returning on fruiting spurs of ‘Lapins’. GA3 reduced stem browning and surface pitting disorder of ‘Sweetheart’ and ‘Lapins’ after 4 weeks of cold storage at 0 °C; however, these effects were optimized at 25 ppm. Respiration rate and weight loss were unaffected by GA3 at harvest or after 2 and 4 weeks of cold storage. Unidentified endogenous factors that regulate FF and are inducible by GA3 appear to be largely responsible for improved resistance to pitting. Collectively, the results demonstrate high sensitivity of cherry FF and skin color to GA3. 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 GA3 applied in a single application at the end of pit hardening.
Todd C. Einhorn, Yan Wang, and Janet Turner
Todd C. Einhorn, Janet Turner, and Debra Laraway
Reflective fabric was installed before bloom in 2009 and 2010 in alleyways of a mature, low-density ‘Anjou’ pear orchard (269 trees/ha). Four treatments were applied to study intracanopy light environments on fruit growth rate and size, cropload, yield, and fruit quality: 1) no fabric (NF); 2) partial-season fabric applied before full bloom (FB) and removed 75 days after full bloom (dafb) (PSF); 3) full-season fabric applied before FB and removed at harvest (FSF); and 4) shadecloth (60%) applied 60 dafb through harvest (SC). PSF and FSF improved yield by 12% and 18%, respectively, over the two-year period relative to NF. The high yields of fabric treatments were attributed to fruit number in the lower (less than 2.4 m) interior, mid-, and exterior zones of the canopy. Photosynthetic active radiation (PAR) was increased by fabric 28%, 95%, and 30% in the lower exterior, mid-, and interior canopy, respectively. Photosynthesis:light response curves indicated improved carbon assimilation of pear leaves developing in the elevated PAR environment of the lower canopy. Fruit growth rate and final size were unaffected by fabric treatments. FSF fruit size was similar to NF despite higher fruit density. Compared with NF, FSF had a small, non-significant effect on fruit maturity (increased softening) at harvest. Yield and fruit size of SC fruit were significantly reduced. The number of fruit in SC trees did not differ from NF in 2009, but the effect of shade reduced fruit number in 2010. Fabric did not affect fruit quality attributes after three and six months of regular atmosphere cold storage. Pears from SC trees did not attain ripening capacity after three months of cold storage and a 7-day ripening period and had lower sugar content compared with other treatments. The cumulative yield advantages associated with FSF support its use in mature pear orchards.
Todd C. Einhorn, Debra Laraway, and Janet Turner
The effect of crop load level on vegetative growth, fruit growth, yield, fruit quality, surface pitting, crop value and return bloom was studied over a 2-year period on 9- and 10-year-old ‘Sweetheart’/‘Mazzard’ sweet cherry (Prunus avium) trees. In early spring, whole-tree crop loads were adjusted to two different levels by removal of reproductive buds (either 50% or all but one) from spurs and compared with an unthinned control. In 2009, heavy crop loads of unthinned trees reduced fruit size by 30 days after full bloom (DAFB). At harvest, fruit diameter of thinned treatments was increased 22% and 27% compared with unthinned fruit. Fruit quality attributes [soluble solids concentration (SS), fruit firmness, and total acids (TA)] were significantly greater for thinned treatments. Thinned treatment yields were reduced 40% to 54% relative to unthinned trees, with greater percentages of fruit in large size classes. Despite significantly fewer fruit per tree, moderately thinned trees had a higher estimated crop value ($142 per tree) than unthinned trees ($125 per tree). Crop value was lowest for the heavily thinned treatment ($107 per tree), reflecting overthinning. In 2010, shoot growth was negatively related to crop load level. Fruit growth of unthinned trees was not significantly affected by higher fruit density until 89 DAFB. Yield of 2010 unthinned trees was 87% of 2009, while thinned tree yields were similar between years. Improved fruit quality and greater percentages of large fruit were observed for thinned treatments in 2010; however, crop value was highest for unthinned trees ($190 per tree), even though 18% of the fruit were too small for fresh market sale. Surface pitting was unaffected by crop load level in either year. Return bloom (flowers per reproductive bud and reproductive buds per spur) was significantly, negatively related to the prior season's crop load in 2010 and 2011. In the current sweet cherry pricing structure, higher crop value is associated with large volumes of medium-sized fruit. Thinning to manage crop load of low-medium density, productive ‘Sweetheart’/‘Mazzard’ trees will not be an annual requirement, though in heavy fruit set years crop load management will improve crop value.
