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i. Erner, Y., R. Goren and S. P. ,\Jonselise. 1976. The rough fruit condition of the 'Shamouti' orange-Connection with endogenous hormonal balance. ]. Ho1·t. Sci. 51:367-374. 8. - - - - - , - - - - - - and ------. 1976. The reduction of peel roughness of 'Shamouti' orange with growth regulators. ]. Amer. Soc. Hart. Sci. 101:513-515. 9. - - - - - - , S. P. ;\lonselise and R. Goren. 1975. Rough fruit condition of the 'Shamouti' orange-Occurrence and patterns of development. Physiol. Veg. 13:435-4-13. 10. Feinstein, B., S. P. :\Ionselise and R. Goren. !975. Studies on the reduction of seed number in mandarins. HortScience 10:385-386. 11. Fishier, ,\lichal and S. P. ;\fonselise. 1971. The use of ethephon (2chloroethyl phosphonic acid) to promote color development of 'Shamouti' orange fruits. Israel]. Agric. Res. 21:67-77. 12. Gibson, A. C. 1975. Developmental studies of pvriform fruits of grapefruit.]. Amer. Soc. Hart. Sci. 100:674-678. 13. Goldschmidt, E. E. and S. P. Monselise. 19i7. Physiological assumptions toward the development of a citrus fruiting model. Proc. Int. Soc. Citriculture 2:668-6/2. 14. Goren, R. and S. P. Monselise. 1965. Interrelations of hesperidin, some other natural components and certain enzyme systems in deYcloping 'Shamouti' orange fruits.]. Hart. Sci. 40:83-99. 15. - - - - - - and - - - - - - . 19il. Effects of ringing on yield of low bearing orange trees. 1- Hart. Sci. 46:435-441. 16. Gustafson, F. G. !939. The cause of natural parthenocarpy. Amer. ]. Bot. 26:135-!38. li. Krezdorn, A. H. 1969. The use of growth regulators to improve fruit set in citrus. Proc. lsi Int. Citrus SymjJ. 3:1113-lll9. 18. Lewin, I. J. and S. P. ,\fonselise. 1976. Further ·t . tion of seeds in mandarins by !\'.-\.-\ spraYs. Scie', ~ldte, r,n th. 19. ;\lonselise, S. P. Growth regulators used to ex ten '1 "' ![,,._ ;'.-~-::'iii of grapefruit. I Congreso Mundial de Citricu~t the t'i<kin;'"'li4. 393-398. '"a, H;; \~ 4 . ' ' -~ 20. ------, R. Goren and Israela 1\'allerstein. Jrt- 0 on orange fruit set and young fruit abscission ·;;· Cirr!;;,., 515. ' rL \ri,·nr: 21. - - - - - - and .-\. Sasson. 19i7. Effects of urch . . . 1 orange fruit quality and storage abilitY. Pwc. 111 ~ d trearn 1rn•· ·.Soc ° r. · "Ill! , 1 ·939 •- -M 23-' ''lTf(•t&lJ 22. ------, ,\L Weiser, ~- Shafir, R. Goren and F : "'~ ~I 1976. Creasing of orange l)eel-l'hniolog.- and cr 1,~- L. Go],!•• ~,..._ _ ·, ; • ... ~ • ' ttroJ. } .... ,~ . .lt ,, ::>1.341-3~1. . . J -/f~rt.\:t''l ' 23. Peled, ~oa, B. Fe1ns~eu~ and S. I . ~I_ons~lisc. 1();G . · . ,, rt' seed number and frmt srze regulatron rn ·I cmpl!'' b; Rcduulf11111 \" homwnal means. (Heb1ew). A ion Hmwtea 3! II" II~ hcllll<~] &tit ::<l 24. Soost, R. K. and R. H. Burnett. 1961. Effect o[.o 11 ;,· h. ', . · · · • mand,u " I'>t·n·lltn '''' ·~ · :• • · c h aracte11strcs an d f rmt o f 'Cl ementme 111 11 Ho1t. Sci. /7:194-201. · "- -~''"'.!.,. !].1 25. vVallerstein, _Israela,_ R._ Goren and S. P. MonscJisc. l'l-· . changes m grbberellm-hke substances of 'Shatnorni' .: d. Sn"lllll sinensis (L.) Osbeck) trees in relation to rinoing ,O'j~ 1 ~"' lf,tr111 75-82. " · · · '"'· \'ci. !l:. j)[l 26. \Vutscher, H. K. 1976. Influence of night tcntpnatur .. on fruit shape of grapefruit. I- A1ner. Soc. Ho,t. Sii. ~O~~~~ •1~!11~ . :.