Chemical Thinning Agents Research Articles

Evaluating the sustainable cultivation of 'Fuji' apples: suitable crop load and the impact of chemical thinning agents on fruit quality and transcription

Evaluating the sustainable cultivation of 'Fuji' apples: suitable crop load and the impact of chemical thinning agents on fruit quality and transcription

EFFECT OF FLOWER THINNING ON YIELD AND QUALITY OF ‘`STANLEY`’ PLUM (PRUNUS DOMESTICA L.)

Plum is the most important fruit crop for cultivation in Serbia. However, a small amount of plum fruit is sold as fresh fruit due to its poor quality. In order to improve fruit quality and obtain regular and high yield chemical blossom thinning agents were applied. The experiment was conducted on seven-year-old plum trees of cultivar `Stanley` which were planted on distance 5x5 m. Ammonium thiosulphate and ethephon were applied in the next treatments: 1) ammonium thiosulphate 1.5% (ATS); 2) ethephon 0.015% (E); 3) ammonium thiosulphate 1.5% + ethephon 0.015% (ATS + E); 4) hand thinning (HT); 5) untreated control treatment (UTC). Chemical thinning treatments were performed once during the phase of full bloom, while hand thinning treatment was performed after the fall of unfertilized fruitlets at the end of May. Parameters analyzed were yield, fruit size, fruit weight, pit weight, fruit firmness, fruit shape index, soluble solids content, total acid content and the amount of harvested fruits per time unit. The obtained results have shown that ATS + E, ATS and HT treatments significantly reduced the number of fruits on the trees compared to the control treatment. However, yield per tree was reduced significantly only in the ATS + E treatment. Other treatments compensated for the smaller number of fruits per tree with a significantly larger fruit size. Since the fruit size was larger on treated trees, the amount of harvested fruits per time unit was significant compared to control treatment. There were no significant differences among the applied treatments in terms of fruit firmness, soluble solids and total acid content.

Crop Load Management in Nashi Pear—A Review

Most nashi cultivars require heavy thinning, and this has traditionally been performed by the time and labour-intensive practice of hand thinning. Crop load management is a key cost driver for nashi production, but there are limited cost-effective options available for nashi growers compared to other pome fruit, especially apples and, to a lesser extent, European pears. There is, however, potential to adapt some of the thinning tools and techniques used in apples and European pears to reduce the labour requirements and high cost of thinning in nashi, thus improving industry profitability. Several chemical thinning agents have potential for nashi, and an understanding of the optimal application rates, times and weather conditions for each chemical, as well as the conditions/factors that impact the tree carbon balance, will improve the predictability of chemical thinning. However, it is difficult to target specific flowers/fruitlets within a cluster with chemicals, and the flowers that produce the preferred fruit shape and size are in the middle of the flower cluster. Mechanical thinning during the flowering period with either Darwin or BAUM-style string thinners has potential, particularly as these devices can be used as early as flower emergence. As for chemical thinning, the issue of non-selectivity needs to be addressed; however, the development of mechatronic systems should overcome most problems that occur with the currently available mechanical thinners. Shading at critical times is an avenue that could be explored further to ascertain the critical stage when developing fruit are susceptible to enable the determination of the optimal timing and duration of shading. Targeted pruning and bud thinning during the dormant winter period to reduce the floral bud numbers is a valuable option for the precise placement of fruit in optimal positions and to set up the required number of clusters. This review highlighted several tools/techniques that, with further work, can be incorporated into a systematic approach to crop load management in nashi while reducing the risk and cost.

Efficacy of an Eco-Friendly Bloom Thinning Formulation on Mango Trees and Its Olfactory Effect on an Insect Pollinator, Apis mellifera

In various orchard fruit trees, thinning of blossoms and fruits is important to increase fruit size and quality and to promote a new bloom in the following season. Several chemical thinning agents are currently commercially available, but they are inconsistent and produce side effects in crop plants and insect pollinators. Because of environmental concerns, developing alternative eco-friendly bloom thinning agents is necessary. We developed an eco-friendly bloom thinning formulation (BTF) using minerals and extracts of various medicinal plants. Our BTF spray (0.1%, w/v) decreased the number of fruits per tree (46.5%) and fruit yield per tree (81.5%) but increased the fruit weight (196.8%) compared with the control treatment; the spray induced a small number of larger mango fruits in the treated trees. We also investigated the effect of BTF on the olfactory behavior of Apis mellifera L. (Hymenoptera, Apidae), a major insect pollinator. We analyzed the behavioral changes of adult workers at two different concentrations (0.1% and 1%) of nine different BTF spray components using a Y-tube olfactometer. The behavioral responses of honey bees to nine BTF components showed significant differences. However, honey bees showed no clear attraction or repellent behavior towards the tested BTF components. Our results suggest that the newly developed eco-friendly BTF is practically applicable in mango orchards without interrupting honey bee behavior.

