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Ido Schechter, J.T.A. Proctor, and D.C. Elfving

Mature apple trees (Malus domestica Borkh.) were studied in 1989 and 1990 to explore the effect of crop load on fruit dry weight (DW), dry-matter concentration (DMC), specific leaf area (SLA), and leaf C exchange, using girdled (G) and nongirdled (NG) limbs. Fruit DW and DMC decreased with heavier fruit loads. Fruit on G limbs had higher fruit DW and DMC than on NG limbs. SLA on NG limbs was unaffected by crop load, but increased dramatically on G limbs with a crop load of less than one fruit per square centimeter limb cross-sectional area. These leaves also had a low photosynthetic rate, high stomatal resistance, and high internal CO2 concentration. The results do not support the concept of feedback inhibition of photosynthesis and demonstrate specific circumstances in which the capacity of sinks to use assimilates was saturated. Exceeding this limit by significantly reducing sink strength resulted in excessive carbohydrate accumulation in leaves. Nutrient levels in leaves on G, nonfruiting limbs were generally lower than for the other treatments.

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Paul T. Wismer, J.T.A. Proctor, and D.C. Elfving

Benzyladenine (BA), carbaryl (CB), daminozide (DM), and naphthaleneacetic acid (NAA) were applied postbloom as fruitlet thinning agents to mature `Empire' apple (Malus domestica Borkh.) trees. BA, NAA, and CB reduced fruit set and yield per tree, and increased fruit size, percent dry weight, soluble solidscontent and return bloom. Fruit size was reduced, return bloom, length: diameter ratio and flesh firmness were increased, and fruit set and yield unaltered by DM. Although fruit set and yield were similar for BA, NAA, and CB, BA treated fruit were larger, indicating that BA increased fruit size beyond the effect attributable to chemical thinning alone. BA increased the rate of cell layer formation in the fruit cortex, indicating that BA stimulated cortical cell division. NAA, CB and DM had no effect on cell division rate. Mean cortical cell diameter at harvest was increased by NAA and CB and reduced by DM. Cell diameter at harvest in BA-treated fruit was similar to the control. These data support the hypothesis that BA-induced fruit size increase in `Empire' apple results from greater numbers of cells in the fruit cortex, whereas the fruit size increase due to NAA or CB is a consequence of larger cell size. Chemical names used: N-(phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)]; 1-napthaleneacetic acid (NM); 1-naphthalenyl methylcarbamate [carbaryl (CB)]; butanedioic acid mono (2,2dimethyl hydrazide) [daminozide (DM)].

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D.C. Elfving, E.C. Lougheed, C.L. Chu, and R.A. Cline

Foliar daminozide (DZ) applications to `McIntosh' apple trees (Malus domestics Borkh.) increased fruit color, reduced preharvest drop, resulted in greater firmness at harvest and after air storage, delayed starch hydrolysis, and reduced fruit ethylene production at harvest and after storage. Foliar paclobutrazol (PBZ) reduced preharvest drop and flesh firmness loss if applied within 5 weeks after full bloom (WAFB). Later applications had no effect. PBZ did not influence the progress of starch hydrolysis or ethylene production at harvest but reduced poststorage ethylene production in one season. Stem-cavity browning and brown core were increased by PBZ applied at 5 and 9 WAFB in 1987. In 1988, fruit soluble solids content (SSC) was reduced by a double application of PBZ and by uniconazole (UCZ). UCZ had little effect on `McIntosh' fruit other than the reduction in SSC. PBZ applications were less consistent in their effects than DZ. Chemical names used: butanedioic acid mono(2,2-dimethylhydrazide) (daminozide); ß-[(4-chlorophenyl) methyl]-α- -(l,l-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol); ß-[(4-chlorophenyl)methylene]-α-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (uniconazole).