•· I ,, I) 0 ..)j3·~)jl ·;t't' j I' _:,: '\[l Il ' !.til :l~ I) PHYSIOLOGICAL ASSUMPTIONS TOWARD THE DEVELOPMENT OF A CITRUS FRUITING MODELl E. E. S. P. MONSELISE Department of Horticulture, Hebrew University of jerusalem, Rehovot, Israel GOLDSCHMIDT AND Additional index words. biomass, flowering, abscission, fruit set, fruit size, plant hormones, photosynthesis. Abstract. The difficulties in assessing the fruiting potential of a perennial plant like citrus are outlined. A biomass balance sheet for citrus trees is compiled using the 'Shamouti' orange as a standard variety. The sequence of biological processes from flowering through abscission and fruit set to the acquisition of full size are analyzed from a biometric point of view and assembled into a relational flow-diagram. Three major "decisions" seem to determine the yield: The no. of flowers, the final percentage of fruit set and the potential for fruit enlargement. The existence of control mechanisms to secure a reasonable crop is indicated. The role of plant hormones and availability of photosynthate as principal regulatory mechanisms of fruiting in citrus is evaluated. The ever increasing basic knowledge of Citrus allows today, more than in the past, to provide a rather detailed description of the host of environmental and endogenous factors which modulate the biological systems of production. The translation of such knowledge into a series of coefficients can be further transformed into equation-models which may eventually be utilized for economical forecast as well as for gaining a broader overall understanding of the biological processes themselves. By doing so, the integrated lThe authors are indebted to Prof. Y. Yaadiah for stimulating discussions during the early phases of this study. This work was supported by the Israeli l\finistry of .-\griculture. 668 - ~ t Pr-oc. Int. Soc. Citriculture. 2:668-672. 1977. tH '\( ! )\l<l 1 behavior and the manifold interactions of dilfcrent iwli~/i:1 vi duals in a large plant community or witlti 11 a populatitln , ''. ~ ' ' f ocus. rl'l 1ese are tl1e .,,to 1' . on t I1e same tree comes Into . o f f nuts biometric aspects which cannot be fully ev~tluated bv mean! 1 :l of physiological investigations at the individual org:;n lml 1 1 Methods of simulation-modeling· have lreen mer! until , ·nplr, now mostly in the study of annual plants such as forage or i ·ttha pasture crops which are consumed in full, or industria! uor~ t a_s ~otton (16) and alfalf~ (6). With trees work ltas hl'rll 1 hmlted to wood productiOn by forest trees (1·1) wherrb1 only part of the peculiar characteristics of perennial pl.ull ! Fi~ productivity are taken into account. Quite recently some · ,.,, o work with peaches has been published (1). Tints, when lJ> ·ut~t proaching the problem of citrus productivity it sccmsa~,.\:>a1 propriate to outline several major differences bctween [rUJt ,, n trees and annual crops. . l1l The complexity of a large tree unit bearing at one wne . 'Ill! a variety of organs differing in age and activity is i1;unctlr· j .t. ately evident. Even more important is the fact that tile rre-: which builds up its skeleton during its early years, a~cumu lates reserves which are not necessarily utili1etl dunng tht. same season. The long time storage of pan of the rcscflli in the tree makes it difficult to prepare a detailed balan(t 1 sheet of annual and cumulative production aml pre1·c~Jt; 1111 direct and reliable estimation of potential lltT producll , ..,., , 0 After years of investigation into the biological aspect\ ; 1 citrus tree productivity it seems now time to pool dcLll ~ physiological knowledge into an integrated lramew_ork ~I .',~; lowing an overall description of the beh:l\ ior ot t!llfete:: j ''" spe_cies of citrus unde1: v_ariou~ condi Lions wit il the prac:~ . -,~·;.' ultimate go~ I- of attami~g higher product!\ IL ~, t~not\It ·:::!, better_ defimt~on of optimum growth conclJtiOm- I~-e"<C ·''" followmg sectwns we will analyze the sequence oi P10 . -~ l"tt involved in fruiting of citrus trees, pointing out eu-n.. 1' '.,[ knowledge as well as missing links. l 't I ! ., II!,) I I I fruiting Potential and Biomass Balance Sheet collected in past years concerning 'Shamouti' or(Citrus sznenszs (L.) Osbeck) have been used for n of the yearly biomass balance sheet which the first step to·wards modeling of a biological our sheet (Table 1) includes some rough approxibut can neYertheless form the basis for the expresbioma'' in a medium, 20-year old 'Shamouti' tree to 5,1·eet lime (Citrus limettioicles Tan.), with a top of 26 m: (11 ). ·we suggest that the balance of antiritY can be calculatedv,;ith a perennial plant havJ( ' . l l l l ' . 