Cluster and Berry Characteristics of Grapevine (Vitis vinifera L.) as Influenced by Thinning Agents and Gibberrelic Acid Applications

Some cultivars of grapevine have very compact cluster which adversely affect its quality and marketability. ‘Yaghouti’ is a commercial early table grape cultivar that has very compact clusters. The present study was conducted to assess the effects of mechanical and chemical thinning as well as GA3 application on cluster morphology and berry size of grapevine cv. ‘Yaghouti’. Treatments were chemical thinning agents including: NAA (7.5 and 15 mg/L), Ethephon (75 and 150 mg/L), Sevin (750 and 1500 mg/L) and mechanical thinning by a comb (comb thinning), applied a week after fruit set and unthinned clusters as control with 3 replications, 10 days after thinning, clusters were immersed in GA3 solution at 60 mg/L. Results indicated that berry numbers were reduced in all thinning treatments. Length, width and weight of berries increased significantly when treated with hand or chemical thinning. GA3 alone and Sevin at 750 mg/L in combination with GA3 treatment improved yield and the quality of berries. Total phenolics compounds, TSS and berries colour were improved in all thinning treatments. In general, application of Sevin at 750 mg/L combined with GA3 and hand thinning along with GA3 are proposed to improve quality of grapevine berries.

Plant-derived nutrient solution improves thinning effect on pear flowers and fruits

Pear (Pyrus pyrifolia (Burm. f.) cv. Nak.) is cultivated worldwide, with China ranking as the top producer of Asian pear. Pear trees often waste a lot of nutrients during the growing process, due to the large number of flowers on the trees. Traditional thinning methods to reduce flower number are often laborious and time-consuming and affect fruit quality. Here, we investigated the use of plant-derived nutrient solution (PDNS), a liquid organic fertilizer that is made from naturally fallen tree fruit, as a thinning agent. PDNS was sprayed onto two pear cultivars, ‘Whangkeumbae’ and ‘Hosui’ during bud, flower and fruit stage. We observed that the thinning effect of PDNS was significantly greater than that of widely used chemical thinning agents and that PDNS can promote pear tree vegetative growth and fruit quality. The work described in this report lays a foundation for developing protocols using environmentally benign thinning agents in organic fruit production.

A comparison of different fruit thinning agents in apple cultivar ‘Golden Delicious’

The natural crop load of cultivar 'Golden Delicious' varies greatly from year to year, but is often too high under Belgian climatological conditions resulting in a high percentage of small fruits. Hence, thinning is necessary to obtain a good fruit size. In the trial discussed here the following chemical thinning agents were compared: Amid-Thin (60 g naphtyl acetamide or NAD ha-1 standard orchard), Ethephon 480 (120 g ethephon ha-1 standard orchard), 6-benzyladenine (6-BA) (150 g active ingredient or a.i. ha-1 standard orchard) and metamitron (330 g a.i. ha-1 standard orchard). All thinning agents except ethephon are registered in Belgium for fruit thinning. Only 6-BA and metamitron effectively thinned 'Golden Delicious' in 2016, while no clear thinning effect of NAD and ethephon was observed.

A two-year study on chemical thinning agents for ‘Rocha’ pear (Pyrus communis L.)

In recent years, new chemical thinners have been introduced in the European market and researchers have tested them on apples and European pears. In Portugal, grower groups and academia jointly organized the thinning trials. Reported here are the results of two years' research (2015 and 2016) carried out in a commercial orchard of 'Rocha' pear planted in 2010. The following active ingredients: 6-benzyladenine (6-BA), naphthalene-acetic-acid (NAA) + 6-benzyladenine (6-BA), abscisic acid (ABA), and metamitron (MT) were tested at different concentrations and time of application. All the tested chemical thinners reduced fruit load, although the efficacy of some of them (NAA and 6-BA) was affected by the climatic conditions. ABA and Metamitron, the two newest chemical thinners introduced in Portugal, performed well and positively affected some fruit quality parameters.