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D.C. Elfving, I. Schechter, R.A. Cline, and W.F. Pierce

Mature `Macspur McIntosh'/MM.106 trees trained to the CL tree form were converted to the PL tree form in 1987 by removal of east- and west-oriented upper scaffold limbs. Control trees were pruned to maintain the CL form. Dormant pruning in later years maintained either tree form. No summer pruning was used in this study. Canopy light levels along horizontal transects at one m above the soil and vertical transects, both through the center of the canopy, were unaffected by tree form or transect direction. Yields were significantly lower for PL trees in 1987 and 1989, while yield efficiency was reduced in PL trees in all 3 years. Fruit size, trunk cross-sectional area, and foliar macro-nutrient content were unaffected by tree form during this study. Fruit color development in both the upper and lower halves of the canopy was uninfluenced by tree form.

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Tory Schmidt, Don C. Elfving, James R. McFerson, and Matthew D. Whiting

Potential strategies against biennial bearing in apple [Malus × sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] include promotion of return bloom with an “on”-year application of ethephon or inhibition of return bloom with an “off”-year application of gibberellic acid (GA), but the influence of initial crop load on the efficacy of these bioregulators is poorly understood. In 2004 and 2005, six total trials were initiated in which whole trees were manually adjusted shortly before anthesis to one of three levels of crop load (100%, 50%, 0%) in ‘Cameo’, ‘Honeycrisp’, and ‘Fuji’; GA4 + 7 was overlaid on trees of each crop level in four trials and ethephon in two. In all trials, initial crop load was the primary determinant of return bloom; proportional influence on flower density, fruit density, and yield was generally most pronounced at the 50% crop level. GA4 + 7 consistently reduced floral initiation, whereas ethephon promoted it. Flowering responses from a historically alternating ‘Cameo’ trial site showed greater sensitivity to ethephon and less sensitivity to GA4 + 7 than did responses from parallel trials established in an annually bearing ‘Cameo’ block, suggesting a predilection of nascent buds to a specific fate before the influence of exogenous bioregulators or gibberellins from seeds produced in developing fruit. Light crop loads and GA4 + 7 applications generally promoted shoot extension, whereas heavy crops and ethephon had the opposite effect.

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D.C. Elfving, I. Schechter, R.A. Cline, and W.F. Pierce

Mature `Macspur McIntosh'/MM.106 apple trees (Malus domestica Borkh.) trained to the central-leader (CL) tree form were converted to the palmette-leader (PL) tree form in 1987 by removal of east- and west-oriented scaffolds in the upper canopy. Control trees were pruned to maintain the CL form. Dormant pruning in subsequent seasons maintained either tree form. No summer pruning was done. Canopy light levels along horizontal transects 1 m above the soil and vertical transects, both through the center of the canopy, were unaffected by tree form or transect direction. Yields were significantly lower for PL trees in 1987 and 1989, while yield efficiency was reduced in PL trees in all 3 years. Fruit size, trunk cross-sectional area growth, and foliar macronutrient content were unaffected by tree form. Fruit color development in both the upper and lower halves of the canopy was not influenced by tree form during the study.

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Torrance R. Schmidt, Don C. Elfving, James R. McFerson, and Matthew D. Whiting

Gibberellins inhibit flowering in apple (Malus domestica) and show promise as tools to promote annual bearing. The authors validated the efficacy of gibberellic acid (GA) to reduce return bloom dramatically in two biennial cultivars. ‘Honeycrisp’ fruit treated in 2004 with GA4+7 at 0, 200, 400, or 600 mg·L−1 demonstrated advanced maturity in terms of starch levels, flesh firmness, and titratable acidity, whereas ‘Cameo’ fruit showed variable treatment effects. In 2005, 0, 300, 600, 900, or 1200 mg·L−1 GA4+7 was applied to ‘Cameo’, and fruit maturity was once again unaffected. Two commercial GA products (GA4, GA4+7) were applied in 2005 to ‘Honeycrisp’ at 400 mg·L−1. Both formulations caused fruit to have less flesh firmness and acidity, and increased levels of starch conversion compared with the untreated control at harvest and after 140 d of common storage. All GA treatments in all four trials profoundly diminished flowering in the season after treatment. Results demonstrate differences in sensitivity to GA between the two cultivars.