Matthew Arrington, Mateus S. Pasa, and Todd C. Einhorn
Postbloom thinning of ‘Bartlett’ pears (Pyrus communis L.) is required to produce fruit of commercially acceptable size. In the Pacific Northwestern United States, low temperatures during early stages of pear fruitlet development often limit the efficacy of commercial thinning compounds. Hand thinning, therefore, remains the standard crop load management practice. Chemical thinning protocols are necessary to reduce the cost and dependence on hand labor. The plant hormone abscisic acid (ABA) was evaluated over multiple years in several ‘Bartlett’ pear orchards. ABA was applied to whole canopies at variable rates (50–500 ppm) when fruit diameter was generally between 10 and 12 mm. In three of four trials, ABA thinned in a dose-dependent manner. The relative degree of thinning for a given dose, however, was inconsistent among trials. Trees treated with ABA had a higher proportion of blank and single-fruited spurs than the control. Net photosynthesis (P n) of single leaves was reduced 75% to 90% within one day of ABA application but gradually returned to ≈80% of control levels within 7 days and fully recovered by ≈14 days. Slightly greater and longer lasting P n inhibition occurred with increasing ABA dose. Fruit weight and return bloom generally increased with increasing ABA rate. Fruit quality, when measured, was unaffected by ABA treatments. Inconsistent thinning response with ABA may be attributed to environmental factors, biological factors, or both.
Mokhles A. Elsysy, Andrew Hubbard, and Todd C. Einhorn
Metamitron is a relatively new postbloom thinning compound for pome fruits that inhibits the photosystem II (PS II) pathway of photosynthesis. Reduced assimilation of carbohydrate by metamitron action may lead to a carbohydrate deficit that promotes fruit abscission. We have evaluated the thinning efficacy of metamitron rate and, to a limited extent, application timing for ‘Bartlett’ pear in five separate trials over 3 years (2015–17) in northern Oregon. Comparisons were made to a nontreated control and, depending on the trial, a commercial standard thinning compound [benzyladenine (6-BA) or abscisic acid (ABA)]. Application timings ranged between 7- and 16-mm fruit diameter depending on the trial. Metamitron markedly inhibited photosynthesis (PN), typically for a duration of 2 to 3 weeks, although longer persistence was observed in two trials. Generally, PN was reduced linearly with increasing metamitron rate, but the effect varied by rate and year and may have been enhanced by high temperatures. Metamitron effectively thinned in four of five trials whereby pear fruit set was negatively and linearly related to rate. Rates of 200 to 300 ppm were efficacious and produced target crop loads. In only one trial, increasing concentrations (600 ppm) led to greater thinning. Metamitron significantly thinned ‘Bartlett’ pears when applied between 10 and 13 mm. In contrast, early application timing (≈7 mm) had little effect on fruit abscission. Fruit size increased with decreasing crop load and thus was significantly improved by metamitron. Interaction between biological and environmental factors likely contributed to year-to-year variability in efficacy.
Todd C. Einhorn, Horst W. Caspari, and Steve Green
Approach-grafted 1-year-old `Gala'/M7 apple trees were grown with both tops for the remainder of the 2003 season in a greenhouse. Trees were supplied with >100% (control, PRD100) or 50% (PRD50, DI50) of daily ETc either applied to one root compartment only (PRD100, PRD50) or divided evenly across both root compartments (control and DI50). ETc was estimated from gravimetric measurements, and irrigation was switched between wet and dry root compartments several times throughout the experiment. Soil moisture was measured both gravimetrically (tripod) and volumetrically (time-domain reflectometry). Predawn leaf water potential (υpd) and single leaf gas exchange (photosynthesis, stomatal conductance, and transpiration) were recorded daily, and sap flow in stems and roots was monitored continuously using the heat-pulse technique. Leaves were collected for abscisic acid (ABA) determination following gas exchange measurements. Regardless of irrigation placement (i.e., PRD or DI), both 50% ETc treatments experienced similar declines in υpd and single leaf gas exchange relative to control levels. In addition, ABA concentrations were similar for PRD50 and DI50, and were significantly higher than the control and PRD100 treatments. PRD100 trees had similar υpd as control trees; however, gas exchange was reduced >25% compared to the control. Bulk leaf ABA concentration did not differ significantly from control levels and does not by itself explain the down regulation of stomata with PRD100.
