The photosynthesis inhibitor metamitron is a highly effective thinner for Golden Delicious apple in a warm climate

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1 Fruits, 215, vol. 7(3), p c Cird / EDP Sciences 215 DOI: 1.151/fruits/2157 Aville online t: Originl rticle The photosynthesis inhiitor metmitron is highly effective thinner for Golden Delicious pple in wrm climte Rphel A. Stern MIGAL Glilee Reserch Institute, P.O. Box 831, Kiryt Shmon 1116, Isrel Deprtment of Biotechnology, Fculty of Life Sciences, Tel-Hi College, Upper Glilee 1221, Isrel Received 13 July 214 Accepted 21 Jnury 215 Astrct Introduction. Smll fruit size is limiting fctor in mrketing pple (Mlus domestic). Severl techniques hve een used to improve fruit size, mong them lossom nd fruit thinning with plnt growth regultors such s uxins nd cytokinins to reduce crop lod. Mterils nd methods. In the present study, crried out in 3 consecutive yers ( ), we evluted the photosynthesis inhiitor metmitron (MM) s thinner for Golden Delicious pple. Results. MM pplied once t g h 1 (15 19 mg L 1 in the form of.1%.125% Brevis r t 12 L h 1 ) t the 6-mm fruit-dimeter stge incresed drk-dpted chlorophyll fluorescence, mesured s reduction in Fv/Fm vlues nd resulted in significnt fruit thinning. Consequently, the verge time spent on hnd thinning ws reduced from 2 3 dys h 1 to 2 5 dys h 1 nd there ws considerle nd significnt shift to lrger fruit size. Discussion. The reltively low effective doses of MM compred to those used in Europe nd the USA were likely due to the higher night tempertures for 3 weeks postppliction, which incresed respirtion nd cused ssimiltion deficiencies during tht criticl period of fruit development. Conclusion. In wrm growing environment the thinner MM is highly effective on pple, since only one ppliction (FB+7) of reltively low dose (19 mg L 1 MM s.125% Brevis r ) is enough for optiml fruitlet thinning. Keywords: Isrel / pple / Mlus domestic / thinning / metmitron / crohydrte stress / photosynthesis / fruit size Résumé Efficcité élevée de l éclircissge du pommier Golden Delicious vec l inhiiteur de photosynthèse métmitron utilisé en climt chud. Introduction. Un fruit trop petit est un fcteur restrictif dns l vente de pommes. Pour contourner ce prolème, plusieurs techniques ont été expérimentées, entre utres l éclircissge des fleurs et des fruits pr des régulteurs de croissnce, pr exemple uxines ou cytokinines, pour réduire l quntité récoltée. Mtériel et méthodes. Dns cette étude étlée sur trois ns ( ), nous vons évlué l efficcité du métmitron (MM) dns l opértion d éclircissge du pommier vr. Golden Delicious. Résultts. Une ppliction de MM à une dose de g h 1 (15 19 mg L 1 sous l forme de,1 %,125 % Brevis r à 1 2 L h 1 ) u stde de croissnce du fruit de 6 mm de dimètre ugmenté l fluorescence de l chlorophylle dptée à l oscurité, mesurée en fonction de l réduction du rpport Fv/Fm, et ugmenté l éclircissge des fruits de fçon significtive. En conséquence, le temps requis pour effectuer l éclircissge de fçon mnuelle été réduit de 2 3 jours h 1 à2 5 jours h 1,etldimension des fruits été ugmentée de fçon significtive. Discussion. L efficcité du MM à file dose en comprison ux doses prtiquées en Europe et ux Étts-Unis est prolement due à l tempérture nocturne élevée pendnt les trois semines consécutives à l emploi du MM. Cette tempérture ccentué l respirtion du fruit et réduit l ssimiltion de glucides durnt l période critique du développement du fruit. Conclusion. Sous climt chud l éclircisseur de pommes MM se montre prticulièrement efficce, puisqu un seul tritement u stde FB+7 à une dose reltivement file (19 mg L 1 MM à,125% du produit commercil Brevis r ) est suffisnt pour ssurer un éclircissge optiml. Mots clés : Isrël / pommier / Mlus domestic / éclircissge / métmitron / stress croné / photosynthèse / clire du fruit 1 Introduction Golden Delicious pple trees (Mlus domestic Borkh.) produce n excessive numer of fruitlets. The higher Corresponding uthor: rffi@migl.org.il production reduces fruit size nd induces lternte ering. To otin lrger fruit every yer, fruit thinning hs ecome n essentil prctice in the commercil production of qulity pple fruit [1]. Since hnd-thinning is lorious nd costly, it is not generl prctice in orchrd mngement. Insted, the use of chemicls to thin flowers or fruitlets is