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S.R. Drake, T.A. Eisele, M.A. Drake, D.C. Elfving, S.L. Drake, and D.B. Visser

This study was conducted over three crop seasons using 'Delicious' (Scarletspur strain) apple trees on MM.111 rootstock. The bioregulators aminoethoxyvinylglycine (AVG) and ethephon (ETH) were applied alone or in combinations at various time intervals before harvest. Fruit response to bioregulators was evaluated at harvest and after storage. AVG applied 4 weeks before first harvest retarded starch loss at harvest, retained greater firmness, and reduced internal ethylene concentration and watercore of fruit at harvest and after both regular and controlled atmosphere storage. AVG did not influence peel color (hue values), but the flesh color of treated apples was more green. AVG in all instances tended to reduce the sensory scores for apples and apple juice. In contrast, ETH enhanced starch hydrolysis, flesh color development (green to more yellow), and soluble solids concentration while reducing titratable acidity levels. ETH had no influence on fruit firmness at harvest, but reduced firmness levels after storage in an inverse relationship to the concentration applied. Sensory values for whole apples were not influenced by ETH treatment, but ETH improved sensory preference for apple juice, particularly at early harvest. Applying AVG before ETH enhanced soluble solids and sensory scores for both fruit and juice. Treating with AVG followed by ETH at 150 mg·L–1 permitted the maintenance of satisfactory firmness values (>53.4 N) after long-term storage along with better quality and sensory perceptions. Using specific combinations of both AVG and ETH permitted ETH-mediated improvements in objective and perceived fruit quality to be obtained without the losses in flesh firmness and storability due to uncontrolled ethylene evolution and ripening typically observed when ETH is applied alone preharvest.

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S.R. Drake, T.A. Eisele, D.C. Elfving, M.A. Drake, S.L. Drake, and D.B. Visser

In a study conducted over three crop seasons, Ethrel (ETH) increased the Brix, sucrose, and sorbitol content of 'Scarletspur Delicious' apple juice while reducing the fructose content. Both longer preharvest exposure to, and higher concentrations of, ETH had a stronger influence than application closer to harvest and/or at lesser amounts. Time of ETH application tended to influence individual carbohydrates more so than amount of ETH applied. ETH reduced total acidity and also reduced apple juice individual acid (quinic and malic) contents with longer preharvest exposure or higher concentrations. Aminoethoxyvinylglycine [AVG (ReTain)] reduced both Brix and sucrose content of 'Scarletspur Delicious' apple juice, but had no influence on either total acidity or individual acid contents. Combinations of AVG with ETH tended to counteract the influence of either used alone on total Brix, carbohydrates, total acidity and individual acids. Mineral content of 'Scarletspur Delicious' apple juice was not strongly influenced by application of either ETH or AVG.

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S.R. Drake, D.C. Elfving, M.A. Drake, T.A. Eisele, S.L. Drake, and D.B. Visser

This study was conducted over two crop seasons using `Scarletspur Delicious' and `Gale Gala' apple trees (Malus ×domestica). The bioregulators aminoethoxyvinylglycine (AVG), ethephon (ETH), and 1-methylcyclopropene (MCP) were applied at various times before or after harvest. Fruit response was evaluated at harvest and after regular atmosphere (RA) and controlled atmosphere (CA) storage [2.0% oxygen (O2) and <2.0% carbon dioxide (CO2) at 0 °C] and quality of whole and juice apple products evaluated. AVG reduced starch loss and ethylene production, enhanced firmness, and reduced cracking in `Gale Gala,' but reduced sensory acceptance of apples and apple juice. ETH intensified starch loss, ethylene production, and reduced firmness, but did not affect `Gale Gala' fruit cracking. AVG followed by ETH reduced starch loss, ethylene production, and cracking and maintained firmness. This combination also aided in sensory acceptance of apples but reduced sensory preference of apple juice. Exposure to postharvest MCP improved flesh firmness retention and reduced ethylene production after both RA and CA storage. MCP either favored or reduced sensory acceptance of whole apples, depending on the particular season, but reduced sensory preference of apple juice. Sensory scores for `Scarletspur Delicious' apples were more strongly modified by bioregulators than were `Gale Gala' apples.