1 . n!ans ,dliC 1 c eve opec In prevwus seasons, )y bnng~tno account the rate of renewal of perennial and other 111 • percentage d ry wt. A mature tree d oes not and their ·tr<re annual addition to its main skeleton; however a l' b branches and roots are produced, ~he volume of existing jun·cascs and starch res~rves bmld up. On the other the turnover of leaves IS almost complete every year [ flowers. frui tlets and fruits are by definition a new 1 product. Dry wt. productio? is. in the rang~ of 40 per yc~n_-, about h~lf of which 1s mature frmt. The is invested Ill productiOn of leaves (10 kg/tree), which build up the yearly dry matter necessary for growth ,ield. Shedding of Bowers and fruitlets is a waste in 'of eflicicnt dry matter utilization. The sheet presented Table l is only the expression of factual evidence but uot tell us if and how the balance can be improved to yield. seems important to ascertain whether the photosyndficiency of leaves is the limiting factor and if the ·nthetic performance of a given variety can be imThe fact that 'Marsh' seedless grapefruit (Citrus i\facL) trees are able to produce up to twice as much 1ti' orange trees of comparable size and more than 'Valencia' orange trees (Table 2) illustrates Grapefruit and oranges do not seem to vary in their net assimilation rates (7) and the reasons for different fruiting potential must therefore be more related perhaps to sink-source control of phototranslocation or to photorespiration (10). Fruiting Processes in Citrus Fig: I depicts .in a very. schematic way the chronological ot processes mvolved m crop production of 'Shamouti' over a period of I c1months. Processes along the time spelled out in rectangles above the two main curves . the no. of reproductive units present at any time thCJr average wt. Using this scheme we will discuss the critical stages of the fruiting system. I. .\ yearly biomass sheet for an average 20 year old 'Shamouti' otange on sweet lime with a top volume of 26 m3. Total Fresh fresh wt.(organ No. of wt.(tree unit (g) units(tree (kg) % dry wt. 40,000 25,000 5,000 100 26 16 5 50 40 15 20 10 100 100 100 5.0 10.5 2.5 1 200.00 500 100 20 100 20 Int. Soc. Citriculture, 1977, Vol. 2. .. Species Yarien Citrus sinensis Source Fruit Yield kg( tree Fresh wt of Jeayes kg(tree Kg fruit per kg leaves Valencia late (li) 60 30 2.0 Citrus sinensis Shamouti (11) 100 26 3.8 Citrus paradisi :\farsh seedless (17) 700 50 14.0 The flower formation stage. This is certainly a critical stage as a hypothetical lack of Bowers would mean no yield at all. Flower no., however, is very rarely a limiting factor as the large ~ajority of Bowers drops anyway and very few of th~m persist on the tree to become mature fruits. Only in margn:al cases (young trees entering the bearing age, offye~r~ m alternately bearing varieties) the no. of Bowers o~·Igmally produced is insufficient to produce satisfactory yields. On the other hancl, many relatively unfruitful types ('Clementine' mandarins, certain 'Navel' strains) produce only a marginal crop despite extremely profuse amounts of flowers (in the range of 2 x 10" and more). In this case it is co~ceivable that excessive Hower production may hamper satisfactory set. . In most varie~ies, when flower population varies over a wide range, we. fa1l to detect a clearcut relationship between Hower ancl fr~It no. At most, we find a moderate optimum curve connectmg