Effect of Some Chemical Thinning Agents on Fruit Quality and Oil Content of Manzanillo and Eggizi Shami Olive Cultivars

The present investigation was conducted for two successive seasons (2009-2010) on two olive cultivars (Manzanillo and Eggizi Shami) at a private orchard in the Alex desert road. The aim of study is to investigate the effect of some chemical thinning agents GA3 at concentrations (100,150 and 200 p.p.m), NAA at (100,150 and 200 p.p.m) and Urea at (4, 6, 8 and 10%) on fruit characteristics (fruit length, diameter and weight) and oil content. Chemicals were sprayed at full bloom, five days after full bloom, ten days after full bloom and fifteen days after full bloom. The obtained results showed that chemical thinning agents GA3, NAA and Urea are effective in improving fruit quality and oil content in both studied cultivars. It is applicable to spray Urea as a chemical thinning agent at 4% at ten days after full bloom because it is cheap, effective and as a source of nitrogen supply.

Reviewing the Commercial Potential of Hand Thinning in Citrus with a Cost-benefit Analysis of Summer Hand Thinning of ‘Nadorcott’ Mandarin

Hand thinning is not often applied as a commercial cultural practice in citriculture due to the practice’s reliance on costly manual labor. However, hand thinning could provide unique benefits such as treatment selectivity and easier control over thinning intensity, as opposed to foliar sprays of chemical thinning agents. In “on-year” ‘Nadorcott’ mandarin (Citrus reticulata) trees, summer (January) and autumn (April) hand-thinning treatments of removal of all fruit <20 and <40 mm diameter were evaluated for effects on leaf carbohydrates and fruit growth rate. Other factors assessed included treatments’ effects on tree total fruit yield, fruit quality, and fruit size distribution. In addition, two different summer hand-thinning treatments (removal of all fruit <20 and <25 mm) were evaluated for effects on fruit size distribution and fruit yield over two seasons to determine their potential financial implications. Summer hand thinning reduced the numbers of small fruit and increased initial growth rate of prevailing fruit. This resulted in larger numbers of premium-sized fruit per tree, without treatments reducing total fruit yield and fruit quality. Additional labor was eliminated by quicker picking of fruit during harvest, and in season 2, fruit yield was higher for summer hand-thinning treatments compared with control. A higher potential income resulted from change in fruit size distribution and the breaking of alternate bearing over the 2-year period. The results provide producers of late mandarin cultivars with an alternative to chemical thinning agents to manage crop load and improve fruit size in individual “on-year” trees.

Effects of S-Abscisic Acid and (+)-8′-Acetylene Abscisic Acid on Fruit Set and Stomatal Conductance in Apple

Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are ≈8 to 15 mm in diameter, indicating that fruit are sensitive to a transient carbohydrate stress during this period. We investigated the potential for S-abscisic acid (ABA) and an ABA analog [(+)-8′-acetylene ABA] to chemically thin apple fruit by causing a stomatal limitation of photosynthesis. Stomatal conductance (gS) of ‘Imperial Gala’/M.7 was reduced by 60% 3 h after application of 250 mg·L−1 ABA or 25 mg·L−1 (+)-8′-acetylene ABA. Stomatal conductance began to recover 4 days after application but did not return to control levels until 19 days after treatment. Application of 250 mg·L−1 ABA combined with 100 mg·L−1 6-benzyladenine (6-BA) when mean fruit diameter was ≈10 mm reduced fruit set of ‘Gala’/M.7 but not ‘Pink Lady™’/M.7 or ‘Morganspur Delicious’/MM.111. Fruit set of ‘Pink Lady™’/M.7 was reduced by application of 20 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at full bloom or 10 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at the 10-mm fruit diameter stage. Fruit set of ‘Morganspur Delicious’/MM.111 was reduced by application of 25 mg·L−1 (+)-8′-acetylene ABA, either alone or in combination with 75 mg·L−1 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8′-acetylene ABA triggered leaf abscission at rates above 250 mg·L−1 and 25 mg·L−1, respectively. Fruit set and gS data from the present studies indicate the biological activity of (+)-8′-acetylene ABA is 10-fold higher than ABA. These results suggest that ABA and (+)-8′-acetylene ABA reduced fruit set by causing a stomatal limitation in photosynthesis that resulted in a transient carbohydrate stress. Thinning responses to ABA and (+)-8′-acetylene ABA at the concentrations used in these experiments were reduced compared with standard concentrations of currently available chemical thinning agents. However, increasing the concentration of ABA or (+)-8′-acetylene ABA to levels that would achieve comparable thinning are also likely to result in unacceptable leaf abscission.

