EFFECT OF TRUNK GIRDLING ON THENUMBER OF FRUIT RETAINED BYBEARING HADEN MANGO TREES

The trunks of Haden mango trees were girdled at monthly intervals from mid-April (four monthsbefore panicle emergence) until the fourth week of October (two months after panicle emer-gence), and the fruit retained per tree after the period of fruit-drop was counted. A numberof non-girdled trees served as controls. Girdling before or at the time of panicle emergenceincreased fruit retention, whereas girdling one or two months after panicle emergence hadno apparent effect. Lateral shoots developed from beneath the girdles a number of weeksafter girdling. An improved method of top-working is suggested in view of this responseand the fact that girdling one or two months before panicle emergence was most effectivein increasing fruit retention.

INTRODUCTIONHaden mango trees bear shyly despitean adequate flowering intensity. Pooryields of Haden trees grown in theeastern Transvaal havebeen ascribed toinadequate pollination and fertilisation,in addition to the negative effect lownight temperatures have on theseevents (De Wet and Robbertse, 1986;Robbertse et aI, 1988).

Healthy and well irrigated mango treesin the eastern Transvaal generally setlarge numbers of fruit directly afterflowering. The number of fruit retaineduntil harvest is determined by the num-ber that subsequently drop as opposedto the number of fruit that are initiallyset. Drop mainly occurs when the fruitare 5 to 25 mm in diameter, and in thecase of most of the mango cultivars cur-rently exported, an average of less thanone fruit eventually remains per panicle.

Chacko (1984) concluded from investi-gations made of the pattern of move-ment and distribution of 14C-photosyn-thates in mango at various stages offlowering and fruit development, thatthe immediate and most direct reasonfor the heavy drop of fruitlets, is thecompetition between them for a limitedsupply of photoassimilates, as well ascompetition from developing shoots forthese substances. Girdling generallycauses an accumulation of carbohy-drates abovethe girdle and a diminutionof carbohydrates in the root system(Fahmy, 1952; Noel, 1970; Wallerstein, etaI, 1974; Wallerstein et aI, 1978; Roperand Williams, 1989). It would thereforebe expected that the number of fruit re-tained by bearing mango trees would beincreased by girdling before fruitlets be-gin to drop.

The present study was performed toascertain whether girdling the trunks ofbearing Haden mango trees increasesthe number of fruit retained by the trees,and to determine if the time of girdling,

relative to that of flowering, is of sig-nificance concerning the effect of gird-ling.

MATERIALS AND METHODS

In early April, 1990, 40 four-year-oldHaden mango trees on Sabre (rootstock)were selected for uniformity of size ina commercial orchard in the LetsiteleValley in the north-eastern Transvaal(grower: M Amm). The trunks of five·trees were girdled every month fromApril 17 until October 23. The girdleswere made 18 cm above the graft unionwith a girdling knife which removed a5 mm-wide strip of bark. Immediately af-ter girdling, the wounds were coveredwith a sealer (Bacseal@). Single treesserved as plots in a completely ran-domised design. Five non-girdled treeswere incorporated as controls.

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To effect uniform flowering, the termi-nal shoots of each tree were topped dur-ing the first week of July (July 4 and 6).Heading cuts were made 30 to 50 mmdeep to remove the terminal whorl ofleaves. This procedure additionallyresulted in the removal of any develop-ing panicles present as a result of ini-tial bud-break, which commenced inmid-June.

On November29,once fruit-drop was nolonger occurring, the fruit retained pertree was counted.RESULTS AND DISCUSSION

Panicles began to re-emergeon August21. Girdling at the time of panicle emer-gence or before increased the numberof fruit retained by the trees (Figure 1).Girdling in mid-July or mid-June, one ortwo months before panicle emergence,Panicle Emergence

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Date of GirdlingAverage number of fruit retained by Haden mangotrees girdled monthly from mid-April, four monthsbefore panicle emergence, until the fourth week ofOctober, two months after panicle emergence (ver-tical bar: ± SE).

was most effective in improving set, thenumber of fruit retained per tree being192% greater, on average, than that ofthe control trees. When trees weregirdled in mid-Mayor mid-April, three orfour months before panicle emergence,set was improved by an average of125%. Girdling at the time of panicleemergence increased set by an averageof 66%, whereas girdling in mid-Sep-tember or during the fourth week of Oc-tober, one or two months after panicleemergence, had no apparent effect onthe number of fruit retained.

The sizable increase in fruit retention inresponse to girdling in mid-winter, indi-cates that the poor cropping ability ofHaden mango trees grown locally is notsolely due to inadequate pollination andfertilisation. That girdling increased thenumber of fruit retained when per-formed at or before, as opposed to af-ter panicle emergence during the periodof initial set and drop, supports the viewthat the supply of photoassimilates af-ter flowering plays an important role.This is stated in light of the expectationthat time would be required for carbo-hydrates to accumulate above thegirdle.