2 128 Rphel A. Stern: Fruits 7 (215) customry [2 4]. Severl compounds hve een used for this purpose, mong them the synthetic uxins 1-nphthlenecetic cid (NAA) or nphthlenecetmide (NAD). These compounds re involved in the formtion of scission lyers nd cuse fruit drop through ctivtion of ethylene, which induces the hydrolytic enzymes polyglcturonse nd cellulse [5]; they lso increse drk respirtion nd decrese photosynthesis [6, 7]. Another compound used for thinning is the cytokinine 6-enzyldenine (BA), which hs een shown to lock emryo development y cusing severe reductions in crohydrte levels; this, in turn, reduces polr uxin trnsport cross the fruit pedicel nd enhnces the scission zone s sensitivity to ethylene, leding to fruit drop [8, 9]. The trizinone hericide metmitron (MM) hs een recently reported to hve thinning ctivity in pple vi inhiition of photosynthesis [1,11]. Specificlly, it is photosystem II (PSII) inhiitor tht disrupts the photosynthetic pprtus for 7 to 1 d fter ppliction, reducing electron trnsport rtes [12]. The normlized rtio of vrile fluorescence to mximum fluorescence (Fv/Fmx) represents the mximum potentil quntum efficiency of PSII, which is c..8 in most plnt leves [13]. Chnges in this rtio fter tretment were used to study the effect of MM on photosynthesis inhiition. MM exhiited thinning ctivity when pplied to pple fruitlets t the 1- to 12-mm dimeter stge ([1,11] or even lter, t 2 mm [12]. The ojective of the present study ws to compre the new thinning gent MM (Brevis r ) to the commercil uxins NAA nd NAD (Agriton r ) nd the commercil cytokinin BA (MxCel r ) pplied to Golden Delicious pple grown in the wrm climte of Isrel. 2 Mterils nd methods 2.1 Experimentl orchrds nd conditions Experiments were conducted over three sesons etween 212 nd 214 on mture Golden Delicious pple trees MM 16 rootstock in two orchrds: (i) t Ortl, locted in the Goln Heights, 95 m ove se level (sl). The trees were 3. m high, t 4.5 m 2.2 m spcing (1,1 trees h 1 ); (ii) t Mtityhu orchrd locted in the Upper Glilee, 7 m sl. The trees were 3.3 m high with the sme spcing s in Ortl. Both orchrds were locted in semirid region with high tempertures (c. 35 C mx.) nd low humidity (< 4% RH) during the summer (My-Octoer). Annul rtes of precipittion (Novemer-April) were pprox 7 mm nd 9 mm t Mtityhu nd Ortl respectively. The soil t oth sites ws m deep, nd clssified s well-drined protogromosol (68% cly) on terr ross soil t Mtityhu, or on sltic rocks t Ortl. The soil ph ws 7.7 t Mtityhu nd 7.3 t Ortl with CCO 3 content of pprox 7% (w/w) t oth sites. The irrigtion system t oth sites consisted of two lterl lines per row, plced 1. m prt, with 1.6 L h 1 pressure-compensted in-line drippers (Netfim, Iftch, Isrel) spced.5 m prt. 2.2 Chemicl ppliction Three commercil products, contining uxin, cytokinin or MM, were pplied: (1) Agriton r, liquid formultion contining 1.8% (v/v) NAD plus.68% (v/v) NAA, mnufctured y Nufrm, Frnce; (2) MxCel r, liquid formultion contining 1.9% (v/v) 6-BA, mnufctured y Vlent Bio- Science USA; (3) Brevis r, contining 15% (w/w) MM, mnufctured y Mkhteshim-Agn, Isrel. Chemicls were pplied s folir spry t 1.2 L tree 1 using high-pressure hndgun [Experiments (Expt.) 1 nd 2] or t 1,2 L h 1 