the extremes. Additional data are needed to learn more about the significance of this curve. Type of inflorescence and flower position. Not all flowers have identical chances of set. There is a considerable no. of abortive Bowers (mostly staminate) or defective because of pest and diseases. About 35% 'Shamouti' Bowers (15) are mcapable of setting. In certain varieties, staminate flowers can attain very high percentages for internal or climatic causes which need elucidation. The type of inflorescence and the position of individual Bowers are also factors in set. It has been shown by different workers (8, 9, 12) that mixed -· ~r~[----~ ~ tg>- FRUIT DEVELOPMENT 1! i MATURATION ~~~_N__j ~N~j ~J -~----~---------.. __j•IL_ ____ I INOUC- 1 [FLUSH&ll TION & i :FLOWER, D.IFFEREN-j DEVELORj j 1 ~-DROPG I I II Ill L .L __j__ _ PRE-I HARVEST I DROP i Annual Annual dry incrematter ment producrate tion/tree (Kg) (%) 0.65 0.65 1.00 247 Table 2. Fresh wt of leaYes and of mature fruits, as well as the efficiency of one kg leaYes as to Yield production in different \·arieties and species. Approximate data from different sources, as indicated. 39.0 Fig. 1. Schematic description of the chancres in the no. of reproduc· tive units (flowers, fruitlets, fruits) and th~ weight of the individual reproductiYe unit during the course of 'Shamouti' orange fruit development. Rect.angles. above the 2 main curves spell out rna jor processes along the t1me ax1s. 669 inflorescences comprising flowers and leaves are most successful in producing fruits. In one specific case (8) initial set was high in 'Shamouti' on leafless inflorescences (which blossom first) but during the progress of subsequent drop tJ1e proportion of fruitlets on these leafless inflorescences decreased in favor of leafy inflorescences. :1\ ew leaves have an apparently important nutritional (9, 10, 12) or hormonal (2, 8) task in ensuring the persistence of frui tlets. ·when several flowers are borne on the same axis, complex interactions will probably occur expressing competition as to rate of flower development and chances of set. Drop of flowers, fruitlets and fntit. The different stages of abscission is the main factor responsible for the fact that the no. of persisting fruit is extremely small if compared with the no. of flowers initially produced. The amount of fruit units is so small that we rna y actually say that the whole flower population abscises at different stages except for a negligible portion eventually developing into the crop we are interested in. It is customary to define the process whereby ovaries adhere and become fruits as "fruit set." The final set is determined, however, only at the end of the abscission process. At earlier stages it is possible to speak of an initial set, though obviously its value changes daily with the progress of abscission. The percentage final set expresses the relationship between flower and fruit no., and relatively small deviations in the set percentage make all the difference between a small, an average or a large yield. Fig. 2 shows the relation between flower no. and percentage set. Flower no., plotted on a logarithmic scale, is based on data collected from 90 9-year old individual 'Shamouti' orange trees (8) budded on sour orange (Citrus aurantiurn L.) growing in heavy soil and having a large variability in flower no., perhaps because they were in the early stages of fruiting. The figure shows quite clearly that when flower no. is small, the tree attains large percentages of final set (up to 11%) and thereby compensates for the initial lack of flowers. On the other hand, when flower no. is excessive, final set percentage progressively decreases clown to 0.1 %. There must therefore be a control system, whose mechanism is not clear, but which cannot be questioned as