SUGAR AND ACID METABOLISM IN CITRUS FRUIT 1 Karen E. Koch Two important horticultural questions in this area are: 1. What affects sugar levels in citrus fruit? 2. What affects acid levels in citrus fruit? Sugar Levels- Photosynthetic activity in ~arby leaves has long been thought to influence levels of soluble so~ds in fruit, and these so~ds are predominantly sugars. Classic work by Sites am Reitz (Fig. 1) showd highest solids were in fruit picked from the outside of the tree on the southeast, south, and west exposures; positions where light intensity am photosynthetic activity would likely have been greatest. N 1)1 s high low Fta. 1. Effect of d1rect10n of 8unl1aht expo8ure aim! 8had1ns on total 801ub1e 8o11d8 (%) of'va1enc1a' Or&ns.8 bar 88ted fr08 port1od8 of the tree 8hown here in cro.. Mct1on. (redom fr08 S1tes aim! leitz, 1949) A still closer tie has recently been foom between photosynthesis in single leaves x-.ar fruit, am sugars translocated into fruit sections vertically aligx-.d with them. The fruit quarter positioned as shown in Figure 2 usually receives over 90% of the photosynthetic material exported from the nearby leaf. Further examination of imividual juice segments has shown that most of these photosynthates fro. a single leaf move into only one or two of the segments. Similar patterns of sugar distribution occur from an imividual leaf even if it is farther away from the fruit or if all other nearby leaves are removed. 1 Assistant Professor, GaiMsvi.l1e, FL 32611. Fruit Crops Depart1Rent, lias, University of Florida, 59
r1l' 2. Label.d product. of pbotoeynth..1. 8OV8 fra l..v.. l8ar fruit 1oto 88ctio- Yerticelly al1l_d with tb_. Greater than 90% of photo-.yuthat.. froa a J1v.n leaf viii tyl'ically be foo- in the 1/4 fruit labe1ad A in thi. f1lure. Tre_port into 1_ividual juice 88ct1o- 1..t1l1.ore.pec1f1c. (troa l.odi, 1984.) Sugar movement into citrus fruit is strikingly affected by the unusual distribution of vascular tissues in the fruit. The peel has a dense ~twork of vascular bumles am receives photosynthetic materials fr~ leaves relatively rapidly (about 3 to 6 hours). Citrus juice tissues, however, differ froa the juice tissue of alldost all other crop species in haviag ~ vascular strams. Three vascular bumles outside each segaent must therefore provide.ill the solids to the juice vesicles inside (Fig. 3). Dorsal vascular bundles are the main path of sugar movement, with some photosynthetic products also entering by septal bundles. Central.traMs supply only seeds. Ptl. J. three.e.clllar b.&1id1.. aupp1y pbot08yatbetic prllduct. (prt- _rt1y auaan) to all juic8 ti.8u8. id8ide a liven.er- _at. C1trv. fruit dlff.r f~ all other alro_lc crop. In be.1.. web exted81v8 DOo-...CIIlar anu. Bach juice...1el. 1. joided to the.ea- -at ep1ami. at a.1te Deer a.a.clllar b.&1id1e by a bel r- like...iel..talk. Figure 3 also shows that each juice vesicle is attached to a point on the inside of the segment epiderais just opposite 0" of the three vascular bundles. The base of.ost juice vesicles il a minute, hair-like strand up to 2 C8 long (about 3/4 inch). Sugar aove.nt through this zo~ is apparently not facilitated by any vascular tissuel or other specialized structures. Photosynthetic materials froa leaves.ove through these vesicle stalks and sepent epidemi8 so slowly. that complete transfer may take 3 to 5 day.. The sugars appear to reach the fruit within le88 than 1 day, but redistribution into juice tissues requires an extended tiae. The primary sugar transported fr~ leaves to froit8 of citrus trees is sucrose (c~on table sugar). which can be broken down into two other smaller sugars. glucoae and fructose (ltg. 4). Even though sucrose is also the..in sugar in citrus juice, it is apparently broken down and resynthesized as it moves into the froit. A possible reason for this could be to maintain a -down-hill- concentration gradient frc8 leaves to points where sugars are -unloaded- frc8 the vascular tissues in the fruit. Sucrose could then contirl.1e to move into the fruit even though very high 1ew18 are accu8ulati~ in the juice vesicles. 60