using n ir-lst Spidet lower spryer (Expt. 3 nd 4, semi-commercil trils). The nonionic surfctnt Triton X-1 ws only included in the commercil control of the Agriton r tretments t.25% (v/v). Control trees were not spryed ut hnd-thinned in mid-june with c. 2 3 dys of work h 1 in ll experiments. Expt. 1 (212). In preliminry experiment, conducted in 212, three concentrtions of Brevis r (MM) pplied t full loom + 7dys(FB+7 = 6-mm dimeter stge; I) FB+14 (1-mm stge; II) or oth (I+II), were exmined: 1..6% = 9 mg L 1 MM = 11 g MM h 1 (MM 11 I or MM 11 II or MM 11 I+II); 2..8% = 12 mg L 1 MM = 145 g MM h 1 (MM 145 I or MM 145 II or MM 145 I+II); 3..1% = 15 mg L 1 MM = 18 g MM h 1 (MM 18 I or MM 18 II or MM 18 I+II). The commercil control ws Agriton r.4% = 73 mg L 1 NAD + 27 mg L 1 NAA t FB + 3. The control tretment did not receive ny chemicl tretment. Expt. 2 (213). In 213, the experiment ws conducted with new set of trees in the sme orchrd s Expt. 1. We incresed the MM concentrtion to 19 mg L 1 = 225gMMh 1 (.125% Brevis r ) exmined only once t FB+7 nd compred it to the highest concentrtion of Expt. 1 (15 mg L 1 MM = 18 g MM h 1 =.1% Brevis r )which ws exmined once t FB+7 orfb+14 or twice (on oth dtes). The commercil controls were Agriton r.4% t FB+3 nd MxCel r.75% = BA (15 mg L 1 )tfb+7. Untreted control trees did not receive ny chemicl tretment. In ddition the return loom of trees from Expt. 1 (212) ws recorded in spring 213 y mesuring the flowering intensity per tree ( = no flowering; 1 = low, 2 = medium, nd 3 = high flowering). Expt. 3 (213). A semi-commercil experiment ws conducted in 213 t Ortl orchrd. One ppliction of the highest concentrtion of 19 mg L 1 MM = 225 g MM h 1 (.125% Brevis r )tfb+7 or two pplictions of.125% Brevis r (225 g MM h 1 ) t FB+7 plus.8% Brevis r (145 g MM h 1 )tfb+14 were exmined in comprison to untreted control trees. Expt. 4 (214). In 214 nother semi-commercil experiment ws conducted in the sme orchrd s Expt. 1 nd 2. One ppliction of 19,225 or 27 g MM h 1 (.1%,.125% or.15% Brevis r, respectively) t FB+7 ws exmined in comprison to untreted control trees. Two new mesurements were performed: 1. Photosynthesis inhiition: Chlorophyll fluorescence mesurements were crried out on five recently fully expnded leves from four trees per tretment (three concentrtions of MM vs. untreted control trees) every other dy for 1 dys fter MM ppliction. Fluorescence ws mesured

3 Rphel A. Stern: Fruits 7 (215) A Numer of fruit tree Metmitron concentr on (g h -1 ) + me of pplic on B Yield of lrge fruit (kg tree -1 ) Metmitron concentr on (g h -1 ) + me of pplic on Figure 1. Effect of single or doule ppliction of.6%,.8% or.1% (v/v) Brevis r [11, 145 or 18 g Metmitron (MM) h 1 ]tfull loom (FB) + 7 dys (I), FB + 14 dys (II) or oth (I+II), or single ppliction of.4% (v/v) Agriton r (73mgL 1 NADplus27mgL 1 NAA) t FB + 3 dys s commercil control on the numer of fruit per tree (A) nd the yield of lrge fruit per tree (B). Tretments were pplied t 1.2 L tree 1 using hndgun spryer. Ech dt point is the men of six trees per tretment. Different letters denote significnt differences etween the mens ccording to Duncn s new multiple rnge test (DMRT) t P.5. (Expt. 1). in the drk etween 5: m nd 6: m, using portle Mini-Pm Photosynthesis Yield Anlyzer (Heinz Wlz GmH, Effeltrich, Germny). 