Fruit self-thinning: a trait to consider for genetic improvement of apple tree.

In apple (Malus×domestica Borkh), as in many fruiting crops, fruit maintenance vs abscission is a major criteria for production profitability. Growers routinely make use of chemical thinning agents to control total fruit load. However, serious threats for the environment lead to the demand for new apple cultivars with self-thinning properties. In this project, we studied the genetic determinism of this trait using a F1 progeny derived from the cross between the hybrid INRA X3263, assumed to possess the self-thinning trait, and the cultivar ‘Belrène’. Both counting and percentage variables were considered to capture the fruiting behaviour on different shoot types and over three consecutive years. Besides low to moderate but significant genetic effects, mixed models showed considerable effects of the year and the shoot type, as well as an interaction effect. Year effect resulted mainly from biennial fruiting. Eight Quantitative Trait Locus (QTL) were detected on several linkage groups (LG), either independent or specific of the year of observation or the shoot type. The QTL with highest LOD value was located on the top third of LG10. The screening of three QTL zones for candidate genes revealed a list of transcription factors and genes involved in fruit nutrition, xylem differentiation, plant responses to starvation and organ abscission that open new avenues for further molecular investigations. The detailed phenotyping performed revealed the dependency between the self-thinning trait and the fruiting status of the trees. Despite a moderate genetic control of the self-thinning trait, QTL and candidate genes were identified which will need further analyses involving other progenies and molecular investigations.

A Hand-Held Mechanical Blossom Thinning Device for Fruit Trees

<abstract> Abstract. Blossom or green fruit thinning is a common practice for producing larger and higher quality fruit in tree fruit production. Mechanical blossom thinning could decrease production costs when compared to hand thinning of either blossom or green fruit. In stone fruit, a few chemical thinning agents are available or reliable. In apple, chemical thinning agents are under regulatory scrutiny, and efficacy is highly dependent on weather. This study aimed to develop a hand-held mechanical thinning device suitable for thinning blossoms on fruit trees pruned to different architectures. Research prototypes of different configurations have been developed, fabricated, and tested in blossom thinning operations in commercial orchards. Results obtained from field tests verified that a hand-held mechanical blossom thinner could effectively remove blossoms from apple, stone fruits, and sweet cherry. Among all evaluated parameters, spindle speed appeared to have the greatest direct effect on the capacity to remove blooms. Field test results indicated that the test device could remove 61.1%, 30.8%, and 18.0% flowers on a single branch with a swipe around 0.5 m·s^-1 under high (2500-3000 rpm), medium (1500-1800 rpm), and low (500-800 rpm) speed settings, respectively. In trees trained to central leader architecture, 85 s was required to remove 50% of blossoms using the hand-held mechanical thinner and at least 180 s when the task was accomplished by hand. Field tests also revealed that a better ergonomic design of the device could further improve operational efficiency.

Regulation of source: sink relationship, fruit set, fruit growth and fruit quality in European plum (Prunus domestica L.)—using thinning for crop load management