to its importance in adjusting the level of yield to the tree's potential. Abscission may be split into several stages showing some degree of overlapping (Fig. 1). It is evident that a very large no. of units abscise during the first weeks; at this stage their individual weight is still small so that the tree has not invested so much in the building of each unit. The no. of units lost progressively decreases toward the end of the abscission period, but each unit is by now much heavier (2-5 g). \Ve also know that the abscission zone . switches from the pedicel to the calyx zone (5).P~o~(·s,h~ during the abscission period changes may a], 0 0 relu;n,,;:! sensitivity of fruits to endogenous and exogenotccur in t~ 1 and growth regulators (4) reflecting the chancring ' l:th;:.< of the fruit to abscise. vVe therefore assum~ tha.~tn~en,:: abscission stages exist although a better character·d 1 ffe:~~: these stages and their control is still wanting. llattQn ~ . Final c?mPor:ents of yield: no. and size of fruit.j ; y1eld obtamed Is actually the product of the no he fil'.,~; vested fruit by their weight. The ability of a tre ·of f1.:;1 ~ruit fluctuates w~thin a giver~ ra?ge, th~ n?. of fr~rito ~ l: mversely proportiOnal to then srze. This IS true .~ ~~ l given variety, but also holds when we compare ci';ulnn i j tivars and species producing widely different frui.t r~H tll ( . man~larins_, oranges and ~rapefruits. ~s an_ exampl~ 1 ;,M, <1 relatiOnship between frmt no. and siZe, F1g. 3 sho 1 .• f lh~·l from sizing experiments with 'vVilking' mandarin (~(ht, nobilis Lour. x Citrus deliciosa Tenore), 'Shamouti' 0 :ltr.,, t and 'Marsh' seedless grapefruit. That data from 3 (!i[t_ng, species should fit naturally onto a line expressing· 111 trrn: 1; · 1· l'b f' d ·1 ere4 tiv~ re_ atw.nsllbiP etweben nut no. an we_I~ 1t is not rtt·c~; ., san y 1mp11e( y t11e a ove genera 1 proposJtwns 'Shar · ·. , , l . ' II<)Uil and Marsh usually possess a remarkab e amount of . , 1 contr~l maintaining the fi.nal_ y~el~ within ~~ narrow ras; ;:. 1 I'" of frmt no. per tree. The vVllkmg mandann on the <l} .1 1 . ule ,., hand, presents ~ st:ong tenuency to a ternat10n as do manv of th~ mandann-hke ~rees, and the dat~ reported reflect . ,~ cher~ucal or agr~techmcal treatments wh1ch reduced fruit no. m a very radical way (2). We do not know about sitU:!· I· · tions in grapefruit or oranges where a reduction in fruir no ·' 1 can cause a two- or three-fold increase in fruit weight. In .,: , the ''Wilking' variety (as in other mandarin-like fruits) ron. '.: trol of fruit no. is rather loose and as a compensation, tht j ~ .. tree is able to vary fruit size within a remarkable ranw·. , '\ A more detailed perusal of 'Wilking's data (Fig. ·1) sh0111 .m that it is possible to subdivide the curve into two portions: a decrease in fruit no. from 3000 to 1300 does not camn .'.~i significant increase in average wt., while additional thinning .. 11 • markedly affects size. The danger of reserve cxhall\tum. i· I 1 l': !. ;( ! 103 r--___, ----,-t f ~ l 0 Pc_ ~ MARSH SEEDLESS ~ GRAPEFRUIT n 0 f- I 6 <C) w 5 flft 6 m f#;4::o. '!1', SHAMOUTI ORANGE l '•)1\1 i "1! l II\( 0 f-- 0 ~102~ lL . ....J I <l: ::J 0 0 o 00 0 0 O 0 0 0 0 Q) 0 WILKING MANDARIN ~0 0 > 0 I, I I :1'11 'H.\'~ ::1)d ·::.\11 'idt z '1\'. f- ~ 4- 101 L___ _--L___ )J2 10 4 NO. OF FLOWERS I TREE Fig. 2. The relationship bemeen no. of ftowersjtree (logarithmic scale) and final percentage of fruit set in 90 'Shamouti' orange x Sour orange trees. (From (8)). 670 _L__L__.L_L_L_LLL 103 NO. OF FRUITS/TREE L , _ __l_ .. r L-"-~ n ,, ; , •.. 11 off.n~:r~: I Fig. 3. Weight of indiYidual fruit plotted against the no. ''•!! 