2. Fruit-set percentge: At the lloon stge, 5 flowers per tretment (5 flowers per cluster 5 clusters per rnch five 3-yer-old rnches with similr spurs 4 trees) were leled t c. 1.5 m height, lwys on the south side of the row. The percentge of initil fruit set ws determined 3 weeks fter tretment (1 My 214), efore June drop. 2.3 Sttisticl nlysis The experiment consisted of rndomized complete lock design, with six replictes (locks) of 1 tree per tretment (Expt. 1 nd 2) or four replictes of 1 trees per tretment (Expt. 3). At hrvest, the yield from ech tree ws weighed nd ll fruit were sorted ccording to dimeter: smll (< 65 mm), medium (65 75 mm) or lrge (> 75 mm). Percentge dt were sujected to rcsine trnsformtion efore nlysis to provide norml distriution. Dt were nlyzed for sttisticl significnce using generl liner model (GLM) procedure. Duncn s new multiple rnge test (DMRT) ws used to compre tretments when ANOVA showed significnt differences mong mens using JMP softwre (SAS Institute Inc., Cry, NC, USA). 3 Results 3.1 Preliminry experiments In the preliminry experiment (Expt ), the MM tretments hd thinning effects (figure 1A). However, there ws only one tretment tht ws significntly different from the control. Nevertheless, there ws trend of response to dose nd time of ppliction. At oth ppliction times (FB+7 nd FB+14) the highest concentrtion of 18 g MM h 1 reduced

4 13 Rphel A. Stern: Fruits 7 (215) Fruit-size distriu on (kg tree -1 ) B 39 A 91 A 5 B 141 A Smll (<65) Medium (65-75) Lrge (>75) Fruit dimeter (mm) MM 18 I Untreted Control Figure 2. Effect of the optiml Metmitron (MM) tretment from Expt. 1: 18 g MM h 1 s.1% (v/v) Brevis r t full loom (FB) + 7 dys on fruit-size distriution. Different lowercse or uppercse letters within ech size group denote significnt differences etween the mens of kg tree 1 or fruit no. tree 1 (mens in the rs), respectively, ccording to DMRT t P.5. the fruit numer compred to the intermedite nd lowest concentrtions (145 nd 11 g MM h 1 respectively) lthough not significntly. However, the efficiency of the tretments t the erly ppliction (FB+7) ws higherleit not significntly so, compred to the lte ppliction t FB+14 (figure 1A); s result the yield of lrge fruit from the erly tretments ws much higher (figure 1B). For exmple: 18 g MM h 1 t FB+7 reduced the numer of fruit per tree from 444 in the control trees to 279 nd s result gve high yield of lrge fruit: 27 kg tree 1 compred to only 1 kg tree 1 in the untreted controls (figure 1B). The sme concentrtion ut t FB+14 reduced the fruit numer to 345 nd the yield of lrge fruit ws only 8 kg tree 1. Two pplictions t ech of these concentrtions did not improve the results of only one ppliction t FB+7. The fruit-size distriution fter the optiml tretment (18gMMh 1 t FB+7) vs. untreted control trees shows tht this tretment lso douled the yield of the medium fruit size nd drmticlly reduced the yield of smll fruit (figure 2). The commercil control Agriton r (NAD+NAA) gve results similr to those of the optiml MM tretment. In the second yer (Expt ), we exmined the optiml concentrtion found in Expt. 1 (18 g MM h 1 )given t FB+7, FB+14 or oth. In ddition we incresed the concentrtion to 225 g MM h 1 ut only t the erlier ppliction of FB+7, which showed etter results thn FB+14.All MM tretments showed strong thinning effect which influenced fruitsize distriution, i.e., c. hlf the yield of smll fruit (pprox. 2 vs. 4 kg tree 1 ) nd out 1-fold tht of lrge fruit with no influence on the yield of medium-size fruit (figure 3). However, 18 g MM h 1 t FB+7 ws enough to produce optiml results. The higher concentrtion of 225 g MM h 1 or two pplictions of 18 g MM h 1 were not etter thn only one erly ppliction (FB+7) t the lower dose (18 g MM h 1 ). As in Expt. 1 the lte ppliction t FB+14 ws less efficient thn the erly one. The commercil controls gve good results, especilly with Agriton r (NAD+NAA). The return loom in spring 213 on trees from Expt. 1 (212) showed no negtive effects compred to the untreted 4B control trees (figure 4). In fct, it ws even slightly, leit not significntly, higher. The optiml tretment of 18 g MM h 1 t FB+7, which reduced fruit numer nd improved fruit-size distriution in 212 (Expt. 1) ws gin the est tretment: it lmost douled the flowering intensity, while the sme concentrtion ut t lter ppliction (FB+14) gve much lower flowering similr to ll other tretments t lte ppliction. 