Following girdling, others found endo-genous increases in the levels of auxinand gibberellin and reductions in thelevel of cytokinin above the girdle,and in each case, the opposite effect onthe hormone level in tissues below thegirdle (Stoltz and Hess, 1966; Wilson,1968; Goren et aI, 1971; Wallerstein, etaI, 1973; Van Staden and Brown, 1978;Wallerstein et aI, 1978; Grierson, et aI,1982; Dan et aI, 1984). Increases in fruitretention following pre-bloom sprays ofauxin and gibberellin have been report-ed (Baghel, et aI, 1987; Mullins, 1985).Hence, the possibility of consequentialhormonal changes being partly or whol-ly responsible for the improvement infruit retention cannot be disreQarded.Lateral shoots began to develop frombeneath the girdle three to six weeks af-ter girdling (Figure 2). Auxin is involvedin maintaining apical dominance inplants, and is synthesised in apicalmeristems from where it is transportedthrough the phloem (Wilkins, 1969).Therelease of buds beneath the girdle fromapical dominance is thus in accordancewith the proposal that the accumulationof auxin above and its reduction belowthe girdle is due to interruption of itsbasipetal movement in the phloem (Wil-son, 1968; Noel, 1970).

Mango trees that yield poorly or producefruit having undesired characteristicsare often top-worked to higher yieldingor desired varieties. Trees are normally

Fig 2 Lateral shoot development frombeneath the girdle.

sawed-off just above the junction ofprimary scaffold branches with thetrunk, and the desired scion grafted onnew shoots that subsequently growfrom beneath the cuts. Exposure ofgrafts to direct sunlight often causesthem to die-back as a result of excessivemoisture loss.

The lateral shoots that grew from be-neath the girdle were noted to be suit-able for the purpose of grafting, their oc-currence indicating an attractive alter-native to conventional top-working. Bygirdling primary scaffold branches inmid-winter, grafting the desired sciononto lateral shoots that subsequentlydevelop from beneath the girdles, andremoving the original canopy at thesites of the gi rdles after harvest andonce the grafts have taken, the appar-ent benefits are an additional and elevat-ed crop, and shading of the newly madegrafts by the tree canopy.

REFERENCES

BAGHEL, B S, R K SHARMA, AND P K RNAIR, 1987. Influence of prefloweringspray of urea and NAA on fruit retentionof mango (Mangifera indica L). Prog Hart,19, 200-202.

CHACKO, E K, 1984. Physiology of vegetativeand reproductive growth in mango (Man-gifera indica L) trees. Proc First Aust Man-go Res Workshop, CSIRO, Melbourne, pp54-70.

DAN, I R, R A WILDES, AND D J CHALMERS,1984. Effects of limb girdling on growthand development of competing fruit andvegetative tissues of peach trees. Aust JPlant Physiol, 11, 49-58.

DE WET, E AND P J ROBBERTSE, 1986. Apreliminary study of the pollen of Man-gifera indica Lev Haden in South Africa.S Atr J Plant and Soil, 3, 87-89.

FAHMY, I, 1952. Grafting studies on macada-mia and sapodilla in relation to carbohy-drates, using pre-girdled scions. ProcFlorida State Hart Sac, 65, 190-192.

GRIERSON, W, J SOULE AND K KAZUHIDE,1982. Physiological stress. Hart Rev, 4,251-252.

GOREN, R, E E GOLDSCHMIDT AND S PMONSELlSE, 1971. Hormone balance inbark and leaves of Shamouti orange treesCitrus sinensis (L) Osbeck in relation toringing. J Hart Sci, 46, 443-451.

MULLINS, P D F, 1985. Delaying of floweringin Haden mango trees. HorticulturalScience, 2, 6-8.

NOEL, A R A, 1970.The girdled tree. Bot Rev,36, 162-195.

ROBBERTSE, P J, E DE WET AND L ACOETSER, 1988. The influence of temper-ature and boron on pollen tube growth inmango. SA Mango Growers' Assoc. Year-book, 8, 4-6.

ROPER, T P AND L E WILLIAMS, 1989. NetCO2 assimilation and carbohydrate parti-tioning of grapevine leaves in response totrunk girdling and gibberellic acid appli-cation. Plant Physiol, 89, 1136-1140.

STOLTZ, L P AND C E HESS, 1966. The ef-fect of girdling upon root initiation: Aux-in and rooting co-factors. Arner Sac HartSci, 89, 744-751.

VAN STADEN, J AND N A C BROWN, 1978.Changes in the endogenous cytokinins ofbark and buds of Salix babylonica as aresult of stem girdling. Physiol Planta, 43,148-153.

WALLERSTEIN, I, R GOREN AND Y BEN-TAL, 1978. Effect of ringing on root star-vation in sour orange seedlings. J HartSci, 53, 109-113.

WALLERSTEIN, I, R GOREN AND S PMONSELlSE, 1973. Seasonal changes ingibberellin-like substances of Shamoutiorange [Citrus sinensis (L) Osbeckl treesin relation to ringing. J Hart Sci, 48, 75-82.

WALLERSTEIN, I, R GOREN AND S PMONSELlSE, 1974. The effect of girdlingon starch accumulation in sour orangeseedlings. Can J Bot, 52, 935-937.

WILKINS, M 8,1969. The physiology of plantgrowth and development. McGraw-Hili,New York.

WILSON, B F, 1968. Effect of girdling on cam-bial activity of white pine. Can J Bot, 46,141-146.

ACKNOWLEDGEMENTS

Thanks are due to Mike Amm for mak·ing trees available for this study.