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Plant Growth Regulation 31: 17–26, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 17 Significance of flower and fruit thinning on fruit quality H. Link Universität Hohenheim, Institut für Obst-, Gemüse- und Weinbau, Fachgebiet Obstbau – Bavendorf, 88213 Ravensburg, Germany Key words: apple (Malus domestica L. Borkh.), colour, crop load, disorders, firmness, flavour, fruit size, russeting Abstract The effects of mechanical or chemical flower and fruit thinning on fruit quality were primarily by altering crop load. However, there were also direct effects of thinning agents. Fruit size was directly related to thinning intensity. In addition to crop load, age of wood, flower bud quality, competition within clusters and canopy were important factors affecting the response to thinning. Short- and long-term thinning studies identified two groups of quality components: Group 1 characteristics include size, colour, skin performance, firmness and sugar and acid content of the fruit. Group 2 characteristics were represented by inorganic components, especially calcium and potassium which are implicated in the susceptibility of fruit to physiological disorders. While group 1 characteristics were improved by increasing thinning intensity, storability of the fruit was better at high than at low crop loads. There- fore, a compromise between all quality requirements must be found for a good economic return. Establishing the trends of thinning on the different quality parameters can help to select a thinning strategy for local or regional conditions typically being determined by growing and market conditions. 1. Introduction Fruit thinning is the most important technique in apple growing for improving fruit quality [22]. Since thin- ning can be performed mechanically or chemically, thinning intensity may vary not only on the method used but also on the physiological condition of the trees and cultural practices employed. For the fresh market, fruit size, appearance, flavour, firmness and storability are of main interest. The grower therefore, will have to focus his orchard practices to satisfy these market demands in order to produce high quality fruit consistently at maximum yields. It is impossible to maximize all quality factors simultaneously, because there are positive and negative interrelations among them. Therefore, a reasonably balanced compromise between quality and quantity must be achieved. The grower must find the correct point of this compromise, based on his management skills, cultivars grown, cultural practices employed and marketing goals. There is little chance to provide a recipe to fit all situations. Understanding the principles of how fruit thinning may improve quality parameters is essential and with the help of research to improve thinning practices and products, growers can develop thinning programs best suited to their orchard conditions. For this purpose, a series of our thinning studies was analysed for common trends mainly by means of correlation and regression analysis, and comparisons were made with results from selected literature. 2. Materials and methods Thinning experiments were performed on uniform and heavily flowering orchards over three decades. Since fruit thinning via alternate bearing influences fruit quality not only in the year of thinning, some exper- iments were carried out for two or three years; one experiment for more than 10 years (1982–1992) on the same site. There, flower density of the trees frequently changed over the years because of irregular bearing of the cultivars and the thinning treatments employed.

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Page 1: Significance of flower and fruit thinning on fruit quality

Plant Growth Regulation31: 17–26, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

17

Significance of flower and fruit thinning on fruit quality

H. LinkUniversität Hohenheim, Institut für Obst-, Gemüse- und Weinbau, Fachgebiet Obstbau – Bavendorf, 88213Ravensburg, Germany

Key words:apple (Malus domestica L. Borkh.), colour, crop load, disorders, firmness, flavour, fruit size, russeting

Abstract

The effects of mechanical or chemical flower and fruit thinning on fruit quality were primarily by altering cropload. However, there were also direct effects of thinning agents. Fruit size was directly related to thinning intensity.In addition to crop load, age of wood, flower bud quality, competition within clusters and canopy were importantfactors affecting the response to thinning. Short- and long-term thinning studies identified two groups of qualitycomponents: Group 1 characteristics include size, colour, skin performance, firmness and sugar and acid contentof the fruit. Group 2 characteristics were represented by inorganic components, especially calcium and potassiumwhich are implicated in the susceptibility of fruit to physiological disorders. While group 1 characteristics wereimproved by increasing thinning intensity, storability of the fruit was better at high than at low crop loads. There-fore, a compromise between all quality requirements must be found for a good economic return. Establishing thetrends of thinning on the different quality parameters can help to select a thinning strategy for local or regionalconditions typically being determined by growing and market conditions.

1. Introduction

Fruit thinning is the most important technique in applegrowing for improving fruit quality [22]. Since thin-ning can be performed mechanically or chemically,thinning intensity may vary not only on the methodused but also on the physiological condition of thetrees and cultural practices employed. For the freshmarket, fruit size, appearance, flavour, firmness andstorability are of main interest. The grower therefore,will have to focus his orchard practices to satisfy thesemarket demands in order to produce high quality fruitconsistently at maximum yields.

It is impossible to maximize all quality factorssimultaneously, because there are positive andnegative interrelations among them. Therefore, areasonably balanced compromise between quality andquantity must be achieved. The grower must findthe correct point of this compromise, based on hismanagement skills, cultivars grown, cultural practicesemployed and marketing goals. There is little chanceto provide a recipe to fit all situations.