Demand for large fruit of uniform size is increasing in the market; thinning is a means to achieve consistently large fruit and to overcome possible alternate (biennial) bearing for the small-fruited European plum (Prunus domestica L.). However, chemical thinning agents for stone fruits are scarce and/or often ineffective. Hence, the objective of this work was to study possibilities of enhancing fruit growth and to improve fruit quality, viz size using plum as a model crop. Nine-year-old ‘Ortenauer’ plum trees, trained to spindles, with maximum flowering intensity (score value 9) near Bonn, Germany were mechanically, chemically or hand-thinned. Un-thinned plum trees in the same rows served as control. Trees were either mechanically thinned at full bloom on 20 April 2009 with a rotor speed of either 300, 400 or 500 rpm, and half of those trees additionally treated with ATS (15 L/ha) and an ethylene releasing compound 35 days after full bloom or manually thinned. The objective of 1/3 flower removal was successfully achieved even with the slowest rotor speed of 300 rpm. The number of fruit per branch was significantly reduced from 152 to 67–76, equivalent to a (source: sink) leaf: fruit ratio of 5:1. Mechanical thinning significantly enlarged fruit mass from 28 g in the un-thinned control to 30–32 g with rotor speeds of 400 or 500 rpm. Additional chemical thinning with ATS and an ethylene-releasing compound resulted in no further increase in fruit mass. Inner fruit quality (sugar) of the plums appeared unaffected by either mechanical or chemical thinning, except for fruit firmness: Plums thinned with an ethylene releasing compound were softer and ripened earlier than respective control fruit. The most efficient method of flower removal and fruit mass enlargement was mechanical blossom thinning with 400 rpm, which may provide a suitable replacement for chemical and/or manual thinning. Alternatively, the mechanical thinning can be combined with either of those options. Mechanical blossom thinning was as effective to overcome or avoid alternate/biennial bearing as the ethylene releasing compound in the previous year. The results are discussed with respect to stone fruit being more difficult to thin than pome fruit, because the former do not exhibit leaves at the time of blossom thinning. Stone fruits develop within a shorter time and a larger number of (smaller) leaves (source) are required for the same fruit growth and final fruit size (sink). A lower threshold of fruit removal has to be exceeded before the remaining stone fruit grow faster and final fruit mass and sugar (and possibly fruit firmness) increase, while acidity remains unaffected by fruit set. An upper saturation threshold is reached fairly quickly without further effects.

APPLE FRUIT CHEMICAL THINNING IN CHIHUAHUA, MÉXICO

Chemical fruit thinning is a common practice in apple (Malus x domestica Borkh.) orchards for achieving high quality fruits, reduced costs of hand-thinning and promote return of flowering. Hand thinning is a common and high cost practice, not only due to the labor involved, but also because seedling and vigorous clonal rootstocks result in large trees. The chemical thinning agents: Ethephon (ETH), Carbaryl (CB), Naphthalene acetic acid (NAA), 6- Benzyladenine (6-BA), and fish oil were evaluated on ‘Golden Delicious’ and ‘RedChief Delicious’ apple trees in 2004 and 2005. Chemical thinning effect was compared with early and late handthinnings. Treatments were sprayed when the biggest fruit in the clusters was 10 mm in equatorial diameter. NAA showed the best effect in reducing final fruit set in both cultivars and years evaluated, with results comparable to those of early hand-thinning (thinned when king-fruit size was 10 mm) in ‘Golden Delicious’ trees. In ‘RedChief Delicious’, fruit set reduction by NAA, fish oil and CB treatments was similar to early hand-thinned trees only in 2005. In 2005, time spent during supplementary hand-thinning in ‘Golden Delicious’ trees was reduced by chemical thinning treatments, except by fish oil; whereas in ‘RedChief Delicious’ only NAA K-salt reduced it in 2004. An increase in fruit equatorial diameter was observed only in 2004 with NAA and fish oil. Alternate bearing, typical of ‘Golden Delicious’, was not diminished by these chemical thinning treatments.

Efficacy of chemical bloom thinning agents to European plums

During the period 1998–2000, thinning trials were conducted using bloom thinners on mature European plum trees at Ullensvang and at fruit growers’ sites in western Norway. In 1998, unsprayed control and hand-thinned ‘Victoria’ trees were compared with trees treated at full bloom with a single application of 1% Armothin® or 1.5% ammonium thiosulphate (ATS). The same programme was conducted in the following two years with the addition of a single full bloom treatment with 250 ppm ethephon and a post-bloom application one month after full bloom with the mixture 10 ppm 1-napththalene acetic acid (NAA) and 75 ppm ethephon. High volume sprays were conducted the first two years and low volume the last year. Thinning trials testing 1–1.5% ATS and 5–7% lime sulphur in comparison with unthinned and hand-thinned trees to the cultivars ‘Opal’ and ‘Victoria’ were conducted in 2000, at growers’ sites. Generally, thinning treatments reduced crop load and enhanced fruit quality (fruit size, soluble solid content, fruit firmness and ground and surface colour), but the results varied from year to year. Fruit set was reduced to about half of control values and the percentage of class 1 fruits was doubled compared to the control trees. High volume sprays to running off were more effective than low volume sprays. The cultivar ‘Opal’ was more sensitive to ATS than ‘Victoria’ and a low dosage is recommended. All thinning compounds caused some minor leaf injury but no fruit damage. The flower thinners were efficient at rather low temperature. No differences in the amount of gummosis (internal disorder of the fruits) were observed as a result of treatments on ‘Victoria’ plums. Return bloom was improved by thinning. In conclusion, a single dilute application at full bloom of 1% Armothin®, 1.5% ATS or 250 ppm ethephon or one application of the mixture 10 ppm NAA-75 ppm ethephon four weeks after bloom reduced fruit set and crop load and increased fruit quality and return bloom of the cultivar ‘Victoria’. Similar results were achieved with one dilute spray with 1% ATS or 5% lime sulphur at full bloom to the cultivar ‘Opal’.