1 (logarithmic 'cale on Loth axes). Data from experiment; pcrfoi':1; 1fl'lll'l) -'"·" each \·ariet\' separatelY. Each point represents an average froiii ' ·1r : of a single tree. (From (2) and unpublished data). • hoc. 1n t. Soc. C;, t icu lMe, !97 ;, rol. ,~, .,, ,--I 0 0 0 0 Oo 0 0 0 oo 0 0 00 ao00 0 r:P /}J 0 00 0 0 0 l.--~~~~--~~~~~~ 1000 2000 3000 the developing fruit. The decision about the partitioning of flow, the actual control of its channeling towards different aims, is crucial with regard to levels of yield. Distributing assimilates between different processes seems even more intricate in a perennial, where distances between organs are large and there is a need to provide for life continuation in following years. The dashed line linking control of partitioning and energy converter represents the reversed control (feed back) on energy conversion. An increased demand for assimilates may accelerate their production by leaves and cause a better expression of potential production by a tree. On the other hand a lack of suitable utilization may congest production centers of leaves. The same clashed line also hints as to possibilities of biochemical and hormonal controls of a more refined type. PRODUCT ENVIRONMENT NO. OF FRUITS/TREE . r'll· 4. \\'ciglu ol individual 'Wilkin&' mandarin fruit plotted.against •, ~t of fruit/ tree. Data fran~ a thmn.mg expenment. Each pomt rep- \ an average !rom all frmts of a smgle tree (2). _____ / FEED BACK alternate bearing in these varieties, probably lies in of excessive fruit no. The tree is apparently unrecluce fruit size, perhaps because of genetic above data deal with the average size of fruit; inclisizcs, however, vary within relatively large limits. behind this variability probably lie in comsource-sink relationships, governed by efficiency of and rei a tive positions of leaves and fruit. Fruit no. vs. size is undoubtedly one of the important ts which will need to be considered when developmodel. It will also be necessary to pay special attention economic importance of size, one of the main paramdctcrmining fruit acceptance by consumers. Diagramming Citrus Fruiting A host of factors has a direct or indirect bearing on yield ritrus trees. A full enumeration should include environ. macro- and micro-climatical elements, water avail' nutrients and physiological data specific for cultivar JTI 0 WILKING MANDARIN ~0 of non-irrigated crops (as natural pasture, winter etc.) place a considerable emphasis on physical com. ls o[ environment and especially on water availability, influence is extremely important under the growing of these plants. agriculture, as found in orchards of our area, a suitable supply of water and nutrients permitting fair expression of their potential productivity. A a citrus orchard is obviously dealing with a well irrigated crop, though there are still ample possi, . tv improve orchard care. At this preliminary stage, r~ 1h~l! presuppose in our discussion normal care as sup~~Ill an average commercial orchard. fhe simplest way to describe in universal terms the of yield production by means of a diagram as used engineering, is depicted in Fig. 5. Environfactors including carbon dioxide, water and solar participate in the photosynthetic process occurring the gTeen leaf. The solar energy is converted into the building blocks of production. Assimilates their different goals: i.e. to build a framework of roots, leaves, new growth, spare buds, flowers and Int. Sor. Citriculture, 1977, Vol. 2. Fig. 5. A simple flow diagram demonstrating major processes (circles) involved in crop production in an agricultural plant. The application of the above principles to the production system of a citrus tree is detailed in Fig. 6. Here each rectangle stands for a different type of organ, the arrows denote organ development, wide-stemmed arrows express direction of assimilate How, the dashed lines stand for possibilities of control and the springs