F1g.5. Effect of direct10n of sunlight expo.ure and shadina on fruit qu8lity of 'Valencia' oranle. harvested fro. portion. of the tree.hown here in cros. section. A. Soluble solids, B. Acid (%), and C. J.tio of.oluble.olid/acid. A.i N 8 B. c. high low 61
SUCROSE c -0 c-c ~ -~.Ac -- ~ c \ JI/, c c ~ IL o -l\c-c/l GLUCOSE 1- FRUCTOSE c Pil.4. Sucro.e (table 8ular) 1. the prtaary sular transported in citru. trees and.tored in fruit, but can be broken d~ into Iluco.e and fracto... A.i.ilar breakdown of sucrose (but with~t resynthesis) has been loom to occur in peel of grapefruit after exposure to cold. A resistance to later chilling injury then develops. Phot08ynthe8is in green citrus peel may be iaportant in reducing losses of respiratory CO frcd fruit. As much as OM third or more of the sugars moving fram leave8 into fruit during a liven day could be lost to the ataosphere after conversion to CO2 duri~ respiration. In the ligqt, ho~ver, s~ of this CO-,- is -trapped- in the peel by photosynthetic processes add recycled back to the fhdt interior as sugars add/or organic acids. The aaount of recycling will vary during fruit developaent because respiration occurs very rapidly during early growth, am peel photosynthesis decreases with color change. Acid Levels - Levels of organic acids in citrus juice (mostly citric acid) differ dependiogon the p08ition of fruit on the tree. Figure 5 (previous page).hows that total acidity, like total soluble.olide i8 highest in fruit on the out.ide of the tree picked from southea8t,.outh, and west exposure8. The ratio of.oluble.olide to acids was therefore found by Site. add Reitz (also shown in Fig. 5) to be highest in the northea8t quadrant of the tree, even though the total solids in these fruit wre low.r. --_8 OCI8 _no -~-" IV s..- "'e".-- 'il.6. Diatr1butioft of total acidity (Oft rilbt) and ratio of 801ubl. aolida/acida (on left) io edible portion of 'Va1eecia' oranle. (redone froa Tinl. 1969) Total acidity within an ora~e can vary dramatically fr~ 0.. part of the juice tissues to another. am the right side of Figure 6 shows the most acid part of Valencia ora~e pulp is in its center. The resulti~ soluble solids/acid ratio (left side of Figure 6) is also lowest in the middle of the fruit. 62
Levels of organic acids have long been known to be reduced in grapefruit (aoo other citrus) by application of arsenic-based c~poum.. Results are often evident as early as Augu8t am the change in 80luble 80lids/acid ratio is due specifically to effects on acidity. HUMID TROPICS W ARM NIGHTS. low auga,. high acid ARID DESERT COOL NIGHTS HIGH RAINFALL./110/1 8UO.' LOW RAINFALL..cl~ droo8 '.Oldly wlt/l ~.v.'oom.nt rl" 7. Loaa-tera cll..tlc effect. ou.uaar aud.cid iu citrua fruit. Lons-term climatic effects on sugar ai i add levels are shown in Figure 7. Additional infotmation bas begun to appear regarding short-term effects of enviroaaental coaditions, but reports are conflicting.. 1. SYNTHESIS ~--r 3. COMPARTMENTA~IZATION (In vacuol..> 2. UTILIZATION 1il' 8. Thr.. proc which cad iaflu.nc. the ext.nt of acid accu8ulat1od in citru8 fruit. All three of; the processes shown in Figure 8 8USt be considered; utilization, and coapartaentalization. synthesis Evidence suggests that at lea8t s~of the organic acids in citrus juice may be synthesized A) in leaves aoo translocated to fruit, B) in peel aoo moved inward to pulp, aoo C) inside juice vesicles. Organic acid8 fr<8 leaves were initially d18coonted a8 a aajor soorce of citru8 fruit acidity years ago after an experiment show.d that fruit of sweet and soor le_on Won still "SWoet" or "soor" if grafted am grown on leafy branche8 of the opp08ite type. Still, 8<8ewbat less draaatic effects of leaves on fruit acidity may still be i_portant. In grapes, for example, at least o~ of the aajor type8 of organic acids in the fruit (tartaric acid) has been foooo to be synthe8ized in 1eave8 add transported to fruit. Acid levels in citrus leaves are also extreaely high, even -ore so than the fruit. 63