Understanding the principles of how fruit thinningmay improve quality parameters is essential and withthe help of research to improve thinning practices andproducts, growers can develop thinning programs bestsuited to their orchard conditions. For this purpose,a series of our thinning studies was analysed forcommon trends mainly by means of correlation andregression analysis, and comparisons were made withresults from selected literature.

2. Materials and methods

Thinning experiments were performed on uniform andheavily flowering orchards over three decades. Sincefruit thinning via alternate bearing influences fruitquality not only in the year of thinning, some exper-iments were carried out for two or three years; oneexperiment for more than 10 years (1982–1992) on thesame site. There, flower density of the trees frequentlychanged over the years because of irregular bearing ofthe cultivars and the thinning treatments employed.

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In the more recent studies the experimental treeswere planted at distances between 0.8 and 1.5 m oreven less within the row. For the very early studies,planting distances within rows varied from 2 to 3 m oreven more.

Fruit thinning was done by hand or chemically.Unthinned trees were used as controls.

Complete flower clusters minus leaves wereremoved between pink bud and full bloom by handthinning (early hand thinning). Late hand thinningwas performed just after the “June drop” by removingsmall and injured fruit selectively. In both methodscrop load was adjusted to a density rating of 8 [41].

The most common chemical treatment was a naph-thaleneacetamide (NAAm) spray at 40 (‘Boskoop’,‘Cox’s Orange Pippin’, ‘Gravenstein’, ‘Idared’) or 60ppm a.i. (‘Glockenapfel’, ‘Golden Delicious’). For‘Golden Delicious’ 0.3–0.5% v/v emulsifiable mineraloil was often added to improve NAAm penetrationthrough the cuticle. Additional thinning chemicals andconcentrations were: naphthaleneacetic acid (15–30ppm a.i. NAA), carbaryl (500–1500 ppm a.i. NMC),benzyladenine (50–200 ppm a.i. BA) and ammoni-umthiosulfate (0.5–1.5% a.i. ATS). A special thinningtreatment was a high concentrated spray with urea(4–8%) evaluated because of the unique situation inGermany, where no thinning chemicals are currentlyregistered.

In most studies a single spray was applied with aknapsack sprayer or high pressure spray gun at 1000–2000 1 per ha, up to ‘run off’.

A complete block design was usually used for theshort-term trials with 8–10 trees per treatment withone- two- or three-tree-plots. In the long-term trial20 trees per plot were used for unthinned and handthinned treatments and 40 trees per plot for chemicalthinning treatments. In this trial no replicate plots wereused.

Yield records were taken on a per tree basis.Size and colour grading was performed on all fruitproduced per tree or on a composite sample of 300 kgfruit (in the long-term trial) using a commercial grader,equipped with an optical colour sorter. After gradingthe fruit into 5 mm classes, subsamples were taken forfruit Ca, Mg, K, P and N analysis, for degree of fruitrusset and for storage trials to assess the incidence ofphysiological disorders.

The data were subjected to correlation and regres-sion analysis or to an analysis of variance when appro-priate. Comparisons of treatment means were madeusing Duncan’s/Tukey’s multiple range test.

3. Influence of thinning on fruit qualityparameters

3.1 Mean fruit weight

Mean fruit weight was negatively correlated with cropload. The regression line was linear in the size rangesof interest to fruit growers. Obviously, the degree ofcorrelation may be rather large or small. In the experi-ments, the correlation coefficients ranged from−0.36to−0.83. Compared with large trees, small trees gavehigher and more significant coefficients. The r2-valuesbetween 0.19 and 0.69 clearly indicate that factorsother than crop load in the year of thinning may alsoinfluence fruit size.

Since carbon-based compounds are the backboneof fruit growth, and the supply of carbon available tothe fruit may be limiting during early fruit develop-ment by competition from too many fruitlets or othersinks [17], a marked influence of the time of thinningon fruit size would be expected.

Thinning results reported in different fruit growingcountries have documented the effect of early thinningon increasing fruit size [6, 34, 37, 40]. Fruit size canbe viewed as the result of cell number, cell size andintercellular space [8]. Pearson and Robertson [26]stated “that variation in fruit size is determined early indevelopment by the amount of cell division, and thatwithin one season cell volume in individual fruit of thesame age was fairly uniform so that the variation infruit size was determined by cell number and, to someextent, by the amount of air space. Between seasons,however, the size of cells may be the overriding factorin determining fruit size”.