Todd C. Einhorn, Horst W. Caspari, Steve Green, and Greg Litus
One-year-old `Gala'/M7 apple trees were potted into 30-L containers and approach-grafted about 45 cm above the graft union in late Spring 2003. Trees were grown with both tops for the remainder of the 2003 season in a greenhouse. In Apr. 2004, one of the tops was removed. Trees were fully watered by an overhead irrigation system until July 2004, when trees were subjected to one of four irrigation regimes: control received >100% of ETc applied evenly to the two pots; PRD100 received >100% ETc applied to one pot only; and two regimes received 50% ETc applied to either one (PRD50) or both pots (DI50). Both gravimetric (tripod) and volumetric (time-domain reflectometry) soil moisture measurements were taken daily prior to and after irrigations. In addition, heavy isotope H2O (18O) was applied to one of the two root compartments and analyzed in the leaves to further determine the validity of the model. Sap flow was monitored in six split-rooted trees using miniaturized heat-pulse probes inserted into the stem above the graft union and into each of the two root systems below the graft union. Under fully irrigated conditions, root sap flow was proportional to root trunk cross-sectional area, and was not a function of root system origin (i.e., roots of mother plant with original top remaining or roots of daughter plant with original top detached). Water uptake from a previously dried root zone was rapid when the irrigated side was switched, but much more gradual when the other side was maintained wet. Interactions between soil moisture and sap flow in relation to factors governing canopy demand will be presented.
Todd C. Einhorn, Mateus S. Pasa, and Janet Turner
Prohexadione-calcium (P-Ca) was applied to ‘Anjou’ pear (Pyrus communis L.) trees in the lower and upper Hood River Valley (HRV), Oregon, to determine its effectiveness for managing the excessive vigor of ‘Anjou’ under different growing climates. Vegetative growth and development (weekly shoot growth rate, total annual extension growth, number of initiated shoots, internodal length, and number of nodes), yield (fruit number and fruit size), and return bloom dynamics were evaluated between 2010 and 2013. P-Ca consistently reduced shoot elongation by ≈40% in all years and at both sites when doses of 250 ppm were applied in early spring (i.e., ≈5 cm of annual shoot extension) compared with untreated trees. Shorter shoots were the result of both reduced internodal growth and fewer nodes. In the cooler, upper HRV, a single P-Ca application controlled shoot elongation for the entire season, but in the warmer, lower HRV, a second flush of growth was generally observed ≈60 days after the first application. A subsequent P-Ca application (250 ppm) provided added growth control in some instances. Yield was unaffected by P-Ca the season of application; however, in one year, an increase in fruit number indirectly led to reduced fruit size; otherwise, fruit size was unaffected by P-Ca. Postharvest fruit quality was not influenced substantially by P-Ca. Return bloom, however, was consistently reduced by P-Ca. Return yield, the year after P-Ca application (recorded in 2013 only), was reduced in proportion to the decrease in return bloom relative to untreated trees. In 2012, ethephon was also evaluated, alone or in combination with P-Ca. When applied on its own either once (150 ppm, 5-cm growth), or twice [150 ppm, 5-cm growth; 300 ppm, 57 days after full bloom (DAFB)], ethephon did not affect vegetative growth or yield components but did improve return bloom and return yield relative to other treatments; however, when combined with P-Ca, ethephon did not reverse reductions in return bloom or return yield induced by P-Ca. The most effective ethephon treatment for promoting flowering and return yield (300 ppm, 57 DAFB) was not tested in combination with P-Ca. We conclude that P-Ca is an effective tool for controlling vigor of ‘Anjou’ trees, but the decrease in return bloom requires additional investigation. Further work testing combinations of ethephon and P-Ca are warranted to optimize growth and productivity of ‘Anjou’ trees.
Todd C. Einhorn, Cecil Stushnoff, Ann E. McSay, Phil L. Forsline, Sam Cox, Joel R.L. Ehrenkranz, and Loretta Sandoval
Phlorizin is known for its role in reducing glucotoxicity and has a long history of use in diabetes research. In addition, its contribution to the pool of total phenolics adds to the overall health benefits attributed to fruit. Phlorizin is limited to Rosaceae family plants, of which apple comprises its current commercial source; however, limited information exists regarding its biodiversity among apple taxa. A subset of 22 taxa from a core collection of apple accessions representative of the global genetic diversity of apple was used to investigate the biodiversity of phlorizin present in apple shoots and in fruit relative to total phenolic content and free radical scavenging capacity. Fruit and shoots were harvested from the USDA Plant Genetic Resources Unit in Geneva, N.Y. Validation and quantification of phlorizin was conducted using a rigorous high-pressure liquid chromatography (HPLC) procedure. Total phenolics in fruit, assayed using a Folin-Ciocalteu method and expressed as gallic acid equivalents, ranged from 227 to 7181 mg·L-1
and were strongly related to 2,2' azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) antioxidant capacity for the core collection (r= 0.778). On a molar basis, phlorizin had lower antioxidant capacity than other major phenolic compounds present in apple fruit, but was more effective than ascorbic acid. Phlorizin yield in dormant apple shoots, expressed as percent weight, ranged from 0.9% to 5.5%. A rapid, 96 well micro-plate spectrophotometric assay was also developed to aid in the screening of multiple samples for selection of high phlorizin yielding apple taxa. Spectrophotometry overestimated phlorizin content as expected, but the calibration curve between HPLC and spectrophotometry was acceptable, r 2 = 0.88.