depict interactions between organs. The horizontal axis describes the time dimension and the continuity of processes within such dimension. The scheme starts with the initiation of flower bud one year and ends with the bud due to initiate flowers next year, and includes all the set of processes pertaining to the formation of yield. Three different types of shoot develop from the flushing bud in close competition: leaves only grow from the vegetative flush, while fruits will develop from the other two. The task of leaves is to provide assimilates (building blocks, energy and regulators) to the developing fruit as well as to other tissues. The importance of leaves becomes evident during set when competition is most severe. The chances of setting are better for a fruitlet borne on a mixed ORGAN DEVELOPMENT - - - 7 FLCJW OF ASSIMILATESZ'LZ$ CONTROL ~ COMPETITION :; _ _ .1 INDUCTION----"> FLUSH 1975 ----'>-~~Jp---~G~~~~H------">- INDUCTION 1977 Fig. 6..-\ flow diagram representing the overall processes involYed in fruiting of citrus trees and the relations between different tree organs during the period between flower bud induction in one year and flower bud induction in the following year. 671 shoot as the proximity of leaves has an important nutritive and/or regulative influence in this connection (8, 9, 10, 12). On the other hand proximity of many fruitlets may have a negative value because of competition. At the stages of fruit development, the scheme is simpler. I\' ow assimilates are partitioned mainly between developing fruits and reserve pools at different tree places, if we disregard the time when summer flush is growing (by far a smaller no. of new vegetative shoots than in spring). The nature of the regulatory systems involved is still greatly unknown. On one hand, there may be some sinksource control from developing fruits to photosynthesizing leaves, as noted above. On the other hand, all organs may exercise nutritional and/ or hormonal control on the fate of buds due to flower in the following season. Points of Decision and Control Mechanisms In conclusion we may assume the existence of 3 points of decision where the fate of yield would be settled. The first is at the level of flower formation. An absolutely negative decision at this point, completely precludes yield production in the present year. On the other hand profuse production of flowers is not sufficient to ensure a high level of productivity in certain cultivars. The alternative between abscission or set, which is a continuous one during a number of weeks, is the second crucial decision period. At the end of complex and elaborate abscission processes the final size of fruit population will be settled. By means of abscission control the tree may correct up to a certain level the first decision: if the no. of flowers is too small abscission can be inhibited, so that yield will still be fair. The third piece of decision-making concerns the final size of the average individual fruit also emphasizing the importance of quantitative differences in the partition of reserves between individuals within the fruit population. If fruits are relatively few they may grow more, if they are numerous they grow less, while the product of no. x wt of fruits tends to be constant within reasonable limits. In the present paper we endeavor to set the basis for a quantitative analysis of_ yield comp<_>nents ~n _a commercial citrus grove. In so clomg we utilized ex1stmg data and knowledge. There are still several links requiring additional logical and experimental elaboration. One of these is the relative importance of photosynthetic production and of hormonal regulation in controlling yield levels. The question of regulative systems in a perennial plant, extends beyond the limits of one particular year and includes