In our thinning trials mean fruit size increased by30 percent maximum when thinning was performedbetween pink bud and full bloom as compared tothinning after the ‘June drop’. Fruit weight duringdifferent years correlated closely with the number ofcells per fruit (r = 0.4–0.97) and with cell size (r= 0.8–0.97). Compared with the unthinned control,blossom thinning increased cell number by 5–35%and cell size by 4–10% [25]. The results also indicateresponse-differences between cultivars. For instance,‘Cox’s Orange Pippin’ may require more cells thanlarger ones for good sized fruit than ‘Golden Deli-cious’ (Table 1), but more work is needed to confirmthis.

Differences in flower bud quality as affected bycompetition within flower clusters and vigour of thefruiting wood also determine fruit size [35]. These

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Table 1. Effect of thinning on fruit quality parameters (means of 3 years)

Cultivar Cells/fruit Cell size Fruit weight % soluble mval acid/ Firmness % fruit affected by mg/100 g fw

× 106 µm g solids 100 ml N Bitter pit Int. breakdown Ca K

Unthinned

Boskoop 63.0 219 155 11.7 13.5 95 8 0.8 5.16 101

Cox 42.4 203 97 11.1 14 89 20 3.9 4.36 136

G Delicious 65.9 182 109 11.5 7.4 80 – – 5.12 111

Hand-thinning

Boskoop 68.9 236 206 13.3 18.6 100 41 10.6 4.77 129

Cox 55.3 212 120 12.4 17.5 99 74 13.1 3.0 171

G Delicious 67.9 200 149 12.7 9.3 79 – – 3.65 128

Data by [25]

factors may indicate fruits that should be thinnedpreferentially [19]. When we picked fruit from indi-vidual trees and separated lateral fruit from one-year wood and terminal fruit from one- to three-yearwood the biggest difference was between lateral andterminal fruit. Compared with the ‘terminals’, meanfruit weight of the ‘laterals’ was−22 g in ‘Elstar’,−35 g in ‘Jonagold’ and−24 to−40 g in ‘GoldenDelicious’, depending on vigour of the fruiting wood.The fruit borne on older wood (more than 3 years old)was of equal size as from lateral inflorescences.

Crop load in the year before thinning may also bea source of fruit size variation. Heavy cropping in theprevious season may reduce the cell number in flowerreceptacles, compared to those in normally croppingtrees [3]. In an experiment in 1995, we selected threegroups of Elstar trees from a population of 250 trees:In 1994, trees in groups 1 and 2 flowered abund-antly while trees in group 3 had only a light bloom.In 1995, group 1 showed moderate bloom intensity,groups 2 and 3 had a ‘snow ball’ bloom (group 2 treesflowered abundantly in both years). In 1995, the group1 trees were not thinned; groups 2 and 3 were blossom-thinned. Although the fruit number per tree within allgroups was comparable at blossom time in 1995, themean fruit weight was 122, 128 and 146 g for groups1 to 3, respectively.

There is widespread view that an effective thin-ning treatment should yield single-fruit clusters, andso favour fruit size. Thinning ‘Gala’ to single fruitclusters as opposed to thinning alternate clusterscompletely increased the mean weight of fruit by 6g. However, in some instances cluster thinning gavesomewhat larger fruit than single fruit thinning. Since‘Elstar’ and ‘Idared’ showed comparable results inadditional experiments, it may be concluded that there

is no necessity for single-fruit clusters to achieveadequate size. This does not correspond to NewZealand experiences, where ‘Gala’ especially mustbe thinned to single-fruit-clusters to get acceptablefruit size (Wünsche J, personal communication). Onthe other hand, our results are consistent with Swissstudies [30] using an unilateral thinning procedure.

Chemical thinning is advantageous for fruit growthbecause the relationship between sink source and sinksize [11] is regulated during the cell division period offruit growth. But, note also that growth regulators mayinfluence the growth of fruit directly – both positivelyand negatively.

A growth retarding effect of NAA and NAAmshortly after application is well known. In our experi-ments this effect was generally overcome quickly. Thiseffect persisted up to picking time on a few occa-sions only, where fruit were then smaller than thosefrom unthinned trees, despite a thinning responseof the trees. Rather often, these fruits showed areduced number of developed seeds. Rather oftenly,we observed that under those conditions the separationlayer induced by the spray was incomplete, and so fruitcontinued to be connected to the xylem vessels, but notto the phloem vessels.

‘Elstar’ appears to be very prone to the growthretarding effects of NAAm and NAA. The occur-rence of retarded ‘pigmy fruit’ can vary from yearto year and from region to region. It is likely that ahigh proportion of pigmy fruit is associated with apoor thinning effect in ‘Elstar’. In ‘Golden Delicious’typical pigmy fruit were not seen at harvest, but some-times a rather high proportion of unacceptable 40–50mm fruit.