3.2 Semi-commercil experiments In the semi-commercil experiment with the ir-lst spryer (Expt t Ortl) we expected high yield. Therefore, we exmined the highest concentrtion from Expt. 2 (225 g MM h 1 )tfb+7compred to the sme tretment plus nother ppliction of 145 g MM h 1 t FB+14. Indeed, the yield ws high (85 kg tree 1 = 85 t h 1 )nd oth tretments reduced fruit numer considerly (dt not shown). As result the fruit-size distriution ws improved, with lower yield of smll fruit (< 65 mm) nd higher yield of medium (7 mm) nd lrge (> 75 mm) fruit (figure 5). As in Expt. 1 nd 2, the second ppliction of MM did not give etter results thn only one ppliction t the erlier time (FB+7). In the third yer (Expt ) we conducted nother semi-commercil experiment to exmine the higher concentrtion of 27 g MM h 1 (.15% Brevis r ) t the optiml time of ppliction (FB+7). All MM tretments reduced the mximum potentil quntum efficiency of PSII from 1 to 7 dys fter tretment, mesured s n increse in drk-dpted chlorophyll fluorescence (reduction in Fv/Fm vlues) compred to the untreted control (figure 6). However, the lowest dose of 18 g MM h 1 (.1% Brevis r ) hd only slight effect compred to the moderte dose of 225 g MM h 1 (.125% Brevis r ), which reduced the Fv/Fm vlues drmticlly from c.8 in control trees to only c..4. The highest dose of 27gMMh 1 (.15% Brevis r ) did not differ from the moderte dose. The initil fruit-set percentges of ll MM tretments were lower thn the control nd showed considerle dose response. The lowest dose hd only slight effect (55.4% vs. 59.4% in the control trees), while the intermedite nd highest doses reduced the fruit-set percentge to 22.6% nd 19.6% respectively i.e. one-third of the control. The correltion etween Fv/Fm vlues s n verge of ll effected period (FB+7 until FB+17) nd the initil fruit-set percentge ws positively nd significntly high (R 2 =.95; P =.5). In control trees 2 3 dys h 1 were needed for hndthinning t the end of June, compred to 2 5 dys h 1 following the optiml tretment in ech yer, depending on crop lod. 4 Discussion 4.1 Plnt metolism involved Smll size is one of the limiting fctors in mrketing pple fruit; therefore, the economic enefits of tretments tht

5 Rphel A. Stern: Fruits 7 (215) Fruit-size distriu on (kg tree -1 ) MM 18 I MM 18 II MM 18 I+II MM 225 I NAD+NAA (FB+3) BA (FB+7) Untreted Control c Smll (<65) Medium (65-75) Lrge (>75) Fruit dimeter (mm) Figure 3. Effect of single or doule pplictions of.1% or.125% (v/v) Brevis r [18 or 225 g Metmitron (MM) h 1 ] t full loom (FB) + 7 dys (I), FB + 14 dys (II) or oth (I+II), single ppliction of.4% (v/v) Agriton r (73mgL 1 NADplus27mgL 1 NAA) t FB + 3 dys, or single ppliction of.75% (v/v) MxCel r (15 mg L 1 BA) t FB + 7 dys on fruit-size distriution (Expt. 2). Ech dt point is the men of six trees per tretment. Different letters within ech size group denote significnt differences etween the mens ccording to DMRT t P.5. c 3,5 3 Flowering intensity (-3) 2,5 2 1,5 1,5 Figure 4. Effect of tretments in 212 (Expt. 1) on the return loom in spring 213. Flowering intensity per tree ws recorded s follows: = no flowering; 1 = low; 2 = medium, nd 3 = high flowering. Different letters denote significnt differences etween the mens ccording to DMRT t P.5. cn improve verge fruit size re potentilly very high [1,14]. Severl techniques hve een used to improve pple fruit size, mong them ppliction of synthetic cytokinins such s CPPU or BA t FB+14, to stimulte cell division in the fruit [15,16]. However, when the numer of fruit is too high, the efficiency of these tretments decreses ecuse of the limited supply of ssimiltes from