~ CO2 PEP VEPCa'bOXy,aao> OAA 1 malic acid Fig. 9. The enzyae PEP earboxyla.e begins a serie. of reaction. l.ading to for8ation of organie acid. fro. CO2' 1 citric acid The possibility of organic acid transport from peel to pulp is also suggested by several observations. The first of these is that the arsenic treatments noted above affect juice acidity without the arsenic ever reaching the pulp. Only leaves and peel retain levels of this compound that can be readily measured. Known effects of arsenate on metabolic pathways may therefore be occurring only in peel and leaves. In addition, transport of labeled materials frm peel to pulp has been measured, and high levels of organic acids occur in peel. Lastly, an enzyme known to synthesize organic acids (PEP carboxylase) is present in peel tissues (see Fig. 9). Juice vesicles may produce their own organic. acids, either by _tabolizing sugars, or through activity of the enzyme shown in Figure 9. Greater activity of this enzyme occcurs in peel, but juice vesicles have an extensive capacity to take up CO2 (presumably as shown in Fig. 9). ff- 11'001,... malic ",~it~~w;' (01"8'. ".,n.h..a..).old...,;, e f. CO2iifP:ep _""~HJt- ".""'1- aconitic acid lil'tic acid cycle',~ ' Pil. 10. The fir.t.te, in citric acid utilization 1. the enay8e aconita...hovn h.re with.n X. Act iyi ty of thi. enz,.. appear. to influence le.el. of citric acid wher. exaained thu. far. Rates of citric acid utilization could ultimately affect levels of this acid which accwaulate. aoo the enzyme aconitase is primarily responsible for its direct breakdown of citrate in.ost tissues (Fig. 10 on previous page). Research in Japan on sweet aoo sour maooarins aoo in Florida on low-acid grapefruit sutants (developed by C. J. Hearn. U.S.D.A. Orlaooo) has iooicated that activity of this enzyme of the citric acid cycle may be low in fruit where the most acid accumulates. 64
Co8part8entalization of organic acids can prevent their utilization. This "8torage" occurs when acids are transported across a me.brat-. into the central vacuole of a cell. Vacuoles are known to be the site of acid storage and occupy over 90% of the volume in most cells. Investigations into properties of the vacuolar.mbrat-. in citrus are just beginning, but may clarify part of the basis for fruit acidity. The ultimate goal of current research at the University of Florida on sugar/acid levels in citrus fruit is to provide alternative means of controlling this ratio. 65
References 1. 2. 3. 4. 5. 6. 7. 8. Erickson, L. C. 1968. The gemral physiology of citrus. pp. 86-127. In: W. Reuther, L. D. Batchelor, H. J. Webber (eds.). The Citrus rndustry: Vol. II. University of California Press. Riverside, CA. Koch, K. E. 1984a. Translocation of photosynthetic products from source leaves to aligned juice segments in citrus fruit. HortScience 19:260-261. Koch. K. E. 1984b. The path of photosynthate translocation into citrus fruit Plant. Cell and Euvirooment. 7:647-653. Koch, K. E. add W. T. Avig~. 1984. Localized photosynthate deposition in citrus fruit segments relative to source-leaf position. Plant and Cell Physiology. 25:859-866. Purvis, A. C. and W. Grierson. 1982. Accuaulation of reducing sugar am resistance of grapefruit peel to chilling injury as related to winter temperatures. J. Amer. Soc. Hort. Sci. 107(1):139-142. Sinclair, W. B. citrus fruits. 1984. The biochemistry and physiology of the lemon and other University of California Press, Riverside, CA. Sites, J. W. am H. J. Reitz. 1949. The variation in imividual Valencia oranges fro. different locations of the tree as a guide to sampling and spot-picking for quality. Part I. Soluble solids in the juice.. Proc. Amer. Soc. Hort. Sci. 54:1-10. Sites,.1. w. am H..1. Reitz. 1950. The variation in individual Valencia oranges from different locations of the tree as a guide to sampling and spot-picking for quality. Part I. Titratable acid and soluble solids! titratable acid ratio of the juice. Proc. Amer. Soc. Hort. Sci. 55:73-80. 9. Ting, S. V. Distribution of edible portion of citrus fruits. soluble ccdpomnts am quality factors in the J. Amer. Soc..Hort. Scl. 94:515-519. 66