Ethephon (CEPA) can also retard fruit growth [7,16]. This was occasionally observed in our studies

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when 250–350 ppm a.i. CEPA was sprayed duringbloom, resulting in fewer and smaller fruit than inthe controls. Using a mixture of 125 ppm a.i. CEPAcombined with NAAm (normal dose) the retardingeffect on fruit growth was tolerable.

Ammoniumthiosulfate (ATS) is one of the newerthinning compounds. From our thinning trials there isan indication that ATS may retard fruit growth to theextent that fruit size does not correspond to the cropload obtained. This was more pronounced when ATS-concentration and spray volume were rather high, orwhen thinning was performed during a rainy period. Inthis case, shoot growth may also be retarded consider-ably.

Benzyladenine (BA) has been shown to increasefruit size even in the absence of fruit thinning [2, 10,24]). Since a (late) NAA Thinning spray may result insmaller apples than expected at a certain crop load, acombination of NAA and BA was used for thinningby several authors [Basak A, personal communica-tion, 40]. Surprisingly, these sprays may inhibit fruitgrowth in ‘Delicious’ [5], but increase fruit size ofother cultivars.

3.2 Fruit size distribution

Fruit size distribution corresponds to a normal distri-bution curve. Every effective thinning treatment shiftsthe curve from the lower size categories to the higherones. Thinning may thus result in fewer kilos of smalland more kilos of large fruit. The difference betweenan efficient thinning treatment and an unsatisfactoryone is, whether or not the potential maximum numberof large fruit will be produced. In addition, a low yieldof small and of oversized fruit is also expected.

An analysis of our fruit size data from indi-vidual ‘Elstar’, ‘Golden Delicious’ and ‘Jonagold’trees showed that the yield of small (<60 mm) and ofmiddle sized apples (60–70 mm) increases exponen-tially with crop load. The yield of large apples can bedescribed by an optimum-curve: too many apples pertree, and overthinning also, will reduce the quantity oflarge apples below the potential value (Figure 1).

In the case of ‘snow-ball’ bloom, the size distri-bution is influenced by a large number of ‘tree-and thinning-factors’ discussed previously. The effectof crop load during the previous year was strikingin an ‘Elstar’ trial already described. In this case,a pronounced concentration in the 75 and 80 mmsize categories was recorded from less productivetrees in the previous year as compared to highly or

Table 2. Effect of thinning on yield (kg/tree) of mid sized –large, well marketable fruit, means from years 1982–1990

Unthinned Chemical Hand-thinning

control thinning Early Late

Boskoop 25.6 29.3 22.8 24.9

Cox’s OP 12 15.2 17.3 16.2

Glockenapfel 17.7 18.5 21.3 23.1

G Delicious 17 22.5 22.8 23.6

Gravenstein 21.9 20.4 23.4 21.2

Idared 19.1 21.7 20.4 23.2

Means 19.46 21.27 21.75 22.03

mid productive trees (Figure 2). Probably, fruit budquality and carbohydrate-reserves [23] contributed tothese differences in fruit-size-distribution. A possibleconclusion from these results is that effective thinningmight be focused on those attributes also.

From the thinning trials of a single year it maybe concluded that the yield of large apples is aboutdoubled, depending on cultivar and fruit thinningintensity. But in reality this does not hold for an indi-vidual orchard over several years. In multi-year trialsbloom density within and between years usually isnot at the potential maximum, but will be related tothe cropping regularity of the orchard, as affected byfruit growing practices and environmental and geneticinfluences.

In our long-term thinning trial under good produc-tion conditions, it was clearly shown that the oppor-tunity to increase the quantity of middle to largesized fruit may be rather restricted. Increases in theproportion of fruit of marketable size (oversized fruitexcluded) ranged from 7–45%, depending on the thin-ning method and cultivar. Best results were achievedwith ‘Cox’s Orange Pippin’ and ‘Golden Delicious’,but poor results with ‘Boskoop’, ‘Gravenstein’ and‘Idared’ (Table 2).

3.3 Influence of thinning on fruit appearance andorganic fruit constituents

Colour. Only fruit well supplied with carbohydratesattain good colour and flavour [30, 36, 37]. Anoverall tendency is that fruit thinning decreases thepercentage of green fruit and increases yellow back-ground colour of yellow cultivars, and the extentand intensity of surface colour in red cultivars. Wefound a positive linear relationship between colour

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Figure 1. Yield of Elstar (1650 trees per ha) in 3 size categories as related to crop load (fruits per tree); data for 90 individual trees in 1997.

Figure 2. Fruit size distribution in Elstar in 1995. All the trees gave comparable yields roughly. Size distribution differences were due todifferences in crop load in the previous year (see 3.1 Mean fruit weight for treatment details).