the leves. Another pproch to increse fruit size is y reducing fruitlet numer from the eginning of fruit growth [2, 4, 14]. This cn e chieved y limiting the production of crohydrtes in the tree during the 2- to 3-weeks period fter loom, precisely when the demnd for ssimiltes y the fruitlets is highest [17]. This is supported y Botton et l. [8], who showed crohydrte deficit in the trees nd fruit cortex immeditely fter tretment with 6-BA, which cused lterl fruit scission, nd y Dsh et l. [18], who showed reduction in erly fruit growth of pple nd thinning fter severe shding which limited crohydrte production nd decresed cell production nd expnsion. Recent reports hve ccumulted from the USA nd Europe on the PSII inhiitor MM, whose reduction of photosynthesis reduces fruit set in pples when pplied t the 6- to 1-mm dimeter fruit stge [1 12]. Indeed, our results from 214 (Expt. 4) support this. However in ll of those reports, the optiml mount of MM ws c g h 1 t the

6 132 Rphel A. Stern: Fruits 7 (215) MM 225 I MM 225 I + MM 145 II Untreted Control Yield (kg tree -1 ) Fruit dimeter (mm) Figure 5. Effect of one ppliction of.125% (v/v) Brevis r [225 g Metmitron (MM) h 1 ] t full loom (FB) + 7 d (I) or two pplictions of.125% (v/v) Brevis r given t FB + 7 d (I) nd.8% (v/v) Brevis r (145gMMh 1 )tfb+ 14 d (II) on fruit-size distriution (Expt. 3). Ech dt point is the men of 4 trees per tretment. Different letters within ech size group denote significnt differences etween the mens ccording to DMRT t P.5.,9,8,7 Fv/Fm,6,5,4, Dys er loom control MM 18 MM 225 MM 27 Figure 6. Effect of single ppliction of.1%,.125% or.15% v/vbrevis r [18, 225 or 27 g Metmitron (MM) h 1 ] t full loom (FB) + 7 dys (1 April 214) on drk-dpted chlorophyll fluorescence (Fv/Fm) in leves (Expt. 4). Tretments were pplied using ir-lst spryer t 1,2 L h 1. Ech dt point is the men of 4 trees x 5 leves per tretment. Different letters for ech dy s mesurements denote significnt differences etween the mens ccording to DMRT t P.5. Arrow indictes ppliction dte. first ppliction (6 mm) nd c g h 1 t the second (1 12 mm). Our findings showed optiml results with the lower concentrtion of 18 g MM h 1 (figures 1 3) givenin only one ppliction (FB+7 = 6-mm stge). Even when yield ws very high, nd the MM concentrtion ws slightly higher (225 g h 1 ) there ws good thinning effect with only one erly ppliction (FB+7; figure 5). Similr results of thinning tht improves fruit-size distriution nd sves working dys on fruit hnd-thinning were otined with Gl, Red Delicious nd Pink Ldy in 212 nd 213 in the sme orchrds (dt not shown). 4.2 The effect of temperture The reson for the strong response, even with the lower concentrtion of MM, could e the higher intensity of drk respirtion in Isrel t the criticl time of fruitlet growth, which uses ssimiltes insted of converting them into new cell mteril in the fruit [19, 2]. The verge night temperture during April-My (from FB until FB+21) over the 3 yers of the experiment (212, 213, 214) ws 13.7 C, 12.8 C nd 14.6 C respectively. These high night tempertures increse respirtion [19, 21] nd my increse the sensitivity of the fruitlets

7 Rphel A. Stern: Fruits 7 (215) to photossimilte deficiency [22 24]. As result, the tree proly needs less MM compred to the colder orchrds in Europe nd the USA. The verge night tempertures in pplegrowing res in Europe like Sud-Tirol in Itly or Est-Mlling in Englnd re c. 1 C lower thn in Isrel during the three weeks fter loom [25]. Growing Empire pples under controlled environmentl tempertures, Yoon et l. [24] discovered the criticl influence of high night tempertures on the consumption of ssimiltes, which incresed fruitlet-scission levels. The min finding ws tht high dy/night tempertures (29/21 C) in the first 3 weeks fter loom decrese