accumulative effects of reserve pools and age effects which are unknown to the annual plant. In future, attempts will be made to deve]o fruiting model into a quantitative tool which P__ the ized in the ~liagnosis and cure of specific low '~ ll! bt 110~ problems. \\ e feel confident that an overall ul~tOduqi~~ of ways to control and increase citrus tree proclu t~t~lltii~-even tuall y ensue. CtJ\·n; ._,; I Literature Cited I. Chalmers, D. J. and B. \"an der Ende. 1975. Proc!uct' . : trees: factor affecting dry weight distribution durin~' It:· ''f ;.,.,~Ann Bot. 39:423-432. tree~'-""< 2. Galliani, S., S. P. Monselise and R. Goren. 19/j_ hn ,,. size and breaking alternate bearing in '\Vilking' 1 ~' 01 '1li: ~"-! ethephon _and ~t~er agents. H or/Science 10:_68. n.rn<fatilll ;, 3. Goldschmrdt, E. E. and S. P. l\Ionselrse. !9,2. Honnon·I . flowering in citrus trees and in other woody perennial;' 10 ~;:')( ,, In Plant Growth Substances 1970. D. J. Carr, ed., Spri;, P: ••_-1-~ ; ,.;: 1 Berlin. gn '<lilt i .~i 4. Greenberg, J., R. Goren and J. Riov. 1975. The role f -~ .and polygalacturonase in abscission of young and matnre\ ctllu)o 1lanu"'-. orange fruits. Pilysiol. Plant. 34:1-7. _Iii ! j . . 110 n o: 5. Bar-Even, E. and S. P. Monselise. 1959 .. \ study on th 1 'Shamouti' oranges from set to harvest. Pror. l·ift/, lilt/ '"'Pff.l terp Mediterr. Citriculture, Catania, p. 181-187. ·"'· C""f ~ 0 G. Holt, D. A., R. J. Brrla, G. E. Miles, M. M. Schreiber a I)\ -~i ;)!!n Peart. 1975. Environmental physiology, modeling anti ,;~Ill ·It OC'ur, of alfalfa gr_owth. I. Concep}ual development of SIME!l. p~~ lll varl Unm., A gr. Sta. Res. Bull. 90,, 26 p. • ., ofrUS 7. K hairi, 1\f. M. Ah. and A. E. _Haii. I'l76. T~tn[JeLtture and hrnnidito lorex 11 e eels on net p otosynt 11esrs an< 1 transptratwn of citrus. Pf,.,,.; P_lant. 36:29-34,- _ . <an 1 . 8. Kolodner, Y. E., I-.. E. Goldschnudl and S. P. Monsdise. 1971 i ttpar. 1 fluence of number of flowers and inflorescence type on yiehl ,1 ~n-ex 'Shamouti' orange. (Hebrew) Alan Hmwtea 2R:!i·10-G·l~. i HPLC 9. Lcnz, F. 1966. Flower and fr~it developmenL in 'Valencia' hll! d 5 orange as affected by type of rnflorescence and nutritional , ~~~, 1 10 I Hurt. Res. 6:65-78. hon r 10. ~~~~~~, P. E._ Kriedemann and H. J. Daunicht. l!ln_ Elf<ttl(l! 111ily roots and emergmg axrllary shoots on photosynthesis hdwi.,r Ill. ~e<tor citrus cuttings. Angew. Bot. 46:2~7-231. . I ibsorl 11. Monselise, S. P. and Lotte Heymann Herschberg. l'l'i:l. lnllurncel!(l It exposure and age on dry matter content, area and mineral rum. ms position of 'Shamouti' orange leaves. Proc. Awer. Soc. 1/ml. S<i. lure, • 62:67-74. A' 12. Moss, G. I., B. T. Steer and P. E. Kriedemann. 1'172. The rcgnl•r.tq quJie 1 role of inllorescence leaves in fruit setting by sweet orange (Ci:;.; i HPLC sinensis). Physiol. Plant. 27:432-438. l· 13. Omer, A. 1969. Internal and external factors alkctin~ ahsri"i<>nolf iriph 'Shamouti' orange leaves. (Hebrew). M.Sc. Thesis, l!ehrew t:ni•. !liner of Jerusalem, Rehovot, 57 p. I !elect 1-l. l'romnitz, L. C. 1975. A photosynthate allocation model for tr~' lows r;row_th. Phot_usynthetica 9_(1):1-15: . , I tissue 15. Shavtt, A. 19~6. An mvesttgatron mto the process of flower ami fllllll abscission in the 'Shamouti' orange. Bull. Res. Couucil hmrl lrt txam 189-199. , 1nd g 16. Stapleton, H. N., D. R. Buxton, F. L. Watson, D . .J. Nolting •1111 1111ay D. :-1. Baker. 1975. Cotton: a computer simulation ol rotton h'Tn•tb.. U. of Ariz., Agr. Exp. Sta. Tech. Bull. 206, 124 p. . 17. Turrell, F. l\I. 1973. Appendix III, Frost Protection. !11 Reutht·r, II \I ed. The Citrus Industry, Vol. 3, p. 506-520. <1d1 ,_,I i:! 111 I j 1 :.:nil { ::!lot I Ull) ·:l.h :an !',1n .. ,,; ·:·.:o ;:.tpl : th ·!'io :lrll) 672 . . -- roL! Proc.lnt. Soc. C1trzculturc, 1911,