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Figure 3. Colour and mean fruit weight in Jonagold (normal strain).Fruits with a colour index of 50 and more are well coloured, but toolarge (≥240 g) in other respects. (Colour index extends from 0 to100.)

and crop load, correlation coefficients were 0.60–0.76in ‘Elstar’, ‘Golden Delicious’ and ‘Jonagold’. Fruitthinning was considerably more effective in improvingfruit colour than picking over trees during an extendedharvesting period which is a common practice in thefruit industry [21].

As for the optimum degree of thinning, problemsmay arise with large-fruited cultivars. For instance,a problem with ‘Jonagold’ may be that a crop loadadjusted to achieve the required fruit size is too largefor good colour development [39], and when optim-ized for an acceptable red coloration a considerableportion of the fruit is too large for the market (Figure3).

In addition to the promoting effect of thinning onfruit coloration, specific agents may be capable ofincreasing fruit colour. The most significant colourpromoting effect by thinning agents was foundafter CEPA treatments. This was due to increasedanthocyanin formation induced by the ethylene pro-duced by CEPA breakdown in the apple tissue [28].

Fruit russeting.When the growth rate of fruitlets islow, small cracks develop in the epidermis leading tofruit russeting. Hand-thinning will usually increase thegrowth rate of fruitlets, and thus reduce the amount offruit russeting.

Chemical thinning is performed during the mostsensitive phase of fruit development for the induc-tion of russeting. It is therefore expected that thinningcompounds might influence fruit russeting specifi-

cally. The classical thinning compounds NAA andNAAm show a smoothing effect on skin quality[31]. On the other hand, carbaryl may increase fruitrusseting to a harmful level when it is applied duringearly fruit development, but less when it is sprayed2–4 weeks later [20]. Different formulations (liquidsinstead of wettable powders) and concentration ofcarbaryl may result in a low or relatively severe russetincidence (Gartner R, personal communication).

For better thinning effectiveness of NAA or NAAmand carbaryl, mineral oil may be added to the spray.However, the skin smoothing effect of auxin-type thin-ning agents may then be eliminated [31]. At timesin our experiments and depending on the kind of oil,fruit russeting was increased when certain fungicideswere applied during the thinning period; adding oilto NAAm or NAA proved highly damaging if dith-ianon was the fungicide used. On the other hand,use of wettable sulphur or captan during that periodincreased the amount of russet-free fruit.

Only limited information about the effects of‘modern’ thinning chemicals on fruit russeting isavailable at present. There are indications that BAmight increase fruit russeting to some extent. AlsoATS in our experiments resulted in more russetingwith certain cultivars in some instances, particularlywhen the ATS concentration was high.

Fruit shape. Normally thinning will favour fruitdevelopment. This means that the calyx lobes willbe flattened so that the calyx end of the fruit growsround instead of somewhat ‘peaky’. The calyx cavitygrows deep and wide instead of flat and narrow. Fruitsymmetry in our experiments was not influencedsignificantly by different thinning chemicals, thoughthere can be a slight tendency of ‘Elstar’ applesto be lop-sided due to unequal development of thecarpels.

Carbohydrate and acid content of the fruit.Theimprovements in fruit size and colour by thinning areaccompanied by higher contents of soluble solids andtitratable acid. Thinning therefore improves taste andalso appearance of the fruit [31]. In our experimentsfruit thinning gave 2–3% more soluble solids. Totaldry matter content correlated positively with solublesolids content (r = 0.76–0.96). Data analysis indicatedthat the increase in sugar content may not be related tofruit size; evaluating fruit from individual trees, smalland large fruit showed no differences in soluble solidsor total dry matter.

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Thinning increased titratable acid content of thefruit by 10–20% compared to unthinned fruit [18].Most often, hand-thinning resulted in higher acidcontent than chemical thinning, because thinning effi-ciency was better. A close relationship between colourand acid content resulted in up to 12% more acid inyellow ‘Golden Delicious’ fruit (at harvest) comparedto green ones.

Plotting acid content of individual fruits againsttheir weight at harvest showed a linear relationship.But the correlation coefficients were both positive andnegative. An increase in titratable acid with increasedweight was found with control fruit, since acid contentmay still have been in the ascending range at thetime of harvest. In contrast, thinned fruit showed adecreasing acid content with increasing fruit weight,probably due to acid decay being faster than acidincrease in the larger fruit.

Acid content decreased linearly with time, andno significant differences in decline of acids duringstorage could be observed in fruit collected fromhand-thinned or chemically-thinned trees.

Firmness of fruit.Considering that fruit firmness isdetermined multifactorially by positive and negativeeffects caused for instance by fruit size, number andsize of cells, volume of intercellular space, specificgravity, harvest maturity, dry matter (pectin) andmineral content, enzyme activity, a wide range ofpossible thinning effects on fruit firmness at harvestand on subsequent softening during storage is to beexpected. On heavily cropping trees carbohydratesupply for cell wall synthesis may become limiting.Since thinning increases the dry matter content ofthe fruit, an opportunity for improving fruit firmnessseems likely. On the other hand, firmness is oftennegatively associated with other thinning effects, suchas mean fruit weight and calcium content of the fruit[33].