Physiological and Horticultural Effects of Sweet Cherry Chemical Blossom Thinners

This article reports on the physiological effects and horticultural benefits of chemical blossom thinners on 9-year-old and 12-year-old `Bing'/`Gisela®5′ sweet cherry trees in 2004 and 2005, respectively. Chemical thinning agents were applied at 20% and 80% full bloom (FB) by air-blast sprayer and were comprised of: 2% ammonium thiosulphate (ATS), 4% vegetable oil emulsion (VOE), 2% fish oil + 2.5% lime sulfur (FOLS), 1% tergitol, and an untreated control. Leaf gas exchange, leaf SPAD meter readings, chlorophyll fluorescence parameters, fruit yield, and fruit quality were evaluated. FOLS, tergitol, VOE, and ATS suppressed leaf net CO2 exchange rate (NCER) by 33%, 30%, 28%, and 18%, respectively, over a variable length recovery period directly after 80% FB treatment. Leaf NCER recovered fully from every thinning treatment. Reductions in leaf NCER were unrelated to gS. VOE reduced estimated leaf chlorophyll content the greatest, suppressing overall levels by 11% for 23 days after treatment. All blossom thinners reduced constant fluorescence (Fo). No thinning agent reduced fruit set or yield in 2004. ATS, FOLS, and tergitol reduced fruit set in 2005. VOE was ineffective as a thinner yet exhibited significant leaf phytotoxicity. Among thinners, there was no relationship between inhibition of leaf NCER and thinning efficacy.

PHOTOSYNTHETIC INHIBITION OF 'SEYVAL BLANC' GRAPEVINES WITH TERBACIL

PHOTOSYNTHETIC INHIBITION OF 'SEYVAL BLANC' GRAPEVINES WITH TERBACIL

Chemicals, Timing, and Environmental Factors Involved in Thinner Efficacy on Apple

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Non-structural carbohydrate status and CO 2 exchange rate of apple fruitlets at the time of abscission influenced by shade, NAA or BA

Four days of 90% shading at the time of fruit setting (1–5 days after petal fall, DAPF) of mature ‘Jonagold’/M.9 apple trees was combined with the application of thinning agents to establish the possible interaction of both main treatments on fruit retention, and the effects of the main treatments on fruitlets CO 2 exchange and their carbohydrate concentration. Shading alone reduced yield by up to 35%. When shading was followed by the application of 15 ppm 1-naphthaleneacetic acid (NAA) or 50 ppm 6-benzyladenine (BA), the interaction between shading and chemical thinners was on the border of significance. Following the assumption that abscission was induced by shading before the application of chemical thinning agents, the same process of diminished dark respiration rate ( R d) was found on fruitlets sampled 3 days after the end of shading and 7 days after NAA and BA spraying, i.e. in the fruitlets that were considered prone to drop. A week after the end of shading the abscission of fruitlets due to shading was completed and on the remaining fruitlets that were not prone to abscission any more, larger negative net CO 2 assimilation rates ( A) and enhanced R d were recorded. Fruitlets prone to abscission sampled 2 days after the end of shading showed a lower concentration of glucose and ascorbic acid and a higher concentration of starch. Eight days after the end of shading, the fruitlets from shaded trees that were not prone to abscission any more contained less fructose while the fruitlets from NAA and BA sprayed trees had a higher concentration of fructose a week after application. The results do not support the hypothesis that a good assimilate supply is needed for the retention of apple fruitlets, because the fruitlets retained on trees expressed even stronger CO 2 loss and because the concentration of non-structural carbohydrates did not decrease in fruitlets prone to drop with the exception of glucose.