net photosynthesis levels, cusing significnt scission in comprison with low dy/night tempertures (24/13 C), regrdless of the different thinning tretments. Another possile reson for the higher efficiency of MM in Isrel is the higher verge dily tempertures (3-yer verge of 28 C in Isrel vs. c. 15 C in Sud-Tirol), which cn increse the efficiency of photosynthesis inhiition y MM compred to the lower tempertures in Europe. The verge mximum dy temperture during the first week fter MM ppliction in 214 (Expt. 4) when photosynthesis ws reduced (figure 6) ws28 C. Together, these oservtions strengthen the hypothesis tht temperture regime during the tretment hs drmtic influence on the efficiency of the chemicl used for thinning [26]. The drmtic reduction in Fv/Fm vlues in Isrel (from.8 to.4; figure 6) nd tht reported in the USA y McArtney nd Oermiller [12] (from.8 to.7) supports this hypothesis. 4.3 Adpting to glol wrming Climte chnges re likely to impct key spects of temperte perennil horticulturl physiology nd production. For exmple recent oservtions of the timing of pome fruit flowering hve shown trend towrds erlier emergence. In Frnce nd South Afric flowering of Golden Delicious pple hs dvnced y c. 3. nd 2. dys per decde respectively [27, 28]. These chnges hve een ssocited with recently oserved wrming in these regions. With projection for wrming to continue into the future [29] it is likely tht the timing of flowering will continue to occur erlier nd the risk for spring frost, or potentil seprtion of the flowering periods of cross-pollinting species tht could led to poor fruit-set will increse. Dryshire et l. [3] in Southern Austrli nd Luedeling et l. [31] demonstrted tht flowering time my move in different directions in different loctions under wrming climtic conditions. For loctions predicted to receive less winter chilling, dely in chieving the chilling requirement my led to n overll dely of flowering. However, those loctions with stle or incresing chill will likely respond to climte chnge with erlier flowering. The influence of glol wrming on spects of plnt physiology such s photosynthesis efficiency or respirtion intensity, s we hve shown in this study will e very significnt in the future. These effects will e especilly importnt in wrm pple-growing res such s Est-Asi, Austrli, Afric, Ner-Est nd even prts of Europe nd North Americ, nd will need to dopt suitle techniques for mintining productivity. 5 Conclusion MM pplied once t g h 1 (15 19 mg L 1 in the form of.1.125% Brevis r t 1,2 L h 1 )tthe 6-mm fruit dimeter stge resulted in significnt fruit thinning which in turn cused considerle shift to lrger fruit size. These reltive low effective doses noted in the wrm pple growing re of Isrel compred to the higher doses needed in Europe were likely due to the higher night tempertures for 3 weeks post ppliction which incresed respirtion nd cused ssimiltion deficiencies during tht criticl period of fruit development. This study, which showed tht higher night tempertures cn increse fruitlet sensitivity to the reduction of ssimiltes y MM, suggests tht lowering the rte of MM used for chemicl thinning could prove to e one wy to dpt to glol wrming. Acknowledgements. I m grteful to Mxi Shulmn from Brm for his vlule ssistnce with the experiments. References [1] Byers R., Flower nd fruit thinning nd vegettive:fruiting lnce, in: Ferree D.C., Wrrington I.J. (Eds.), Apples: Botny, production nd uses, CABI Pulishing, Wllingford, UK, 23, pp [2] Dennis F., The history of fruit thinning, Plnt Growth Regul. 31 (2) [3] Greene D., Cost G., Fruit thinning in pome- nd stone-fruit: stte of the rt, Act Hort. 998 (213) [4] Morn R., Southwick S., Chemicl loom thinning of pome nd stone fruits, In: Bsr A. 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