The interpretation of the results on fruit firm-ness from our thinning trials was difficult althoughnumerous pressure tests with fruit from thinned plotswere made. On the whole, thinning increased fruitfirmness, but in some instances a decrease could alsobe observed. In 1995 for instance, firmness variation in‘Elstar’ and ‘Jonagold’ fruit from flower-thinned treeswas−3.9 to +4.5 N (1 cm2 probe) compared to controlfruit. Fruit of three different size categories (65–70,70–75, 75–80 mm) from the same trees showed a0.5–5.1 N reduction of fruit firmness at maturity from

small to large fruit (the initial firmness ratings variedbetween 63 and 71 N).

Results from different years indicate a potential2–18% increase in firmness by thinning as comparedto unthinned. ‘Elstar’ exhibited the largest firmnessincrease, while ‘Jonagold’ showed very little response,possibly due to different textural properties of the twocultivars. Also there was an indication that early fruitthinning before petal fall (by hand and chemically)might have improved fruit firmness more than thinninglater, a finding similar to thinning results in ‘Cox’sOrange Pippin’ [13, 14].

From the fruit industry’s standpoint moderate thin-ning adjusted to fruit quality and quantity did notimprove fruit firmness significantly in our experi-ments. Rarely, the gain of firmness was more than 5 N,and this was also found in other thinning experiments[13].

Since thinning may advance fruit maturity by 10–14 days [14, 32, 38], fruit in our trials was notpicked at the same time but the harvest was adjustedto crop load of the trees. As a result, thinning didnot significantly change the loss of firmness duringCA-storage.

3.4 Storage quality of the fruit

Apart from decay, physiological disorders may causeunacceptable losses during fruit storage, bitter pit andbreakdown being the most important ones. Since bitterpit, internal breakdown, lenticel blotch pit and someother disorders are mainly calcium-related disorders,bitter pit will be taken as an example for the evaluationof the impact of thinning on fruit storability.

Fruit thinning generally increased bitter pit, partic-ularly treatments applied near bloom as opposed toafter the ‘June drop’ (Figure 4). Greater severityof thinning resulted in higher levels of bitter pit.However, for the same crop load, untreated fruit weremore susceptible than fruit from thinned trees.

Frequently bitter pit incidence is positively relatedto fruit size, especially when composite samples arebeing investigated [14, 32]. This relationship wasnot confirmed frequently with fruit from individualtrees [27, 29]. In our experiments with ‘Boskoop’,‘Cox’s Orange Pippin’ and ‘Gravenstein’ with 50–100 individual trees of each cultivar for 3 years, thecorrelation coefficients for the fruit size/bitter pit rela-tionship ranged from−0.25 to +0.45. Possibly, treesize and fruit position within the canopy were signifi-

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Figure 4. Bitter pit incidence in Boskoop as related to time and intensity of hand-thinning.

cant reasons for the highly variable correlation [27,29].

Crop load influenced bitter pit incidence in indi-vidual trees more consistently and impressively (r =−0.25 to−0.95), than fruit size.

It is commonly accepted that the incidence of bitterpit is related to an imbalance of calcium [1] andpotassium and to a wide potassium calcium ratio ofthe fruit; positive correlation coefficients were foundwith potassium and negative ones with calcium. Forboth elements our experiments gave linear regressionlines.

The effect of fruit thinning was to increase phos-phorus and potassium content of the fruit and todecrease calcium within the fruit. In contrast toother findings [12] phosphorus did not increase butdecreased storability of the fruit in our experiments.This may be due to phosphorus contents being alreadyoptimal for unthinned fruit. Heavy fruit thinningincreased P-content up to 16 mg/100 g fresh weight,but it is questionable, whether this high P content wasthe principle factor which decreased storability of thefruit.

Also nitrogen content of the fruit was increased bythinning to some extent. But, we generally recordednot more than 50 mg N/100 g fresh weight, and nocorrelation between nitrogen content of the incidenceof fruit disorders could be found.

Calcium and potassium content of composite fruitsamples and fruit from individual trees showed asimilar trend as to bitter pit incidence related to fruitsize or crop load.

There were no strong indications that fruit calciumcontent might be influenced specifically by thinningcompounds. When urea was used as a thinning agent,calcium content was expected to decrease [15, 35],since severe scorching of the leaves was commonlyobserved some days after application. But results werethe same as with the other thinning chemicals; fruitcalcium and potassium concentration at harvest wererelated to crop load mainly.

An occasional exception could be found withNAAm; though it had decreased crop load andincreased fruit size, calcium concentration wasincreased by 1–1.5 mg/100 g fresh weight as comparedto control fruit. This may provide a means for growinglarge storable fruit, provided these findings can besubstantiated by additional trials.

4. Concluding remarks

Undoubtlessly, flower and fruit thinning is one of themost effective techniques in apple growing. Increasingthe intensity of flower and fruit thinning mainlyimproves fruit quality characteristics important for the

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fresh market, such as size and colour of the fruit.Concommitantly, the taste of the fruit representedby sugar and acid content and fruit firmness will beincreased. But the potential of improving fruit qualityis limited, because fruit storability besides total cropwill be impaired when fruit thinning will be performedtoo heavily.

A major difficulty of thinning is to find the optimalthinning intensity. Adjusting it to the requirementsof the market to fruit size, oftenly will not yieldsufficient fruit colour in certain cultivars, and whenoptimized for a perfect fruit colour, too many over-sized fruit susceptible to physiological disorders willbe recorded.

The same holds, when thinning is applied toimprove flower bud quality and to overcome alternatebearing. In the last case especially, thinning should beperformed very early and intensively. To avoid over-sized fruit then, adjusted cultural practices to inducefor instance some water stress or a slight nitrogen defi-ciency, should support the thinning treatment. Thus,the growers can find an individual orchard manage-ment for their local or regional conditions.

Acknowledgements

I would especially like to thank Prof. Dr. M. Bukovac,Michigan State University, Dr. D. Johnson, Horti-culture Research International, East Malling and Dr.J.D. Quinlan, East Malling Trust For HorticulturalResearch, not only for correcting the English but alsofor improving the presentation by valuable sugges-tions.

References

1. Bangerth F (1979) Calcium related physiological disorders.Ann Rev Phytopath 17: 97–122

2. Basak A (1996) Benzyladenine (BA) as an apple fruitletthinning agent – preliminary results. Hort Sci 28: 54–57

3. Bergh O (1985) Effect of the previous crop on cortical cellnumber of Malus domestica cv. ‘Starking Delicious’ appleflower primordia, flowers and fruit. SA J Plant Soil 2: 191–196

4. Bergh O (1990) Effect of time of hand-thinning on apple fruitsize. SA J Plant Soil 7: 7–10

5. Bukovac MJ, Black BL and Hull J Jr (1994) Interactionbetween NAA and BA on cropping and fruit size in ‘Delicious’and ‘Empire’ apples. Hort Sci 29: 472

6. Denne MP (1960) The growth of apple fruitlets and the effectof early thinning on fruit development. Ann Bot 24: 397–406

7. Ebert A and RJ Bender (1986) Influence of an emulsifi-able mineral oil on the thinning effects of NAA, NAAm,

carbaryl and ethephon in the apple cultivar Gala grown in theconditions of southern Brazil. Acta Hort 179: 667–672

8. Goffinet MC, Robinson TL and Lakso AN (1995) A compar-ison of ‘Empire’ apple fruit in unthinned and hand-thinnedtrees. J Hort Sci 70: 375–387

9. Greene DW and Autio WR (1989) Evaluation of benzylad-enine as a chemical thinner on ‘McIntosh’ apples. J Amer SocHort Sci 114: 68–73

10. Greene DW, Autio WR and Miller P (1990) Thinning activityof benzyladenine on several apple cultivars. J Amer Soc HortSci 115: 394–400

11. Hansen P (1982) Assimilation and carbohydrate utilization inapple. Proc XXIst Int Hort Congr, Hamburg, pp 257–268

12. Johnson DS and Yogaratnam N (1978) The effects of phos-phorus sprays on the mineral composition and storage qualityof Cox’s Orange Pippin apples. J Hort Sci 58: 171–178

13. Johnson DS (1992) The effect of flower and fruitlet thinningon the firmness of ‘Cox’s Orange Pippin’ apples at harvest andafter storage. J Hort Sci 67: 95–101

14. Johnson DS (1994) Influence of time of flower and fruitthinning on the firmness of ‘Cox’s Orange Pippin’ apples atharvest and after storage. J Hort Sci 69: 197–203

15. Jones HG and Samuelson TJ (1983) Calcium uptake by devel-oping apple fruit. II. The role of spur leaves. J Hort Sci 58:267–282

16. Jones KM, Graham B, Bound SA and Oakford MJ (1993) Pre-liminary trials to examine the effects of ethephon as a thinnerof ‘Gala’ and ‘Jonagold’ apples. J Hort Sci 68: 139–147

17. Lakso AN, Bepete M, Goffinet MC and Corelli Crappadelli L(1998) Aspects of carbon supply and demands in apple fruits.Acta Hort 460: 13–18

18. Link H (1967) Der Einfluß der Ausdünnung auf Fruchtqual-ität und Erntemenge bei der Apfelsorte ‘Golden Delicious’.Gartenbauwiss 32: 423–444

19. Link H (1968) Welche Früchte fallen beim chemischenAusdünnen ab? Erwerbsobstbau 10: 190–193

20. Link H (1971) Untersuchungen über das Auftreten derRauhschaligkeit bei ‘Golden Delicious’ nach chemischemAusdünnen mit Carbaryl. Erwerbsobstbau 13: 145–148

21. Link H (1986) Einfluß von Ausdünnungsmitteln auf dieFruchtqualität bei Apfel. VDLUFA 20: 797–808

22. Looney NE (1993) Improving fruit size, appearance, andother effects of fruit crop “quality” with plant bioregulatingchemicals. Acta Hort 329: 120–127

23. Martin D, Lewis TL and Cerny J (1964) Apple fruit cellnumbers in relation to cropping alterations and certain treat-ments. Aust J Agric Res 15: 905–919

24. McLaughlin JM and Greene DW (1984) Effects of BA,GA4+7, and daminozide on fruit set, fruit quality, vegeta-tive growth, flower initiation and flower quality of ‘GoldenDelicious’ applies. J Amer Soc Hort Sci 109: 34–39

25. Nelgen N (1982) Über Beziehungen zwischen vegeta-tiver Entwicklung, Fruchtentwicklung und Fruchtqualitä beiden Apfelsorten ‘Cox Orange’, ‘Golden Delicious’ und‘Boskoop’. Ph.D-thesis Univ Hohenheim

26. Pearson JA and Robertson RN (1953) The physiology ofgrowth in apple fruits. IV Seasonal variation in cell size,nitrogen metabolism, and respiration in developing GrannySmith apple fruits. Austr J Biol Sci 6: 1–20

27. Perring MA and Jackson CH (1975) The mineral compositionof apples. Calcium concentration and bitter pit in relation tomeans mass per apple. J Sci Agric 26: 1493–1502

28. Saure MC (1990) External control of anthocyanin formationin apple. Sci Hort 42: 181–218

Page 10: Significance of flower and fruit thinning on fruit quality

26

29. Schumacher R und Fankhauser F (1974) Stippebildung undMineralstoffgehalt von Äpfeln in Abhängigkeit von ihrerStellung in der Baumkrone. Schweiz Landw Forsch 13: 353–364

30. Schumacher R and Stadler W (1985) Einfluss der Fruchtfarbeund verschiedener Ausdünnungsmethoden auf die Fruchtqual-ität. Schweiz Zeitschr Obst-Weinbau 121: 478–483

31. Schumacher R and Stadler W (1987) Zusatzpräparateverbessern die Ausdünnungswirkung von Naphthylacetamid.Schweiz Zeitschr Obst-Weinbau 123: 248–252

32. Sharples RO (1968) Fruit-thinning effects on the developmentand storage quality of ‘Cox’s Orange Pippin’ apple fruits. JHort Sci 43: 359–371

33. Siddiqui S and Bangerth F (1995) Effect of pre-harvestapplication of calcium on flesh firmness and cell-wallcomposition of apples – influence of fruit size. J Hort Sci 70:263–269

34. Silbereisen R (1976) Über die Wirkungen früher undgrößenselektiver Ausdünnungen von Fruchtbehängen auf dasWachstum von Äpfeln. Angew Bot 50: 285–300

35. Volz RK, Ferguson IB, Hewett EW and Woolley DJ (1994)Wood age and leaf area influence fruit size and mineralcomposition of apple fruit. J Hort Sci 69: 385–395

36. Walter TE (1967) Factors affecting fruit colour in apples: Areview of world literature. Ann Rep East Mall Res Sta: 70–82

37. Wertheim SJ, Nijsse F and Joosse ML (1977) Benoni telenzonder doorplucken en beurtjaren. Fruitteelt 67: 475–478

38. Wertheim SJ, Scholtens A and Bootsma JH (1978) Hetchemisch dunnen van enkele beurtjaargevoelige rassen. Fruit-teelt 68: 558–560

39. Wertheim SJ (1987) Jonagold kleurt pas bij veel blad pervrucht. Fruitteelt 77: 15–16

40. Westwood MN, Batjer LP and Billingsley HS (1967) Cell size,number, and fruit density of apples as related to fruit size,position in cluster, and thinning method. Proc Amer Soc HortSci 91: 51–62

41. Winter F (1969) Die “Behangdichtenmethode”, ein Modellzur Analyse und Prognose von Kernobsterträgen. Luxemburg,Agrarstatistische Studien 5: 138S

42. Wismer PT, Proctor JTA and Elfving DC (1995) Benzylad-enine affects cell division and cell size during apple fruitthinning. J Amer Soc Hort Sci 120: 802–807